What is in a seat post?
James
some bicycles ride more or less harsh than others.
Just considering the seat post on my MTB, I've got an aluminium post
that sticks out of the frame 10", 27.2mm dia. (I think) and a similar
one I measured from the junk box has a 3.2mm wall thickness.
From
http://www.calculatoredge.com/civil%20engg%20calculator/beam.htm#tube
-or-
http://tinyurl.com/5udxhcj
With 150 lb force applied, I get 0.112047" deflection, or 2.8mm .
Obviously this is a force at right angles to the unloaded pipe.
Considering the human form on a traditional bicycle, and that an
acceleration up tends to force the hands and butt away from each
other, causing the seat to be forced backward, not just down.
The fact that the seat post might be at 70 degrees to vertical,
doesn't mean all forces are straight down on it.
Is it possible that with the frame flexing to some degree as well,
that the seat might move up to, say 5mm?
JS.
Chalo
>
> Is it possible that with the frame flexing to some degree as well,
> that the seat might move up to, say 5mm?
Surely more than that-- but it's not all the post. The saddle rails
and shell move quite a bit under load, and the seat tube flexes when
pried by the post. None of these things occurs in isolation.
Chalo
James
Agreed, however I'm interested in your estimations of frame and seat
post movement under loads from lumps and bumps in the road.
There's a notion of a frame that gives a hard jarring ride compared
with those that yield a more comfortable ride. Aluminium frames with
large oversize diameter tubes were said to be more harsh than
traditional steel frames, and obviously offered more stiffness in the
BB area.
I've heard folks say that the bicycle frame doesn't flex noticeably in
plane to create the effect of suspension, however if it was to move
5mm at the seat end of the seat post under transient loads, I'd say
that was significant enough to be noticed, wouldn't you?
Taking that one step further, if frame A flexed 3mm with a short seat
post and oversize tubes, where as frame B flexed 8mm with a long seat
post and 1" steel tubes of 0.5mm wall thickness, would that be
noticeable to most people? I think it would.
JS.
AMuzi
I don't know that, but a change of tire pressure (+/-
10~20psi) is likely more significant.
--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971
andre...@aol.com
I usually ride with my tires (700/23c) up to 120psi. Im about 190. Its
not that I think that they roll better, but that they get less flats.
The few times that I pump them to 100psi, they flatted. I think that
it was probably coincidence, but know Im paranoid about flatting with
lower psi. Is there anything to this theory?
I never believed that Al frames were harsher. Then I got a Javelin
bike on sale with one of the modern inexpensive Al frames. Boy, what a
nasty surprise. On roads with cracks or grindy pavement, it was an
awful experience. I quit going down one particular road all together.
After a few months, I took the components out and sold the frame on
ebay. Put the components on a steel frame and it all became nice
again.
Frank Krygowski
> On Jan 17, 5:46 pm, Chalo <chalo.col...@gmail.com> wrote:
>
> > James wrote:
>
> > > Is it possible that with the frame flexing to some degree as well,
> > > that the seat might move up to, say 5mm?
>
> > Surely more than that-- but it's not all the post. The saddle rails
> > and shell move quite a bit under load, and the seat tube flexes when
> > pried by the post. None of these things occurs in isolation.
>
> Agreed, however I'm interested in your estimations of frame and seat
> post movement under loads from lumps and bumps in the road.
>
> There's a notion of a frame that gives a hard jarring ride compared
> with those that yield a more comfortable ride. Aluminium frames with
> large oversize diameter tubes were said to be more harsh than
> traditional steel frames, and obviously offered more stiffness in the
> BB area.
>
> I've heard folks say that the bicycle frame doesn't flex noticeably in
> plane to create the effect of suspension, however if it was to move
> 5mm at the seat end of the seat post under transient loads, I'd say
> that was significant enough to be noticed, wouldn't you?
