Wheel Theory for Jobst
Glen
> > I invite commentary.
> Brian wrote:
> This is almost laughable, Jobst. No thank you. I realize that I've hugged
> a cactus and I'm not going to do it again. I'll discuss this with other
> engineers who I know will have the courtesy to give me an understandable
> explanation, rather than a knee-jerk defensive reaction. I have no problem
> being told I'm wrong, or accepting it, I'd just like to come away with a
> better understanding, rather than the feeling I've irritated "the master"
> by having the audacity to ask questions about his work.
>
> I always thought that the very essence of science was to question what you
> see, hear and read, in the hope that you might learn something in the
> process. Well, I did learn something, but it wasn't about bicycle wheels.
>
> --
> Regards
>
> Brian
Brian,
Having had the same experience as you not long ago, I can tell you that
Mark Antonavich?(sorry, forgot the spelling) is patient, polite, and
will couch the explaination in lay terms. I was eventually able to
visualize the concept with the help of the models he presented in the
non-offensive method common to good teachers. Unfortunately, there is
not necessarily a correlation between great knowledge and having the
skills to communicate that knowledge to others.
I've made my living with science for the last 26 years and have only
rarely encountered a personality like Jobst's. I can assure you it is
not a pre-requisite for competence in any field.
Regards, G. Byrns
bnys...@bit-net.com
I recently re-read The Bicycle Wheel (third edition). While I understand the
concepts of wheel construction, I still have a bit of a problem with the the
statement "The wheel stands on its spokes". After banging my head on the wall
for a while trying to understand how a spoke in tension can exert a
compressive force, I have come up with a model that seems equally plausible
and is, if nothing else, more intuitive. Perhaps it's just another way of
saying the same thing, or perhaps it's wrong. However, if it's correct, I
think it's easier for the average non-engineer to grasp.
Please pardon the excessive detail, but I'm trying to make sure of my logic
and make it easier for others who read this to follow.
UNDERLYING FACTS
A) A rim is essentially a circular spring.
B) The stiffness of a spring of a given cross section is proportional to its
length.
THE ACTIONS/REACTIONS OF A RIM ALONE
For the purposes of this model, we will assume that the rim in question is not
stiff enough to support the weight of the rider by itself.
If a force is applied to a rim at the top and bottom, the rim compresses
vertically and expands horizontally. The amount of vertical compression is
equal to the amount of horizontal expansion at at points 90 degees from the
axis of the applied force.
ASSUMPTION - In order for the rim to compress in one area, it must expand in
another.
THE RIM ACTIONS/REACTION WHEN BUILT INTO A WHEEL. In a spoked wheel, the rim
can still be compressed (though the force is now applied at the hub and rim,
instead of the top and bottom of the rim). However, the tension of the spokes
restricts the rim's ability to expand outward horizontally as it is
compressed vertically. This restriction forces nearly all rim deformation to
take place in along a smaller section of the rim, the "load affected zone"
(LAZ). This has the effect of "shortening the spring" of the rim. Now,
instead of the stiffness of the rim being equal to its circumference, it is
equal to the length of the LAZ. Our "shorter spring" is now stiff enough to
support the weight of the rider.
CONCLUSIONS The wheel doesn't stand on its spokes, it stand on a "spring",
the load affected zone of the rim.
I think this model better explains the function of the wheel, as you have
directly opposing forces at the point where pressure is applied. It differs
from the "standing on the spokes" model in that the tension in the spokes in
the compressed portion of the LAZ does not help support the rim, it works
against it, pulling it inward. The tension in the spokes at the end of the
LAZ - where the rim is trying to expand to counter the forces in the
compressed section - does help support the rim by resisting expansion. The
rest of the spokes in the wheel support the rim to a lesser degree.
I didn't see anything in your spoke tension measurements that would
contradict this theory. In fact, it seems to corroborate it. I realize that
this has probably been argued to death somewhere else, so thanks for bearing
with me. If I've screwed up somewhere, please help me to see the error of my
ways.
Thanks
Brian Nystrom
-----------== Posted via Deja News, The Discussion Network ==----------
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Jobst Brandt
> I recently re-read The Bicycle Wheel (third edition). While I
> understand the concepts of wheel construction, I still have a bit of
> a problem with the the statement "The wheel stands on its spokes".
> After banging my head on the wall for a while trying to understand
> how a spoke in tension can exert a compressive force, I have come up
> with a model that seems equally plausible and is, if nothing else,
> more intuitive. Perhaps it's just another way of saying the same
> thing, or perhaps it's wrong. However, if it's correct, I think
> it's easier for the average non-engineer to grasp.
> Please pardon the excessive detail, but I'm trying to make sure of
> my logic and make it easier for others who read this to follow.
> UNDERLYING FACTS
> A) A rim is essentially a circular spring.
> B) The stiffness of a spring of a given cross section is
> proportional to its length.
Not a good start. The rim is not a circular spring, whatever that is
supposed to imply. Part (B) is even more vague. All structural
materials have an elastic modulus that describes their response to
loads and this is the ratio of stress to strain. Calling your tenets
"facts" does not make them factual or underlying.
> THE ACTIONS/REACTIONS OF A RIM ALONE
> For the purposes of this model, we will assume that the rim in
> question is not stiff enough to support the weight of the rider by
> itself.
You needn't make such an assumption. How about using a real rim and
real spokes. There is no need to make an artificially different model
to visualize what happens. The book develops this step by step,
without any theoretical structure or material properties.
> If a force is applied to a rim at the top and bottom, the rim
> compresses vertically and expands horizontally. The amount of
> vertical compression is equal to the amount of horizontal expansion
> at at points 90 degrees from the axis of the applied force.
What the rim does by itself is not the same as what a complete wheel
does. Although the load is applied between hub and ground, it could
be applied by sitting on the wheel. That works as well, except that
now you have two load affected zones with symmetry about the
horizontal center of the wheel. This model just neutralizes the hub,
it being at equilibrium in your choice of loading.
> ASSUMPTION - In order for the rim to compress in one area, it must
> expand in another.
Your use of terms is so vague as to make this sort of discussion
impossible. You are changing the shape of the unspoked rim, it is not
compressing and that is what makes its circumference remain constant.
Your example is making the rim oval by bending. Compression of the
rim is something that practically does not occur in a wheel other than
when the spokes are originally tensioned.
You are constructing an invalid deflection diagram. This is all
covered in great detail in the book, although in the book the wheel is
loaded normally as it would be in a bicycle. As I said, this does not
change anything except that your model has top to bottom symmetry
about the horizontal diameter.
> THE RIM ACTIONS/REACTION WHEN BUILT INTO A WHEEL. In a spoked
> wheel, the rim can still be compressed (though the force is now
> applied at the hub and rim, instead of the top and bottom of the
> rim). However, the tension of the spokes restricts the rim's
> ability to expand outward horizontally as it is compressed
> vertically.
> This restriction forces nearly all rim deformation to take place in
> along a smaller section of the rim, the "load affected zone" (LAZ).
> This has the effect of "shortening the spring" of the rim. Now,
> instead of the stiffness of the rim being equal to its
> circumference, it is equal to the length of the LAZ. Our "shorter
> spring" is now stiff enough to support the weight of the rider.
> CONCLUSIONS The wheel doesn't stand on its spokes, it stand on a
> "spring", the load affected zone of the rim.
Why do you go to such lengths to build an invalid model with invalid
assumptions to hypothesize about the forces acting in a bicycle wheel?
Just come right out and say you do not believe the finite element
analysis, the tensiometer measurements, and the acoustic response that
you yourself can test, that show agreement with the deflection model
presented in the book. This analysis has been reviewed by the
scientific community and found valid.
You sound like the review of the book in Bicycling by Wheelsmith when
the book first appeared. Wheelbuilders generally attacked it as
though it threatened their livelihoods. I often wonder whether the
challenges that appear here on wreck.bike are from wheelbuilders who
feel defrocked or just non engineers who believe there is nothing to
science or the scientific method.
> I think this model better explains the function of the wheel, as you
> have directly opposing forces at the point where pressure is
> applied. It differs from the "standing on the spokes" model in that
> the tension in the spokes in the compressed portion of the LAZ does
> not help support the rim, it works against it, pulling it inward.
> The tension in the spokes at the end of the LAZ - where the rim is
> trying to expand to counter the forces in the compressed section -
> does help support the rim by resisting expansion. The rest of the
> spokes in the wheel support the rim to a lesser degree.
I think you have a preconceived notion of how a bicycle wheel works,
one that has been held for more than 100 years without question for
reasons that escape me. In any event, that perception is incorrect
and has been thoroughly disproven. The model you have created at your
keyboard cannot have been verified by any measurement and arises
purely from biased perspective. I ask you to re-read what you claim
to have read (I strongly doubt you did) because all these points are
answered there including methods by which you can prove them to
yourself. It helps to have a bicycle with wheels at hand.
> I didn't see anything in your spoke tension measurements that would
> contradict this theory. In fact, it seems to corroborate it. I
> realize that this has probably been argued to death somewhere else,
> so thanks for bearing with me. If I've screwed up somewhere, please
> help me to see the error of my ways.
I can't. I wrote the book at a time when the concepts were even more
foreign to not only some, but all readers, than they are to you now.
In that atmosphere, I spent great effort to thoroughly explain what,
how, and why the wheel acts as it does in more detail that I want to
repeat here. It is there in print for you and others to examine. If
you think you find support for your perception in the book, then you
are constructing that from wishful thinking. It is not there.
Jobst Brandt <jbr...@hpl.hp.com>
Emilio Castelli
Emilio
Jobst Brandt <jbr...@hpl.hp.com> wrote in message
news:7ga4jo$57s$6...@hplms2.hpl.hp.com...
Jobst Brandt
> You really should be nicer to your customers...
Customers? Several times per year someone like Brian writes that the
book is wrong and its wheel analysis is wrong. On the one hand, I
find it hard to believe that these people read what they say they read
because their proposals usually go directly against what is explained
with self verifying examples and computations, and on the other hand,
they restate beliefs that prevailed before the wheel was analyzed. I
am at a loss to explain this any more directly or indirectly than does
the posting that you cite. Maybe you can explain it more clearly by
understanding what motivates these writers. Most preface their
dissertations with a disclaimer of not being a scientist.
I invite commentary.
Jobst Brandt <jbr...@hpl.hp.com>
velo_souffrance
post, Mr. Nystrom simply wrote, "If I've screwed up somewhere, please help
me to see the error of my
weren't sound, he just needed some help getting an arm around them. Anyway,
you don't have any _obligation_ to help explain your book -- it is what it
is, I guess. But I took his post to basically just mean, "Cool book!! ...
but I'm not sure I fully understand this one thing ... and since you're out
there in cyberspace Mr. Brandt and I reckon you've thought about this from a
zillion angles, maybe if it's not too big a favor you could prod me in the
right way so that I'll get it!" So why the fuss?
Jobst Brandt wrote in message <7gafib$k3l$3...@hplms2.hpl.hp.com>...
>Emilio Castelli writes:
>
>> You really should be nicer to your customers...
>
>Customers? Several times per year someone like Brian write that the
>book is wrong and that the analysis is all wrong. On the one hand, I
Joe Joey Joe Joe Jr. Shabadoo
<emi...@metro.net> wrote:
>You really should be nicer to your customers...
>
>Emilio
>
Is this another case of, "Ooh, jobst is being mean again. Ooh, he's
picking on some helpless person." Get real.
I read the bicycle wheel for the first time many years ago, and I go
back to it as a reference. No, I don't agree with everything in it;
or more accurately, I don't UNDERSTAND everything in it, but then
again, I'm not an engineer, so most of the things I don't agree with
are things that I have no basis of judgement on.
Brian was trying to conceptualize certain instances in the book, and
asking the author himself to clarify (how many of us wish that we had
that luxury with other books? Everybody in social sciences, raise
your hand!). Brandt very systematically picks apart Brian's argument,
not necessarily from a, "no you're stupid" standpoint, but more from
a, "here is where you are wrong, these assumptions are invalid, and
the conclusions are incorrect." Phrased as politely? No, but the
scientific method is there, if the art of rhetoric is not. And as far
as politeness goes, check it at the door. I'm here to learn tech
issues.
Let me ask you, Emilio, do you build your own wheels? I do. Do you
own a copy of this book? I do. Has it helped be build wheels?
Moreso than any other single publication. Do you understand finite
element analysis? Well, I don't. But MY world isn't going to change
if I don't.
Frankly, the science and all that is neat, but I only really use and
understand part two, which is the section that is the step by step in
building and repairing. The rest of the book is Brandt's way of
letting the reader know there is more than "voodoo and trickery" in
wheel building; there's actual physics involved (imagine that!). The
science validates, and it justifies the book costing $20. I have yet
to see a decent discussion on Brandt's terms; if he is wrong, somebody
whip out a slide rule and come up with other equations. And to the
rest of us who don't care, we can skip the thread.
I build my own wheels, I've built maybe six sets. I know master
mechanics that are more skilled and practiced than I am in wheel
building, who often ask, "Who's this Jobst Brandt guy?" I'm not one
to preach that Brandt is the authority on wheels, especially to these
guys who can outbuild me with both hands covering their eyes (and
because I don't entirely belive it myself; however, it is published,
and how many other wheelbuilders can say that?). And most of these
guys don't want to hear it anyway. Because THEIR world isn't going to
change if it is proved that it's more than witchcraft.
I guess my point being is if you can't take critical analysis of YOUR
critical analysis, you shouldn't try to point them out as definitive.
Be ready to rebut, to accept, and to analyze some more. But don't
ever make the mistake that someone is wrong just because they are a
bit rude; in fact, the reason they ARE rude is often because they have
been proved correct over time.
Do I think Jobst is god? Hell no. Do I believe there is sound
mechanical and science behind his book? Yes, absolutely. Do i
understand it all? No, otherwise I'd be working as an engineer. Do I
think Jobst is definitive? Well, I have yet to come across something
that approaches this topic from such a non-sensical way. Brandt isn't
the authority on everything, but he is very knowledgeable about
wheels, and I respect his knowledge on this subject matter. This is
not to say I respect him as a person (and before I dig myself into a
hole, it's not to say that I DON'T), but the issue here is that people
have a tendency to assume too much about a person's personality when
they really have no reason (or right) to do so.
I've sat through too many of these threads, and have come across only
a couple of threads that had good points. The rest were a waste of my
time (and probably Jobst's as well). At least he has the patience to
answer (however rude it may come off as); I typically don't even do
that...
It still cracks me up that people try to refute the numbers given; it
would be irresponsible to publish data as scientific without any
research done. To those ends, there is very little doubt in my mind
that Jobst (or his research team or whoever) has done this, and people
try to refute the claims based solely on intuition...let's see some
numbers. He's given them.
Wait, I'm slipping off my soapbox now. Any comments I'd rather field
in private, unless you're going to go on a profane rant, in which case
I can't think of a quicker way to get on my killfile. But I'm always
looking for INTERESTING and INTELLIGENT comments to why I am wrong.
--
Joe Joey Joe Joe Junior, Shabadoo
jn...@quads.uchicago.edu
obsquecious (ob skwee shus) - adj. Obscure to the point of alienation.
Usage: If you're trying to hold my interest, don't be obsquecious.
DD861
masses understand. I don't recall any teacher ever saying, "... well, go
back and read your textbook again. The answer is already in there." I've
never heard of such a pedagogical technique.
Nyaaah, I don't buy this high and mighty science stuff. My family,
excepting me, is a bunch of engineers and scientists -- so I know a few of
'em -- the best make it anything but high and mighty. It's so easy to
flimflam on der blinkenlights and make the whole thing seem like a
mumbo-jumbo incomprehensible to civilians. That type of behavior's for
third-tier hacks. Science poseurs, as it were ... lacking in clarity and
most generally accepted forms of social grace.
Ben wrote in message ...
>The fuss is because the proposed alternative model for laypeople is
>incorrect and as such clouds the waters even more. There will always be
>difficulties in laypeople understanding engineering concepts. Even with
>great logic and mathematical understanding, there are areas that require
>specific training to fully grasp. This is why engineers study at college.
>
>Please don't get me wrong: I have nothing against people asking
>questions/posing theories here. However, often the rebuttal seems quite
>rude when in fact it is to the point. There is no room for niceties in
>science (mostly...).
>
>Cheers,
>
>Ben
>
>velo_souffrance <velo_so...@hotmail.com> wrote in message
>news:7gaoi6$bge$1...@ash.prod.itd.earthlink.net...
>> E-mail seems to do a lousy job of conveying vibes. As I read the
original
>> post, Mr. Nystrom simply wrote, "If I've screwed up somewhere, please
help
>> me to see the error of my
>> ways" -- it sure didn't sound like he was alleging that your theories
>> weren't sound, he just needed some help getting an arm around them.
>Anyway,
>> you don't have any _obligation_ to help explain your book -- it is what
it
>> is, I guess. But I took his post to basically just mean, "Cool book!!
>...
>> but I'm not sure I fully understand this one thing ... and since you're
>out
>> there in cyberspace Mr. Brandt and I reckon you've thought about this
from
>a
>> zillion angles, maybe if it's not too big a favor you could prod me in
the
>> right way so that I'll get it!" So why the fuss?
>>
>>
>>
>> Jobst Brandt wrote in message <7gafib$k3l$3...@hplms2.hpl.hp.com>...
>> >Emilio Castelli writes:
>> >
>> >> You really should be nicer to your customers...
>> >
E Harp
<benco...@ozemail.com.au> wrote:
Oh phooey. If there is one thing that gets under my skin, it is the notion
that there are concepts that one cannot understand unless they have the
"proper training."
In my experience, it is often a question of terminology. If you explain
something in terms your audience does not understand, you might as well be
speaking a foreign language.
As a software engineer, I have worked with a lot of very talented people.
Over the years, I have found that the best of them can explain the most
sophisticated of concepts in language the average high school sophmore
could easily understand. I strive for this myself when communicating.
Granted, there are realms for which this is probably impossible - quantum
theory for instance. The path to those concepts seems to me to be too long
for lay terms to travel. Even so, we can all understand the consequences.
In any case, it is not Jobst's ability to speak in engineering terms that
I trust. It is his vast experience.
That said, I am ordering a copy of Jobst's book. Time to find out what all
the fuss is about (not to mention how to build a decent wheel).
Ed Harp
San Jose, CA
> The fuss is because the proposed alternative model for laypeople is
> incorrect and as such clouds the waters even more. There will always be
> difficulties in laypeople understanding engineering concepts. Even with
> great logic and mathematical understanding, there are areas that require
> specific training to fully grasp. This is why engineers study at college.
>
> Please don't get me wrong: I have nothing against people asking
> questions/posing theories here. However, often the rebuttal seems quite
> rude when in fact it is to the point. There is no room for niceties in
> science (mostly...).
>
> Cheers,
>
> Ben
>
><snip previous stuff>
Ben
spokes" may not be the clearest way to express it, but JB is trying to
communicate the results of finite element analysis (force analysis) which
has nothing to do with springs. I know it sounds pompous however if you
wish to delve into engineering fields, you should gain some engineering
knowledge. A quick glance at many cable supported strucrutes (eg Cetrepoint
Tower, Sydney) will show the reliance on tension reduction to support what
would otherwise be compression loads is not a new engineering concept.
Springs are a mechanical device to store energy, which is not the desired
function of a rim.
Ben
<bnys...@bit-net.com> wrote in message
news:7g9v3h$klo$1...@nnrp1.dejanews.com...
> Jobst,
>
> I recently re-read The Bicycle Wheel (third edition). While I understand
the
> concepts of wheel construction, I still have a bit of a problem with the
the
> statement "The wheel stands on its spokes". After banging my head on the
wall
> for a while trying to understand how a spoke in tension can exert a
> compressive force, I have come up with a model that seems equally
plausible
> and is, if nothing else, more intuitive. Perhaps it's just another way of
> saying the same thing, or perhaps it's wrong. However, if it's correct, I
> think it's easier for the average non-engineer to grasp.
>
> Please pardon the excessive detail, but I'm trying to make sure of my
logic
> and make it easier for others who read this to follow.
>
> UNDERLYING FACTS
> A) A rim is essentially a circular spring.
> B) The stiffness of a spring of a given cross section is proportional to
its
> length.
>
>
> THE ACTIONS/REACTIONS OF A RIM ALONE
> For the purposes of this model, we will assume that the rim in question is
not
> stiff enough to support the weight of the rider by itself.
>
> If a force is applied to a rim at the top and bottom, the rim compresses
> vertically and expands horizontally. The amount of vertical compression is
> equal to the amount of horizontal expansion at at points 90 degees from
the
> axis of the applied force.
>
> ASSUMPTION - In order for the rim to compress in one area, it must expand
in
> another.
>
>
> THE RIM ACTIONS/REACTION WHEN BUILT INTO A WHEEL. In a spoked wheel, the
rim
> can still be compressed (though the force is now applied at the hub and
rim,
> instead of the top and bottom of the rim). However, the tension of the
spokes
> restricts the rim's ability to expand outward horizontally as it is
> compressed vertically. This restriction forces nearly all rim deformation
to
> take place in along a smaller section of the rim, the "load affected zone"
> (LAZ). This has the effect of "shortening the spring" of the rim. Now,
> instead of the stiffness of the rim being equal to its circumference, it
is
> equal to the length of the LAZ. Our "shorter spring" is now stiff enough
to
> support the weight of the rider.
>
>
> CONCLUSIONS The wheel doesn't stand on its spokes, it stand on a "spring",
> the load affected zone of the rim.
>
> I think this model better explains the function of the wheel, as you have
> directly opposing forces at the point where pressure is applied. It
differs
> from the "standing on the spokes" model in that the tension in the spokes
in
> the compressed portion of the LAZ does not help support the rim, it works
> against it, pulling it inward. The tension in the spokes at the end of the
> LAZ - where the rim is trying to expand to counter the forces in the
> compressed section - does help support the rim by resisting expansion. The
> rest of the spokes in the wheel support the rim to a lesser degree.
>
>
> I didn't see anything in your spoke tension measurements that would
> contradict this theory. In fact, it seems to corroborate it. I realize
that
> this has probably been argued to death somewhere else, so thanks for
bearing
> with me. If I've screwed up somewhere, please help me to see the error of
my
Ben
incorrect and as such clouds the waters even more. There will always be
difficulties in laypeople understanding engineering concepts. Even with
great logic and mathematical understanding, there are areas that require
specific training to fully grasp. This is why engineers study at college.
