frame deflection measurements - any numbers?
whi...@hotmail.com
some numbers about the vertical stiffness of frames. If there aren't,
is there anybody with access to bikes and a viable testing setup? All
the arguments about the stiffness of the bicycle structure, and how
small the differences in materials, s-bends, "zertz" inserts etc... are
based on deduction, but not real numbers. I believe, and would wager
untold amounts that the "tire supremacy" argument is sound, and that
Jim Beam blows a lot of hot air (though I'm not convinced he isn't just
trying to make a big joke by going against all sense and consensus for
fun) about how huge an effect it has (dental disaster vs. sofa from his
al to steel frames...), though I'm not convinced he isn't really Jobst
with an alternative account having everyone on.
So, a Rinard type deflection test for frames, but in the other plane?
Anyone know of anything?
Whitfit
diann...@yahoo.com
> So, a Rinard type deflection test for frames, but in the other plane?
> Anyone know of anything?
>
> Whitfit
>From http://draco.acs.uci.edu/rbfaq/FAQ/8e.2.html
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Subject: 8e.2 Frame Stiffness
From: Bob Bundy <bo...@ico.isc.com>
As many of you rec.bicycles readers are aware, there have been
occasional,
sometimes acrimonious, discussions about how some frames are so much
stiffer than others. Cannondale frames seem to take most of the abuse.
The litany of complaints about some bike frames is long and includes
excessive wheel hop, numb hands, unpleasant ride, broken spokes,
pitted headsets, etc. I was complaining to a friend of mine about how
there
was so much ranting and raving but so little empirical data - to which
he replied, "Why don't you stop complaining and do the measurements
yourself?". To that, I emitted the fateful words, "Why not, after all,
how hard can it be?". Following some consultation with Jobst and a few
other friends, I ran the following tests:
The following data were collected by measuring the vertical deflection
at
the seat (ST), bottom bracket (BB) and head tube (HT) as a result of
applying 80lb of vertical force. The relative contributions of the
tires, wheels, fork, and frame (the diamond portion) were measured
using
a set of jigs and a dial indicator which was read to the nearest .001
inch. For some of the measures, I applied pressures from 20 to 270 lbs
to check for any significant nonlinearity. None was observed. The
same
set of tires (Continentals) and wheels were used for all measurements.
Note that these were measures of in-plane stiffness, which should be
related to ride comfort, and not tortional stiffness which is something
else entirely.
Bikes:
TA - 1987 Trek Aluminum 1200, this model has a Vitus front fork, most
reviews describe this as being an exceptionally smooth riding bike
SS - 1988 Specialized Sirus, steel CrMo frame, described by one review
as
being stiff, hard riding and responsive
DR - 1987 DeRosa, SP/SL tubing, classic Italian road bike
RM - 1988 Cannondale aluminum frame with a CrMo fork, some reviewers
could not tolerate the rough ride of this bike
TA SS DR RM
---------- ---------- ---------- ----------
ST BB HT ST BB HT ST BB HS ST BB HT
diamond 1 1 0 2 2 0 2 2 0 1 1 0
fork 3 11 45 3 9 36 4 13 55 3 10 40
wheels 2 2 2 2 2 2 2 2 2 2 2 2
tires 68 52 66 68 52 66 68 52 66 68 52 66
total 74 66 113 75 65 104 76 69 123 74 65 108
What is going on here? I read the bike mags and this net enough to
know
that people have strong impressions about the things that affect ride
comfort. For example, it is common to hear people talk about rim types
(aero vs. non-aero), spoke size, butting and spoke patterns and how
they
affect ride. Yet the data presented here indicate, just a Jobst
predicted,
that any variation in these factors will essentially be undetectable to
the rider. Similarly, one hears the same kind of talk about frames,
namely, that frame material X gives a better ride than frame material
Y, that
butted tubing gives a better ride that non-butted, etc. (I may have
even
made such statements myself at some time.) Yet, again, the data
suggest
that these differences are small and, perhaps, even undetectable. I
offer
two explanations for this variation between the data and subjective
reports
of ride quality.