It might be noticeable. As I recall, seatposts flex more than we
might think, and if other things remain equal, long ones certainly
flex more than short ones. But I'd guess that 5mm is a bit beyond
realistic.
If you're curious, you could do some static measurements pretty
easily. You could measure from the rear dropout to the saddle clamp
of a seatpost with the bike unloaded, then loaded. If you're
expecting deflections on the order of 5mm, you should be able to see
something using an ordinary tape measure. If you can't...
> Taking that one step further, if frame A flexed 3mm with a short seat
> post and oversize tubes, where as frame B flexed 8mm with a long seat
> post and 1" steel tubes of 0.5mm wall thickness, would that be
> noticeable to most people? I think it would.
Whether any such change is noticeable depends partly on the other
factors that contribute. If you had a bike with wide, soft tires and
a cushy saddle, those might give so much deflection that seatpost flex
would be lost in the noise.
- Frank Krygowski
kolldata
comparative dimensions.
Lou Holtman
TOUR magazine tests the comfort of a frameset. It is defined as the
vertical deflection of the frame under load without wheels and WITH the
seatpost the manufacturer includes with the bike.
The values vary from 70 N/mm to 125 N/mm. Most of the deflection is from
a skinny seatpost with a setback.
Lou
Chalo
>
> There's a notion of a frame that gives a hard jarring ride compared
> with those that yield a more comfortable ride.
There is that notion, yes. I believe it will remain a notion rather
than a measured and documented effect.
> I've heard folks say that the bicycle frame doesn't flex noticeably in
> plane to create the effect of suspension, however if it was to move
> 5mm at the seat end of the seat post under transient loads, I'd say
> that was significant enough to be noticed, wouldn't you?
Sure. Nobody has demonstrated that a diamond frame deflects that much
under tolerable riding conditions, though.
> Taking that one step further, if frame A flexed 3mm with a short seat
> post and oversize tubes, where as frame B flexed 8mm with a long seat
> post and 1" steel tubes of 0.5mm wall thickness, would that be
> noticeable to most people? I think it would.
I can certainly feel the difference between a long seatpost and a
short seatpost when I hit a sharp bump in the road. Several times in
the past, I could feel a difference because the post bent. That
implies a significant and palpable amount of deflection on those bumps
that are not quite severe enough to bend the post.
I have pointed out before (years ago, though), that the axis of
deflection in a seatpost may make it more important for rider comfort
than some widely touted forms of passive suspension, e.g. the wag in a
CFRP fork. When the rear wheel of a bicycle hits a bump, the bike
frame will tend to pivot around the front axle (with some variation in
the "virtual pivot point" due to movement of the front tire and
fork). Deflection of the seatpost is in a direction that is better
aligned with the frame's rotation around the front axle than a
telescoping post or swingarm rear suspension. Thus it's apt to offer
more benefit per millimeter of effective travel.
If you intend to use a long seatpost to enhance comfort, take care to
use a good strong one. The Thomson Elite post is a proven model,
light enough to flex noticeably but also very strong, with a low
failure rate in the field.
Chalo
James
Ah! I should have Googled more intently. "Komfort" didn't kross my
mind ;-) I tried reading some of the Tour Magazine site, but I need to
pass it through a translator.
Like these?
http://cozybeehive.blogspot.com/2010/01/bicycle-frame-stiffness-numbers.html
(scroll down to "Komfort")
212 - 421 N/mm
Wow. Very different figures. The one you have at 70 N/mm is
intriguing. My example 150 lb force is 669 N, so 9.6 mm of total
deflection, compared to the 421 N/mm gives just 1.6 mm.
Chalo described the "virtual pivot point", which sounds logical to me.
I suspect that when we hit a bump there is also a tendency for hands and
butt to push apart horizontally, thereby exerting a force that is not
only straight down on the seat in the direction of gravity.
I wonder what could be done to better estimate the movement at the seat
end of the seat post relative to the rear axle under normal bicycle
operating conditions? Or to estimate the force exerted at least?