Please don't get me wrong: I have nothing against people asking
questions/posing theories here. However, often the rebuttal seems quite
rude when in fact it is to the point. There is no room for niceties in
science (mostly...).
Cheers,
Ben
velo_souffrance <velo_so...@hotmail.com> wrote in message
news:7gaoi6$bge$1...@ash.prod.itd.earthlink.net...
> E-mail seems to do a lousy job of conveying vibes. As I read the original
> post, Mr. Nystrom simply wrote, "If I've screwed up somewhere, please help
> me to see the error of my
> ways" -- it sure didn't sound like he was alleging that your theories
> weren't sound, he just needed some help getting an arm around them.
Anyway,
> you don't have any _obligation_ to help explain your book -- it is what it
> is, I guess. But I took his post to basically just mean, "Cool book!!
...
> but I'm not sure I fully understand this one thing ... and since you're
out
> there in cyberspace Mr. Brandt and I reckon you've thought about this from
a
> zillion angles, maybe if it's not too big a favor you could prod me in the
> right way so that I'll get it!" So why the fuss?
>
>
>
> Jobst Brandt wrote in message <7gafib$k3l$3...@hplms2.hpl.hp.com>...
> >Emilio Castelli writes:
> >
> >> You really should be nicer to your customers...
> >
Brian Nystrom
Jobst Brandt wrote:
> Emilio Castelli writes:
>
> > You really should be nicer to your customers...
>
> Customers? Several times per year someone like Brian writes that the
> book is wrong and its wheel analysis is wrong.
I didn't say that the book was wrong, only that I read the data in it and
came to a different conclusion. I re-read it and I still don't seen
anything in the FEM data or your other explanations that contradicts that
conclusion. The way you reacted, one would think that I was espousing the
"hub hangs from the spokes" idea, which I'm not.
> On the one hand, I
> find it hard to believe that these people read what they say they read
> because their proposals usually go directly against what is explained
> with self verifying examples and computations, and on the other hand,
> they restate beliefs that prevailed before the wheel was analyzed.
I don't appreciate your inference that I am lying about reading your book.
Yes, I had preconcieved notions before I re-read it - I BELIEVED WHAT YOU
WROTE AND WAS TRYING TO DEEPEN MY UNDERSTANDING OF IT. Instead, when I
went over it again, I had more questions than answers.
> I am at a loss to explain this any more directly or indirectly than does
>
> the posting that you cite. Maybe you can explain it more clearly by
> understanding what motivates these writers. Most preface their
> dissertations with a disclaimer of not being a scientist.
My motivation was to try and understand where I was right or wrong in my
interpretation of your data. I was looking for information, not
condescention and attitude. I expected that if I presented a theory in a
respectful manner, I might get a respectful reply. What was I thinking?
I've seen how you treat other people on this forum, so I guess I should
have known better.
> I invite commentary.
Ray Kennedy
> Nyaaah, I don't buy this high and mighty science stuff. My family,
> excepting me, is a bunch of engineers and scientists -- so I know a few of
> 'em -- the best make it anything but high and mighty. It's so easy to
> flimflam on der blinkenlights and make the whole thing seem like a
> mumbo-jumbo incomprehensible to civilians. That type of behavior's for
> third-tier hacks. Science poseurs, as it were ... lacking in clarity and
> most generally accepted forms of social grace.
Hear hear. Science serves not rules.
If you can't communicate your knowledge to some one else
whats the point?
'Know it alls' hide behind language barriers and live in fear of
being exposed one day, for the frauds they are.
Sincerely
Ray
David Stanley
>
>Nyaaah, I don't buy this high and mighty science stuff. My family,
>excepting me, is a bunch of engineers and scientists -- so I know a few
of
>'em -- the best make it anything but high and mighty. It's so easy to
>flimflam on der blinkenlights and make the whole thing seem like a
>mumbo-jumbo incomprehensible to civilians. That type of behavior's for
>third-tier hacks. Science poseurs, as it were ... lacking in clarity
and
>most generally accepted forms of social grace.
>
The best hundred non-fiction books of the last hundred years list was in
this morning's paper.
A quick glimpse saw Lewis Thomas's "Lives of a Cell" and Stephen Jay
Gould's "Mismeasure of Man" in the list. They both manage to put
extremely technical work into graceful prose that anyone with interest in
the subjects at hand can read.
And both men do(did) real science, just like plenty of people around
here.
DnF
"I am satisfied- I see, dance, laugh, sing..."
-Walt Whitman
David Stanley MXQ...@prodigy.com
Bin Tan
After reading all the debate, a few observations:
Does it really matter whether a wheel stands on 4 or 8 spokes or it
hangs on 4 or 8 spokes in motion, as long as we know round and true wheels
are fast wheels?
We know we can't just have 4 or 8 spokes there even if
someone can prove it actually stands or hangs on those few spokes at a
certain moment. (I am not talking about spinergies)
A wheel definitely hangs on upper spokes in the sense that there is
tension instead of compression in those spokes. And it does not seem too
likely that tension in lower spokes will turn into compression when that
section of the wheel meets the ground. Otherwise, the spoke would press
the nipple into the tire.
I am prepared to say that every spoke contributes a certain amount of
tension to the hub, which combine to support the bike.
Jobst Brandt
> I didn't say that the book was wrong, only that I read the data in
> it and came to a different conclusion. I re-read it and I still
> don't seen anything in the FEM data or your other explanations that
> contradicts that conclusion. The way you reacted, one would think
> that I was espousing the "hub hangs from the spokes" idea, which I'm
> not.
>> On the one hand, I find it hard to believe that these people read
>> what they say they read because their proposals usually go directly
>> against what is explained with self verifying examples and
>> computations, and on the other hand, they restate beliefs that
>> prevailed before the wheel was analyzed.
> I don't appreciate your inference that I am lying about reading your
> book. Yes, I had preconcieved notions before I re-read it - I
> BELIEVED WHAT YOU WROTE AND WAS TRYING TO DEEPEN MY UNDERSTANDING OF
> IT. Instead, when I went over it again, I had more questions than
> answers.
I didn't see this as a question but an alternate perception of the
deflections and stresses that parallels what has been the status quo
for so many years.
I think that, had you asked specifically how you reconcile the
displacements shown in graphics and numerical tables supports this or
that description, I could have answered your question. The whole book
is written to dispel the theory you proposed. It IS truly difficult
to believe you read the book from your contentions that are entirely
at odds with what is presented in great detail and in lay terms in the
book.
>> I am at a loss to explain this any more directly or indirectly than
>> does the posting that you cite. Maybe you can explain it more
>> clearly by understanding what motivates these writers. Most
>> preface their dissertations with a disclaimer of not being a
>> scientist.
> My motivation was to try and understand where I was right or wrong
> in my interpretation of your data. I was looking for information,
> not condescention and attitude. I expected that if I presented a
> theory in a respectful manner, I might get a respectful reply. What
> was I thinking? I've seen how you treat other people on this forum,
> so I guess I should have known better.
As I have often stated, these "questions" come out as dissertations on
technology with some trailing or leading "isn't it true". This is not
asking a question but rather is telling the net how it really is. As
should be evident, my responses to people who ask questions go to
great lengths to clarify the issues.
>> I invite commentary.
> This is almost laughable, Jobst. No thank you. I realize that I've
> hugged a cactus and I'm not going to do it again.
The commentary I sought was not yours but rather that of people who
understand your manner of "questioning" and how to respond to it. I
recall the last technical exchange with you was about the dynamics of
cornering in which you authoritatively told us how it is done, only to
reveal that you read it in a book, a book written by a bicycle racer
who wasn't too sharp on physics, although apparently good at winning,
which makes believers of his readers. This belief is strengthened by
the wishful thinking that even though not physically strong, one can
win races by fast cornering. It has appeal.
> I'll discuss this with other engineers who I know will have the
> courtesy to give me an understandable explanation, rather than a
> knee-jerk defensive reaction. I have no problem being told I'm
> wrong, or accepting it, I'd just like to come away with a better
> understanding, rather than the feeling I've irritated "the master"
> by having the audacity to ask questions about his work.
I don't buy it. Throwing yourself to the sympathy of the net, having
been mercilessly attacked for your pursuit of the truth, is a regular
ploy. You may fool some of the folks but those who read your
presentations with a critical eye will not be taken in.
> I always thought that the very essence of science was to question
> what you see, hear and read, in the hope that you might learn
> something in the process. Well, I did learn something, but it wasn't
> about bicycle wheels.
> Regards
Touching! You have to try hard to make this sound sincere. However,
your true sentiments leak out between the lines. Don't scowl so.
Jobst Brandt <jbr...@hpl.hp.com>
Jeffrey J. Potoff
>
> After reading all the debate, a few observations:
>
> Does it really matter whether a wheel stands on 4 or 8 spokes or it
> hangs on 4 or 8 spokes in motion, as long as we know round and true wheels
> are fast wheels?
>
> We know we can't just have 4 or 8 spokes there even if
> someone can prove it actually stands or hangs on those few spokes at a
> certain moment. (I am not talking about spinergies)
>
> A wheel definitely hangs on upper spokes in the sense that there is
> tension instead of compression in those spokes. And it does not seem too
> likely that tension in lower spokes will turn into compression when that
> section of the wheel meets the ground. Otherwise, the spoke would press
> the nipple into the tire.
>
The wheel is a prestressed structure. The spokes in the load zone,
the bottom of the wheel in contact with the ground, are under
less tension than the rest of the spokes in the wheel; they
are being compressed. The tension of the rest of the spokes in the
wheel does not increase in any measurable way.
It seems the difficult part is visualizing how something can be
compressed if it's still under tension. Time to head to the
library.
> I am prepared to say that every spoke contributes a certain amount of
> tension to the hub, which combine to support the bike.
I am prepared to make dinner. Tonight is going to be Kung Pao Chicken.
Jeff
Rick Denney
>
>After reading all the debate, a few observations:
>
>Does it really matter whether a wheel stands on 4 or 8 spokes or it
>hangs on 4 or 8 spokes in motion, as long as we know round and true wheels
>are fast wheels?
Yes, it really matters. Because when you understand that spokes carry
the load of the rider in compression, then you understand why the
tension in the spoke must exceed the compressive load that it carries
to keep from buckling. And you you'll understand that spokes
experience their highest stress while unloaded. If you understand
those things, then you'll understand why the strongest wheel has
spokes as tight as the rim will allow.
If you miss the point along the way, you won't understand why spokes
need to be so tight, and you will build wheels (or support those who
build wheels) looser than they need to be. And you'll never understand
why these wheels can't stay in true, which will eventually lead you to
your need for round the true wheels. You will believe that thicker
spokes make stronger wheels, or even that thicker spokes can add the
strength that a lighter rim takes away. These myths stem from thinking
that wheels hang from the spokes.
>
>We know we can't just have 4 or 8 spokes there even if
>someone can prove it actually stands or hangs on those few spokes at a
>certain moment. (I am not talking about spinergies)
The fewer the spokes, the stronger the rim must be in bending and
compression, to allow those few spokes to be tight enough to carry the
compressive load. If you understand the superposition of the
compressive load on the tensile stress in the spoke, then you'll be
able to analyze wheels whether or not they even have tension in the
spoke. When you look at a Tri-Spoke, are the spokes in tension or not?
You can't tell by looking, and you can't tell by feeling. If the wheel
stands on the bottom spokes, then it doesn't matter.
>
>A wheel definitely hangs on upper spokes in the sense that there is
>tension instead of compression in those spokes. And it does not seem too
>likely that tension in lower spokes will turn into compression when that
>section of the wheel meets the ground. Otherwise, the spoke would press
>the nipple into the tire.
The word "hang" only has meaning when gravity is the force being
constrained. Turn the wheel on its side, and there's no "hanging" in
the plane of the rim. But when you push the rim up against a wall, the
compressive load being carried by the tensile spokes nearest the wall
is just as real in that orientation as it is when a vertical wheel is
loaded against gravity.
>
>I am prepared to say that every spoke contributes a certain amount of
>tension to the hub, which combine to support the bike.
What if the wheel has no tension, but spokes that can withstand
compressive stress? The boundary between tensile and compressive
stress in the spoke is just a sign change on stress, not a sea change
in the structure. If you understand why wheels stand on their spokes,
you can understand any spoked wheel, from 36-spoke bike wheels to
Spinergys to Tri-Spokes to wooden wagon wheels.
Rick "We've trod this path before" Denney
Ben
Bicycle Wheel myself, so I can't comment on the style or try to explain it.
I agree with the sentiments of your posts. However if one encounters a
poorly explained/written report/analysis in their field of expertise, they
are more likely to understand it than a layperson...similarly if the
layperson were to gain some grounding in the subject in question, they also
would be more likely to understand.
Ben....recalling that this is a bicycle NG
E Harp <rgb...@aol.com> wrote in message
news:rgb555-2904...@17.219.157.28...
> In article <Oa7W2.4599$116....@news2.ozemail.com.au>, "Ben"
> <benco...@ozemail.com.au> wrote:
>
> Oh phooey. If there is one thing that gets under my skin, it is the notion
> that there are concepts that one cannot understand unless they have the
> "proper training."
>
> In my experience, it is often a question of terminology. If you explain
> something in terms your audience does not understand, you might as well be
> speaking a foreign language.
>
> As a software engineer, I have worked with a lot of very talented people.
> Over the years, I have found that the best of them can explain the most
> sophisticated of concepts in language the average high school sophmore
> could easily understand. I strive for this myself when communicating.
>
> Granted, there are realms for which this is probably impossible - quantum
> theory for instance. The path to those concepts seems to me to be too long
> for lay terms to travel. Even so, we can all understand the consequences.
>
> In any case, it is not Jobst's ability to speak in engineering terms that
> I trust. It is his vast experience.
>
> That said, I am ordering a copy of Jobst's book. Time to find out what all
> the fuss is about (not to mention how to build a decent wheel).
>
> Ed Harp
> San Jose, CA
>
> > The fuss is because the proposed alternative model for laypeople is
> > incorrect and as such clouds the waters even more. There will always be
> > difficulties in laypeople understanding engineering concepts. Even with
> > great logic and mathematical understanding, there are areas that require
> > specific training to fully grasp. This is why engineers study at
college.
> >
> > Please don't get me wrong: I have nothing against people asking
> > questions/posing theories here. However, often the rebuttal seems quite
> > rude when in fact it is to the point. There is no room for niceties in
> > science (mostly...).
> >
> > Cheers,
> >
> > Ben
> >
> ><snip previous stuff>
Rick Knowlan
> E-mail seems to do a lousy job of conveying vibes. As I read the original
> post, Mr. Nystrom simply wrote, "If I've screwed up somewhere, please help
> me to see the error of my
> ways" -- it sure didn't sound like he was alleging that your theories
> weren't sound, he just needed some help getting an arm around them. Anyway,
> you don't have any _obligation_ to help explain your book -- it is what it
> is, I guess. But I took his post to basically just mean, "Cool book!! ...
> but I'm not sure I fully understand this one thing ... and since you're out
> there in cyberspace Mr. Brandt and I reckon you've thought about this from a
> zillion angles, maybe if it's not too big a favor you could prod me in the
> right way so that I'll get it!" So why the fuss?
>
The Bicycle Wheel (the book) functions like a finely constructed internet
troll. The unknowing, in an attempt to understand, set out their incorrect
understanding of how a wheel functions. They invite comment or discussion.
Unless their tone is suitably modest and self-effacing, Jobst pounces, thrashes
them, then gloats over the corpse of their brashness and naďveté.
IMHO, there's a difference between debating the ideas of a colleague engaged in
scientific inquiry and responding to an obviously misinformed poster on RBT. The
temper of Jobst's responses are acceptable for the former but not the latter.
Rick Knowlan
Rick Knowlan
> The wheel is a prestressed structure. The spokes in the load zone,
> the bottom of the wheel in contact with the ground, are under
> less tension than the rest of the spokes in the wheel; they
> are being compressed.
I feel like that comedian (is it Steven Wright?) who says he is experiencing
deja vu and senility at the same time--I think I've forgotten this before. :-)
People often refer to the lower spokes as being "compressed". In my view, that
unfortunate choice of wording creates misunderstanding. Tension and compression
are absolute states, not relative ones. An unloaded, unstressed spoke is in a
neutral state--neither in tension nor compression. When it is loaded axially
with a net tension load, it is in tension. If the net tension load is reduced
to a lower net tension load, the spoke is still in tension.
Mark McMaster
That is one defintion of Tension and Compression, but not the only
(correct) one. When I mount my bike, my weight makes the bottoms of my
tires compress - no wait, they tire casings in their natural state are
limp and flat until they became expanded under pneumatic pressure - I
guess my tires weren't compressed, they were simply less expanded.
Tension and compression can be used as relative terms as well as
absolute. To make a distinction, you have to add a qualifier, like
"net" or "delta". For a wheel, loads are supported by a delta
compression of the bottom spokes, although these spokes remain in net
tension.
Mark McMaster
MMc...@ix.netcom.com
Jobst Brandt
> People often refer to the lower spokes as being "compressed". In my
> view, that unfortunate choice of wording creates misunderstanding.
> Tension and compression are absolute states, not relative ones. An
> unloaded, unstressed spoke is in a neutral state--neither in tension
> nor compression. When it is loaded axially with a net tension load,
> it is in tension. If the net tension load is reduced to a lower net
> tension load, the spoke is still in tension.
I think this is a diversion from the initial "question". The hub
clearly does not hang from the upper spokes or they would show an
increase in tension. The four or so spokes between the hub and ground
are the only ones that show any significant change by becoming shorter
when the wheel is loaded. I believe that most readers are familiar
with algebra and know that numbers can be negative or positive and
that these numbers can be added and subtracted. Choosing an arbitrary
convention, tension can be labeled negative and compression positive,
or the converse. Adding compression to the bottom spokes (compressing
them) leaves them with less tension.
Not knowing that the spokes are in tension, as in one example in the
book does that shows a cast aluminum wheel that looks from the side
like a conventional cross spoked wheel but is in reality a rigid
casting. We do not know whether the spokes are in tension or
compression as a result of the cooling process when the wheel was
made. Unless annealed in an oven afterwards, the spokes are
definitely stressed one way or the other, most likely tension,
considering the cooling rate of the rim and much thicker hub.
However, in this model, in spite of the possibility of tension, most
readers agree that this wheel stands on its bottom spokes and that
strain gauges on those spokes would show a shortening when loaded.
Getting back to the wire spoked wheel, and having determined that the
bottom spokes are in fact the only ones that change tension
(detectable by plucking the spokes and noting a change in tone) the
reader agrees that the wire spoked wheel would show a similar reading
if strain gauges (resistance gauges glued to the surface of the metal
to measure change in length [strain]) were attached to its spokes.
At the same time most people still insist that the wire spoked wheel
is hanging from the upper spokes while the cast wheel is standing on
the bottom ones. The above derivation is contained in the book, but
with more carefully chosen words than I can muster at the stroke of a
KBD. Likewise, the book treats the presentations that doubters
present on wreck.bike at length by showing the deflection that occur.
You wonder why I am short with some of these folks. I expect someone
who doesn't understand something to ask "you say... how does that
agree with..." Instead, I regularly get a dissertation on how the
wheel "really" works and get challenged to refute that, while the book
refutes exactly that point. It is hard for people who haven't read
the book, to believe that someone would be making that up. Whether
Brian is one of these is not certain but to me all the indications
appeared no different from previous ones.
I may have jumped on his case without cause, but having had a previous
exchange on another technical thread, biased my opinion for this one.
I'll take it that I was wrong about Brian.
I apologize for unloading my frustrations on a reader who sincerely
sought to get clarification.
Jobst Brandt <jbr...@hpl.hp.com>
J & R
pains to be concise and present technical ideas in easily understandable
terms. I've also had people come back to me repeatedly, claiming to have read
the documents and that "they don't make any sense." They ask for my help then
end up telling me I'm full of shit. It's at that point that I begin thinking
to myself that they are either brain-damaged, ill-educated, or lying, that
sufficient understanding is achieveable by anyone who can a) read and b)
reason. Therefore these people either cannot read, or they cannot reason.
QED.
Of course the world is not that simple. And even though one can thoroughly
understand a bicycle wheel, complex software, and the scientific method, the
variety of human thought, interaction and intentions is far more complex. I
don't understand how everyone else thinks, or even how I think for that
matter. Scientific method, logic, and reasoning provide a good framework for
meaningful discourse, some would say the only reasonable framework. Others
think differently. It can take great patience and tolerance for a dialogue to
succeed between a person who adheres to a certain brand of metaphysics and
another who distrusts the term. But that tolerance should be expected from
both sides.
"Joe Joey Joe Joe Jr. Shabadoo" wrote:
> On Thu, 29 Apr 1999 12:53:40 -0700, "Emilio Castelli"
> <emi...@metro.net> wrote:
>
> >You really should be nicer to your customers...
> >
> >Emilio
> >
>
Rick Knowlan
> You wonder why I am short with some of these folks. I expect someone
> who doesn't understand something to ask "you say... how does that
> agree with..." Instead, I regularly get a dissertation on how the
> wheel "really" works and get challenged to refute that, while the book
> refutes exactly that point. It is hard for people who haven't read
> the book, to believe that someone would be making that up. Whether
> Brian is one of these is not certain but to me all the indications
> appeared no different from previous ones.
>
> I may have jumped on his case without cause, but having had a previous
> exchange on another technical thread, biased my opinion for this one.
> I'll take it that I was wrong about Brian.
>
> I apologize for unloading my frustrations on a reader who sincerely
> sought to get clarification.
I guess it might feel a bit like being a big-name gunslinger in the old
west--with no shortage of young guns itchin' to take you on, it's easy to
develop a quick trigger finger.
Your gracious apology speaks volumes.
Rick Knowlan
> That is one defintion of Tension and Compression, but not the only
> (correct) one. When I mount my bike, my weight makes the bottoms of my
> tires compress - no wait, they tire casings in their natural state are
> limp and flat until they became expanded under pneumatic pressure - I
> guess my tires weren't compressed, they were simply less expanded.
>
> Tension and compression can be used as relative terms as well as
> absolute. To make a distinction, you have to add a qualifier, like
> "net" or "delta". For a wheel, loads are supported by a delta
> compression of the bottom spokes, although these spokes remain in net
> tension.
I offer this not to refute what you have offered, but simply to state my
(uncharacteristic) desire for purity of concept.