Engineering:
These data are all static measurements and perhaps only applicable at
the
end of the frequency spectrum. Factors such as frequency response, and
damping might be significant factors in rider comfort.
Psychology:
There is no doubt that these bikes all look very different, especially
the
Cannondale. They even sound different while riding over rough
roads. These factors, along with the impressions of friends and
reviews
in bike magazines may lead us to perceive differences where they, in
fact,
do not exist.
Being a psychologist, I am naturally inclined toward the psychological
explanation. I just can't see how the diamond part of the frame
contributes
in any significant way to the comfort of a bike. The damping of the
frame
should be irrelevant since it doesn't flex enough that there is any
motion to actually dampen. That the frame would become flexible at
some important range of the frequency spectrum doesn't seem likely
either.
On the other hand, there is plenty of evidence that people are often
very
poor judges of their physical environment. They often see
relationships
where they don't exist and mis-attribute other relationships. For
example,
peoples' judgement of ride quality in automobiles is more related to
the
sounds inside the automobile than the ride itself. The only way to get
a good correlation between accelerometers attached to the car seat and
the rider's estimates of ride quality is to blindfold and deafen the
rider (not permanently!). This is only one of many examples of mis-
attribution. The role of expectation is even more powerful. (Some
even
claim that whole areas of medicine are built around it - but that is
another story entirely.) People hear that Cannondales are stiff and,
let's face it, they certainly *look* stiff. Add to that the fact that
Cannondales sound different while going over rough roads and perhaps
the rider has an auditory confirmation of what is already believed to
be true.
Unless anyone can come up with a better explanation, I will remain
convinced that differences in ride quality among frames are more a
matter of perception than of actual physical differences.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Sandy
news:1143085757.6...@i40g2000cwc.googlegroups.com,
diann...@yahoo.com <diann...@yahoo.com> a réfléchi, et puis a déclaré
:
> whi...@hotmail.com wrote:
>> So, a Rinard type deflection test for frames, but in the other plane?
>> Anyone know of anything?
>>
>> Whitfit
>
>> From http://draco.acs.uci.edu/rbfaq/FAQ/8e.2.html
>
> ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> Subject: 8e.2 Frame Stiffness
> From: Bob Bundy <bo...@ico.isc.com>
>
Kind of useful, if you only ride your bike coasting in a perfectly straight
line downhill on glass.
The article offers nothing on frame deflection in isolation. Tour Magazine
and Le Cycle both do regular bench evaluations of frame flex. Are there
really _no_ English language publications that do the same ?
Perhaps there is also a bit of professional self-serving inquiry and
explanation, also.
--
Sandy
Verneuil-sur-Seine
*******
La vie, c'est comme une bicyclette,
il faut avancer pour ne pas perdre l'équilibre.
-- Einstein, A.
M-gineering
>
> Unless anyone can come up with a better explanation, I will remain
> convinced that differences in ride quality among frames are more a
> matter of perception than of actual physical differences.
>
> ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
>
The human body is pretty good at perceiving accellerations in three
dimensions over a wide frequency range.
You have measured in only one direction and at a steady state.
Why do you feel you have to introduce psychology if your experiments
don't match the expected outcome?
--
---
Marten Gerritsen
INFOapestaartjeM-GINEERINGpuntNL
www.m-gineering.nl
Road Man
"Sandy" <leu...@frree.fr> wrote in message
news:44225a14$0$20650$636a...@news.free.fr...
Sandy, the only English-language bike mag (of any size) I could
imagine addressing this today is "Cycling Plus," from the UK.
"Bicycling" has long been mainly a fashion magazine. Last issue I
noticed great difficulty telling the articles from the advertisements.
Presumably we English-speakers are mostly known to be color-obsessed
consumption bovines, presenting no market demand for hard information
and test results. Very sad!
Ken Freeman
Dan
been involved in the accident reconstruction business For many years
wherein I do instrumentation and measurement. In my experience, people
are not very good at quantifying or qualifying physical phenomena. They
very often describe events that violate Newtonian physics. (litespeed
bikes come to mind here)
My degrees are in civil/structural engineering. A welded, triangular
frame (really a truss) is an extremely stiff structure. The vertical, in
plane deformation of a bicycle frame must be very small - the same goes
for the wheels. Out of plane stiffness is another issue and probably
varies quite a bit with bicycle frame type.