Cheers,
James.
James
> James wrote:
>> There's a notion of a frame that gives a hard jarring ride compared
>> with those that yield a more comfortable ride.
>
> There is that notion, yes. I believe it will remain a notion rather
> than a measured and documented effect.
Seems like some komfort tests are being performed!
>> I've heard folks say that the bicycle frame doesn't flex noticeably in
>> plane to create the effect of suspension, however if it was to move
>> 5mm at the seat end of the seat post under transient loads, I'd say
>> that was significant enough to be noticed, wouldn't you?
>
> Sure. Nobody has demonstrated that a diamond frame deflects that much
> under tolerable riding conditions, though.
>
>> Taking that one step further, if frame A flexed 3mm with a short seat
>> post and oversize tubes, where as frame B flexed 8mm with a long seat
>> post and 1" steel tubes of 0.5mm wall thickness, would that be
>> noticeable to most people? I think it would.
>
> I can certainly feel the difference between a long seatpost and a
> short seatpost when I hit a sharp bump in the road. Several times in
> the past, I could feel a difference because the post bent. That
> implies a significant and palpable amount of deflection on those bumps
> that are not quite severe enough to bend the post.
Indeed. Hope it wasn't painful. I know a dude who broke the seat clamp
off, about 40 km from the finish of a 130km ride. He rode back standing
up, which was tiring to say the least.
> I have pointed out before (years ago, though), that the axis of
> deflection in a seatpost may make it more important for rider comfort
> than some widely touted forms of passive suspension, e.g. the wag in a
> CFRP fork. When the rear wheel of a bicycle hits a bump, the bike
> frame will tend to pivot around the front axle (with some variation in
> the "virtual pivot point" due to movement of the front tire and
> fork). Deflection of the seatpost is in a direction that is better
> aligned with the frame's rotation around the front axle than a
> telescoping post or swingarm rear suspension. Thus it's apt to offer
> more benefit per millimeter of effective travel.
That's an interesting idea. If you could make a seat post as a damped
bendable spring, you'd save weight on the average MTB for sure.
> If you intend to use a long seatpost to enhance comfort, take care to
> use a good strong one. The Thomson Elite post is a proven model,
> light enough to flex noticeably but also very strong, with a low
> failure rate in the field.
No, I don't intend to push my seat posts into the plastic region of
deflection, thanks. It was more for illustrative purposes that I was
interested in the amount of deflection one might expect.
Until Lou pointed out the komfort tests, I was not aware that anyone in
the industry took the notion of vertical frame compliance very seriously.
Cheers,
James.
Lou Holtman
Like I said the numbers I found included the seatpost (without saddle).
I looked it up in old issues I have of TOUR magazine. Maybe the numbers
you found are those of the frames alone.
Lou
Frank Krygowski
>
>
> I wonder what could be done to better estimate the movement at the seat
> end of the seat post relative to the rear axle under normal bicycle
> operating conditions? Or to estimate the force exerted at least?
If you want to know about the movement of the seatpost end, rig up a
displacement transducer to measure the distance from the rear dropout
to the seatpost top, and a system to record the data.
At the low end, this could consist of (say) a couple pieces of
telescoping tubing and a jury-rigged pen and paper setup to record
extremes of travel, with the pen fixed to one member and the paper to
another. If you wanted to learn more, a string potentiometer (or
perhaps an LVDT) and a portable data recording system would tell you
more, but at much greater cost.
If you got such displacement information from a test ride, you could
use a static test to get the effective spring constant in N/mm, and
from that get decent force information.
- Frank Krygowski
thirty-six
> I was wondering on frame materials and geometry, and the notion that
> some bicycles ride more or less harsh than others.
>
> Just considering the seat post on my MTB, I've got an aluminium post
> that sticks out of the frame 10", 27.2mm dia. (I think) and a similar
> one I measured from the junk box has a 3.2mm wall thickness.