20 or so years ago, when I used to analyze structural failures for a living, it was
imperative to differentiate between tension and compression in structural members
because long, thin members (like spokes) behave so differently in these conditions.
You know the saying, when all you know is a hammer, everything begins to look like a
nail? Well, all I know (in engineering) is static structures, so I think of bicycle
wheels as structures and want to apply my early lessons to them.
Brian Nystrom
Jobst Brandt wrote:
> You wonder why I am short with some of these folks. I expect someone
> who doesn't understand something to ask "you say... how does that
> agree with..." Instead, I regularly get a dissertation on how the
> wheel "really" works and get challenged to refute that, while the book
> refutes exactly that point. It is hard for people who haven't read
> the book, to believe that someone would be making that up. Whether
> Brian is one of these is not certain but to me all the indications
> appeared no different from previous ones.
Actually, I have both the first and third editions of the book and have
read both of them more than once.
> I may have jumped on his case without cause, but having had a previous
> exchange on another technical thread, biased my opinion for this one.
> I'll take it that I was wrong about Brian.
Our last exchange got a bit more heated than necessary (my fault), but by
the end, I thought we had essentially agreed that we were more or less
arguing whether the cornering technique under discussion was necessary or
not. You took the position that it wasn't and I countered that for someone
with less than perfect bike handling skills (like myself), it was useful.
At that point, it seemed that we agreed to disagree and left it at that.
> I apologize for unloading my frustrations on a reader who sincerely
> sought to get clarification.
Apology accepted.
The whole point of these newsgroups (at least for me) is to learn and,
in-turn, pass on whatever useful knowledge I may posess to others. If I
didn't feel like I was doing one or the other, I wouldn't waste the time.
If all I wanted to do was argue, I could call my ex-wife. ;-)
BTW, Mark McMaster gave me a very interesting and understandable example
of why the "rim as a spring" model doesn't work. Perhaps you might want to
incorporate it into an FAQ (as you've done on other subjects) so that the
next time that someone brings it up (it's inevitable), you'll have an
answer handy that even the technically inept can understand.
--
Regards
Brian
Rick Denney
>20 or so years ago, when I used to analyze structural failures for a living, it was
>imperative to differentiate between tension and compression in structural members
>because long, thin members (like spokes) behave so differently in these conditions.
>You know the saying, when all you know is a hammer, everything begins to look like a
>nail? Well, all I know (in engineering) is static structures, so I think of bicycle
>wheels as structures and want to apply my early lessons to them.
>
The strength of the wheel si defined by the following equation:
Maximum load = compressive strength of spokes in load-affected zone -
pretension of spokes in load-affected zone.
The load-affected zone is a function mostly of the strength and shape
of the rim, and is the bottom portion of the wheel, starting at the
contact point with the ground and radiating from there.
With Tri-Spokes, the pretension is presumably zero (though this is not
obvious from inspection). With wire-spoked wheels, the compressive
strength is zero. With Jobst's cast wheel example, it's somewhere in
between. But the point of using the terms that we use is that the
equation works for all spoked wheels.
If the cast wheel gains half its load-carrying capacity from
pretension due to differential cooling, and half from compressive
strength, then the compressive strength need only be half what it
would need to be without the pretension. That means the designer can
make it much lighter.
An important point is that the equation, though it tells us everything
we need to know about carrying radial loads, only describes the spokes
in the load-affected zone. If we think about hubs hanging from top
spokes, we are thinking about spokes that don't even enter into the
equation. The purpose of spoke tension is to provide the pretension
needed for that equation--nothing else.
Rick "It is a static equation" Denney
JCMWeb
>There will always be
>difficulties in laypeople understanding engineering concepts. Even with
>great logic and mathematical understanding, there are areas that require
>specific training to fully grasp. This is why engineers study at college.
The old: 'It's too complicated for you mere mortals to understand' excuse. It
has a rich tradition of use by religious leaders, kings, philosophers,
scientists, engineers, and auto mechanics.
It never ceases to amaze me when experts try to defend their lack of
communication skills with that excuse. Using the term 'Layperson' just adds to
it- as if engineering (in this case) is somehow mystical and only initiates may
have an understanding of the principles involved. Engineers may go to school to
'Fully' grasp engineering concepts, but a full grasp of a concept is rarely
needed to understand how or why a system functions. Just because you may not
be able to communicate an idea simply and succinctly, don't try to tell the
listener that it's his lack of intelligence and training that prevents him from
understanding you. How many people here have found an idea impossible to
understand with one teacher, and simplistic from another?
>There is no room for niceties in science (mostly...)
Why is _Science_ exempt from courtesy and tact? Can you not tell someone that
they are in error nicely? Or is that you feel so above everyone else that you
can't be bothered other people's feelings?
People with specialized skills are often rude and uncommunicative. Why?
Because the rest of us are too overawed to call them on their boorishness. Is
it acceptable for a french waiter to be rude just because you aren't fluent in
french? The fact that one person has an area of expertise outside of
another's, doesn't give them the right to be a jerk.
Regards,
John Marcos
Jm1226
>
>Why is _Science_ exempt from courtesy and tact? Can you not tell someone
>that
>they are in error nicely? Or is that you feel so above everyone else that
>you
>can't be bothered other people's feelings?
Well said.
I'd like to point out that regardless of what I think of the communication
skills of the people on both sides of this wheel theory debate I have learned
enough to know I want to know more about the subject. I plan on purchasing
Jobst's book the next time I see it. So in the long run Jobst has succeded in
his objective of publishing a book on Wheels. I'm willing to spend money on
his product to hopefully gain a better understanding of wheel construction.
Even if he did call me a spammer for sending a post to his e-mail and the net
at the same time.
Regards
John
"Talk is cheap, supply always exceeds demand."
Sam Cadby
It is interesting that Brian cannot see he has offended Jobst by
saying that he either disagrees with Jobsts analysis or, alternatively,
that Jobsts book is incomprehensible. One offends Jobst as an engineer,
the other as a writer.
I found the book clear and readable. However, I have a grounding in both
physics and engineering. I have read many engineering texts and many
texts written by engineers for public consumption. There is a difference.
Engineers easily forget that many concepts they take for granted are
alien to the general reader. Most engineers are familiar with the concept
of pre-stressed structures. To a non engineers it is a sophisticated and
counterintuitive concept.
The fact that Brian has not fully understood the concepts and models in
the book can't be blamed on his failing to read it properly, nor on the
book being too complicated. The complexity of the subject doesn't lend
itself to nontechnical explanation and that Jobst has produced a book
that is so readable is a feat that shouldn't be overlooked.
sam
PS Can we start talking about bikes again soon?
Alex Rodriguez
bt...@columbia.edu says...
>A wheel definitely hangs on upper spokes in the sense that there is
>tension instead of compression in those spokes. And it does not seem too
>likely that tension in lower spokes will turn into compression when that
>section of the wheel meets the ground. Otherwise, the spoke would press
>the nipple into the tire.
I don't think you quite understand the way a wheel works. The load of
a bicycle wheel is supported by the bottom spokes. A spoke that is in
tension can be compressed until there is no more tension in the spoke.
When the load exceeds the tension in the spoke, then the spoke tends to
collapse.
>I am prepared to say that every spoke contributes a certain amount of
>tension to the hub, which combine to support the bike.
If this was the case, then the spoke tension in all the spokes would change
when you place a load through the hub. If you check the tension of the
spokes on a wheel that is unloaded and then loaded you will find that only
the bottom spokes have a change in tension. This can be done by plucking
spokes and listening for a change in the tone before and after a load is
applied.
--
-----------------
Alex __O
_-\<,_
(_)/ (_)
Jobst Brandt
>> That is one definition of Tension and Compression, but not the only
>> (correct) one. When I mount my bike, my weight makes the bottoms
>> of my tires compress - no wait, they tire casings in their natural
>> state are limp and flat until they became expanded under pneumatic
>> pressure - I guess my tires weren't compressed, they were simply
>> less expanded.
>> Tension and compression can be used as relative terms as well as
>> absolute. To make a distinction, you have to add a qualifier, like
>> "net" or "delta". For a wheel, loads are supported by a delta
>> compression of the bottom spokes, although these spokes remain in
>> net tension.
> I offer this not to refute what you have offered, but simply to
> state my (uncharacteristic) desire for purity of concept.
> 20 or so years ago, when I used to analyze structural failures for a
> living, it was imperative to differentiate between tension and
> compression in structural members because long, thin members (like
> spokes) behave so differently in these conditions.
Structural analysis has not changed in 20 years although computational
capabilities have. Compression of tensioned elements has remained
unchanged since the time (20 years ago) of writing "The Bicycle
Wheel". I sense an implication that the "hanging from the top"
concept is valid in under your terminology. Could you clarify that.
> You know the saying, when all you know is a hammer, everything
> begins to look like a nail? Well, all I know (in engineering) is
> static structures, so I think of bicycle wheels as structures and
> want to apply my early lessons to them.
The bicycle wheel is a static structure. Its dynamics have nothing
to do with load distribution or its analysis. You'll not find any
difference in the stress distribution whether standing or rolling.
Jobst Brandt <jbr...@hpl.hp.com>
Rick Knowlan
> snip
> I sense an implication that the "hanging from the top"
> concept is valid in under your terminology. Could you clarify that.
Not intended. My point is simply that using compression as a relative
term misleads some into thinking that the user is implying that spokes
carry compressive axial loads, which in turn sparks debate. I've been in
that one before.
I'm not debating your analysis of the wheel, and anyone who has tackled
the old "beam on an elastic foundation" problem from undergrad statics can
appreciate the load distribution in a spoked bicycle wheel.
> The bicycle wheel is a static structure. Its dynamics have nothing
> to do with load distribution or its analysis. You'll not find any
> difference in the stress distribution whether standing or rolling.
Even a lowly civil engineer can fathom that--many "static" structures have
moving loads that impose varying member stresses.
Rick Knowlan
**************************************
The Periodic Table for Civil Engineers:
Fire-----Air-----Water-----Earth
(And we control three of them!)
Mike Bales
compression on the bottom spokes happen? The spokes push out from the
inner side of the wheel don't they? So if the hub is coming down a tiny
bit, wouldn't the spokes on the bottom just poke up that tiny bit? How
could they come under compression, wouldn't they just move?
Rick Knowlan
You're correct.
The problem is semantic--those who speak of the spoke being in "compression"
mean it as a relative, not absolute term. As a loaded wheel rotates and the
spokes go from top to bottom, the load on them changes from a high tension
load (due to the pre-tensioning of the spokes) to a lower tension load.
They don't actually go into true compression--but they do shorten a little
as the tension is reduced, in effect "compressing" compared to the
"stretched" state they are in anywhere but at the bottom of a loaded wheel.
For anyone who questions the misunderstandings caused by using the term
"compression" when they really mean a decrease in tension, I rest my case.
Rick Knowlan
I'm getting really Compressed about this.
Jobst Brandt
>> I just took apart my first wheel in prep to build my first. How
>> does compression on the bottom spokes happen? The spokes push out
>> from the inner side of the wheel don't they? So if the hub is
>> coming down a tiny bit, wouldn't the spokes on the bottom just poke
>> up that tiny bit? How could they come under compression, wouldn't
>> they just move?
> You're correct.
That is not correct. Spokes do not protrude from the rim when the
wheel is loaded unless it is in a state of collapse.
> The problem is semantic--those who speak of the spoke being in
> "compression" mean it as a relative, not absolute term.
No one said that spokes are in compression semantically or otherwise.
You are misrepresenting what has been said and are muddling the matter
for the reader who apparently did not see the earlier part of this
thread.
> As a loaded wheel rotates and the spokes go from top to bottom, the
> load on them changes from a high tension load (due to the
> pre-tensioning of the spokes) to a lower tension load.
You present an incorrect picture of load distribution in that there
is no gradual change from higher to lower tension. The change is
fairly abrupt and is concentrated in a short zone comprising about
four to five spokes that carry the load in a 36 spoke wheel.
> They don't actually go into true compression--but they do shorten a
> little as the tension is reduced, in effect "compressing" compared
> to the "stretched" state they are in anywhere but at the bottom of a
> loaded wheel.
> For anyone who questions the misunderstandings caused by using the
> term "compression" when they really mean a decrease in tension, I
> rest my case.
None too soon. I believe that such explanations serve only to confuse
the issue rather than clear up any misconceptions. With help like
this the task of clarifying the structure of the wire spoked wheel doe
not get easier.
> I'm getting really Compressed about this.
I can tell you are taking this seriously.
Jobst Brandt <jbr...@hpl.hp.com>
Rick Denney
>I just took apart my first wheel in prep to build my first. How does
>compression on the bottom spokes happen? The spokes push out from the
>inner side of the wheel don't they? So if the hub is coming down a tiny
>bit, wouldn't the spokes on the bottom just poke up that tiny bit? How
>could they come under compression, wouldn't they just move?
You are making the very common mistake of confusing compressive
*loads* with compressive *stress*. A spoke with no compressive
strength will always be under either tensile stress or it will be
buckling. But a tensile spoke can carry a compressive load.
Try this experiment. Stand toe-to-toe with a mate, and lock hands.
Lean backward until your arms are outstretched. Your arms will be in
tension, and will be the only thing keepin the two of you from falling
over. Now, do this again, but this time have your friend lean back
against a wall. He should just graze the wall when your arms are
outstretched. In this position, no force is being exerted on the wall.
Now, if a third person comes and pushes against your back toward the
wall, your friend will be pushed up against the wall. The force on
your back, which is compressive, is being carried through your arms to
the wall. The force against the wall is compression, also. But your
arms are still in tension. And they will stay in tension as long as
the compressive force against your back is less than the tension in
your arms. You will feel the compression at your back and the tension
in your arms at the same time.
The trick is that your arms are pretensioned, such that the tension is
always greater than the external compressive load. Spokes work in
exactly the same way. We pretension them, using the rim to resist that
pretension, so that they will have enough tension to stay tensile
under compressive loads.
The above experiment does not say everything about how wheels work,
but it does illustrate how a tensile member, under only tensile
stress, can carry a compressive load.
Rick "Shall we dance?" Denney
Mike Bales
I don't think you quite understand the way a wheel works. The load of
a bicycle wheel is supported by the bottom spokes.
How is the load supported by the bottom spokes? What prevents them from
pushing outward into the tire? Being attached to the hub? What
prevents the hub from moving down to let the spokes push outward into
the tire? The hub being attached to the top spokes that don't allow the
hub to go down that doesn't allow the bottom spokes to go down into the
tire.
If the load of a bicycle wheel is supported by the bottom spokes, then
you should be able to cut all the top spokes and you would be alright
until you rolled, right? I'll bet all the bottom spokes would poke
through though.
Jobst Brandt
That's fair enough since that is the way the bicycle wheel has been
perceived for the last 100 or more years but in light of what is known
today the bicycle wheel does not work that way. Rather than try to
say it in a "nutshell" and start a new contentious thread, please drop
in on your local bike shop or library and read "the Bicycle Wheel".
You will find that the wheel, in fact, does stand on the bottom spokes.
You can also get this book at:
----------------------------------------------------------------------
"the Bicycle Wheel" is available in most bicycle shops in the US.
ISBN 0-9607236-6-8) English
ISBN 0-9607236-4-1) German
Author: Jobst Brandt, Publisher: Avocet.
Avocet:
It is available through several USA mailorder houses, such as Colorado
Cyclist whose number is (1) 800 688 8600. Velo News Books also lists
it (1) 800 234 8356 ext 6. They accept MASTER & VISA cards.
Amazon Books:
http://www.amazon.com/exec/obidos/ISBN%3D0960723668/002-0079281-4025034
Bokus.com (Finland and Sweden):
The best UK source of books on cycling is 'Bicycling Books' in London,
0181 993 3484, who have in stock most everything available on bicycles
in the English language.
http://www.bikebook.demon.co.uk/catalog/310Scien.html
------------------------------------------------------------------------
velo_souffrance
this semantic scheme which has the lower spokes being in "compression" and
carrying the load themselves, because it sounds to some people like you're
saying that the hub is held up by the lower spokes and that the others are
just along for the ride, as it were. Nobody is arguing with the conclusions
reached in the book, at least in this thread. But why isn't it possible
once and for all to simply start saying that the lower spokes are in
tension, albeit lessened by a compressive force -- but the friggin' things
are of course still in tension, net-net, as your book quantifies. I am sure
you wouldn't disagree with this fellow's statement to the effect that if you
cut the uppermost 16 spokes (in a 32' wheel), it's not going to hold much in
the way of a load. You are taking a fairly simple concept and persistently
rendering it all but impenetrable.
Jobst Brandt wrote in message <7gndom$fk7$3...@hplms2.hpl.hp.com>...
Mike Bales
still sounds interesting.
David T. Blake
>Oh brother, here we %*&^ing go again. It pisses me off that you
>perpetuate this semantic scheme which has the lower spokes being in
>"compression" and carrying the load themselves, because it sounds to
>some people like you're saying that the hub is held up by the lower
>spokes and that the others are just along for the ride, as it were.
Well, no one ever contends that the wheel is anything but a
prestressed structure.
But replace the spokes with something that does not buckle,
such as a wagon wheel without spoke tension, and the load
changes are conceptually the same. The bottom spokes are
compressed when a load is applied. Use a cast spoked wheel,
or a trispoke, or just about any spoked wheel, and the changes
are conceptually the same as long as the spokes are a lot stiffer
than the rim.
The statement "The hub stands on the bottom spokes" is uniquely
true of all spoked wheels and does not require that the spokes
be made of thin stainless steel - although it is equally true for
those wheels.
If you instead insist on stating that the hub hangs from the
top spokes due to a decrease in lower spoke tension, you are
wrong for entire classes of spoked wheels. Besides, it is difficult
for me to imagine how one can state the upper spokes are supporting
the applied load when their tension does not change. The bottom
spokes compress under load with a net force equal in magnitude
and opposite in direction to the applied load - they support
the hub.
--
Dave Blake
dbl...@phy.ucsf.edu
David T. Blake
>True, they just said the spokes were compressed, which is I believe
>contributes to misunderstanding. To avoid the confusion, why not just say
>the spokes are "shortened" when they move to a state of reduced tension?
>That avoids the potential for confusion.
Spokes that compress, compress. If you knew nothing about
the spokes, but could watch their grain structure as they
rotated, you would say the spokes compressed while they were
below the hub. What other term are you supposed to use when
the spokes experience a load change related to their shortening
by their elastic modulus ?
>> The change is fairly abrupt and is concentrated in a short zone
>> comprising about four to five spokes that carry the load in a 36
>> spoke wheel.
>Is it really these 4 or 5 that "carry" it, or is the rest of the
>spokes that carry it in tension?
If you repeatedly load and unload a wheel, only the bottom four spokes
will experience a significant strain cycle. If you carry this
to absurd numbers of loads, the bottom four spokes will break
in fatigue long before the others. They are straining a lot harder
for the load being carried. Their strain is linearly related
to the load. They carry the load.
If you expand this to a wagon wheel, the spokes experience
analogous changes in tension/compression. They also shorten
when they cross the LAZ by an amount proportional to the load.
But they are in true compression at the bottom. Or are they ?
How would you even know ? The load changes are the same;the spokes
also compress in the LAZ.
--
Dave Blake
dbl...@phy.ucsf.edu
Kiwi Jack
wrote:
>"velo_souffrance" <velo_so...@hotmail.com> writes:
>>Oh brother, here we %*&^ing go again. It pisses me off that you
>>perpetuate this semantic scheme which has the lower spokes being in
>>"compression" and carrying the load themselves, because it sounds to
>>some people like you're saying that the hub is held up by the lower
>>spokes and that the others are just along for the ride, as it were.
>Well, no one ever contends that the wheel is anything but a
>prestressed structure.
[snip]
>The statement "The hub stands on the bottom spokes" is uniquely
>true of all spoked wheels and does not require that the spokes
>be made of thin stainless steel - although it is equally true for
>those wheels.
By stating (continuously) that the hub "stands" on the bottom spokes,
you are lending to the confusion, and suggesting that the upper spokes
are not necessary, when in fact the spoked wheel is a system, and you
cannot discount the contribution of the other spokes.
Rick Knowlan
> Rick Knowlan writes:
>
> >> I just took apart my first wheel in prep to build my first. How
> >> does compression on the bottom spokes happen? The spokes push out
> >> from the inner side of the wheel don't they? So if the hub is
> >> coming down a tiny bit, wouldn't the spokes on the bottom just poke
> >> up that tiny bit? How could they come under compression, wouldn't
> >> they just move?
>
> > You're correct.
>
> That is not correct. Spokes do not protrude from the rim when the
> wheel is loaded unless it is in a state of collapse.
They would if they were required to carry a net compression load--which is
what I believe the writer inferred by (understandably, in my opinion)
thinking about the NET load on the spokes, rather than the direction of
the delta in the load affected zone.
>
> > The problem is semantic--those who speak of the spoke being in
> > "compression" mean it as a relative, not absolute term.
>
> No one said that spokes are in compression semantically or otherwise.
True, they just said the spokes were compressed, which is I believe
contributes to misunderstanding. To avoid the confusion, why not just say
the spokes are "shortened" when they move to a state of reduced tension?
That avoids the potential for confusion.
>
> You are misrepresenting what has been said
That's not my intention.
> and are muddling the matter
> for the reader who apparently did not see the earlier part of this
> thread.
I think the misunderstanding was very apparent in the writer's post, and
in several earlier threads, some of which resulted in a lot more "heat"
than "light".
> > As a loaded wheel rotates and the spokes go from top to bottom, the
> > load on them changes from a high tension load (due to the
> > pre-tensioning of the spokes) to a lower tension load.
>
> You present an incorrect picture of load distribution in that there
> is no gradual change from higher to lower tension.
Who said anything about the change being gradual? Not I. Now who's
"misrepresenting"? :-)
> The change is
> fairly abrupt and is concentrated in a short zone comprising about
> four to five spokes that carry the load in a 36 spoke wheel.
Is it really these 4 or 5 that "carry" it, or is the rest of the spokes
that carry it in tension?
>
> > They don't actually go into true compression--but they do shorten a
> > little as the tension is reduced, in effect "compressing" compared
> > to the "stretched" state they are in anywhere but at the bottom of a
> > loaded wheel.
I note that you did not comment on this paragraph, and I'm curious as to
why not. Does it correctly explain the physical situation? Or is the
root of my disagreement here based on my own misunderstanding of what
happens in the wheel? Please comment.