Fork deflection is due to cantilever bending, I would expect a lot of
variation between forks and frames since the attachment of the fork to
the frame may be important.
Tires deform a relatively large amount - that is what they are designed
to do, isolate the bike and rider from the road. Put solid tires on a
bike or ride on the rims and it will be stiff.
Most bicycle purchases (mine included) are made on the basis of fashion
and hype. After working as an engineer for many years, I have concluded
that real engineering does not sell and does not have much of a market.
diann...@yahoo.com
> Dans le message de
> news:1143085757.6...@i40g2000cwc.googlegroups.com,
> diann...@yahoo.com <diann...@yahoo.com> a réfléchi, et puis a déclaré
>
> >
> > ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> > Subject: 8e.2 Frame Stiffness
> > From: Bob Bundy <bo...@ico.isc.com>
> >
>
Not sure where you get this idea; the way I read it each frame's
contribution to deflection was also reported (in isolation from the
other components).
> Tour Magazine
> and Le Cycle both do regular bench evaluations of frame flex.
I haven't seen Tour Magazin or Le Cycle measure vertical frame
deflection, only lateral and torsional. The OP was asking for vertical.
> Are there
> really _no_ English language publications that do the same ?
Bicycling used to have the "Tarantula" frame flex fixture, but it's
been years since then.
diann...@yahoo.com
> diann...@yahoo.com wrote (actually quoted Bob Bundy's article):
>
> > Unless anyone can come up with a better explanation, I will remain
> > convinced that differences in ride quality among frames are more a
> > matter of perception than of actual physical differences.
>
> The human body is pretty good at perceiving accellerations in three
> dimensions over a wide frequency range.
I've often wondered how good the human body is at detecting; can you
point to any on line references?
> You have
Actually I only quoted from the faq; Bob Bundy did the measurements,
not me.
> measured in only one direction
Right, the direction the OP asked about. He already knew about Rinard's
lateral measurements.
> and at a steady state.
I'd be interested in reading about non-steady state measurements. Any
references?
> Why do you feel you have to introduce psychology if your experiments
> don't match the expected outcome?
Well, I only quoted the FAQ, so it was really Bob Bundy who introduced
psychology, not me. I suppose he might have introduced it because it
helped him understand the difficult proposition that such small
differences should be perceptible or not.
SYJ
Bob wrote:
---snip---
you can observe some interesting things
> humping with your favorite bike in a trainer
---/snip---
I would imagine so, but I'd be afraid that my wife and I would lose our
balance. Or do you mean 'humping my bike, while it is in a trainer'
(in which case, I'll decline as I think of my relationship with my bike
in more platonic terms)?
Sorry...couldn't resist.
SYJ
Sandy
news:1143146074.6...@u72g2000cwu.googlegroups.com,:
> Sandy wrote:
>> Dans le message de
>> news:1143085757.6...@i40g2000cwc.googlegroups.com,
>> diann...@yahoo.com <diann...@yahoo.com> a réfléchi, et puis a
>> déclaré
>>
>>> From http://draco.acs.uci.edu/rbfaq/FAQ/8e.2.html
>>>
>>> ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
>>> Subject: 8e.2 Frame Stiffness
>>> From: Bob Bundy <bo...@ico.isc.com>
>>>
>>
>> The article offers nothing on frame deflection in isolation.
>
> Not sure where you get this idea; the way I read it each frame's
> contribution to deflection was also reported (in isolation from the
> other components).
Isolated, as in nude of all components, stressed by testing equipment on a
slab. I didn't make myself clear, sorry.
>
>> Tour Magazine
>> and Le Cycle both do regular bench evaluations of frame flex.
>
> I haven't seen Tour Magazin or Le Cycle measure vertical frame
> deflection, only lateral and torsional. The OP was asking for
> vertical.