>
> Fromhttp://www.calculatoredge.com/civil%20engg%20calculator/beam.htm#tube
>
> With 150 lb force applied, I get 0.112047" deflection, or 2.8mm .
> Obviously this is a force at right angles to the unloaded pipe.
>
> Considering the human form on a traditional bicycle, and that an
> acceleration up tends to force the hands and butt away from each
> other, causing the seat to be forced backward, not just down.
>
> The fact that the seat post might be at 70 degrees to vertical,
> doesn't mean all forces are straight down on it.
>
> Is it possible that with the frame flexing to some degree as well,
> that the seat might move up to, say 5mm?
>
> JS.
If you can feel differences in frames, then your saddle and wheels
arn't working.
James
thirty-six
James
Have you got a video of the one on your bike?
James
> On Jan 17, 6:07 am, James <james.e.stew...@gmail.com> wrote:
>> On Jan 17, 5:46 pm, Chalo <chalo.col...@gmail.com> wrote:
>>
>>> James wrote:
>>>> Is it possible that with the frame flexing to some degree as well,
>>>> that the seat might move up to, say 5mm?
>>> Surely more than that-- but it's not all the post. The saddle rails
>>> and shell move quite a bit under load, and the seat tube flexes when
>>> pried by the post. None of these things occurs in isolation.
>> Agreed, however I'm interested in your estimations of frame and seat
>> post movement under loads from lumps and bumps in the road.
>>
>> There's a notion of a frame that gives a hard jarring ride compared
>> with those that yield a more comfortable ride. Aluminium frames with
>> large oversize diameter tubes were said to be more harsh than
>> traditional steel frames, and obviously offered more stiffness in the
>> BB area.
>>
>> I've heard folks say that the bicycle frame doesn't flex noticeably in
>> plane to create the effect of suspension, however if it was to move
>> 5mm at the seat end of the seat post under transient loads, I'd say
>> that was significant enough to be noticed, wouldn't you?
>
> It might be noticeable. As I recall, seatposts flex more than we
> might think, and if other things remain equal, long ones certainly
> flex more than short ones. But I'd guess that 5mm is a bit beyond
> realistic.
Depends on the length, angle, force and material. For long ali seat
posts in MTBs I think it's possible. Then there's the RANS bike like
this http://www.ransbikes.com/Fusion10.htm
> If you're curious, you could do some static measurements pretty
> easily. You could measure from the rear dropout to the saddle clamp
> of a seatpost with the bike unloaded, then loaded. If you're
> expecting deflections on the order of 5mm, you should be able to see
> something using an ordinary tape measure. If you can't...
I think there's more to it than a static load test with assumed force
directions would accurately model.
>> Taking that one step further, if frame A flexed 3mm with a short seat
>> post and oversize tubes, where as frame B flexed 8mm with a long seat
>> post and 1" steel tubes of 0.5mm wall thickness, would that be
>> noticeable to most people? I think it would.
>
> Whether any such change is noticeable depends partly on the other
> factors that contribute. If you had a bike with wide, soft tires and
> a cushy saddle, those might give so much deflection that seatpost flex
> would be lost in the noise.
Note that I am not complaining about ride quality or comfort of my
bikes, I'd just rather people didn't dismiss the frame and seat post
from the equation.
JS.
James
Perhaps, one day.
For the time being it is sufficient to get people thinking and to show
that bicycle frames (with the seat post), are not totally rigid.
Sure tyres deform and wheels and the seat too, but the frame and seat
post also, enough to make a perceived difference to ride komfort.
Some would have it that "As mentioned in other threads on this subject,
ride comfort does not come from frame flexibility but rather its
wheelbase, saddle position between front and rear wheels and handlebar
position. The fork of a bicycle is angled to receive significant
(occurrence times amplitude) axially. Therefore the fork can be ruled
out as well as the rear triangle. That leaves only the flex in top and
downtube. That is easily measured by applying a static load."