> > For anyone who questions the misunderstandings caused by using the
> > term "compression" when they really mean a decrease in tension, I
> > rest my case.
>
> None too soon. I believe that such explanations serve only to confuse
> the issue rather than clear up any misconceptions. With help like
> this the task of clarifying the structure of the wire spoked wheel doe
> not get easier.
Based on the number of threads on this topic I've seen here in 3 months,
it wasn't particularly easy before I joined in. I suspect there is any
easier way to explain the structure, and I'll keep trying.
>
> > I'm getting really Compressed about this.
>
> I can tell you are taking this seriously.
Not seriously enough to get upset about it, but the prospect of finding a
better explanation for the physical phenomenon is enticing.
Rick Knowlan
Mark Hickey
>Oh brother, here we %*&^ing go again. It pisses me off that you perpetuate
>this semantic scheme which has the lower spokes being in "compression" and
>carrying the load themselves, because it sounds to some people like you're
>saying that the hub is held up by the lower spokes and that the others are
>just along for the ride, as it were. Nobody is arguing with the conclusions
>reached in the book, at least in this thread. But why isn't it possible
>once and for all to simply start saying that the lower spokes are in
>tension, albeit lessened by a compressive force -- but the friggin' things
>are of course still in tension, net-net, as your book quantifies. I am sure
>you wouldn't disagree with this fellow's statement to the effect that if you
>cut the uppermost 16 spokes (in a 32' wheel), it's not going to hold much in
>the way of a load. You are taking a fairly simple concept and persistently
>rendering it all but impenetrable.
Jobst is right, and there would be no confusion if the entire r.b.t.
"community" was made up of nothing but other engineers...
However, it's sometimes necessary (dare I say, prudent) to
occasionally couch concepts such as this one in terms that are
understandable, even when they don't necessarily describe a phenomenon
in exactly the same way an engineer might.
Describing the "compression" of the lower spokes as "reduction of
tension" really does speak to the same phenomenon, and is easier for
the layman to understand - no harm done.
Another analogy to help illustrate the principle...
Picture a see-saw, with a spring vertically attached from the ground
to one end. On the other end of the see-saw sits a bicycle rider. It
has to be a bicycle rider because this is about bicycle wheels...
Anyway, the weight of the rider causes the spring to extend until the
"system" achieves equilibrium. This is similar to the tension on the
lower spokes in a bicycle wheel at rest.
Now, because the rider on the see-saw rides MTBs, it's only a matter
of time before his end of the see-saw hits a bump (the fact that a
non-moving see-saw can hit a bump calls for suspension of disbelief,
but go with it, OK?). When this happens, the rider moves up, causing
the other end of the see-saw to move down, *compressing* the spring.
This compression of the spring is really the only change in the
see-saw "system". The spring is analagous to the lower spokes of a
bicycle wheel, in that it "supports" the rider's weight, and that it
"compresses" to absorb the impact. In both a bicycle wheel, and in
this silly example, the "system" reacts in a non-linear way once all
the spring's tension is overcome.
In a wagon wheel, the analagous see-saw model would place the spring
UNDER the rider, and the bump would *compress* the spring as well.
But it's easy to see it's effectively the same mechanism whether the
spring starts in tension or in compression.
Mark Hickey
Habanero Cycles
http://www.cynetfl.com/habanero/
Home of the $695 ti frame
Alex Rodriguez
mike_...@americancentury.com says...
>
>Alex wrote:
>I don't think you quite understand the way a wheel works. The load of
>a bicycle wheel is supported by the bottom spokes.
>
>How is the load supported by the bottom spokes? What prevents them from
>pushing outward into the tire? Being attached to the hub? What
>prevents the hub from moving down to let the spokes push outward into
>the tire? The hub being attached to the top spokes that don't allow the
>hub to go down that doesn't allow the bottom spokes to go down into the
>tire.
As long as the load on the hub does not cause the spokes to go completely
slack, the spokes will not push outward.
>If the load of a bicycle wheel is supported by the bottom spokes, then
>you should be able to cut all the top spokes and you would be alright
>until you rolled, right? I'll bet all the bottom spokes would poke
>through though.
Not quite. The way the bottom spokes support the load placed on the wheel
is by losing tension, also called being compressed. When you cut the top
spokes, the bottom spokes no longer are tensioned so they cannot lose tension.
So once you cut the top spokes, the bottom ones can no longer support a load.
You have to understand that being compressed and losing tension are the same.
Stergios Papadakis
>
snip
>
> On Fri, 30 Apr 1999, Rick Denney wrote:
> > >After reading all the debate, a few observations:
> > >
> > >Does it really matter whether a wheel stands on 4 or 8 spokes or it
> > >hangs on 4 or 8 spokes in motion, as long as we know round and true wheels
> > >are fast wheels?
> >
> > Yes, it really matters. Because when you understand that spokes carry
> > the load of the rider in compression, then you understand why the
> > tension in the spoke must exceed the compressive load that it carries
> > to keep from buckling. And you you'll understand that spokes
> > experience their highest stress while unloaded. If you understand
>
> Well,that I am not sure. the top spokes in the middle have greater
> tension under load.
No, they do not.
>
> > those things, then you'll understand why the strongest wheel has
> > spokes as tight as the rim will allow.
>
> Now I know everything. So WHAT?!
So explain WHY tight spokes make stronger wheels. From
your comments, I think you are unable to do so.
> >
> > If you miss the point along the way, you won't understand why spokes
> > need to be so tight, and you will build wheels (or support those who
> > build wheels) looser than they need to be. And you'll never understand
>
> Who supports those who build loose wheels?
>
> > why these wheels can't stay in true, which will eventually lead you to
> > your need for round the true wheels. You will believe that thicker
> > spokes make stronger wheels, or even that thicker spokes can add the
>
> Of course thicker spokes make stronger wheels, They can stand greater
> tension, can't they?
They can stand greater tension without breaking, but
that does NOT make the wheel stronger because the tensile stress
in each spoke of a loaded wheel is either equal to or
LESS THAN the tensile stress in the spokes of a wheel that is
not loaded. The wheel collapses when the tensile stress in the
spokes goes to zero, not when the tensile stress increases.
>
> > strength that a lighter rim takes away. These myths stem from thinking
> > that wheels hang from the spokes.
> >
>
> Again, whether wheels hang or stand depend on the way people put it.
>
> > >
>
snip
>
> > spoke. When you look at a Tri-Spoke, are the spokes in tension or not?
>
> They are not, of course. I can tell by looking. Because they are not
> spokes in the sense we talk aobut. They are more like beams in car wheels.
> Spinergy is different. SPinergy and ordanary wheels keep sructure
> integrity by a triangular structure in the cross section. And that takes
> tension.
No, you cannot tell by looking whether or not the
spokes of a tri-spoke are in tension. Why do
you think you can? Can you tell by looking that
a regular spoked wheel is undertensioned?
>
> > You can't tell by looking, and you can't tell by feeling. If the wheel
> > stands on the bottom spokes, then it doesn't matter.
> >
> ??
>
> > >
Stergios
Andy Hong
> Now, because the rider on the see-saw rides MTBs, it's only a matter
> of time before his end of the see-saw hits a bump (the fact that a
> non-moving see-saw can hit a bump calls for suspension of disbelief,
^^^^^^^^^^^^^^^^^^^^^^^
Suspension of disbelief? Is that some kind of new, single-pivot,
full-suspension technology made from un-obtainium?
Oh, I couldn't resist.
> Mark Hickey
> Habanero Cycles
> http://www.cynetfl.com/habanero/
> Home of the $695 ti frame
(BTW Mark, your whole see-saw analogy reminds me of the Softride seat
suspension system.)
--
andyhong <666 at whatev dot com> -- replace " at " and " dot " with @ and .
Jobst Brandt
>>>> I just took apart my first wheel in prep to build my first. How
>>>> does compression on the bottom spokes happen? The spokes push
>>>> out from the inner side of the wheel don't they? So if the hub
>>>> is coming down a tiny bit, wouldn't the spokes on the bottom just
>>>> poke up that tiny bit? How could they come under compression,
>>>> wouldn't they just move?
>>> You're correct.
>> That is not correct. Spokes do not protrude from the rim when the
>> wheel is loaded unless it is in a state of collapse.
> They would if they were required to carry a net compression
> load--which is what I believe the writer inferred by
> (understandably, in my opinion) thinking about the NET load on the
> spokes, rather than the direction of the delta in the load affected
> zone.
No one said that these spokes are n compression. You added that
although all those who understand this matter have repeatedly
explained that no spokes are in compression... besides the fact that
you know that and, if you have followed this, have plucked spokes and
found that the bottom spokes give an audible tone that proves they are
in tension. I think you are belaboring what Mark McMasters wrote.
>>> The problem is semantic--those who speak of the spoke being in
>>> "compression" mean it as a relative, not absolute term.
>>
>> No one said that spokes are in compression semantically or otherwise.
> True, they just said the spokes were compressed, which is I believe
> contributes to misunderstanding. To avoid the confusion, why not
> just say the spokes are "shortened" when they move to a state of
> reduced tension? That avoids the potential for confusion.
>> You are misrepresenting what has been said
> That's not my intention.
>> and are muddling the matter for the reader who apparently did not
>> see the earlier part of this thread.
> I think the misunderstanding was very apparent in the writer's post,
> and in several earlier threads, some of which resulted in a lot more
> "heat" than "light".
I think the misunderstanding is apparent by your posting that spokes
are in compression and poke out of the rim.
>>> As a loaded wheel rotates and the spokes go from top to bottom,
>>> the load on them changes from a high tension load (due to the
>>> pre-tensioning of the spokes) to a lower tension load.
>> You present an incorrect picture of load distribution in that there
>> is no gradual change from higher to lower tension.
> Who said anything about the change being gradual? Not I. Now who's
> "misrepresenting"?
Changing from top to bottom, is gradual. The change does not occur
over that span but AT the bottom.
>> The change is fairly abrupt and is concentrated in a short zone
>> comprising about four to five spokes that carry the load in a 36
>> spoke wheel.
> Is it really these 4 or 5 that "carry" it, or is the rest of the spokes
> that carry it in tension?
Now you ask, after telling how it is. The rest of the spokes allow
the wheel to be tensioned but the load is carried by the bottom
spokes. Sometimes I wonder whether contributors like you know the
difference between asking and telling. Just because there is a
question mark at the end of the sentence doesn't necessarily make it a
question. I think this is part of MAS (male answer syndrome) that puts
everything in a statement of fact even when asking.
>>> They don't actually go into true compression--but they do shorten a
>>> little as the tension is reduced, in effect "compressing" compared
>>> to the "stretched" state they are in anywhere but at the bottom of
>>> a loaded wheel.
> I note that you did not comment on this paragraph, and I'm curious
> as to why not. Does it correctly explain the physical situation?
> Or is the root of my disagreement here based on my own
> misunderstanding of what happens in the wheel? Please comment.
It conflicts with the rest of your explanation and is convoluted in a
way that makes the point you are trying to make difficult to discern.
It seems to be in counterpoint to some other statement that I cannot
find. Commenting on such a statement is difficult without getting
into its irrelevance. The stress in the spokes has been explained in
concise technical detail and in plain English many times in this
thread. What point are you making with this statement?
>>> For anyone who questions the misunderstandings caused by using the
>>> term "compression" when they really mean a decrease in tension, I
>>> rest my case.
>> None too soon. I believe that such explanations serve only to
>> confuse the issue rather than clear up any misconceptions. With
>> help like this the task of clarifying the structure of the wire
>> spoked wheel does not get easier.
> Based on the number of threads on this topic I've seen here in 3
> months, it wasn't particularly easy before I joined in. I suspect
> there is any easier way to explain the structure, and I'll keep
> trying.
Most of these threads are from people who do not believe in reading
the material, at best scanning over it to present a case for "the
wheel hangs from the top spokes" over and over again. It is not from
people who are truly interested in grasping the concepts although such
people are drawn into the discussion inadvertently. Having not heard
about the subject these readers often think that this is a matter not
yet established and that the conventional beliefs of the past are
current. They are not.
>>> I'm getting really Compressed about this.
>> I can tell you are taking this seriously.
> Not seriously enough to get upset about it, but the prospect of
> finding a better explanation for the physical phenomenon is enticing.
I don't think you'll find one from the people who understand it. The
best explanation that I can suggest is in "the Bicycle Wheel" that was
written with great care, and was reviewed by many readers who were
first exposed to the concept, before it went to print. The book
contains computed graphs that make the effects graphically visible.
Some technical people who have publicly expressed an opinion to the
effect that the wheel hangs from the upper spokes, feel compelled to
blindly defend that position, trying to turn back the clock to the
"nobody really knows" times when one could get away with that. These
are the most difficult argumenters, because they put forth their
credentials and fill their prose with related jargon. It doesn't work.
Jobst Brandt <jbr...@hpl.hp.com>
velo_souffrance
with the implication of your statement:
>If you instead insist on stating that the hub hangs from the
>top spokes due to a decrease in lower spoke tension, you are
>wrong for entire classes of spoked wheels.
Seems to me it's either-or. By that I mean, in the wagon wheel, spokes are
compression-capable members so they can take the load without consequence to
the other spokes. In the wire-spoked wheel, you're only dealing with
tension members so the hub must by definition indeed hang from some of the
spokes. But I don't mean to be simplistic; that is pretty obvious to anyone
with phy.ucsf.edu in their email address!
Another way to think of it, which I was trying to allude to in the last post
(one of the earlier people in this thread sort of set this up) is a wheel
with only the lower 50% of the spokes present. Sure, it'll hold _some_
load, but without the upper 50% of the spokes, not much. That's why I was
suggesting that the "hanging from the upper spokes" analogy isn't too far
afield. (Anyhow, here I go yapping about a wheel with only half the spokes,
when it would probably be distinctly impossible to create such a thing. So
whether you have any patience left probably depends on whether you're an
engineer or a scientist.)
In sum:
> The bottom spokes compress under load with a net force equal
> in magnitude and opposite in direction to the applied load
... right on, sir ...
> - they support the hub.
... but I am still having trouble swallowing this as such. Although you've
gotten me closer than other commentators in this thread.
Rick Denney
>In article <372F2A79...@americancentury.com>,
>mike_...@americancentury.com says...
>>
>>Alex wrote:
>>I don't think you quite understand the way a wheel works. The load of
>>a bicycle wheel is supported by the bottom spokes.
>>
>>How is the load supported by the bottom spokes? What prevents them from
>>pushing outward into the tire? Being attached to the hub? What
>>prevents the hub from moving down to let the spokes push outward into
>>the tire? The hub being attached to the top spokes that don't allow the
>>hub to go down that doesn't allow the bottom spokes to go down into the
>>tire.
>
>As long as the load on the hub does not cause the spokes to go completely
>slack, the spokes will not push outward.
>
>
And even then, the spokes won't go into the tire. The pressure in the
tube is quite enough to keep that from happening, and the rim won't
deflect enough to create that kind of a problem unless the wheel
collapses. When tensile stress is lost due to inadequare pretension or
excessive compressive load, the spoke buckles the way all thin columns
do.
Rick "Remember, they have no compressive strength--they can't push
anything without pretension" Denney
Rick Knowlan
> Rick Knowlan writes:
>
> >>>> I just took apart my first wheel in prep to build my first. How
> >>>> does compression on the bottom spokes happen? The spokes push
> >>>> out from the inner side of the wheel don't they? So if the hub
> >>>> is coming down a tiny bit, wouldn't the spokes on the bottom just
> >>>> poke up that tiny bit? How could they come under compression,
> >>>> wouldn't they just move?
>
> >>> You're correct.
>
> >> That is not correct. Spokes do not protrude from the rim when the
> >> wheel is loaded unless it is in a state of collapse.
>
> > They would if they were required to carry a net compression
> > load--which is what I believe the writer inferred by
> > (understandably, in my opinion) thinking about the NET load on the
> > spokes, rather than the direction of the delta in the load affected
> > zone.
>
> No one said that these spokes are n compression. You added that
> although all those who understand this matter have repeatedly
> explained that no spokes are in compression... besides the fact that
> you know that and, if you have followed this, have plucked spokes and
> found that the bottom spokes give an audible tone that proves they are
> in tension. I think you are belaboring what Mark McMasters wrote.
No, just trying to clarify why so many misunderstand.
> >>> The problem is semantic--those who speak of the spoke being in
> >>> "compression" mean it as a relative, not absolute term.
> >>
> >> No one said that spokes are in compression semantically or otherwise.
>
> > True, they just said the spokes were compressed, which is I believe
> > contributes to misunderstanding. To avoid the confusion, why not
> > just say the spokes are "shortened" when they move to a state of
> > reduced tension? That avoids the potential for confusion.
>
> >> You are misrepresenting what has been said
>
> > That's not my intention.
>
> >> and are muddling the matter for the reader who apparently did not
> >> see the earlier part of this thread.
>
> > I think the misunderstanding was very apparent in the writer's post,
> > and in several earlier threads, some of which resulted in a lot more
> > "heat" than "light".
>
> I think the misunderstanding is apparent by your posting that spokes
> are in compression and poke out of the rim.
Please read more closely. That was the previous poster, not I. I'd ask
whether you actually read my post, or whether you were "at best scanning
over it to present a case"?
>
>
> >>> As a loaded wheel rotates and the spokes go from top to bottom,
> >>> the load on them changes from a high tension load (due to the
> >>> pre-tensioning of the spokes) to a lower tension load.
>
> >> You present an incorrect picture of load distribution in that there
> >> is no gradual change from higher to lower tension.
>
> > Who said anything about the change being gradual? Not I. Now who's
> > "misrepresenting"?
>
> Changing from top to bottom, is gradual. The change does not occur
> over that span but AT the bottom.
I don't disagree. My wording wasn't precise enough. You inferred that I
meant a gradual change--I accept responsibility for misleading you about my
intentions with imprecise wording.
>
> >> The change is fairly abrupt and is concentrated in a short zone
> >> comprising about four to five spokes that carry the load in a 36
> >> spoke wheel.
>
> > Is it really these 4 or 5 that "carry" it, or is the rest of the spokes
> > that carry it in tension?
>
> Now you ask, after telling how it is. The rest of the spokes allow
> the wheel to be tensioned
This may be the crux of my misunderstanding. I am sincerely asking for your
clarification here, not arguing with you. Please give this some thought:
In a freebody diagram of a hub in a (loaded) conventional bicycle wheel,
drawn with the actual forces acting on it (including the tensions in all the
spokes), what would balance the downward force exerted by the forks?
> but the load is carried by the bottom
> spokes. Sometimes I wonder whether contributors like you know the
> difference between asking and telling. Just because there is a question
> mark at the end of the sentence doesn't necessarily make it a question. I
> think this is part of MAS (male answer syndrome) that puts everything in a
> statement of fact even when asking.
Sorry, I haven't had the operation yet. :-)
If it will make it easier to respond to my questions, post a template for the
kind of sentence that doesn't offend you and I'll reconfigure my questions to
take all the "MAS" out of them.
> >>> They don't actually go into true compression--but they do shorten a
> >>> little as the tension is reduced, in effect "compressing" compared
> >>> to the "stretched" state they are in anywhere but at the bottom of
> >>> a loaded wheel.
>
> > I note that you did not comment on this paragraph, and I'm curious
> > as to why not. Does it correctly explain the physical situation?
> > Or is the root of my disagreement here based on my own
> > misunderstanding of what happens in the wheel? Please comment.
>
> It conflicts with the rest of your explanation
Mine, or the previous poster's?
> and is convoluted in a
> way that makes the point you are trying to make difficult to discern.
See freebody diagram question above. This may clarify your correctness and
my mistake once and for all.
snip
>> I can tell you are taking this seriously.
>
> > Not seriously enough to get upset about it, but the prospect of
> > finding a better explanation for the physical phenomenon is enticing.
>
> I don't think you'll find one from the people who understand it. The
> best explanation that I can suggest is in "the Bicycle Wheel" that was
> written with great care, and was reviewed by many readers who were
> first exposed to the concept, before it went to print. The book
> contains computed graphs that make the effects graphically visible.
>
> Some technical people who have publicly expressed an opinion to the
> effect that the wheel hangs from the upper spokes,
Not my position. It doesn't help when you attribute mistakes made by others
to me.
> feel compelled to
> blindly defend that position, trying to turn back the clock to the
> "nobody really knows" times when one could get away with that. These
> are the most difficult argumenters, because they put forth their
> credentials and fill their prose with related jargon. It doesn't work.
Please answer the freebody diagram question above, directly and without
insulting me, labelling it as an irrelevant question, or questioning my
objectivity, and maybe you'll win a convert to your explanation.
Rick Knowlan
Rick Knowlan
> Rick Knowlan <rickk...@home.com> writes:
>
> >True, they just said the spokes were compressed, which is I believe
> >contributes to misunderstanding. To avoid the confusion, why not just say
> >the spokes are "shortened" when they move to a state of reduced tension?
> >That avoids the potential for confusion.
>
> Spokes that compress, compress. If you knew nothing about
> the spokes, but could watch their grain structure as they
> rotated, you would say the spokes compressed while they were
> below the hub. What other term are you supposed to use when
> the spokes experience a load change related to their shortening
> by their elastic modulus ?
I guess that depends on what you're trying to describe. If you're talking
about net load or stress, spokes in a stable wheel don't go into compression.
And that's what's confusing to some.
> >> The change is fairly abrupt and is concentrated in a short zone
> >> comprising about four to five spokes that carry the load in a 36
> >> spoke wheel.
>
> >Is it really these 4 or 5 that "carry" it, or is the rest of the
> >spokes that carry it in tension?
>
> If you repeatedly load and unload a wheel, only the bottom four spokes
> will experience a significant strain cycle. If you carry this
> to absurd numbers of loads, the bottom four spokes will break
> in fatigue long before the others. They are straining a lot harder
> for the load being carried. Their strain is linearly related
> to the load. They carry the load.
Perhaps you can answer the question I put to Jobst.
In a freebody diagram of a hub in a (loaded) conventional bicycle wheel,
drawn with the actual forces acting on it (including the tensions in all the
spokes), what would balance the downward force exerted by the forks?
> If you expand this to a wagon wheel, the spokes experience
> analogous changes in tension/compression. They also shorten
> when they cross the LAZ by an amount proportional to the load.
> But they are in true compression at the bottom. Or are they ?
> How would you even know ?
Maybe by drawing a freebody diagram of the hub?
