Frame wobble - custom built bike

wrote:

FWIW, when I discussed my bike with a local builder and mentioned 73
degrees/50 rake, his comment was "Nobody does that...".

Now I have ridden many bikes and this was the first that shimmied. If
previous bikes haven't shimmied and this one does, how can you blame
the rider?

Faulty logic! If your first green bike was the first to shimmy, would you
blame the color? BTW, 73 degrees and 50mm offset is not unusual (produces
about 2" of trail).

And no one is "blaming" the rider. It's a combination of factors. For a
given model bike, a few people may experience shimmy, while most don't.

Art Harris

"David L. Johnson" wrote:

If I were a builder, I would cringe at every customer who comes in
claiming that because the headtube is 74 degrees rather than 73, he (the
builder) designed the bike poorly and so it shimmies. That builder has
hundreds of other bikes out in there with the same damn angles, and no
shimmy. The OP did not specify unusual angles or dimensions, so the
builder did what he had done previously -- which worked. But he will
probably have to change something to satisfy the customer, who wrongly
blames the builder. Easiest to replace the fork, and that will change the
system enough that it'll probably work.

I think David's assumptions above are probably spot on - the frame
itself probably isn't "defective", but it doesn't fulfill the mission
statement of the owner, so something needs to be done. I (like David)
suspect that a new fork with a little less rake will probably solve
the problem. In fact, I wonder if it's possible that the current fork
might have been bent somewhat - happened to an OLD MTB of mine (from
too many hard landings in the pre-shock days) and did some funky
things to the handling (taking the trail to almost zero).

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame

Andrew Muzi writes:

As David Johnson noted, shimmy is an harmonic resonance in a
bike/rider system. As machinists say of chatter "change the feed or
change the speed'. Any significant change will open one of the
loops that make his bike shudder. You can't blame the builder any
more than you can blame the rider. Even stock frames with thousands
of iterations where every frame is identical shimmy occasionally
under some riders in some riding positions. It's a phenomenon, an
annoying phenomenon, but not a defect.

I won't go so far as to absolve the frame builder entirely. I have
ridden bicycles that would shimmy at almost any speed, no-hands. The
owner of one of these told me outright that the bicycle could not be
ridden no-hands as we were moseying along at about9-10mph. I was
trying to swing my shoulder bag around and sat up for a moment while
letting go of the bars. This was a long frame with a low rake angle,
a Holland Bike as they call them in Europe.

On the other hand, my tall frame and all the ones I have had, from
Cinelli, Ritchey, or P. Johnson all shimmy if I let them but, I can
easily ride them no hands pedaling or coasting at any speed. I ride
high speed descents with my hands on the stem and tucked in.

This is convenient because when Rolf Dietrich claimed in his patent
that paired spoking prevented shimmy, I could easily demonstrate that
36-spoke conventional, Bontrager, paired spoke and Rolf Vector Pro
wheels all shimmied identically, something that was verifiable with
instrumentation showing both frequency and amplitude development from
releasing the hands from the bars.

As I mentioned here in another shimmy discussion, to demonstrate that
it is frame related, I filled my front tire with water (excluding
residual air) and produced visibly the same shimmy but did not measure
the frequency. My impression was that the frequency was slightly
lower.

Jobst Brandt

Arthur Harris writes:

Does the bike seem to pull to one side? Have you done the string
test to check frame alignment? See:

http://www.sheldonbrown.com/frame-spacing.html

See also:

http://www.sheldonbrown.com/brandt/shimmy.html

Jobst Brandt

On Thu, 24 Feb 2005 22:32:38 -0600, Tom Sherman
wrote:

How about wrapping the tubes in the head tube/down tube/top tube are
with carbon fiber composite to add some stiffness to the frame?

Wrapping tubes with carbon *does* add stiffness. In 1990 a friend
wrapped his alu swing arm with carbon and doubled the stiffness.

David L. Johnson wrote:
On Fri, 25 Feb 2005 14:22:33 +0000, RonSonic wrote:


On Fri, 25 Feb 2005 06:06:29 -0800, jim beam wrote:


David L. Johnson wrote:

On Thu, 24 Feb 2005 20:13:37 -0800, jim beam wrote:



well, the good news is that you've got what should be a shimmy-resistant
wheel setup - the xtr hub has a much better bracing angle than a
standard road hub. beefy rim too.

the bad news, imo, is that unless you're running a funky rack on the
rear or some other unusual device, i think it's a function of tube set
and therefore not something that can be addressed easily without getting
a different frame.