Yes, that's what they measure. And they have results. If you measure for
the infinitessimal, and don't find it, that does not mean that other
elements of flex are not to be found. I spent an hour on the trainer today.
Easy to see lateral deflection, so we look for it and measure it and
evaluate it. I have seen, at our cycling expositions, frame abuse machines
do their thing. But why look for pure vertical deflection, unless, as I
suggested, you ride only downhill on a perfect surface, never pedaling.
>> Are there
>> really _no_ English language publications that do the same ?
>
> Bicycling used to have the "Tarantula" frame flex fixture, but it's
> been years since then.
That's the Cannondale thing, I believe. Could be wrong. Anyway,
approximately that machine was used for the crankset and bottom bracket
testing in Le Cycle. Looked cool, but I don't vouch for it, of course.
--
Bonne route !
Sandy
Verneuil-sur-Seine FR
Mike Reed
thread:
I have a theory (let's call it "q-flex") that while vertical frame flex
is very little, lateral frame flex dampens the bumps vertically as felt
through our feet. When you are pedalling, some percentage of your
weight is supported by your feet through the cranks.
Given a nonzero q-factor, we are pushing a lever displaced laterally
from the center of the frame, and it's not symmetrically supported if
we're under power. While one pedal is pushing down, the other pulling
up -- which both flex the frame in the same direction laterally. If
there is lateral flex, the pedalling with resistance will work the
flex. This would soften the bumps through the pedals as the rider's
weight is supported ~6" off plane. It may even dlightly dampen the
bumps to the saddle and handle bars too since the structure is out of
alignment during the flex.
This assumes a cyclist under power. The more power being put out, the
more of a difference q-flex will make. Also, at the end of a hard ride,
when form starts to fade, the pedal stroke will suffer and be less
consistent, and q-flex would make an even bigger difference.
To see the flex I'm talking about, get on your bike close to a wall.
Sit on the saddle and support yourself with your left hand. Grab the
rear brake hard. Spin your cranks so they are horizontal, then push and
pull hard with your feet like you're trying to start at a light. Watch
the bottom-bracket area as it flexes.
-Mike
diann...@yahoo.com
> Dans le message de
> news:1143146074.6...@u72g2000cwu.googlegroups.com,
> diann...@yahoo.com <diann...@yahoo.com> a réfléchi, et puis a déclaré
> :
> > Sandy wrote:
> >> Dans le message de
> >> news:1143085757.6...@i40g2000cwc.googlegroups.com,
> >> diann...@yahoo.com <diann...@yahoo.com> a réfléchi, et puis a
> >> déclaré
> >>
> >>> From http://draco.acs.uci.edu/rbfaq/FAQ/8e.2.html
> >>>
> >>> ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
> >>> Subject: 8e.2 Frame Stiffness
> >>> From: Bob Bundy <bo...@ico.isc.com>
> >>>
> >>
> >> The article offers nothing on frame deflection in isolation.
> >
> > Not sure where you get this idea; the way I read it each frame's
> > contribution to deflection was also reported (in isolation from the
> > other components).
>
> Isolated, as in nude of all components, stressed by testing equipment on a
> slab. I didn't make myself clear, sorry.
Thanks, I think I understand now. Still, as I read Bob's FAQ, it seems
to me he's saying the measurements were taken in a manner to isolate
the deflection of the frame alone (as if it were nude?), fork alone,
tires alone, etc., no? He then also gives the total.
> But why look for pure vertical deflection, unless, as I
> suggested, you ride only downhill on a perfect surface, never pedaling.
Why measure infintesimal vertical deflection? I used to imagine a
frame's vertical flex was significant, and having the measurements in
Bob's FAQ puts that in perspective. And since the OP specifically asked
about it (vertical deflection), I posted Bob's article to provide the
same measurements that helped me.
[Re: Bicycling Magazine's "Tarantula"]
> That's the Cannondale thing, I believe. Could be wrong.
Looks similar to me, too.
> Anyway,
> approximately that machine was used for the crankset and bottom bracket
> testing in Le Cycle.
Yeah, that was a fantastic report. Even included x-ray images of all
the carbon cranks!