Rick Knowlan
Tense, not compressed. :-)
Rick Denney
>I guess that depends on what you're trying to describe. If you're talking
>about net load or stress, spokes in a stable wheel don't go into compression.
>And that's what's confusing to some.
>
Let's forget about stress for a moment. Let's only talk about total
load.
L = C + T
where
L = Load-carrying capacity.
C = Compressive strength (maximum load, not stress) of spokes in
load-affected zone.
T = Total pretension of spokes in load-affected zone.
All units are force.
This works for all spoked wheels.
For wire-spoked wheels,
L = 0 + T, or L = T
This is because wire spokes cannot bear compressive stress, so C is
zero.
For Tri-Spokes (assuming the spokes are not tensioned),
L = C + 0, or L = C
This equation also works for wooden wagon wheels.
But these are boundary conditions, and anything in between will also
work.
So, the purpose of the spokes outside the load-affected zone is to
supply pretension to the spokes in the load-affected zone and nothing
more. By loading the rim symmetrically, they keep the structure
stable. Because of elastic deflection of the rim, their tension does
not change under any radial loading conditions.
You can also represent the reserve load-carrying capacity (Lr) as
Lr = C + T - La
where
Lr = unused load-carrying capacity,
C = compressive strength of the spokes in the load-affected zone
T = pretension of spokes in load-affected zone, and
La = Applied radial load.
Again, this equation works for all spoked wheels.
If we think of the applied load as having the effect of reducing the
tension in a wire spoke, then we can say that
Lr = 0 + T - La
Lr = T - La
From this, we can see that when C=0, the pretension must always be
greater than the applied load, or the reserve load-carrying capacity
will go negative (the spokes will buckle). We also see that C=0 is a
special case, and when we write things down in this way, and describe
them using the terms that we do, we can be more general, and we stay
focused on what's really important.
Note that I can use this simple relationship to understand everthing I
need to know about what makes wheels strong, and I never once deal
with any spokes but the spokes in the load-affected zone. The only
purpose of having tension in the other spokes is to supply pretension
to those spokes.
Rick "Unstressed" Denney
Rick Knowlan
> snip
>
> Note that I can use this simple relationship to understand everthing I
> need to know about what makes wheels strong, and I never once deal
> with any spokes but the spokes in the load-affected zone. The only
> purpose of having tension in the other spokes is to supply pretension
> to those spokes.
>
I don't disagree, although I admit I haven't gone over it critically.
I'm not disputing the analysis Jobst has done--just the misleading (I think)
statement that a bicycle wheel is "supported by the bottom spokes".
You may be able to help clear this up.
In a freebody diagram of a hub in a (loaded) conventional bicycle wheel,
drawn with the actual forces acting on it (including the tensions in all the
spokes), what would balance the downward force exerted by the forks?
Let's make it simple. Consider a radially laced wheel with 12 spokes as the test
case. We can then extrapolate the findings to more spokes.
Rick Knowlan
Mark Hickey
>mhi...@cynetfl.com (Mark Hickey) writes:
>
>> Now, because the rider on the see-saw rides MTBs, it's only a matter
>> of time before his end of the see-saw hits a bump (the fact that a
>> non-moving see-saw can hit a bump calls for suspension of disbelief,
>
> ^^^^^^^^^^^^^^^^^^^^^^^
>Suspension of disbelief? Is that some kind of new, single-pivot,
>full-suspension technology made from un-obtainium?
It's the latest thing.... just gotta figure out how to make a 2x12"
board corner better.
>Oh, I couldn't resist.
I'll bet you didn't try too hard.... ;-)
>(BTW Mark, your whole see-saw analogy reminds me of the Softride seat
>suspension system.)
Ooops, hope I didn't infringe on any patents....
Rick Denney
Rick, you complain about the way it's describe being confusing to
normal people, and then you support your argument with a free-body
diagram? Have you ever tried to explain a free-body diagram to a lay
person? I have, but I was unsuccessul.
Even so, I can explain to non-technical people (and have done) how
tensile spoke carry compressive loads, and they understand it. They
also understand why the spokes, therefore, have to be tighter than the
loads they carry. When they understand that, they start tensioning
their wheels properly. And that's the objective of using words to
descibe these things--so that normal people can learn to build their
own wheels, and understand why they build them the way they do.
Free-body diagrams don't help here.
But, what the hell. You have force vectors radiating out fromt the hub
representing the tension in the spokes. Without the external load,
they are equal and large. You have a relatively small force vector in
the downward direction representing load. The force vector for the
spoke at the bottom is smaller by the same amount as the load, and you
have your balanced free-body. But remember that compression = negative
tension. If the vectors for the unloaded wheel's spokes were magnitude
zero (representing a wagon wheel), then the reduction in tension would
be negative (which equals compression), and the force vector from the
bottom spoke would point upward instead of pointing downward less as
it does with a prestressed wire spoke. But in both cases, the story is
still told by the bottom spoke.
Rick "Standing on my bottom spokes" Denney
Jobst Brandt
>>> Is it really these 4 or 5 that "carry" it, or is the rest of the
>>> spokes that carry it in tension?
>> Now you ask, after telling how it is. The rest of the spokes allow
>> the wheel to be tensioned
> This may be the crux of my misunderstanding. I am sincerely asking
> for your clarification here, not arguing with you. Please give this
> some thought:
> In a freebody diagram of a hub in a (loaded) conventional bicycle
> wheel, drawn with the actual forces acting on it (including the
> tensions in all the spokes), what would balance the downward force
> exerted by the forks?
An increase in upward force in the bottom spokes. You can test this by
plucking spokes to see which ones change and in which direction (more
or less tension)... but of course you knew that because, as you say,
you read "the Bicycle Wheel" where all these points are explained and
graphically shown.
I don't want to be rude but did you read the book and why are you
asking exactly the questions that I spent much effort writing to
explain exactly the questions you pose here?
Jobst Brandt <jbr...@hpl.hp.com>
David T. Blake
>Perhaps you can answer the question I put to Jobst.
>
>In a freebody diagram of a hub in a (loaded) conventional bicycle wheel,
>drawn with the actual forces acting on it (including the tensions in all the
>spokes), what would balance the downward force exerted by the forks?
Start with a freebody diagram of the unloaded wheel. All spoke
tensions cancel. For each spoke call this the zero point, since
the operating point is irrelevant as long as we stay within
the linear range for the member.
Then load the wheel, and draw a freebody diagram of the
hub using the changes in force due to the load.
There is a downward force applied by the fork.
There are compression forces from the spokes below the
hub ie: they compress by shortening, and they act with
an upward force on the hub. These forces very very nearly are
equal in magnitude and opposite in direction.
If you can't think in terms like these because you are uncomfortable
with negative numbers, you may not want to do much engineering
design work. Because you will forever insist that these are
decreases in tension and the hub must hang from the upper spokes.
Unless the spokes are in absolute compression, in which case the
force changes are identical, but the hub is then standing on the
bottom spokes. But these cases are identical except for the
tensile operating point of the spokes.
>> If you expand this to a wagon wheel, the spokes experience
>> analogous changes in tension/compression. They also shorten
>> when they cross the LAZ by an amount proportional to the load.
>> But they are in true compression at the bottom. Or are they ?
>> How would you even know ?
>Maybe by drawing a freebody diagram of the hub?
We can conceptually create a wagon wheel in which the spokes
can thread into either the hub or rim, and an arbitrary
compression state for the spokes can be used. In this wheel,
the spokes can work in compression, or in tension, or in an
unstressed state. I present you with this wheel without telling
you the tensile state of the spokes. Then I load it, and ask
you how the load is being supported. Clearly the bottom spokes
are compressed, and this compression is proportional to the force
supported at the hub. But is the hub hanging from the upper spokes
or standing on the bottom spokes ?? I would argue it stands on the
bottom spokes since they are compressed, and this compression generates
a force equal in magnitude and opposite in direction to the
applied load.
The spokes are acting in a similar manner at the rim. The rim flattens
in the LAZ. This flattening is also related to the applied load. The
compressing of the spokes transfers the load from the spokes to the
rim. This portion of the rim has the only significant strain due to
the applied load. The spokes in this portion of the rim have the role
of transferring the load. It is incredibly unclear to me how changes
in load at the other spokes could lead to a flattening of the rim
in the LAZ, if the other spokes were supporting the load.
--
Dave Blake
dbl...@phy.ucsf.edu
David T. Blake
>David, that is doggone well said, except I gotta respectfully take issue
>with the implication of your statement:
>
>>If you instead insist on stating that the hub hangs from the
>>top spokes due to a decrease in lower spoke tension, you are
>>wrong for entire classes of spoked wheels.
>
>Seems to me it's either-or. By that I mean, in the wagon wheel, spokes are
>compression-capable members so they can take the load without consequence to
>the other spokes. In the wire-spoked wheel, you're only dealing with
>tension members so the hub must by definition indeed hang from some of the
>spokes.
Well, a well thought out analysis of the wheel might say that as long
as the spokes cannot buckle or break in tension, the tensile operating
point of the spokes is irrelevant to describing the changes in tension
that occur with a load application. You apply a load, and the spokes
below the hub compress with a sum force equal in magnitude and
opposite in direction to the applied load.
>Another way to think of it, which I was trying to allude to in the
>last post (one of the earlier people in this thread sort of set this
>up) is a wheel with only the lower 50% of the spokes present.
That is then not a wheel at all. A better argument which is sometimes
presented in these threads runs as follows.
Take a tensioned wheel, and load it until the two bottommost spokes
have no tension left at all. Then remove those two spokes. How can
spokes that are not there support the load ??
If you unload such a wheel you will find that it is also no longer a
wheel.
The wheel is a pretensed structure. With thin stainless steel spokes,
it has to be. You can use fatter structures to support a spoked
wheel though, and the mechanics are the same. An applied load
is supported by spokes below the hub as they compress (assuming
the rim compliance is much greater than that of the spokes).
--
Dave Blake
dbl...@phy.ucsf.edu
Jobst Brandt
> I guess that depends on what you're trying to describe. If you're
> talking about net load or stress, spokes in a stable wheel don't go
> into compression. And that's what's confusing to some.
What are you alluding to? Do you mean it is confusing to you or is
this a hypothetical audience for whom you are arguing?
> In a freebody diagram of a hub in a (loaded) conventional bicycle
> wheel, drawn with the actual forces acting on it (including the
> tensions in all the spokes), what would balance the downward force
> exerted by the forks?
In a free body diagram, you can delete all uniformly balanced loads
and have a bicycle wheel that supports the load on the bottom spokes.
The only reason for the tension in the diagram is to allay your
perception of flexible wires. Tension only serves to remove the
buckling effect from these wires, nothing more. Tension is not
necessary for stress analysis of the wheel if you leave out buckling
equations for slender beams.
>> If you expand this to a wagon wheel, the spokes experience
>> analogous changes in tension/compression. They also shorten when
>> they cross the LAZ by an amount proportional to the load. But they
>> are in true compression at the bottom. Or are they ? How would
>> you even know ?
> Maybe by drawing a freebody diagram of the hub?
The same is true for the hub because doubling or halving tension
changes nothing in the net forces, so long as the load does not
slacken any spokes. Spoke tension has no effect on the problem, its
only purpose being to prevent buckling.
Jobst Brandt <jbr...@hpl.hp.com>
Rick Knowlan
> Rick Knowlan writes:
>
>
> > This may be the crux of my misunderstanding. I am sincerely asking
> > for your clarification here, not arguing with you. Please give this
> > some thought:
>
> > In a freebody diagram of a hub in a (loaded) conventional bicycle
> > wheel, drawn with the actual forces acting on it (including the
> > tensions in all the spokes), what would balance the downward force
> > exerted by the forks?
>
> An increase in upward force in the bottom spokes.
>
I'm not sure I get it. See my question below.
> You can test this by
> plucking spokes to see which ones change and in which direction (more
> or less tension)...
Now please explain . . .how can a bottom-of-wheel spoke, which you say
above is in tension, provide an upward force to balance the downward force
imposed by the forks? Wouldn't a spoke that is in tension pull DOWN on the
hub?
Have I missed something?
> but of course you knew that because, as you say,
> you read "the Bicycle Wheel" where all these points are explained and
> graphically shown.
I didn't say I read it.
>
> I don't want to be rude but did you read the book and why are you
> asking exactly the questions that I spent much effort writing to
> explain exactly the questions you pose here?
See my questions above--they'll explain why I'm asking.
Rick Knowlan
I thought I knew up from down, but now I'm wondering.
Rick Knowlan
> Rick Knowlan <rickk...@home.com> wrote:
>
> Rick, you complain about the way it's describe being confusing to
> normal people, and then you support your argument with a free-body
> diagram? Have you ever tried to explain a free-body diagram to a lay
> person? I have, but I was unsuccessul.
>
> Even so, I can explain to non-technical people (and have done) how
> tensile spoke carry compressive loads, and they understand it. They
> also understand why the spokes, therefore, have to be tighter than the
> loads they carry. When they understand that, they start tensioning
> their wheels properly. And that's the objective of using words to
> descibe these things--so that normal people can learn to build their
> own wheels, and understand why they build them the way they do.
> Free-body diagrams don't help here.
Despite protestations, Rick Denny show's he's a real trooper--having a go
at the freebody diagram. I truly appreciate it.
>
>
> But, what the hell. You have force vectors radiating out fromt the hub
> representing the tension in the spokes. Without the external load,
> they are equal and large. You have a relatively small force vector in
> the downward direction representing load. The force vector for the
> spoke at the bottom is smaller by the same amount as the load, and you
> have your balanced free-body.
That's my picture, too.
> But remember that compression = negative
> tension. If the vectors for the unloaded wheel's spokes were magnitude
> zero (representing a wagon wheel), then the reduction in tension would
> be negative (which equals compression), and the force vector from the
> bottom spoke would point upward instead of pointing downward less as
> it does with a prestressed wire spoke. But in both cases, the story is
> still told by the bottom spoke.
I guess my "problem" is that I don't cancel out the equal spoke tension in
the unloaded wheel before I look at what happens when a downward force is
imposed by the forks.
Please bear with me. It sounds to me as though the expression "the wheel
stands on its spokes" is a metaphor chosen to summarize a fairly complex
situation rather than a literal description of which spokes actually
supply that net upward force. Does that make sense to you?
Rick "Euler won't let you do that" Knowlan
Jobst Brandt
>>> This may be the crux of my misunderstanding. I am sincerely asking
>>> for your clarification here, not arguing with you. Please give this
>>> some thought:
>>> In a freebody diagram of a hub in a (loaded) conventional bicycle
>>> wheel, drawn with the actual forces acting on it (including the
>>> tensions in all the spokes), what would balance the downward force
>>> exerted by the forks?
>> An increase in upward force in the bottom spokes.
> I'm not sure I get it. See my question below.
You asked a straight forward question and I gave a precise and direct
answer. You speak of free body diagrams so I assume you know what
that is and understand what force vectors are. If not then I think
you should stay away from engineering jargon that someone feeds you as
a suitable question, otherwise you are just playing word games with
the engineers who are attempting to respond to your questions.
>> You can test this by plucking spokes to see which ones change and
>> in which direction (more or less tension)...
> Now please explain... how can a bottom-of-wheel spoke, which you say
> above is in tension, provide an upward force to balance the downward
> force imposed by the forks? Wouldn't a spoke that is in tension
> pull DOWN on the hub?
I think you are taking me and others who have responded for fools:
"Now please explain... [again]". I don't think so.
> Have I missed something?
Not a bit unless you forgot what you read and responded to.
>> but of course you knew that because, as you say, you read "the
>> Bicycle Wheel" where all these points are explained and graphically
>> shown.
> I didn't say I read it.
Most of it has been repeated in words here by me and other technically
versed people who have taken your postings as sincere questions. I am
not convinced that that is the case.
>> I don't want to be rude but did you read the book and why are you
>> asking exactly the questions that I spent much effort writing to
>> explain exactly the questions you pose here?
> See my questions above--they'll explain why I'm asking.
I think you would be better off if you read it if you are truly
interested in the stress distribution in a wheel, rolling, braking or
pedaling. I can see that no progress is being made here.
> I thought I knew up from down, but now I'm wondering.
I am not amused.
Jobst Brandt <jbr...@hpl.hp.com>
Rick Knowlan
> Rick Knowlan <rickk...@home.com> writes:
>
> >Perhaps you can answer the question I put to Jobst.
> >
> >In a freebody diagram of a hub in a (loaded) conventional bicycle wheel,
> >drawn with the actual forces acting on it (including the tensions in all the
> >spokes), what would balance the downward force exerted by the forks?
>
> Start with a freebody diagram of the unloaded wheel. All spoke
> tensions cancel.
> For each spoke call this the zero point, since
> the operating point is irrelevant as long as we stay within
> the linear range for the member.
>
> Then load the wheel, and draw a freebody diagram of the
> hub using the changes in force due to the load.
You could do that, but you'd misrepresent the true loads. And then you'd think
the loads are carried by the . . . but I'm getting ahead of myself here.
> There is a downward force applied by the fork.
Agreed. Draw the freebody, but don't cancel out the spoke tensions. Now, apply
a load to the forks. What offsets the downward load of the forks? Which spokes
provide the actual upward force, not the delta?
>
>
> There are compression forces from the spokes below the
> hub ie: they compress by shortening, and they act with
> an upward force on the hub. These forces very very nearly are
> equal in magnitude and opposite in direction.
>
> If you can't think in terms like these because you are uncomfortable
> with negative numbers, you may not want to do much engineering
> design work.
Gee, was that really necessary?
> Because you will forever insist that these are
> decreases in tension and the hub must hang from the upper spokes.
> Unless the spokes are in absolute compression, in which case the
> force changes are identical, but the hub is then standing on the
> bottom spokes. But these cases are identical except for the
> tensile operating point of the spokes.
That sounds a little like "the wheel stands on its bottom spokes when you ignore
the tension in spokes".
snip the wagon wheel stuff--I'm talking about a bicycle wheel, in which we know
that the spokes all begin in tension.
Why not draw a full forces freebody diagram, leave the "all forces cancel" step
out, and show the actual forces.
Now, what supports the downward force imposed by the forks?
Rick Knowlan
mark v. hillman
to me. He said that a knowledgeable engineer who truly grasps the concepts
can explain it to a layperson without any problem. I have been in bike
shops and found the salespersons able to be rude with their "specialized
technical knowledge" that they assume I couldn't understand, even though I
have been a daily commuter as long as they've been alive. If you know it,
really understand it, love it, you can explain it if you choose to.....
JCMWeb <jcm...@aol.com> wrote in message
news:19990503081402...@ng-fa1.aol.com...
benco...@ozemail.com.au wrote:
>There will always be
>difficulties in laypeople understanding engineering concepts. Even with
>great logic and mathematical understanding, there are areas that require
>specific training to fully grasp. This is why engineers study at college.
The old: 'It's too complicated for you mere mortals to understand' excuse.
It
has a rich tradition of use by religious leaders, kings, philosophers,
scientists, engineers, and auto mechanics.
It never ceases to amaze me when experts try to defend their lack of
communication skills with that excuse. Using the term 'Layperson' just adds
to
it- as if engineering (in this case) is somehow mystical and only initiates
may
have an understanding of the principles involved. Engineers may go to school
to
'Fully' grasp engineering concepts, but a full grasp of a concept is rarely
needed to understand how or why a system functions. Just because you may
not
be able to communicate an idea simply and succinctly, don't try to tell the
listener that it's his lack of intelligence and training that prevents him
from
understanding you. How many people here have found an idea impossible to
understand with one teacher, and simplistic from another?
>There is no room for niceties in science (mostly...)
Why is _Science_ exempt from courtesy and tact? Can you not tell someone
that
they are in error nicely? Or is that you feel so above everyone else that
you
can't be bothered other people's feelings?
People with specialized skills are often rude and uncommunicative. Why?
Because the rest of us are too overawed to call them on their boorishness.
Is
it acceptable for a french waiter to be rude just because you aren't fluent
in
french? The fact that one person has an area of expertise outside of
another's, doesn't give them the right to be a jerk.
Regards,
John Marcos
David T. Blake
>"David T. Blake" wrote:
>
>> Rick Knowlan <rickk...@home.com> writes:
>>
>>>Perhaps you can answer the question I put to Jobst. In a freebody
>>>diagram of a hub in a (loaded) conventional bicycle wheel, drawn
>>>with the actual forces acting on it (including the tensions in all
>>>the spokes), what would balance the downward force exerted by the
>>>forks?
>>
>> Start with a freebody diagram of the unloaded wheel. All spoke
>> tensions cancel.
>
>> For each spoke call this the zero point, since
>> the operating point is irrelevant as long as we stay within
>> the linear range for the member.
>>
>> Then load the wheel, and draw a freebody diagram of the
>> hub using the changes in force due to the load.
>
>You could do that, but you'd misrepresent the true loads. And then
>you'd think the loads are carried by the . . . but I'm getting
>ahead of myself here.
You are not misrepresenting anything. Your choice of the unstressed
state as the zero point will only serve to complicate the
freebody analysis, if we assume that neither plastic
strain or buckling are possible.
>> There is a downward force applied by the fork.
>Agreed. Draw the freebody, but don't cancel out the spoke tensions.
>Now, apply a load to the forks. What offsets the downward load of
>the forks? Which spokes provide the actual upward force, not the
>delta?
The bottom spokes.
On a rear bicycle wheel, when a driving force is applied at the
pedals, half the drive side spokes increase in tension slightly, the
other half compresses. Which spokes are transferring the load ? If you
say, only the 'pulling' spokes, what do you say when the forces in the
pulling spokes are only half (a little less actually) than the load
applied by the chain ? Which spokes oppose the applied load ? Can a
compression of a pretensed spoke lead to a net force ?
>> There are compression forces from the spokes below the
>> hub ie: they compress by shortening, and they act with
>> an upward force on the hub. These forces very very nearly are
>> equal in magnitude and opposite in direction.
>>
>> If you can't think in terms like these because you are uncomfortable
>> with negative numbers, you may not want to do much engineering
>> design work.
>
>Gee, was that really necessary?
Yes. Because these misunderstandings come about because the
concept of the negative numbers is never appropriately taught to
some people, and this is a prime example.
>That sounds a little like "the wheel stands on its bottom spokes when
>you ignore the tension in spokes".
Pretensed spokes can generate a compressive force.
>snip the wagon wheel stuff--I'm talking about a bicycle wheel, in
>which we know that the spokes all begin in tension.