Ref.
http://www.cyclingforums.com/forum/thread/53446/measuring-comfort-in-bicycle-frames
I believe there is more too it.
JS.
kolldata
right but you're the size of a small Rhino. who welds for Rhino's ?
the steel vs tires, now steel tubing vs steel tubing or ?
wires....civilization's end fersure.
thirty-six
The "Supertweel" description sounds like what could have been applied
to the Ariel wheel of J K Starley but for the cross spoking, much
improved by Charles Palmer http://www.google.com/patents/about?id=r9ZuAAAAEBAJ
What you are looking for already stares you in the face and has been
around for more than a century. Fear of liability prevents
manufacturers using aluminium in sufficiently thin section for bicycle
rims to flex properly and steel is out of favour in the "Western
world". The denial of the superiority of the tied and soldered
tangentially spoked wheel also allows manufacturers to sell an
inferior product.
Look again to Paris-Roubaix for the wheel which provides suspension
where it's wanted.
Tºm Shermªn™ °_°
Attach a couple of strain gauges to your seat post that are connected to
a data recorder. Add an accelerometer if your budget allows.
--
Tºm Shermªn - 42.435731,-83.985007
I am a vehicular cyclist.
James
> Attach a couple of strain gauges to your seat post that are connected to
> a data recorder. Add an accelerometer if your budget allows.
>
Merida seems to have done this sort of thing..
http://www.merida-bikes.com/en_int/cms/63
"The already excellent CARBON FLX comfort (7.6 mm vertical deflection;
best result Carbon Hardtail Direct Competition in German BIKE magazine
12/2008) is also dramatically outshined by the 9.4 mm of our new MERIDA
O.NINE!"
Hmm, 9.4 mm vertical deflection from a hardtail MTB.
JS.
Frank Krygowski
> Tºm Shermªn™ °_° > wrote:
>
> > Attach a couple of strain gauges to your seat post that are connected to
> > a data recorder. Add an accelerometer if your budget allows.
>
>
> "The already excellent CARBON FLX comfort (7.6 mm vertical deflection;
> best result Carbon Hardtail Direct Competition in German BIKE magazine
> 12/2008) is also dramatically outshined by the 9.4 mm of our new MERIDA
> O.NINE!"
>
> Hmm, 9.4 mm vertical deflection from a hardtail MTB.
I looked for more info on that, but their site locked up for me.
I imagine the 9.4 mm was at the top of a long seatpost (as we've
discussed) but when landing a big jump. Did anyone see anything
different - like, for example, the force generating that deflection?
- Frank Krygowski
James
I don't sit on the seat while landing a big jump. I doubt many people
do. Do you?
My guess is more likely this was just a large bump, or possibly a nasty
dip, so you drop into a hole and fall on the seat as the bike comes back up.
JS.
Frank Krygowski
> Frank Krygowski wrote: me.
>
> > I imagine the 9.4 mm was at the top of a long seatpost (as we've
> > discussed) but when landing a big jump. Did anyone see anything
> > different - like, for example, the force generating that deflection?
>
> I don't sit on the seat while landing a big jump. I doubt many people
> do. Do you?
Not deliberately. But in the past, I've hit the seat pretty hard. It
happens.
> My guess is more likely this was just a large bump, or possibly a nasty
> dip, so you drop into a hole and fall on the seat as the bike comes back up.
Maybe so. But did you find anything on the force involved to give
that bike 9.4 mm deflection? Deflection information without force
information doesn't tell us much.
- Frank Krygowski
Chalo
>
> Chalo wrote:
> >
> > When the rear wheel of a bicycle hits a bump, the bike
> > frame will tend to pivot around the front axle (with some variation in
> > the "virtual pivot point" due to movement of the front tire and
> > fork). Deflection of the seatpost is in a direction that is better
> > aligned with the frame's rotation around the front axle than a
> > telescoping post or swingarm rear suspension. Thus it's apt to offer
> > more benefit per millimeter of effective travel.