> Looked cool, but I don't vouch for it, of course.
It seems a reasonable simulation of one load case, though there are
also others I suppose.
vesel
<whi...@hotmail.com> wrote in message
news:1143033004.2...@e56g2000cwe.googlegroups.com...
Ride magazine (Australia) have built up a jig for testing lateral frame
deflection. They do the usual riding impression tests then try to do some
quantify of lateral rigidity.
http://www.ridemedia.com.au/
They have only been doing this for a short period so have only done 10-15 or
so frames
Peter
3rd...@willets.org
> Why do you feel you have to introduce psychology if your experiments
> don't match the expected outcome?
>
Some people will do anything to avoid a little engineering. This site
seems to round up the relevant facts:
http://www.bikethink.com/ride_feel.htm
Another interesting fact I found is that Aluminum and Steel have almost
the same speed of sound, so the transmission of vibrations between
steel and aluminum bike parts must be pretty efficient :-(.
Peter Cole
>> Why do you feel you have to introduce psychology if your experiments
>> don't match the expected outcome?
>>
>
> Some people will do anything to avoid a little engineering. This site
> seems to round up the relevant facts:
>
> http://www.bikethink.com/ride_feel.htm
"The three clips below are ->exaggerated audio<- representations of the
first 12 modes of natural frequency superimposed on each other. They
give an audible demonstration of what the frame's ride feel is like."
No numbers except vertical deflection of frames (nominal), not very
compelling or informative -- less "engineering" than most advertising copy.
David Wagner
In considering how a bicycle's construction contributes or detracts from
comfort, what matters is how shocks and vibrations are transmitted, first
through the tires, then the wheels, then the frame. Tires have significant
potential for damping vibrations. Wheels, I don't know.
The rake of a fork, and the length of chain stays, means that impact forces
on the tires are directed somewhat transverse to the frame. This can result
in flex, particularly in the fork, which is not part of a triangle, as are
the chainstays.
Vibrations perpendicular to the fork, I think, would not have a huge impact
on the handlebars. The vibrations that would tend to rattle your teeth
would be compression waves travelling up the fork, or up the seat stays (I
presume these are still within linear elasticity, at least for steel).
All of which means, I think, that frame geometry matters more than how much
the frame flexes in a static test. I think I would make an exception to this
rule for the fork tips, because the amount of flex at the fork tips
translates directly into forces not being transmitted up the fork. Perhaps
one could say something similar for the rear triangle.
Last year's Trek 2300C had a shock absorber of some kind in the seat stay.
I wonder how well that worked? I could not find it on the Trek website just
now.
David Wagner
jobst....@stanfordalumni.org
> My 2 cents worth:
I think you said that because you know this is all BS and part of the
myth and lore of bicycling.
> In considering how a bicycle's construction contributes or detracts
> from comfort, what matters is how shocks and vibrations are
> transmitted, first through the tires, then the wheels, then the
> frame. Tires have significant potential for damping vibrations.
> Wheels, I don't know.
> The rake of a fork, and the length of chain stays, means that impact
> forces on the tires are directed somewhat transverse to the frame.
> This can result in flex, particularly in the fork, which is not part
> of a triangle, as are the chainstays.
> Vibrations perpendicular to the fork, I think, would not have a huge
> impact on the handlebars. The vibrations that would tend to rattle
> your teeth would be compression waves traveling up the fork, or up
> the seat stays (I presume these are still within linear elasticity,
> at least for steel).
Do you mean impact or effect? The fork rake (angle) is chosen to load
the most significant frequent shocks axially in the fork blades,
loading them primarily in compression. There isn't much elasticity in
that mode regardless of material. Add to that, that the "rear
triangle" is in reality a tetrahedron (including the rear axle) and is
ultimately rigid. That is why the tubes back there can be so slender.
> All of which means, I think, that frame geometry matters more than
> how much the frame flexes in a static test. I think I would make an
> exception to this rule for the fork tips, because the amount of flex
> at the fork tips translates directly into forces not being
> transmitted up the fork.