It is important. I offer that as an example because the only
difference in the analysis is the steady state point. The
wheel is supported by identical changes in force, and in
tension/compression of its spokes.
--
Dave Blake
dbl...@phy.ucsf.edu
Rick Knowlan
> Rick Knowlan writes:
>
> >>> This may be the crux of my misunderstanding. I am sincerely asking
> >>> for your clarification here, not arguing with you. Please give this
> >>> some thought:
>
> >>> In a freebody diagram of a hub in a (loaded) conventional bicycle
> >>> wheel, drawn with the actual forces acting on it (including the
> >>> tensions in all the spokes), what would balance the downward force
> >>> exerted by the forks?
>
> >> An increase in upward force in the bottom spokes.
>
> > I'm not sure I get it. See my question below.
>
> You asked a straight forward question and I gave a precise and direct
> answer. You speak of free body diagrams so I assume you know what
> that is and understand what force vectors are.
You want me to establish my bona fides before continuing? I understand
force vectors and freebody diagrams. I was a professional engineer for a
decade and analyzed structural failures for a living. I wrote my own
structural analysis (stiffness method) program in Fortran IV and used it
for years. I rewrote it in basic and migrated it to one of the early
desktop computers in 1977. I was good at it then, but I wouldn't dare to
hold myself out as a professional today because I'm rusty--I haven't done
it for 18 years.
> If not then I think
> you should stay away from engineering jargon that someone feeds you as
> a suitable question, otherwise you are just playing word games with
> the engineers who are attempting to respond to your questions.
No, I'm not playing word games.
> >> You can test this by plucking spokes to see which ones change and
> >> in which direction (more or less tension)...
>
> > Now please explain... how can a bottom-of-wheel spoke, which you say
> > above is in tension, provide an upward force to balance the downward
> > force imposed by the forks? Wouldn't a spoke that is in tension
> > pull DOWN on the hub?
>
> I think you are taking me and others who have responded for fools:
> "Now please explain... [again]". I don't think so.
How could I play you for a fool? My question is very straightforward. Why
not answer it? How can that make you look foolish?
Wouldn't a bottom spoke that is in tension pull down on the hub?
If so, what balances the downward force imposed by the forks?
Rick Knowlan
Rusty, but neither joking nor hostile.
Ben
tension force is equivalent to an increase in compression force in a
prestressed structure.
The wheel is a prestressed structure in equilibrium, the tension in the
spokes countered by compression in the rim.
The bottom spokes have a reduction in tension due to the downward load of
the forks at equilibrium. The remainder of the spokes have the same tension
force as before the load was applied.
An experiment has been quoted where you pluck your front wheel spokes before
and after applying the load. The only spokes whose pitch changes when
plucked are the bottom spokes.
The concept has been explained in engineering terms and a practical example
has shown that it holds true in the real world. I think it is up to you to
understand it now.
Ben
Rick Knowlan <rickk...@home.com> wrote in message
news:37322845...@home.com...
> "David T. Blake" wrote:
>
> > Rick Knowlan <rickk...@home.com> writes:
> >
> > >Perhaps you can answer the question I put to Jobst.
> > >
> > >In a freebody diagram of a hub in a (loaded) conventional bicycle
wheel,
> > >drawn with the actual forces acting on it (including the tensions in
all the
> > >spokes), what would balance the downward force exerted by the forks?
> >
> > Start with a freebody diagram of the unloaded wheel. All spoke
> > tensions cancel.
>
> > For each spoke call this the zero point, since
> > the operating point is irrelevant as long as we stay within
> > the linear range for the member.
> >
> > Then load the wheel, and draw a freebody diagram of the
> > hub using the changes in force due to the load.
>
> You could do that, but you'd misrepresent the true loads. And then you'd
think
> the loads are carried by the . . . but I'm getting ahead of myself
here.
>
> > There is a downward force applied by the fork.
>
> Agreed. Draw the freebody, but don't cancel out the spoke tensions. Now,
apply
> a load to the forks. What offsets the downward load of the forks? Which
spokes
> provide the actual upward force, not the delta?
>
> >
> >
> > There are compression forces from the spokes below the
> > hub ie: they compress by shortening, and they act with
> > an upward force on the hub. These forces very very nearly are
> > equal in magnitude and opposite in direction.
> >
> > If you can't think in terms like these because you are uncomfortable
> > with negative numbers, you may not want to do much engineering
> > design work.
>
> Gee, was that really necessary?
>
> > Because you will forever insist that these are
> > decreases in tension and the hub must hang from the upper spokes.
> > Unless the spokes are in absolute compression, in which case the
> > force changes are identical, but the hub is then standing on the
> > bottom spokes. But these cases are identical except for the
> > tensile operating point of the spokes.
>
> That sounds a little like "the wheel stands on its bottom spokes when you
ignore
> the tension in spokes".
>
> snip the wagon wheel stuff--I'm talking about a bicycle wheel, in which we
know
> that the spokes all begin in tension.
>
> Why not draw a full forces freebody diagram, leave the "all forces cancel"
step
> out, and show the actual forces.
>
> Now, what supports the downward force imposed by the forks?
>
> Rick Knowlan
>
>
>
Rick Knowlan
Ben wrote:
> Rick, you should (by your qualifications) understand that a reduction in
> tension force is equivalent to an increase in compression force in a
> prestressed structure.
>
> The wheel is a prestressed structure in equilibrium, the tension in the
> spokes countered by compression in the rim.
I understand and agree.
> The bottom spokes have a reduction in tension due to the downward load of
> the forks at equilibrium. The remainder of the spokes have the same tension
> force as before the load was applied.
I understand and agree.
> An experiment has been quoted where you pluck your front wheel spokes before
> and after applying the load. The only spokes whose pitch changes when
> plucked are the bottom spokes.
>
> The concept has been explained in engineering terms and a practical example
> has shown that it holds true in the real world. I think it is up to you to
> understand it now.
I understand the concept: I think the often made statement that "a wheel stands
on its (bottom) spokes" is a misleading metaphor rather than an accurate
description of the phenomenon you have described above. That's my point.
Thanks for your clear, concise and respectful post.
Rick Knowlan
Snoopy
wrote:
>>
>>> Now please explain... how can a bottom-of-wheel spoke, which you say
>>> above is in tension, provide an upward force to balance the downward
>>> force imposed by the forks? Wouldn't a spoke that is in tension
>>> pull DOWN on the hub?
>>
>
>
>If so, what balances the downward force imposed by the forks?
>
The unchanged upward component of tension in the rest of the spokes
outside the load effected zone. SNOOPY
-----------------------------------------------------
Message posted by SNOOPY using 'Forte'
Free Agent V1.11/32- http://www.forteinc.com
------------------------------------------------
David T. Blake
>On Fri, 30 Apr 1999, Rick Denney wrote:
>> Yes, it really matters. Because when you understand that spokes carry
>> the load of the rider in compression, then you understand why the
>> tension in the spoke must exceed the compressive load that it carries
>> to keep from buckling. And you you'll understand that spokes
>> experience their highest stress while unloaded. If you understand
>
>Well,that I am not sure. the top spokes in the middle have greater
>tension under load.
So do the spokes running at 3 o'clock and 9 o'clock. This occurs
because the rim flattens in the load affected zone, making the
rest of the rim take on a larger radius of curvature. The strain
changes of these other spokes are quite small compared to
the strain changes in the LAZ spokes.
>> why these wheels can't stay in true, which will eventually lead you to
>> your need for round the true wheels. You will believe that thicker
>> spokes make stronger wheels, or even that thicker spokes can add the
>
>Of course thicker spokes make stronger wheels, They can stand greater
>tension, can't they?
That is not relevant in the cases of most standard spokes bicycle
wheels. A standard type rim with 36 or 32 spokes can take only a
fraction of the yield strength of the spokes. They never get close
to a failure load, so their strength is irrelevant to the durability
of the wheel. Thicker spokes do NOT make a stronger wheel unless the
wheel strength is limited by the spoke strength - which is not true.
Larger cross-section rims allow higher spoke tensions. They allow
the builder to make a stronger wheel - if he uses a higher spoke
tension.
>> strength that a lighter rim takes away. These myths stem from thinking
>> that wheels hang from the spokes.
>Again, whether wheels hang or stand depend on the way people put it.
That is patently not true. A pre-tension of the spokes does not
change the way that the wheel supports loads.
>> The fewer the spokes, the stronger the rim must be in bending and
>> compression, to allow those few spokes to be tight enough to carry the
>> compressive load. If you understand the superposition of the
>> compressive load on the tensile stress in the spoke, then you'll be
>> able to analyze wheels whether or not they even have tension in the
>
>That is correct. I agree.
Well, the rim needs to be stiffer, not stronger.
>> The word "hang" only has meaning when gravity is the force being
>> constrained. Turn the wheel on its side, and there's no "hanging" in
>> the plane of the rim. But when you push the rim up against a wall, the
>> compressive load being carried by the tensile spokes nearest the wall
>> is just as real in that orientation as it is when a vertical wheel is
>> loaded against gravity.
>
>I don't ride a bike without gravity, so I am not interested in what
>happens without gravity.
Well, let's just dismiss out of hand any points that are inexplicable
by our model.
Take a wheel, mount it in the horizontal plane, and apply a load to
the rim at one side. Are the spokes between the load and hub supporting
the load, or are the spokes on the opposite side of the hub
supporting the load ?
--
Dave Blake
dbl...@phy.ucsf.edu
Mark Atanowicz
Rick Knowlan <rickk...@home.com> writes:
> Jobst Brandt wrote:
>
> > You can test this by
> > plucking spokes to see which ones change and in which direction (more
> > or less tension)...
>
> Now please explain . . .how can a bottom-of-wheel spoke, which you say
> above is in tension, provide an upward force to balance the downward force
> imposed by the forks? Wouldn't a spoke that is in tension pull DOWN on the
> hub?
>
> Have I missed something?
Yes, it's call "superposition of loads". Spokes are preloaded in
tension. If they are sujected to a compressive load which is not
greater than their initial tensile load, they are carrying a
compressive (updward) load but the net load is still tension. As an
illustration of this, hang a weight from a rubber band. Push up on
the weight with your finger enough to raise the weight, yet keep the
rubber band from going slack. This is how a spoke can carry a
compressive load. Anotoher thing to consider is that a bare rim is
not very stiff; you can easily move it radially in and out. If the rim
is not very stiff, it cannot transfer a load from the bottom to the top
of the rim, where "rim hanging" theorists insist, unless there is
significant deflection of the rim, and clearly there is not.
--
Mark Atanowicz
"Good judgement comes from experience. Experience comes from bad
judgement."
Peter Jones
Indian rope trick in its ability to puzzle us? A seemingly simple
structure, yet well
able to support both
a) a load a hundred times its own weight, and
b) a week`s weighty argument as to how it does so.
I, and I expect many others, have found the exchanges here on this thread,
"Wheel
theory for Jobst", and on the other thread, "More developments with my bike
wheel", quite fascinating, and the concepts quite challenging.
But, gentlemen, gentlemen, are we not making the understanding of this
matter more difficult than needs be?
Do we not need to grasp just two main points about a sound wheel?
1) That the spokes are always in tension, and
2) that the rim, like an arch (albeit in this case circular), is always
in compression.
Jobst Brandt has written :-
"In no case does loading in the plane of the wheel (the kind encountered
while riding) increase tension in any spoke. Loads are carried by relaxing
(compressing) spokes." (27 April 1999)
"The hub clearly does not hang from the upper spokes or they would show an
increase in tension. The four or so spokes between the hub and ground
are the only ones that show any significant change by becoming shorter
when the wheel is loaded." (1 May 1999)
"You present an incorrect picture of load distribution in that there
is no gradual change from higher to lower tension. The change is
fairly abrupt and is concentrated in a short zone comprising about
four to five spokes that carry the load in a 36 spoke wheel. (4 May 1999)
"Compression of the rim is something that practically does not occur in
a wheel other than when the spokes are originally tensioned."
Rick Denney has written :-
"If you understand those things, then you'll understand why the strongest
wheel has spokes as tight as the rim will allow." (30 April 1999)
Alex Rodriguez has written :-
"If you check the tension of the spokes on a wheel that is unloaded and then
loaded you will find that only the bottom spokes have a change in tension.
This can be done by plucking spokes and listening for a change in the tone
before and after a load is applied." (3 may 1999)
Rick Denny has written :-
"The strength of a wheel is defined by the following equation :-
Maximum load = compressive strength of spokes in load-affected zone -
pretension of spokes in load-affected zone.
The load-affected zone is a function mostly of the strength and shape
of the rim, and is the bottom portion of the wheel, starting at the
contact point with the ground and radiating from there."
I know words can mean different things to different people. The contentious
statement seems to be "The wheel stands on its spokes".
Well, that needs thinking about, but the wheel certainly doesn't do it in
quite the same way that I "stand on my legs", with my weight thrusting my
feet firmly into my shoes. A load thrusting along the spokes in such a way
(even if the spokes were sufficiently
rigid as columns) would quickly displace the nipples, pushing them out from
their seatings in the rim. That this nipple displacement does not happen
shows that spokes and nipples are always in tension between hub and rim, in
whatever segment of the wheel they may find themselves
I have read much of the "load affected zone", and the 4 or so spokes in that
zone. I have tried plucking the spokes as my son, 160lbs, loaded and
unloaded the bike - as Alex Rodriguez has suggested - and was surprised to
find no detectable change of note - but then neither my son nor I have
musical ears. In cycling sharps usually give me flats.
But giving it further thought I realised I would have been surprised if I
had detected a change in note.
Sitting there on the saddle and looking down I could see that the rear tyre
contact patch
had increased in area, so I knew the load had reached the ground. How had
it done so? What path had it followed? Tension? Compression?
We know that the load is carried into the wheel through the hub. Are we
saying it is carried out of the wheel through the "load affected zone"?
Well, I know the load path from my butt to my hub (one of compression down
the column of the seat stays) - but from the hub to the ground? Tension,
compression or pressure?
(Rick Denny has said - "The load-affected zone ... of the rim ... is the
bottom portion of the wheel, starting at the contact point with the ground
....")
Gentlemen no portion of my wheel is in contact with the ground. My wheels
wear tyres, so the rim itself never contacts the ground. The rim is in
contact only with the rubber and fabric of the tyre, and between my wheel
and the ground there is a cushion of air - my tyre is inflated to 45 psi.
What do we know about pressure in a gas?
1) That at any given point it acts equally in all directions, and
2) that its action is perpendicular to the surfaces constraining it.
This means that every square inch of area inside that annular black cavern
of my tyre, be it either the rubber of the tyre or the metal of the rim, is
subject to the same pressure - 45 lbs acting normal to the surface.
Indeed, when I think about it, it seems that every part of my rim, in
whatever sector of the wheel, is subject to pretty much indentical
conditions :-
1) spoke tension
2) compression as in an arch, stemming from the loading in 1) above,
3) contact with the bead of the tyre, and
4) air pressure at 45 psi.
So why is the LAZ in the lower part of the wheel? How does any one sector of
the wheel "know" if it is in the LAZ or not? What conditions prevail in the
LAZ to distinguish it from the unaffected zones?
How is it that the load affects the LAZ in a manner different from any other
part of the wheel - the unaffected zones? If you speak of a "load-affected
zone", do you not need to show also how the load is passed on? In this case
to the road.
Has the load been thrust downwards through the rubber? I think not - it
would seem far too flimsy; the rubber is merely there to keep the air in
place. Then has the load been thrust downwards through the fabric threads
of the tyre? Again, I think not. Cloth will not carry a load in thrust, it
will only carry a load in tension.
Air pressure beneath the rim? It is not only beneath the rim. It is equal at
45 psi normal to the rim in all parts throughout the circumference.
So how does the load get from the LAZ to to road?
Gentlemen, is it perhaps an error to think of a wheel as a system of three
parts only, hub spokes and rim? A bare wheel may not be the same thing as a
wheel wearing a tyre. And none of us ride nude wheels.
May the wind always be with you
Peter Jones
bnys...@bit-net.com wrote in message <7g9v3h$klo$1...@nnrp1.dejanews.com>...
>Jobst,
>
>I recently re-read The Bicycle Wheel (third edition). While I understand
the concepts of wheel construction, I still have a bit of a problem with the
the statement "The wheel stands on its spokes". After banging my head on the
wall for a while trying to understand how a spoke in tension can exert a
>compressive force, I have come up with a model that seems equally plausible
>and is, if nothing else, more intuitive. Perhaps it's just another way of
>saying the same thing, or perhaps it's wrong. However, if it's correct, I
>think it's easier for the average non-engineer to grasp.
>
>Please pardon the excessive detail, but I'm trying to make sure of my logic
>and make it easier for others who read this to follow.
>SNIP
Tho X. Bui
>
> Isn't a wire-spoked bike wheel a truly wonderful contrivance, rivalling the
> Indian rope trick in its ability to puzzle us? A seemingly simple
> structure, yet well
> able to support both
> a) a load a hundred times its own weight, and
> b) a week`s weighty argument as to how it does so.
> >Please pardon the excessive detail, but I'm trying to make sure of my logic
> >and make it easier for others who read this to follow.
> >SNIP
:-)
What really amazes me is that there is somebody who actually read _the
whole thread_
I usually go the easy way, just pick out one sentence from a poster I
don't like and immediately turn-on the flame thrower...
Tho
Jobst Brandt
> Isn't a wire-spoked bike wheel a truly wonderful contrivance, rivaling the
> Indian rope trick in its ability to puzzle us? A seemingly simple
> structure, yet well able to support both
> a) a load a hundred times its own weight, and
> b) a week`s weighty argument as to how it does so.
> I, and I expect many others, have found the exchanges here on this
> thread, "Wheel theory for Jobst", and on the other thread, "More
> developments with my bike wheel", quite fascinating, and the
> concepts quite challenging.
> But, gentlemen, gentlemen, are we not making the understanding of this
> matter more difficult than needs be?
You go on to cite appropriate sections of this discourse that clarify
the subject and then go on to add the following that is not all that
clear or enlightening because it drags in other concepts that are
equally obscure to many readers.
> I know words can mean different things to different people. The
> contentious statement seems to be "The wheel stands on its spokes".
Most of what was written in explanation focused on why that wording
is correct and accurate. It has been presented by several people each
using various methods to make the concept clear. In fact, surprising
to me, people who claim to be mechanical or structural engineers seem
to have the most trouble with it.
> Well, that needs thinking about, but the wheel certainly doesn't do
> it in quite the same way that I "stand on my legs", with my weight
> thrusting my feet firmly into my shoes.
Let's look at that. You get on the wheel and your weight appears on
the scale on which the wheel stands. The bottom four spokes become
shorter and the tire shows a belly at that place. Your shoes on the
scale register your weight on the scale. Your legs get a bit shorter
from the load and the rubber soles of your tennis shoes bulge out a
bit.
In fact, if you didn't have prior knowledge about flexible things like
string and wire, you would not be able to say anything about the
tension in these spokes. That is why the aluminum die cast bicycle
wheel is introduced as an example. Asked what deformation one should
expect, most people see the rigid spokes and say that the wheel
deforms with a slight flattening at the bottom and a shortening of the
downward spoke(s) and that the wheel stands on that spoke.
This is correct and strain gauges reflect that change precisely. Now
consider that die castings have cooling stresses caused by sequential
cooling and that the thicker part of the spokes near the hub and the
hub itself cool last causing shrink. This puts the die cast spokes in
tension. That castings do this is known and that is why S-shaped
spokes are used on cast iron hand wheels of old to prevent cracks that
occur if they were not S-shaped. All cast railway wheels also have
such ribs on the inside. When confronted by this in spite of a strain
gauge's inability to detect built-in spoke tension, some people who
originally said it stood on the bottom spoke now say it hangs from the
top spokes. The point is that the built-in tension has no effect on
the outcome, deflection load transmission or otherwise.
> A load thrusting along the spokes in such a way (even if the spokes
> were sufficiently rigid as columns) would quickly displace the
> nipples, pushing them out from their seatings in the rim. That this
> nipple displacement does not happen shows that spokes and nipples
> are always in tension between hub and rim, in whatever segment of
> the wheel they may find themselves.
That is obvious and consistent with what has been presented here.
What seems to be amiss is that (-10) + (+2) = (-8) and that
compression taken as positive can be added to either a negative
(tension) or positive (compression) number, the change being the same
(+2) compression. I think many people do this without question in
algebra in school, never considering that it has practical and actual
representation in nature, this being an example.
> I have read much of the "load affected zone", and the 4 or so spokes
> in that zone. I have tried plucking the spokes as my son, 160lbs,
> loaded and unloaded the bike - as Alex Rodriguez has suggested - and
> was surprised to find no detectable change of note - but then
> neither my son nor I have musical ears. In cycling sharps usually
> give me flats.
I have difficulty imagining not hearing an octave difference yet
recognizing people's voices. You must not have plucked it in a way
that reveals its characteristic tone. This requires plucking the
spoke with the fingernail close to the nipple. A soft pluck with the
ball of the finger at midspan does little good.
> But giving it further thought I realised I would have been surprised
> if I had detected a change in note.
Please explain what you mean by that.
> Sitting there on the saddle and looking down I could see that the
> rear tyre contact patch had increased in area, so I knew the load
> had reached the ground. How had it done so? What path had it
> followed? Tension? Compression?
> We know that the load is carried into the wheel through the hub. Are
> we saying it is carried out of the wheel through the "load affected
> zone"?
If you don't believe so, please explain how you think the load force
reaches to the ground.
> Well, I know the load path from my butt to my hub (one of
> compression down the column of the seat stays) - but from the hub to
> the ground? Tension, compression or pressure?
> (Rick Denny has said - "The load-affected zone ... of the rim ... is
> the bottom portion of the wheel, starting at the contact point with
> the ground ....")
> Gentlemen no portion of my wheel is in contact with the ground. My
> wheels wear tyres, so the rim itself never contacts the ground. The
> rim is in contact only with the rubber and fabric of the tyre, and
> between my wheel and the ground there is a cushion of air - my tyre
> is inflated to 45 psi.
You quibble. Take the tire off the rim if you like, it changes
nothing. My wheels have tires. Do you have a different definition of
a bicycle wheel?
> What do we know about pressure in a gas?
> 1) That at any given point it acts equally in all directions, and
> 2) that its action is perpendicular to the surfaces constraining it.
> This means that every square inch of area inside that annular black
> cavern of my tyre, be it either the rubber of the tyre or the metal
> of the rim, is subject to the same pressure - 45 lbs acting normal
> to the surface.