>
> That's an interesting idea. If you could make a seat post as a damped
> bendable spring, you'd save weight on the average MTB for sure.
http://www.thudbuster.com/details_dynamics.html
Chalo
Andre Jute
> Merida seems to have done this sort of thing..http://www.merida-bikes.com/en_int/cms/63
>
> "The already excellent CARBON FLX comfort (7.6 mm vertical deflection;
> best result Carbon Hardtail Direct Competition in German BIKE magazine
> 12/2008) is also dramatically outshined by the 9.4 mm of our new MERIDA
> O.NINE!"
>
> Hmm, 9.4 mm vertical deflection from a hardtail MTB.
And their hard work is rewarded with... piles? -- AJ
Andre Jute
I liked the Thudbuster concept well enough to spend quite some time
investigating it before I decided on the balloon model of suspension.
The thing about the Thudbuster is that it needs considerable vertical
space to work, so virtually enforcing a steeply sloped top tube, which
in turn compromises the stiffness of the entire bike (all other things
being equal) and makes for a seat on top of a very tall unbraced tube.
Super, thoroughly worked-out, idea though.
In the end, this in 60x622 size
http://www.bike-components.de/products/language/en/info/p20308_Big-Apple-Liteskin-Kevlar-Guard-Performance-Faltreifen-Modell-2010-.html
and this
http://www.chainreactioncycles.com/Models.aspx?ModelID=27883
in combination give me painless seated five-inch kerb-stepping
ability even when I'm caught out and don't manage to go light over the
seat at the right fraction of a second.
Andre Jute
James
Ooo! The nut cracker, sweet!
A friend made a thing he called jarbars, back in the late 80's, or early
90's. I don't think they ever went commercial, but basically it was
handlebars on linear bearings with springs and a damper. They moved
more horizontal than vertical, for similar reasons.
JS.
James
> On Jan 19, 7:40 am, Chalo <chalo.col...@gmail.com> wrote:
>> James wrote:
>>
>>> Chalo wrote:
>>>> When the rear wheel of a bicycle hits a bump, the bike
>>>> frame will tend to pivot around the front axle (with some variation in
>>>> the "virtual pivot point" due to movement of the front tire and
>>>> fork). Deflection of the seatpost is in a direction that is better
>>>> aligned with the frame's rotation around the front axle than a
>>>> telescoping post or swingarm rear suspension. Thus it's apt to offer
>>>> more benefit per millimeter of effective travel.
>>> That's an interesting idea. If you could make a seat post as a damped
>>> bendable spring, you'd save weight on the average MTB for sure.
>> http://www.thudbuster.com/details_dynamics.html
>>
>> Chalo
>
> I liked the Thudbuster concept well enough to spend quite some time
> investigating it before I decided on the balloon model of suspension.
> The thing about the Thudbuster is that it needs considerable vertical
> space to work, so virtually enforcing a steeply sloped top tube, which
> in turn compromises the stiffness of the entire bike (all other things
> being equal) and makes for a seat on top of a very tall unbraced tube.
> Super, thoroughly worked-out, idea though.
For the LT model, minimum insertion is 144mm. That doesn't sound like a
lot to me, for 3" of travel. How much seat post do you have sticking out?
The pivots are a nuisance IMO. They'll get loose over time with
moisture and dirt affecting them. The effect being a loose seat feeling.
> In the end, this in 60x622 size
> http://www.bike-components.de/products/language/en/info/p20308_Big-Apple-Liteskin-Kevlar-Guard-Performance-Faltreifen-Modell-2010-.html
> and this
> http://www.chainreactioncycles.com/Models.aspx?ModelID=27883
> in combination give me painless seated five-inch kerb-stepping
> ability even when I'm caught out and don't manage to go light over the
> seat at the right fraction of a second.
>
> Andre Jute
Don't forget the B190. It might develop a complex.
JS.