This is hogwash. No one can predict which frame/wheel/component/rider
combination will shimmy, and which won't. The biggest contributer to
shimmy is that last bit, the rider. Both in terms of position/mass, and
in terms of how the rider handles the bike, it can make a huge difference.
A degree or two of "bracing angle" is totally irrelevant.


really? how did your math lead you to arrive at that opinion?

Do you have math that will predict shimmy and permit one to design it out? If so
how much are you earning consulting with frame builders?


Thanks for answering for me. You're right. Though a bike/rider seems to
be a simple mechanical system, it isn't. It is, technically, a complex
system, to use the latest jargon. Small changes -- practially any change
-- can affect things like shimmy. That is part of the problem with people
claiming one thing, or another, "cures" shimmy, because that change
probably did on their bike. It wouldn't necessarily on the OP's bike. But
the placement of the wheel flanges is one of the least likely fixes, since
the wheel acts essentially as a rigid piece --- especially at 16mph.

So, yeah, "my" math did lead me to this conclusion. Shimmy is a resonance
response, produced by unpredictable interactions of the various components
of the system. Change whatever you want, and it just might go away.

well, with respect, a couple of millimeters on bracing angle /does/ make
a difference. the smaller the bracing angle, the greater elasticity
will be experienced by the wheel. 1/sin theta & all that stuff.

harmonics, sure, that's complex, especially for two angled tubes, but
you can still make rough guesstimates based on tube torsions. bigger
diameter tubes with a reciprocating torsional load have different
[higher] resonant frequencies to a smaller diameter tube of same wall
thickness.


The shimmy described, being worse no-hands or one-handed, is what Jobst
has described in his FAQ on the subject.

that's the same faq that mistakenly claims gyro forces are relevant,
right? "bikes" without any wheels at all still shimmy.


If I were a builder, I would cringe at every customer who comes in
claiming that because the headtube is 74 degrees rather than 73, he (the
builder) designed the bike poorly and so it shimmies. That builder has
hundreds of other bikes out in there with the same damn angles, and no
shimmy. The OP did not specify unusual angles or dimensions, so the
builder did what he had done previously -- which worked. But he will
probably have to change something to satisfy the customer, who wrongly
blames the builder. Easiest to replace the fork, and that will change the
system enough that it'll probably work.

wrote:
FWIW, when I discussed my bike with a local builder and mentioned 73
degrees/50 rake, his comment was "Nobody does that...".

Now I have ridden many bikes and this was the first that shimmied. If
previous bikes haven't shimmied and this one does, how can you blame
the rider?

Another builder on another list commented that he 'cures' shimmy
problems by putting in a stiffer top tube. It doesn't really cure the
shimmy, but it does make it happen at a higher speed.

and that's the right way to do it! "tune" the resonances of the frame &
wheels out of sync & you've pretty much eliminated the problem.


Orin.

wrote:

Tom Sherman writes:


One of the methods of increasing shear resistance in existing
elevated roadway piers lacking (remember the section of freeway that
collapsed during the 1989 Loma Prieta earthquake) is to wrap the
piers with a fiberglass reinforced polymer (FRP) blanket.


All the retrofit ones I have seen have a full metal jacket bolted
around the concrete piers with the interstitial filled with concrete
slurry.

One of my material's engineering professors made up some fiberglass
wrapped concrete cylinders. They exceeded 25,000 psi compressive
strength (at the load frame limit) while the unwrapped specimens
fractured at less than 5,000 psi.

--
Tom Sherman - Earth

Tim McNamara wrote:

"jcb1973" writes:


No, I didn't spec either geometry or tubing. My requirements were
strength, comfort, and good handling - I commute in London daily, and
the bike was built purely for that. I left the design entirely to
Roberts - and that's why I'm particularly disappointed that the
handling is not what I hoped for. Their response hasn't been great so
far - when I mentioned wobble with no hands, the answer was "don't ride
with no hands". Point taken, but I feel that's ducking the issue
somewhat...!


This is an unacceptable answer. Every bike should be safely rideable
no-handed and one-handed....

I feel reasonably safe on my bike, but I can not ride it no-handed -
there is a little bit of pedal steer that wants to make the handlebars
oscillate (but this is so minor that even a very light grip damps it out
completely).

--
Tom Sherman - Earth

Jobst Brandt wrote:

See also:

http://www.sheldonbrown.com/brandt/shimmy.html

The end of the last sentence of the third paragraph appears to be
missing. It is also missing he
http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html.

--
Tom Sherman - Earth