You seem to be introducing an old topic that only helps muddle the
subject of the wheel, and that is how the tire supports a load.
> Indeed, when I think about it, it seems that every part of my rim,
> in whatever sector of the wheel, is subject to pretty much identical
> conditions :
> 1) spoke tension
> 2) compression as in an arch, stemming from the loading in 1) above,
> 3) contact with the bead of the tyre, and
> 4) air pressure at 45 psi.
> So why is the LAZ in the lower part of the wheel? How does any one
> sector of the wheel "know" if it is in the LAZ or not? What
> conditions prevail in the LAZ to distinguish it from the unaffected
> zones?
I find difficulty in separating questions from rhetorical questions
the way you put all this. I suggest you take the tire off the rim and
analyze the wheel (rim and spokes+nipples and hub) alone. This is the
essence of the system under discussion and does not rely on a tire.
> How is it that the load affects the LAZ in a manner different from
> any other part of the wheel - the unaffected zones? If you speak of
> a "load-affected zone", do you not need to show also how the load is
> passed on? In this case to the road.
> Has the load been thrust downwards through the rubber? I think not
> - it would seem far too flimsy; the rubber is merely there to keep
> the air in place. Then has the load been thrust downwards through
> the fabric threads of the tyre? Again, I think not. Cloth will not
> carry a load in thrust, it will only carry a load in tension.
> Air pressure beneath the rim? It is not only beneath the rim. It is
> Air pressure beneath the rim? It is not only beneequal at
> 45 psi normal to the rim in all parts throughout the circumference.
> So how does the load get from the LAZ to to road?
The pressure is uniform throughout the tire so it's not a difference
or imbalance in inflation pressure that supports the rim above the
road. The tire casing is also not able to support any load in
compression as you know from flat tires. Tension in the casing
supports the rim and it does so by being reduced in the LAZ as well as
by the angle at which it pulls away from the rim.
> Gentlemen, is it perhaps an error to think of a wheel as a system of
> three parts only, hub spokes and rim? A bare wheel may not be the
> same thing as a wheel wearing a tyre. And none of us ride nude
> wheels.
Maybe you should, just for technical curiosity. It really doesn't
change the description of the system and it may be easier to grasp.
Let me propose another example. Holding a wheel horizontally with the
hub attached to a rigid fixture, push inward toward the hub and
visualize what part of the wheel might deflect from that. I don;t
know anyone who claims it is on the opposite side of the wheel. Maybe
you also can see that the deflection is where you are pushing and that
that shortens the spoke at that place.
Jobst Brandt <jbr...@hpl.hp.com>
Tim McNamara
<velo_so...@hotmail.com> wrote:
.....................In the wire-spoked wheel, you're only dealing with
> tension members so the hub must by definition indeed hang from some of the
> spokes. But I don't mean to be simplistic; that is pretty obvious to anyone
> with phy.ucsf.edu in their email address!
Simplistic and obvious, perhaps, but false.
The trick is to see what happens to the rim in the load affected zone:
it flexes, reducing the radius between the hub and the rim. This flex
is what compresses lower the spokes; in a perfectly rigid rim, as I
understand it, the load would be equally distributed between the upper
and lower spokes. Because the rim is not rigid, the load is borne
under compression by the lower spokes. I don't know a numerical value,
but I would imagine that the deformation of the rim at the contact
point would be very small- probably not observable to the naked eye.
> ... but I am still having trouble swallowing this as such. Although you've
> gotten me closer than other commentators in this thread.
It's simple enough to test. Pluck the upper and lower spokes of an
unloaded wheel. Load the wheel. Pluck the same spokes again and
listen for cvhanges in pitch; pitch in a vibrating whire is directly
related to tension. The lower spokes will reduce in pitch while the
pitch of the upper spokes remains unchanged. If the load was hanging
from the upper spokes, the pitch of those spokes when plucked would be
higher because their tension would have been increased.
The spokes are not compressed by the hub sinking towards the rim under
load; they are compressed by the rim flexing towards the hub at the
Jobst Brandt
> Now back to the loaded spoke. Is not the net load on the spoke still
> one of(reduced) tension? And has the other part of the load
> vanished? If not, what is carrying the missing share?
> I have a little difficulty with this concept. We add a load (160
> lbs?) to the hub of a wheel. No spokes experience an increase in
> tension, and 4 or 5 spokes experience a reduction in tension. We've
> added a load yet so far we've only had reductions. Where is there a
> balancing increase of some type?
I think you've got it. The increase is the added compression in the
spokes that "lost tension". THAT IS AN INCREASE equal and opposite
the load on the hub.
>> "If the rim is not very stiff, it cannot transfer a load from the
>> bottom to the top ... unless there is significant deflection of the
>> rim, and clearly there is not."
> Quite so. However, to stiffen the rim we support it with well tensioned
> spokes so that it can transfer a great deal of load around its circumference
> without deflection ...
Not rally, because it is weak in bending and stiff in compression. It
can only transmit compression around the rim and because the diameter
does not change appreciably (only the 0.1 mm or so at the LAZ) there
is also no change in tension in the spokes. The bottom spokes give up
some tension but the compressive force in the rim remains unchanged
because the road pushes inward where the spokes lost tension.
> In the same way that a stone bridge arch can transfer load along its
> curve to its abuttments.
There is a disparity in deflections here. Take an unspoked rim and
deflecting it a couple of mm it takes no significant force. In
bending, the rim is a wet noodle in comparison to spoke rigidity. It
is more like a piece of sheet metal glued to foam rubber on the floor
to walk on. The far ends don't do much to support your load. In
fact, this model is called the elastically supported beam (as in RR
track) and the bicycle wheel is a circular form of it.
Jobst Brandt <jbr...@hpl.hp.com>
DD861
stupid tree and hit every branch on the way down. The above finally drives
the point home. I was thinking that if some of the spokes are compressed,
something _must_ happen to the other
spokes (their tension would increase, etc.), notwithstanding the book's
specific test data to the contrary. But there it is. Sometimes with us
non-engineers it just
takes saying it a few different ways, be patient, sooner or later we'll get
it. Aaaaahhhh.
Jobst Brandt wrote in message <7gvufr$9l8$1...@hplms2.hpl.hp.com>...
Ben
person. In email or NG, it is double: verbal and face-to-face explanations
are much easier.
Personally, I don't owe anyone here a physics lesson: if they are
sufficiently interested, resources are widely available to gain a basic
grounding in the subject (in this case, a good starting point would be to
read the book in question).
If it were a friend, relative or associate, I am sure I could communicate
the idea in 10 minutes tops. These people are not in this NG.
Ben
mark v. hillman <swed...@email.msn.com> wrote in message
news:OSooleDm#GA.174@cpmsnbbsa03...
Peter Jones
Mark Atanowicz wrote in message <7gv14n$9ns$1...@newsgate.sps.mot.com>...
>Yes, it's call "superposition of loads". Spokes are preloaded in
>tension. If they are sujected to a compressive load which is not
>greater than their initial tensile load, they are carrying a
>compressive (updward) load but the net load is still tension. As an
>illustration of this, hang a weight from a rubber band. Push up on
>the weight with your finger enough to raise the weight, yet keep the
>rubber band from going slack. This is how a spoke can carry a
>compressive load. Another thing to consider is that a bare rim is
>not very stiff; you can easily move it radially in and out. If the rim
>is not very stiff, it cannot transfer a load from the bottom to the top
>of the rim, where "rim hanging" theorists insist, unless there is
>significant deflection of the rim, and clearly there is not.
>
>--
>Mark Atanowicz
" ... hang a weight from a rubber band. Push up on
the weight with your finger enough to raise the weight,
yet keep the rubber band from going slack.
This is how a spoke can carry a compressive load."
Or is it just that the load is now being shared? Your finger is carrying a
compressive load (you can feel it), and the rubber band is still carrying a
(reduced) load under tension. The total load is unchanged.
Now back to the loaded spoke. Is not the net load on the spoke still one
of(reduced) tension? And has the other part of the load vanished? If not,
what is carrying the missing share?
I have a little difficulty with this concept. We add a load (160 lbs?) to
the hub of a wheel. No spokes experience an increase in tension, and 4 or 5
spokes experience a reduction in tension. We've added a load yet so far
we've only had reductions. Where is there a balancing increase of some type?
"If the rim is not very stiff, it cannot transfer a load from
the
bottom to the top ... unless there is significant deflection
of the rim, and clearly there is not."
Quite so. However, to stiffen the rim we support it with well tensioned
spokes so that it can transfer a great deal of load around its circumference
without deflection ... in the same way that a stone bridge arch can
transfer load along its curve to its abuttments.
May the wind always be with you
Peter Jones
Peter Jones
Jobst Brandt wrote in message <7gvpcn$t0u$2...@hplms2.hpl.hp.com>...
>Peter Jones writes:
>
>> Isn't a wire-spoked bike wheel a truly wonderful contrivance, rivaling
the
>> Indian rope trick in its ability to puzzle us? A seemingly simple
>> structure, yet well able to support both
>> a) a load a hundred times its own weight, and
>> b) a week`s weighty argument as to how it does so.
>
>> matter more difficult than needs be?
>
>You go on to cite appropriate sections of this discourse that clarify
>the subject and then go on to add the following that is not all that
>clear or enlightening because it drags in other concepts that are
>equally obscure to many readers.
Well, I'm certainly sorry if I seem to be adding obscurity. Followers of
this discourse on how a bicycle wheel supports its load seem to fall into
two camps, the "hangers" and the "standers". I'm a newcomer to the net, (but
not to cycling) and I broke in at the middle of this discourse, but I have
read the thread back to 28 April (that's already seventy postings!), and I
ordered THE book four days ago. I am as yet neither a "hanger" nor a
"stander", I am still just a fence-sitting asker of questions. If I drag in
other concepts it is because I perceive some relevance in their
consideration. I can see the quick appeal of the "hanging" argument, but
I've no wish to readily dismiss the "standing" argument merely because it
needs to be wrestled with. After all, there are some very nice people in the
"standing" camp.
>
>> I know words can mean different things to different people. The
>> contentious statement seems to be "The wheel stands on its spokes".
>
>Most of what was written in explanation focused on why that wording
>is correct and accurate. It has been presented by several people each
>using various methods to make the concept clear. In fact, surprising
>to me, people who claim to be mechanical or structural engineers seem
>to have the most trouble with it.
Yes, I've read the thread.
>
"The wheel stands on its spokes"
OK, let me try my grasp of that, using your figures above. The load in the
spoke (together with 3 or 4 adjacent spokes?) has gone from (-10) to (-8).
Those spokes have experienced a reduction in their tension. I expect this
reduction (a compression of +2) is reflected by the road now sharing the
rider's weight?
>
>> I have read much of the "load affected zone", and the 4 or so spokes
>> in that zone. I have tried plucking the spokes as my son, 160lbs,
>> loaded and unloaded the bike - as Alex Rodriguez has suggested - and
>> was surprised to find no detectable change of note - but then
>> neither my son nor I have musical ears. In cycling sharps usually
>> give me flats.
>
>I have difficulty imagining not hearing an octave difference yet
>recognizing people's voices. You must not have plucked it in a way
>that reveals its characteristic tone. This requires plucking the
>spoke with the fingernail close to the nipple. A soft pluck with the
>ball of the finger at midspan does little good.
>
>> But giving it further thought I realised I would have been surprised
>> if I had detected a change in note.
>
>Please explain what you mean by that.
Yes. The trains of "further thought" I spoke of gave rise to, and were
outlined in, the remainder of my posting.
The presence of the tyre, and its readiness to deflect made me realise we
were neglecting a link in the load path that needed explaining.
>> Sitting there on the saddle and looking down I could see that the
>> rear tyre contact patch had increased in area, so I knew the load
>> had reached the ground. How had it done so? What path had it
>> followed? Tension? Compression?
>
>> We know that the load is carried into the wheel through the hub. Are
>> we saying it is carried out of the wheel through the "load affected
>> zone"?
>
>If you don't believe so, please explain how you think the load force
>reaches to the ground.
Yes - and that is precisely the question I am wrestling with, and why I
believe consideration of the pneumatic tyre is relevant - but let me
wrestle with questions before I wrestle with explanations.
>
>> Well, I know the load path from my butt to my hub (one of
>> compression down the column of the seat stays) - but from the hub to
>> the ground? Tension, compression or pressure?
>
>> (Rick Denny has said - "The load-affected zone ... of the rim ... is
>> the bottom portion of the wheel, starting at the contact point with
>> the ground ....")
>
>> Gentlemen no portion of my wheel is in contact with the ground. My
>> wheels wear tyres, so the rim itself never contacts the ground. The
>> rim is in contact only with the rubber and fabric of the tyre, and
>> between my wheel and the ground there is a cushion of air - my tyre
>> is inflated to 45 psi.
>
>You quibble. Take the tire off the rim if you like, it changes
>nothing. My wheels have tires. Do you have a different definition of
>a bicycle wheel?
I quibble? I think not. I rather think the presence of a pneumatic tyre is
crucial, and I fancy its presence or absence changes everything.
>> What do we know about pressure in a gas?
>> 1) That at any given point it acts equally in all directions, and
>> 2) that its action is perpendicular to the surfaces constraining it.
>
>> This means that every square inch of area inside that annular black
>> cavern of my tyre, be it either the rubber of the tyre or the metal
>> of the rim, is subject to the same pressure - 45 lbs acting normal
>> to the surface.
>
>You seem to be introducing an old topic that only helps muddle the
>subject of the wheel, and that is how the tire supports a load.
Sorry, but it seemed relevant to wonder how the tyre might take the load on
from the LAZ to the road. The path from hub and spokes to the rim at the LAZ
has been analysed greatly, so it seemed too easy to dismiss the last inch by
saying simply, "Its goes through the tyre." I wondered if it might do this
by tension or compression.
>
>> Indeed, when I think about it, it seems that every part of my rim,
>> in whatever sector of the wheel, is subject to pretty much identical
>> conditions :
>> 1) spoke tension
>> 2) compression as in an arch, stemming from the loading in 1) above,
>> 3) contact with the bead of the tyre, and
>> 4) air pressure at 45 psi.
>
>> So why is the LAZ in the lower part of the wheel? How does any one
>> sector of the wheel "know" if it is in the LAZ or not? What
>> conditions prevail in the LAZ to distinguish it from the unaffected
>> zones?
>
>I find difficulty in separating questions from rhetorical questions
>the way you put all this.
Sorry about the rhetoric, but I find it a useful way to work out the
questions before I begin to seek out the answers.
> I suggest you take the tire off the rim and
>analyze the wheel (rim and spokes+nipples and hub) alone. This is the
>essence of the system under discussion and does not rely on a tire.
I have no difficulty at all with this. I have in my garage a wheel which was
ridden with a flat tyre into a kerb at speed. The "load-affected zone" has
been most clearly affected - neither the rim nor the spokes were able to
cope with the compressive loads. Without an effective tyre the compression
of the wheel parts between the roadside kerb and the hub quickly rendered
the whole thing useless.
However, I do not yet believe the load-path through the tyre is irrelevant.
Those among us in the "standing" camp must be able to explain how the load
(which has reached from the hub to the lower arc of the rim, by compression
down the spokes), goes the extra inch to the ground.
Similarly, those among us in the "hanging" camp must be able to explain how
the load can pass along their chosen route from hub to rim to the ground.
>
>> How is it that the load affects the LAZ in a manner different from
>> any other part of the wheel - the unaffected zones? If you speak of
>> a "load-affected zone", do you not need to show also how the load is
>> passed on? In this case to the road.
>
>> Has the load been thrust downwards through the rubber? I think not
>> - it would seem far too flimsy; the rubber is merely there to keep
>> the air in place. Then has the load been thrust downwards through
>> the fabric threads of the tyre? Again, I think not. Cloth will not
>> carry a load in thrust, it will only carry a load in tension.
>
>> Air pressure beneath the rim? It is not only beneath the rim. It is equal
at
>> 45 psi normal to the rim in all parts throughout the circumference.
>
>> So how does the load get from the LAZ to to road?
>
>The pressure is uniform throughout the tire so it's not a difference
>or imbalance in inflation pressure that supports the rim above the
>road. The tire casing is also not able to support any load in
>compression as you know from flat tires. Tension in the casing
>supports the rim and it does so by being reduced in the LAZ as well as
>by the angle at which it pulls away from the rim.
Yes, there is a casing angle change at the LAZ, (and I fancy that angle
change may be significant) but no, there is no reduction I think in casing
tension. Casing tension, being a function of pressure, must be uniform from
bead to bead across the section of the tyre and throughout the circumference
of the wheel, must it not?
So what supports the rim above the road?
>> Gentlemen, is it perhaps an error to think of a wheel as a system of
>> three parts only, hub spokes and rim? A bare wheel may not be the
>> same thing as a wheel wearing a tyre. And none of us ride nude
>> wheels.
>
>Maybe you should, just for technical curiosity. It really doesn't
>change the description of the system and it may be easier to grasp.
No thank you. If cyclists generally took to riding without tyres I fancy
much would change in the construction of hubs spokes and rims. Those parts
are as they are today because tyres are part of the system, and at some
point the system must be viewed as a whole.
>
>Let me propose another example. Holding a wheel horizontally with the
>hub attached to a rigid fixture, push inward toward the hub and
>visualize what part of the wheel might deflect from that. I don;t
>know anyone who claims it is on the opposite side of the wheel. Maybe
>you also can see that the deflection is where you are pushing and that
>that shortens the spoke at that place.
Yes, this inward push would mimic the load borne by the road, or by the
"wall of death" for a horizontal cyclist, wouldn't it?
>
Thank you Jobst for your insightful responses, both here and on another
thread. They help clarify my concepts, and (forgive me), they lead me
sometimes to new questions to fill gaps in my grasp of this subject. I hope
other readers gain too.
May the wind always be behind you
Peter Jones.
Peter Jones
Tim McNamara wrote in message <070519991740547761%tim...@minn.net>...
>The spokes are not compressed by the hub sinking towards the rim under
>load; they are compressed by the rim flexing towards the hub at the
>contact point with the ground.
I too am wrestling with this. May I butt in?
The rim is not in contact with the ground. What is it that imposes this
force (or resistance) that cause this upward flexure, and how does it do
it - via a tensile or a compressive force?
May the wind always be with you
Peter Jones
Peter Jones
Snoopy wrote in message <37329f0f...@news.caverock.net.nz>...
>>
>The unchanged upward component of tension in the rest of the spokes
>outside the load effected zone. SNOOPY
May I ask a poorly phrased question, please?
From the hub .... but to whence do these spokes carry this force? How is
this upward tension resolved, what is its path to its resolution?
Thank you for tolerating an intrusion
Peter Jones
Peter Jones
>> vanished? If not, what is carrying the missing share?
>
>> I have a little difficulty with this concept. We add a load (160
>> lbs?) to the hub of a wheel. No spokes experience an increase in
>> tension, and 4 or 5 spokes experience a reduction in tension. We've
>> added a load yet so far we've only had reductions. Where is there a
>> balancing increase of some type?
>
>spokes that "lost tension". THAT IS AN INCREASE equal and opposite
>the load on the hub.
Thank you, let me try that. Let's have a wheel with say seven spokes, each
tensioned (that's a -) to a 100 lbs, that's a sum -700lbs. A load through
the axle of 160 lbs (that's a +) will be carried by the two lower spokes
(that's -100lbs +80lbs, a net -20lbs each), they are still in tension, but
much reduced. Now the sum of all loadings is 5 spokes @ -100 each and 2
@ -20 each, that's -540 lbs.
I was expecting something different. Bike wheels like this remind me of an
unsteady donkey I once had - the greater load I put on him, the lighter
on his feet he became!
Is that it, or am I still missing something? Sorry. What is the missing
number, and on what path will I find it?
I was expecting that if we add a load of 160 lbs to the hub we must be able
somewhere to trace that through the spokes, rim, and tyre to the ground,
either as a tension or a compression. Mustn't we? Must not the sum of all
loadings in the 36 spokes increase by 160 lbs? Or not? Are we not looking
for more (160lbs more) than a mere equal and balancing load? After all, the
tyre contact patch is now deforming.
>>> "If the rim is not very stiff, it cannot transfer a load from the
>>> bottom to the top ... unless there is significant deflection of the
>>> rim, and clearly there is not."
>
>> Quite so. However, to stiffen the rim we support it with well tensioned
>> spokes so that it can transfer a great deal of load around its
circumference
>> without deflection ...
>
>Not really, because it is weak in bending and stiff in compression. It
>can only transmit compression around the rim
Yes, of course, and I should have said so, but that is all we need it to do
isn't it, transmit compression around the arc?.
>and because the diameter does not change appreciably (only the 0.1 mm or so
at the >LAZ) there is also no change in tension in the spokes. The bottom
spokes give up
>some tension but the compressive force in the rim remains unchanged
>because the road pushes inward where the spokes lost tension.
I expect this "inward push" (a converse but very useful way of looking at
it) balances the loss of tension, does it not?
Incidentally, how does that "inward push" of the road reach the rim at the
LAZ?
>
>> In the same way that a stone bridge arch can transfer load along its
>> curve to its abuttments.
>
>There is a disparity in deflections here.
OK, forget the stonework, but we're both talking of the transfer of a
compressive load around an arc, aren't we? The bridge has enough lateral
stiffness to not buckle, the wheel rim needs well-tensioned spokes to keep
it in plane.
>Take an unspoked rim and
>deflecting it a couple of mm it takes no significant force. In
>bending, the rim is a wet noodle in comparison to spoke rigidity. It
>is more like a piece of sheet metal glued to foam rubber on the floor
>to walk on. The far ends don't do much to support your load. In
>fact, this model is called the elastically supported beam (as in RR
>track) and the bicycle wheel is a circular form of it.
Thank you for this last idea, I like it. It's why I like 40 spokes instead
of 32 or 28 - a shorter length of unsupported r/way span between spokes.
Thank you also for your rapid responses to my two postings.
May the wind always be behind you
Peter Jones.
Snoopy
<avco...@btinternet.com> wrote:
>>>
>>>If so, what balances the downward force imposed by the forks?
>>>
>>The unchanged upward component of tension in the rest of the spokes
>>outside the load effected zone. SNOOPY
>
>
Yes that is correct. We were considering the free body diagram of a
particular component - the hub
>
>but to whence do these spokes carry this force? How is
>this upward tension resolved,
>
take one spoke as an example.