Michael Press
<7afc5a0a-6e7f-4bba...@w17g2000yqh.googlegroups.com>,
Chalo <chalo....@gmail.com> wrote:
> "virtual pivot point"
The term of art is `instantaneous pole of rotation';
but there was no need for you to put quotation marks
around your phrase because you were using the notion,
not talking about the words.
I asked that people make this notion clear to
themselves when I proposed that they work
out the impact speed of a rider's head striking
the ground in two circumstances:
1. Bicycle falls over.
2. Bicycle wheels slide out laterally.
--
Michael Press
Chalo
>
> Chalo wrote:
> >
> > "virtual pivot point"
>
> The term of art is `instantaneous pole of rotation';
Thanks.
Peter Cole
I rode one of the original Thudbuster seat posts for several years. It
worked fine.
AMuzi
Even before that, Thudbuster, were "Moxy" seatposts (made in
Wisconsin !) and those worked just fine as well. Still, for
some local riders.
--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971
Peter Cole
Yes, mine was a Moxey.
Frank Krygowski
> In article
> <7afc5a0a-6e7f-4bba-8d0c-80d089eae...@w17g2000yqh.googlegroups.com>,
>
> Chalo <chalo.col...@gmail.com> wrote:
> > "virtual pivot point"
>
> The term of art is `instantaneous pole of rotation'
Another synonym is "centro."
- Frank Krygowski
addre...@invalid.invalid
Same here until mine failed catestrophically. I consider myself lucky tb
alive.
Also corresponded with another happy user who lost his job, underwent
multiple surgeries, and won a civil suit over his ThudBuster's failure.
-- PeteCresswell
James
All I've found so far is a pretty picture, and;
"The Merida R&D department holds testing equipment that is also used
by the European bike magazines Bike and Tour who currently do the most
advanced stiffness testing in the market place. "
http://www2.merida-bikes.com/es_INT/Technology.RD-STW
JS.
thirty-six
"Fork testing / comfort – working in both planes to be stiff enough
for precise steering whilst compliant enough to eliminate road buzz."
Te-he.
Wheel stability is the key ingredient for precise steering. A well-
built wheel will be vertically compliant yet torsionally stiff. A
poorly built wheel will be hard (generating high fork stresses) and
torsionally loose (resulting in less than precise steering).
I don't believe that light carbon forks should be used without serious
consideration given to the shock absorbing capabilities of the wheel.
A hard wheel will always tend to oveload such a forkset. It's not
surprising that internal cracks can develop in forks subjject to the
jarring of the modern style of hard wheels.#
Stiffer is not necessarily better, despite it's mascularising
connotation. You can't perform when splattered all over the road.
James
I am not interested in forks or wheels particularly. I am interested in
the amount of vertical (or near vertical) deflection between the top of
a seat post and the rear axle, and the force required to attain it.
JS.
thirty-six
If you insist on using a hard back wheel that will continue to be a
concern, unless you use an Allsop Softride.
James
I insist.
JS.
thirty-six
While I believe there is something in your suggestion, the longer you
make a seatpost the less stable the seat position and less efficient
the pedalling action as the post sways from side to side as well as up
and down. Talking to one of the early users of the Allsop road frame,
much energy is wasted bouncing around on the Softride beam until
controlled pedalling was perfomed. Without the influence of UCI the
frame was determined as unsuitable for racing by its users, but a damn
good training frame. Softening the seatpost in a diamond frame I see
creating a similar effect (although not as pronounced). At least with
the Softride the saddle always points forward.
James
> On Jan 25, 4:05 am, James <james.e.stew...@gmail.com> wrote:
>> thirty-six wrote:
>>> On Jan 25, 3:16 am, James <james.e.stew...@gmail.com> wrote:
>>>> I am not interested in forks or wheels particularly. I am interested in
>>>> the amount of vertical (or near vertical) deflection between the top of
>>>> a seat post and the rear axle, and the force required to attain it.
>>>> JS.