The curl at the end of the fork does not bend as a spring. It is
there to give a straight fork the offset required for a given rake
angle needed for steering dynamics. Today, many forks are made
straight with the offset achieved by angling the fork crown.
> Perhaps one could say something similar for the rear triangle.
You have already said too much about elasticity of the frame for ride
comfort. Use Google to search the web on this.
Frames don't flex in the vertical plane. That is a myth that gets
stronger with each repetition, something of which you are guilty.
> Last year's Trek 2300C had a shock absorber of some kind in the seat
> stay. I wonder how well that worked? I could not find it on the
> Trek website just now.
Please don't "think aloud" and especially not in writing. This has
all been discussed and discredited often in this forum. Don't try to
resuscitate it from the grave.
Jobst Brandt
David Wagner
jobst....@stanfordalumni.org at jobst....@stanfordalumni.org wrote on
3/31/06 7:15 PM:
> David Wagner writes:
>
>> My 2 cents worth:
>
> I think you said that because you know this is all BS and part of the
> myth and lore of bicycling.
I said that because I felt I lacked expertise, but wanted to try out some
opinions. Be merciful!
>> Vibrations perpendicular to the fork, I think, would not have a huge
>> impact on the handlebars. The vibrations that would tend to rattle
>> your teeth would be compression waves traveling up the fork, or up
>> the seat stays (I presume these are still within linear elasticity,
>> at least for steel).
>
> Do you mean impact or effect? The fork rake (angle) is chosen to load
> the most significant frequent shocks axially in the fork blades,
> loading them primarily in compression. There isn't much elasticity in
> that mode regardless of material. Add to that, that the "rear
> triangle" is in reality a tetrahedron (including the rear axle) and is
> ultimately rigid. That is why the tubes back there can be so slender.
I agree with what you say about rake below--rake is primarily for
steering--similar to the caster angle in a car's front end. But here you are
saying that the fork rake is chosen to load shocks axially in the fork
blades. If there is no elasticity (bending?) in that mode, why does it
matter? You confused me here.
But when I ride on a rough road, I'm sure I see some vibration in the
fork--mostly I see the front axle vibrating. And I don't think I have a
loose headset.
> > The curl at the end of the fork does not bend as a spring. It is
> there to give a straight fork the offset required for a given rake
> angle needed for steering dynamics. Today, many forks are made
> straight with the offset achieved by angling the fork crown.
>
> Frames don't flex in the vertical plane. That is a myth that gets
> stronger with each repetition, something of which you are guilty.
But impulses are surely transmitted through the frame as waves, right?
Maybe that is not measureable as "flex". But it remains conceivable that
different materials or different construction or geometry could have an
impact on how these waves propagate through the frame.
Does chain stay length make a difference? That is at least one folk lore
item that I picked up in the last year.
>> Last year's Trek 2300C had a shock absorber of some kind in the seat
>> stay. I wonder how well that worked? I could not find it on the
>> Trek website just now.
>
> Please don't "think aloud" and especially not in writing. This has
> all been discussed and discredited often in this forum. Don't try to
> resuscitate it from the grave.
Sorry, I missed that discussion. Thanks for your reply.
David Wagner
jobst....@stanfordalumni.org
>>> My 2 cents worth:
>> I think you said that because you know this is all BS and part of
>> the myth and lore of bicycling.
> I said that because I felt I lacked expertise, but wanted to try out
> some opinions. Be merciful!
>>> Vibrations perpendicular to the fork, I think, would not have a
>>> huge impact on the handlebars. The vibrations that would tend to
>>> rattle your teeth would be compression waves traveling up the
>>> fork, or up the seat stays (I presume these are still within
>>> linear elasticity, at least for steel).
>> Do you mean impact or effect? The fork rake (angle) is chosen to
>> load the most significant frequent shocks axially in the fork
>> blades, loading them primarily in compression. There isn't much
>> elasticity in that mode regardless of material. Add to that, that
>> the "rear triangle" is in reality a tetrahedron (including the rear
>> axle) and is ultimately rigid. That is why the tubes back there
>> can be so slender.