The spoke exerts a tensile force on the hub.
which means...
The hub exerts an equal and opposite tensile force on the spoke.(1)
Meanwhile at the other end of the spoke, the spoke is pulling the rim
inwards.
which means...
There is an equal and opposite force which the rim is applying to the
end of the spoke outwards (i.e. the rim is putying the spoke in
tension at the rim end) (2).
Since the spoke doesn't fly off into space somewhere we know that (1)
and (2) must exactly balance. The tension in the spoke is resolved.
>
>what is its path to its resolution?
>
Spoke tension is not a force applied externally to the wheel system.
It is an internal force created inside the wheel and felt entirely
inside the wheel- and has no 'external' consequences. SNOOPY
Rick Knowlan
> Well, I'm certainly sorry if I seem to be adding obscurity. Followers of
> this discourse on how a bicycle wheel supports its load seem to fall into
> two camps, the "hangers" and the "standers".
You missed the "inappropriate metaphorers". :-)
Stergios Papadakis
>
> Peter Jones
All right, turn this into a "how does the tire support the
rim" thread! We don't get as
many of these. The answer is : Almost the same way the spokes support
the hub, but the geometry is slightly more complicated
because the tire deforms significantly. The air pressure
in the tire puts tension on the casing. At the
bottom, where it touches the road, the casing is
compressed (although it is in net tension so it
doesn't collapse completely). That compression
(reduction in tension if you insist)
AND the change in angle where the casing leaves the rim
both provide an upward force to the rim.
Stergios
Stergios Papadakis
snip
>
> >The pressure is uniform throughout the tire so it's not a difference
> >or imbalance in inflation pressure that supports the rim above the
> >road. The tire casing is also not able to support any load in
> >compression as you know from flat tires. Tension in the casing
> >supports the rim and it does so by being reduced in the LAZ as well as
> >by the angle at which it pulls away from the rim.
>
> Yes, there is a casing angle change at the LAZ, (and I fancy that angle
> change may be significant) but no, there is no reduction I think in casing
> tension. Casing tension, being a function of pressure, must be uniform from
> bead to bead across the section of the tyre and throughout the circumference
> of the wheel, must it not?
>
> So what supports the rim above the road?
>
>
> Peter Jones.
No, there is a reduction in tension because of the
change in shape of the tire. Let's look at a
cross section of the tire, and assume for a moment
that it is circular when not loaded. Now, the
tension in that ring = (Pressure*2*radius) per unit length.
If you now flatten that ring until it is half of the height it
was, now the tension at 12 o'clock and 6 o'clock is clearly reduced,
because now the pressure has half the area to act on, so the
tension is down by a factor of two at those two points. If the
tire is made of a non-strechty material, the width has now increased
since the height decreased, and the tension at 3 o'clock and 9 o'clock
as increased. Note that this assumes the pressure stays constant,
which it effectively does in a real tire because the LAZ is a fraction
of the total
volume.
This may seem counterintuitive at first, but remember that it
is an external load that deforms the tire from its unloaded, inflated
shape, and that external load can change the tension.
Stergios
David T. Blake
>Sorry, but it seemed relevant to wonder how the tyre might take the
>load on from the LAZ to the road. The path from hub and spokes to the
>rim at the LAZ has been analysed greatly, so it seemed too easy to
>dismiss the last inch by saying simply, "Its goes through the tyre."
>I wondered if it might do this by tension or compression.
This was explained briefly in the post to which you
replied, with the words
Jobst wrote:
The pressure is uniform throughout the tire so it's not a difference
or imbalance in inflation pressure that supports the rim above the
road. The tire casing is also not able to support any load in
compression as you know from flat tires. Tension in the casing
supports the rim and it does so by being reduced in the LAZ as well as
by the angle at which it pulls away from the rim.
*************EndQuote*************
To clarify a little, the gas pressure forces have radial symmettry
and can have no contribution to directly supporting a load. The tire
bears directly on the clinching section of the rim. The radius of
curvature of this contact determines the force from the bead to
the rim. In the LAZ, there is a systematic change in this loading
angle so that the rim is transferring force to the tire greater
at the LAZ than elsewhere in the tire. This increased force transfers
the load and keeps the rim up.
....
>No thank you. If cyclists generally took to riding without tyres I
>fancy much would change in the construction of hubs spokes and rims.
>Those parts are as they are today because tyres are part of the
>system, and at some point the system must be viewed as a whole.
That speaks to a different point. You can learn almost exactly
how the rim supports a load with a tire if you consider
how it supports a load without a tire. The forces are very very
neasrly the same.
--
Dave Blake
dbl...@phy.ucsf.edu
tku...@diabloresearch.com
"Peter Jones" <avco...@btinternet.com> wrote:
>
> Tim McNamara wrote in message <070519991740547761%tim...@minn.net>...
>
> >The spokes are not compressed by the hub sinking towards the rim under
> >load; they are compressed by the rim flexing towards the hub at the
> >contact point with the ground.
>
> I too am wrestling with this. May I butt in?
>
> The rim is not in contact with the ground. What is it that imposes this
> force (or resistance) that cause this upward flexure, and how does it do
> it - via a tensile or a compressive force?
You sit upon the bike. Part of your weight is on the front wheel and
part is on the rear wheel. The tires touch the ground and compress. It
there any question whatsoever that it is a compression load on the tires?
The compression loading is transferred to the rims. In the load affected
zone the rim will deform slightly -- flattening with the compression load.
OK, now how to you explain the compression being converted, somehow, into
tension? In fact, it isn't. The total tension on the spokes remains very
nearly the same on all but those adjacent or in the load affected zone.
Take a hoola hoop and holding one side press the hoop against the ground.
What happens? Right -- the thing goes oval. The rim tries to do the same
thing and is held round by the spokes.
The only thing that can happen is that the bottom side touching the road
bends slightly upwards absorbing the compression load while shedding some
part of the tension.
-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
Stergios Papadakis
>
> Jobst Brandt wrote in message <7gvufr$9l8$1...@hplms2.hpl.hp.com>...
> >Peter Jones writes:
> >
> >I think you've got it. The increase is the added compression in the
> >spokes that "lost tension". THAT IS AN INCREASE equal and opposite
> >the load on the hub.
>
> Thank you, let me try that. Let's have a wheel with say seven spokes, each
> tensioned (that's a -) to a 100 lbs, that's a sum -700lbs. A load through
> the axle of 160 lbs (that's a +) will be carried by the two lower spokes
> (that's -100lbs +80lbs, a net -20lbs each), they are still in tension, but
> much reduced. Now the sum of all loadings is 5 spokes @ -100 each and 2
> @ -20 each, that's -540 lbs.
>
The sum of the magnitudes of the tensions has no meaning because
the tensions are applied in different directions. Go back
to a 32 spoke wheel with 200 lbs tension on each spoke. This is
a more reasonable estimate. Now, for simplicity, assume that
there are 4 equally loaded spokes in the LAZ. Each had 200 lbs
originally, but now with 160 lbs load, divided 40 lbs/spoke, each
had 160 lbs of tension. Look at what has changed: before you
added the load, the hub was in equilibrium, which means it had
no net force on it. After you added the load, tension change
in the four lower spokes was 160 lbs, and the hub was in equilibrium
again. Those four spokes support the hub.
Stergios
> I was expecting something different. Bike wheels like this remind me of an
> unsteady donkey I once had - the greater load I put on him, the lighter
> on his feet he became!
>
> Is that it, or am I still missing something? Sorry. What is the missing
> number, and on what path will I find it?
>
> I was expecting that if we add a load of 160 lbs to the hub we must be able
> somewhere to trace that through the spokes, rim, and tyre to the ground,
> either as a tension or a compression. Mustn't we? Must not the sum of all
> loadings in the 36 spokes increase by 160 lbs? Or not? Are we not looking
> for more (160lbs more) than a mere equal and balancing load? After all, the
> tyre contact patch is now deforming.
>
>
snip
> May the wind always be behind you
>
> Peter Jones.
Jobst Brandt
>> I suggest you take the tire off the rim and analyze the wheel (rim
>> and spokes+nipples and hub) alone. This is the essence of the
>> system under discussion and does not rely on a tire.
> I have no difficulty at all with this. I have in my garage a wheel
> which was ridden with a flat tyre into a kerb at speed. The
> "load-affected zone" has been most clearly affected - neither the
> rim nor the spokes were able to cope with the compressive loads.
That isn't a whole(some) wheel and not a good subject for analysis.
Just a ridable good wheel with no tire responds exactly the same to
loading as it would with a tire except that it cannot absorb shock.
The load path is identical and so is the load distribution with only a
slight narrowing of the load affected zone, so slightly that you
cannot detect the difference.
> Without an effective tyre the compression of the wheel parts between
> the roadside kerb and the hub quickly rendered the whole thing
> useless.
Not so. You may not have good traction but if you do this on a
carpet there is no difference between this and a wheel with inflated
tire.
> However, I do not yet believe the load-path through the tyre is
> irrelevant. Those among us in the "standing" camp must be able to
> explain how the load (which has reached from the hub to the lower
> arc of the rim, by compression down the spokes), goes the extra inch
> to the ground.
As I explained, the load path is visible and is the bulging part of
the tire, nothing else. I suspect you are looking for some transfer
to the far side of the wheel so that the wheel can "hang". Please do
the load detection by tone experiment before insisting that the tire
changes the problem.
> Similarly, those among us in the "hanging" camp must be able to
> explain how the load can pass along their chosen route from hub to
> rim to the ground.
When you find another path than the direct route I presented let me
know about it, but I don't believe thinking out loud (in print) is
helpful.
Casing tension is proportional to the radius of curvature of the
casing, at any pressure, so there are two effects. One is that the
downward pull of the casing is reduced and the other is that the angle
is more to the side so that the downward component of that pull is
reduced. It is this reduction in tension that gives upward force. As
you see we are at a second impasse for those who cannot visualize
algebraic superposition of forces the same as they are in the spokes.
> So what supports the rim above the road?
Let's just take the tire off the wheel and solve the wheel problem
first. The tire problem is the same as the wheel problem and has had
almost as much discussion.
>>> Gentlemen, is it perhaps an error to think of a wheel as a system of
>>> three parts only, hub spokes and rim? A bare wheel may not be the
>>> same thing as a wheel wearing a tyre. And none of us ride nude
>>> wheels.
>> Maybe you should, just for technical curiosity. It really doesn't
>> change the description of the system and it may be easier to grasp.
> No thank you. If cyclists generally took to riding without tyres I
> fancy much would change in the construction of hubs spokes and rims.
> Those parts are as they are today because tyres are part of the
> system, and at some point the system must be viewed as a whole.
I don't want you to ride without tires, only to analyze the stresses
in the wheel. Riding doesn't solve any of these questions or Coppi
would have written "the Bicycle Wheel" long ago or others before him.
In fact this argument is often presented in defense of hanging hubs...
"who knows more about bicycles anyway, the racers or some engineer?"
>> Let me propose another example. Holding a wheel horizontally with
>> the hub attached to a rigid fixture, push inward toward the hub and
>> visualize what part of the wheel might deflect from that. I don't
>> know anyone who claims it is on the opposite side of the wheel.
>> Maybe you also can see that the deflection is where you are pushing
>> and that that shortens the spoke at that place.
> Yes, this inward push would mimic the load borne by the road, or by
> the "wall of death" for a horizontal cyclist, wouldn't it?
> Thank you Jobst for your insightful responses, both here and on
> another thread. They help clarify my concepts, and (forgive me),
> they lead me sometimes to new questions to fill gaps in my grasp of
> this subject. I hope other readers gain too.
I seldom get the impression that I am making headway but I hope so.
Jobst Brandt <jbr...@hpl.hp.com>
Jobst Brandt
>> The spokes are not compressed by the hub sinking towards the rim
>> under load; they are compressed by the rim flexing towards the hub
>> at the contact point with the ground.
> I too am wrestling with this. May I butt in?
> The rim is not in contact with the ground. What is it that imposes
> this force (or resistance) that cause this upward flexure, and how
> does it do it - via a tensile or a compressive force?
It depends on your frame of reference and that may be a sticking
point. The wheel is more easily analyzed without the bicycle, using
the hub as a fixed point and looking at the road as applying loads.
Not that this changes anything but it makes analysis far easier
because the hub vanishes from the problem. That is the essence of the
horizontal wheel example, with its hub located in a rigid fixture.
Pressing against this wheel, deflects it, there where pressure is
applied, not on the far side. For many observers, this picture is far
easier to visualize than a moving bicycle where everything is moving
and rotating. Dynamics only add confusion, while stress distribution
is identical to the static fixed hub model.
Jobst Brandt <jbr...@hpl.hp.com>
Jobst Brandt
>>> Now back to the loaded spoke. Is not the net load on the spoke
>>> still one of (reduced) tension? And has the other part of the
>>> load vanished? If not, what is carrying the missing share?
>>> I have a little difficulty with this concept. We add a load (160
>>> lbs?) to the hub of a wheel. No spokes experience an increase in
>>> tension, and 4 or 5 spokes experience a reduction in tension.
>>> We've added a load yet so far we've only had reductions. Where is
>>> there a balancing increase of some type?
>> I think you've got it. The increase is the added compression in
>> the spokes that "lost tension". THAT IS AN INCREASE equal and
>> opposite the load on the hub.
> Thank you, let me try that. Let's have a wheel with say seven
> spokes, each tensioned (that's a -) to a 100 lbs, that's a sum
> -700lbs. A load through the axle of 160 lbs (that's a +) will be
> carried by the two lower spokes (that's -100lbs +80lbs, a net -20lbs
> each), they are still in tension, but much reduced. Now the sum of
> all loadings is 5 spokes @ -100 each and 2 @ -20 each, that's -540
> lbs.
That the rim load does not change can prove shown by analyzing the
upper half of the wheel. The compressive force must be the same as
for the unloaded wheel because the tension in these spokes has not
changed. The wheel must be in equilibrium before and after the load
is applied.
I don't see why you want to reduce the spoke count to discontinuity.
The problem only gets more confusing when doing that. A tri-spoke
wheel has substantially different rim and spoke loads during its
rotation although it also works like the conventionally spoked wheel.
More complex bending and load sharing effect enter into the problem.
> I was expecting something different. Bike wheels like this remind
> me of an unsteady donkey I once had - the greater load I put on him,
> the lighter on his feet he became! Is that it, or am I still
> missing something? Sorry. What is the missing number, and on what
> path will I find it?
You are counting backwards and forwards at the same time. This
reminds me of an old joke we used, to prove that we had 11 fingers.
Counting down on one hand 10-9-8-7-6 and five on the other hand make
eleven.
> I was expecting that if we add a load of 160 lbs to the hub we must
> be able somewhere to trace that through the spokes, rim, and tyre to
> the ground, either as a tension or a compression. Mustn't we? Must
> not the sum of all loadings in the 36 spokes increase by 160 lbs?
> Or not? Are we not looking for more (160lbs more) than a mere equal
> and balancing load? After all, the tyre contact patch is now
> deforming.
I think we are back-sliding. The load is balanced by a reduction in
tension in the few load affected spokes. Please do the tone by
plucking experiment... without a tire if you think it makes a
difference.
>>>> "If the rim is not very stiff, it cannot transfer a load from the
>>>> bottom to the top ... unless there is significant deflection of
>>>> the rim, and clearly there is not."
>>> Quite so. However, to stiffen the rim we support it with well
>>> tensioned spokes so that it can transfer a great deal of load
>>> around its circumference without deflection ...
>> Not really, because it is weak in bending and stiff in compression.
>> It can only transmit compression around the rim
> Yes, of course, and I should have said so, but that is all we need
> it to do isn't it, transmit compression around the arc?.
As I explained above. The compressive load around the rim is fixed
when the wheel is built. It does not change. That is apparent from
the half wheel analysis in which the compressive load in the rim is
given by the spoke tension in half the wheel (the two halves being
identical and opposite) [N*T/(2 pi) = compressive force.
>> and because the diameter does not change appreciably (only the 0.
>> 1 mm or so at the >LAZ) there is also no change in tension in the
>> spokes. The bottom spokes give up some tension but the compressive
>> force in the rim remains unchanged because the road pushes inward
>> where the spokes lost tension.
> I expect this "inward push" (a converse but very useful way of
> looking at it) balances the loss of tension, does it not?
No. It IS the loss in tension. The inward push of this magnitude
takes almost no force as you notices when deflecting an empty rim 100
times as much by hand. At least that's what was mentioned previously
and appears in a cited paragraph below.
> Incidentally, how does that "inward push" of the road reach the rim
> at the LAZ?
Let's not get into the tire here, just look at the problem with no
tire on the wheel. And DON'T RIDE ON THAT either!
>>> In the same way that a stone bridge arch can transfer load along its
>>> curve to its abuttments.
>>There is a disparity in deflections here.
> OK, forget the stonework, but we're both talking of the transfer of
> a compressive load around an arc, aren't we? The bridge has enough
> lateral stiffness to not buckle, the wheel rim needs well-tensioned
> spokes to keep it in plane.
As I pointed out, the rim doesn't do that, the spokes do. The rim is
an elastically supported beam that is far more elastic than the beam.
The stone arch stands alone and has no spokes. Let us recall that the
contention is that the hub hangs from the top spokes and we are trying
to clarify that it is not so. The stone arch bridge has no hub no
spokes and does not hang from the top.
>> Take an unspoked rim and deflecting it a couple of mm it takes no
>> significant force. In bending, the rim is a wet noodle in
>> comparison to spoke rigidity. It is more like a piece of sheet
>> metal glued to foam rubber on the floor to walk on. The far ends
>> don't do much to support your load. In fact, this model is called
>> the elastically supported beam (as in RR track) and the bicycle
>> wheel is a circular form of it.
Jobst Brandt <jbr...@hpl.hp.com>
Peter Jones
Snoopy wrote in message <37340e41...@news.caverock.net.nz>...
>On Sat, 8 May 1999 09:43:32 +0100, "Peter Jones"
><avco...@btinternet.com> wrote:
>>>>
>>>>If so, what balances the downward force imposed by the forks?
>>>>
>>>The unchanged upward component of tension in the rest of the spokes
>>>outside the load effected zone. SNOOPY
( snip )
>Since the spoke doesn't fly off into space somewhere we know that (1)
>and (2) must exactly balance.
Thank you, yes, I see that if a thing doesn't move any forces acting on it
must be equal and opposite, any one force (or combined forces) being
balanced
by others, hence the spoke remains in situ.
>Meanwhile at the other end of the spoke, the spoke is pulling the rim
>inwards. (or downwards, in the arc we are considering)
>which means...
>There is an equal and opposite force which the rim is applying to the
>end of the spoke outwards
The rim (or any arc of it) also remains in situ, although subjected (as you
point out) to the inward pull of the spoke. How so? What balances this
inward pull?
Is it that this tension from the spoke(s) is resolved by a compressive force
following a path around the arc of the rim?
Peter Jones.
Peter Jones
Stergios Papadakis wrote in message <37348A70...@princeton.edu>...
>> >load; they are compressed by the rim flexing towards the hub at the
>> >contact point with the ground.
>> The rim is not in contact with the ground. What is it that imposes this
>> force (or resistance) that cause this upward flexure, and how does it do
>> it - via a tensile or a compressive force?
>>
>All right, turn this into a "how does the tire support the
>rim" thread! We don't get as many of these.
Sorry! OK, I've started a different thread, "Load Path ... etc ... "
> The answer is : Almost the same way the spokes support
>the hub, but the geometry is slightly more complicated
>because the tire deforms significantly.
Why significant?
>The air pressure in the tire puts tension on the casing.
I agree. Tensíon in the "cloth" casing is purely a function of air pressure
>At the bottom where it touches the road, the casing is
>compressed
I query this. Certainly the casing is deformed, but nothing is compressed.
(Well, the thin rubber tread maybe, but nothing significant that affects our
discussion of the rim) The air pressure is unchanged, thus the tension is
unchanged.
(although it is in net tension so it
>doesn't collapse completely). That compression
>(reduction in tension if you insist)
My comment above still applies here. Nothing reduces tension, except a
reduction in air pressure.
>AND the change in angle where the casing leaves the rim
>both provide an upward force to the rim.
What sort of force is this that the casing provides? It can only be tensile,
can it not? And in direction it can only lie along the line of the thread.
Stergios Papadakis
>
> Stergios Papadakis wrote in message <37348A70...@princeton.edu>...
> >Peter Jones wrote:
> >>
> >> Tim McNamara wrote in message <070519991740547761%tim...@minn.net>...
> >>
> >> >The spokes are not compressed by the hub sinking towards the rim under
> >> >load; they are compressed by the rim flexing towards the hub at the
> >> >contact point with the ground.
>
> >> The rim is not in contact with the ground. What is it that imposes this
> >> force (or resistance) that cause this upward flexure, and how does it do
> >> it - via a tensile or a compressive force?
> >>
>
> >All right, turn this into a "how does the tire support the
> >rim" thread! We don't get as many of these.
>
> Sorry! OK, I've started a different thread, "Load Path ... etc ... "
>
> > The answer is : Almost the same way the spokes support
> >the hub, but the geometry is slightly more complicated
> >because the tire deforms significantly.
>
> Why significant?
>
> >The air pressure in the tire puts tension on the casing.
>
> I agree. Tensíon in the "cloth" casing is purely a function of air pressure
>
> >At the bottom where it touches the road, the casing is
> >compressed
>
> I query this. Certainly the casing is deformed, but nothing is compressed.
> (Well, the thin rubber tread maybe, but nothing significant that affects our
> discussion of the rim) The air pressure is unchanged, thus the tension is
> unchanged.
>
I was imprecise. The tire is compressed. BUT, this does change
the tension, I have explained this more fully in another post
in this thread.
> (although it is in net tension so it
> >doesn't collapse completely). That compression
> >(reduction in tension if you insist)
>
> My comment above still applies here. Nothing reduces tension, except a
> reduction in air pressure.
>
Changing the shape of the tire changes the casing tension,
see my other post.
> >AND the change in angle where the casing leaves the rim
> >both provide an upward force to the rim.
>
> What sort of force is this that the casing provides? It can only be tensile,
> can it not? And in direction it can only lie along the line of the thread.
>
> May the wind always be behind you
>
> Peter Jones
When the casing is deformed in the LAZ, the force with which it
pulls on the rim changes, due to the change in tension and
the change in angle. The difference between the new force
and the old is exactly equal in magnitude and
opposite in direction to the load applied to the wheel.
Stergios