>>> If you insist on using a hard back wheel that will continue to be a
>>> concern, unless you use an Allsop Softride.
>> I insist.
>>
>> JS.
>
> While I believe there is something in your suggestion, the longer you
> make a seatpost the less stable the seat position and less efficient
> the pedalling action as the post sways from side to side as well as up
> and down.
Likewise with soft wheels and tires. Bouncing like a pogo stick doesn't
help anyone.
> Talking to one of the early users of the Allsop road frame,
> much energy is wasted bouncing around on the Softride beam until
> controlled pedalling was perfomed.
Likewise a MTB with suspension absorbs much pedaling force when you
stand on the pedals.
> Softening the seatpost in a diamond frame I see
> creating a similar effect
<snip>
If taken to the extreme.
The Merida folks suggest the construction of the stays also plays a
large part in the amount of vertical compliance of a bicycle frame - not
just the seat post.
My guess is that the frame tubing diameter, wall thickness, material,
size, angles and seat post length all contribute to vertical compliance,
be it 1 mm or 10 mm under some typical load.
Some people don't believe the frame offers any perceivable vertical
compliance, and thus can be discounted when someone says "My carbon
frame seems to absorb road buzz better than my aluminium frame.",
putting the difference down to tyre pressure, wheel base and seat
padding - or imagination.
JS.
thirty-six
Perhaps it was another thread where I mentioned oscillatory
frequencies of individual frame members. Tubes resonate at
frequencies depending on their dimensions. If two frame elements are
naturally harmonic, they will self-excite each other. Classic butted
steel frames with their variable wall thickness and effective length
fixed by the tubing manufacturer along with taper stays do not as a
rule self resonate. This is possibly because the manufacturers
understand these issues. There are exceptions.
As well as the major effect that the wheel and saddle plays in the
comfort of the rider, self-excitation of the frame will cause 'road
buzz' or whatever is your prefered term of the moment. A softer this
that or the other is not necessarily the best way to go about removing
this buzz, simply changing the seatpost to a different length or
thickness can change the chamber dimensions sufficiently to prevent
excitation. Try stuffing the seatpost (or seat-tube) with foam or
plugging the end of the seatpost with a cork.
Another option is to use a rear luggage rack, most people reckon this
kills the liveliness of a frame (in a negative way). It's all really
what are your expectations, if you want a 'lively' race frame you may
successfully ignore resonance where if you want a easy riding tour
bike, that resonance is annoying and needs eradication, fortunately
loading the bike usually cures any buzzy tour bikes. The ommision of
buzz may make the road bike user complain of a dead frame. It may
indeed be that manufacturers are purposely creating frames that are
naturally buzzy to make them feel exciting. Exciting is tiring, I'll
corner a little faster if I want exciting. I can do that well enough
on my steel bike.
I'll hazard a guess that taper gauge stays do much to eliminate
resonance as do taper gauge forks. I don't think these are
commonplace in aluminium alloy stays and possibly think this idea is
overlooked with a carbon-fibre rear triangle because they are built
for unit construction and lighter is better, right? Not always.
Resonance is a killer in disc drives as it is in long bridges, it is
designed out of manufacturing machinery but sometimes ignored in
consumer products. When you are manufacturing for pictures and prices
(ooh, isn't the internet great?) , sometimes engineering goes out the
window.
James
> Maybe so. But did you find anything on the force involved to give
> that bike 9.4 mm deflection? Deflection information without force
> information doesn't tell us much.
I emailed Merida and they replied.
They fix the rear axle. The fork is straight and rigid with the front
axle fixed vertically, but can slide horizontally.
The BB to top of seat post (clamp) is set to 750 mm.
They apply 100 kg of force (981 N) vertically down at a distance of
70mm behind the centre axis of the seat post.
The vertical deflection is measured from the original seat post clamp
position to that under the load conditions described above.
JS.
Frank Krygowski
That's good information. It's what we needed to understand their
"travel" specification.
- Frank Krygowski