> I agree with what you say about rake below--rake is primarily for
> steering--similar to the caster angle in a car's front end. But
> here you are saying that the fork rake is chosen to load shocks
> axially in the fork blades. If there is no elasticity (bending?) in
> that mode, why does it matter? You confused me here.
That's why they are at that angle, to try to keep the principal force
axial. Obviously bumps on the road come in all heights so some will
bend the fork forward and some rearward. If you are familiar with the
cosine function you will notice that for small deflection angles,
there is practically no foreshortening, like tilting a walking cane
plus or minus 10 degrees from vertical. That doesn't change its
height but a smidgen.
> But when I ride on a rough road, I'm sure I see some vibration in the
> fork--mostly I see the front axle vibrating. And I don't think I have a
> loose headset.
Cosine error. No palpable length change.
>> The curl at the end of the fork does not bend as a spring. It is
>> there to give a straight fork the offset required for a given rake
>> angle needed for steering dynamics. Today, many forks are made
>> straight with the offset achieved by angling the fork crown.
>> Frames don't flex in the vertical plane. That is a myth that gets
>> stronger with each repetition, something of which you are guilty.
> But impulses are surely transmitted through the frame as waves,
> right? Maybe that is not measurable as "flex". But it remains
> conceivable that different materials or different construction or
> geometry could have an impact on how these waves propagate through
> the frame.
Cast bronze bells ring, and if they weren't elastic and deformed from
circular to elliptical not much of their principal frequency would
occur. You cannot see these deformations. There are many things
argued here by people unfamiliar with the technical aspects but
instead of asking how something works, they tell all about it in an
authoritative manner and at the very end, add "that's right isn't it"
in some indirect way. Of course it isn't, but the cat
(misinformation) is already out of the bag.
> Does chain stay length make a difference? That is at least one folk
> lore item that I picked up in the last year.
Of course wheelbase makes a difference, but it is geometry rather than
elasticity. You may have noticed that road graders are long to smooth
the irregularities in the surface being graded, the blade being in the
middle of the wheelbase. Beyond that, they have tandem axles in the
rear that divide those bumps in half as well. That's why railroad
cars have two axle bogies and Talgo trains ride more roughly with
single axles and short coaches.
>>> Last year's Trek 2300C had a shock absorber of some kind in the
>>> seat stay. I wonder how well that worked? I could not find it on
>>> the Trek website just now.
>> Please don't "think aloud" and especially not in writing. This has
>> all been discussed and discredited often in this forum. Don't try
>> to resuscitate it from the grave.
> Sorry, I missed that discussion. Thanks for your reply.
Well, thanks probably weren't in order but that's OK. I think we can
rise above it. You may have noticed that a parallel discussion about
rolling resistance in tires is being dueled between fact and sophism.
That helps prepare the playing field for other misinformation.
Jobst Brandt
41
jobst....@stanfordalumni.org wrote:
> That's why they are at that angle, to try to keep the principal force
> axial. Obviously bumps on the road come in all heights so some will
> bend the fork forward and some rearward.
I've seen you mention this before and I can't help wondering: how was
this done? If through trial and error by the builders, based on
failures, I would have thought the process would have been too slow to
keep up with changes in roads over the years, and without sufficient
resolution to arrive at eg 73° instead of 72°. If through mechanical
analysis of impact with a representative bump, how do they choose the
bump? I can't imagine the bicycle industry figuring anything like this
out scientifically.m
jobst....@stanfordalumni.org
If you look at pictures of bicycle when most roads were unpaved or had
cobbles, the rake was much greater than today while track bicycles are
about as steep as stability allows. They also have (or had) round
cross section fork blades. Today, road forks that fail in fatigue
generally break forward even though the greatest (but numerically
small) forces come from braking that would cause the opposite failure.
Jobst Brandt
James Thomson
> If you look at pictures of bicycles when most roads were
> unpaved or had cobbles, the rake was much greater than
> today while track bicycles are about as steep as stability
> allows.
Couldn't that adequately be explained by the requirement of stable handling
in those different situations, rather than fork longevity?
James Thomson