jmsmech-obra@yahoo.com
There is stuff written on shimmy out there.. jobst brandt has a nice write up. also I think Wikipedia has a write up.
my terminology might not be exact..
In general shimmy is a resonance issue. Resonance occurs when a forcing frequency matches a natural frequency.
Natural frequency is function of stiffness, mass, and damping. often if there is enough damping (is shock absorbers in a car) you will never "see" the natural frequency. Most steel or metal structures are lowly damped, so then the goal is to keep Forcing frequencies from matching the natural frequencies.
If you stick a mass on one end of a beam and the other end fixed in a bench vise or etc (cantilevered beam), then pluck or bend the beam back, and let go, it vibrates or goes back and forth at it's natural frequency.
so on your bicycle, your body is the "bench vise" , the top and down tube, and to some degree the fork, are the "beam". The handlebars and the mass on it and the wheel are the "mass".
nat freq = square root [ stiffness(k) / mass (m) ]
All bikes will shimmy at some speed.
so looking at the simple eqn above, the bicycles that shimmy at lower speeds (frequencies) have more handlebar and or wheel mass. AND/OR are have a less stiff top tube and down tube(stiffness near the bb and st have the main effect), fork, etc. An increased rider mass will have a slight decreasing effect too on the nat freq. (obviously not shown in the simple eqn above)
853 tubing although strong, because it is thinner is typically less stiff then other equal dia steel tubes. Yes the 853 is perhaps tougher but certainly not stiffer than your track bike tubes.
Headset is in the stiffness path. once, I changed to a stronglight cylindrical roller bearing headset and got an extra 3-4 mph before no hands shimmy started (ie no hands shimmy started at 28 mph instead of 25).
Obviously when you grab the handlebar you add damping and stiffness and some mass too! but mainly increase the stiffness (so now shimmy will occur at a higher speed)..
How about the forcing frequency ? The simple answer, that is easy for me to explain, is that it is the 1x speed unbalance of the wheel. Any rotating mass will have unbalance. this creates a force "pulse" that occurs every revolution of the wheel. [Obviously the better balanced the wheel the less the force will be. ] When the frequency of this unbalance force(ie the bicycles speed) starts matching the natural frquency of the bicycle's front end, shimmy will start.
Although the above explanation holds true regardless, more complicated things could also be occurring, adding to, some sort of combination. (and here's where my knowledge is limited and thus slightly skeptical).
1. Some sort of tire "scrub" wheel stiffness... ie the front tire could steer one direction loading the wheel/fork and frame up like a spring, when a certain force is reached, the spring could unload, the tire could "scrub" in the other direction, repeating at some frequency...
2. As a wheel spins up in speed it becomes a gyro. This gyro, creates its own inertia, resisting change in direction. so basically this creates a "mass" . when you have a mass, and a spring (wheel, fork, frame,etc), one can have a natural frequency. A heavier rimmed wheel would create more inertia.
3. Also on a cold ride, a rider's shivering has been proposed/indentified as a forcing frequency... The harder the rider grips, the worse things get ??
Alignment in my opinion doesn't have much affect. But i don't have much experience with this. Some have said some misalignment is better then dead on, but i really doubt it has much affect. .
maybe try the following:
(might find a nice long "safe" hill close by ride down with no hands and watch speed and see where shimmy kicks in). obviously try to keep conditions, other than what you are testing, the same from test to test, same road surface, etc...
- Balance your wheel. I doubt this will help much but you never know. (it will make the wheel faster though too )
- Test out different wheel(s) with more spokes, stiffer rim, etc.
- stiffen things. Different headset, different fork, different frame.
- less overhung mass (weight of and on handlebar) this is practically impossible though to really make much of a difference? unless you have some older super heavy sti shifters..?
- pinch top tube with knees will help stiffen too.
-different tires, different pressures ?? a little too much probably..
-check your headset again. Not just bearings, but really you want a good fit of the cups into the frame and unto the fork. Just a snug fit is probably not good enough, anaerobic cylindrical retaining compound works wonders and and any good bike shop should have some. a little bearing preload is good too.
Let us know, i am curious. In my opinion, the thing that makes the most difference on shimmy is the stiffness of the top tube and down tube.... ie just like a cantilevered beam, its stiffness near the fixed end, has the most effect. although torsional stiffness has some effect too.
--- On Mon, 2/2/09, hrdtduck wrote:
> From: hrdtduck
> Subject: Re: [OBRA Chat] Fork/Frame Wobble-shimmy ....HELP!
> To: "'Max Kirchoff'" , obra@list.obra.org
> Date: Monday, February 2, 2009, 12:11 PM
> If your fork is tweaked (may not be visible even on a bench)
> in any way that
> could cause a shimmy.try trading out the fork and see how
> it handles after
> the swap
>
>
>
> From: obra-bounces@list.obra.org
> [mailto:obra-bounces@list.obra.org] On
> Behalf Of Max Kirchoff
> Sent: Monday, February 02, 2009 11:48 AM
> To: obra@list.obra.org
> Subject: Re: [OBRA Chat] Fork/Frame Wobble-shimmy ....HELP!
>
>
>
> Sorry forgot headset (thanks for reminding me craig)
>
> Headset - threaded and seems to be tightened correctly.
> Feel fine and has no
> detectable problems, pits, skips, loose-ness, or gaps.
>
>
> Thanks
>
>
> Max Kirchoff
> http://www.maxmeansmore.com
> http://www.maxisnow.com
>
>
>
> On Mon, Feb 2, 2009 at 11:38 AM, Max Kirchoff
> wrote:
>
> OBRAland,
> Perhaps someone can give me some advice from experience or
> as a fitter.
> I've got some bad wobble in my bike, specifically the
> frontend and mostly if
> i move weight (remove my hands) from the handlebars. But I
> can feel it in
> the bike slightly when on the bars as well. At certain
> speeds it gets real
> crazy without my hands on the bars.
> Of course the simple answer is, "Don't take your
> hands off the bars, dummy!"
> But I feel like I should investigate this as it might
> become a problem at
> higher speeds on distance rides.
>
> My Setup:
> 1999 Schwinn Peloton
> (probably not the culprit as alignment seems fine - also a
> 853 tubing bike,
> which should be tougher than my problem-free track bike
> tubes right?)
> Schwinn Peloton stock aluminum fork
> (kinda sketchy - at about 250lbs total, im pushing my luck
> with this fork i
> think - seems to have flex which I'm pretty sure it
> shouldn't but I don't
> normally run aluminum forks - possible problem cause)
> Mavic Cosmos 9sp rear
> (another possible candidate although I cannot find problems
> with it when its
> on the stand - mind you that's without 250lbs on it)
> Xero 16spoke font
> (rode this on my track bike previously, which felt solid as
> a rock, so not a
> suspect)
>
> Fit note - I'm used to a much more aggresive position
> and this frame is like
> a LazyBoy comparitavley.
>
> Could this just be a fit issue? Weight distrobution issue?
> Did I just put
> together some sort of frankenstein of parts that all have
> bad resonance when
> touching? Does the universe want me to give up riding bikes
> lighter than
> 20lbs? Do I just need to cough up for a new fork and see
> what happens?
> Anyone got any low-costs tests or ideas?
>
> Thanks.
>
>
>
>
>
>
>
> Max Kirchoff
> http://www.maxmeansmore.com
> http://www.maxisnow.com
>
>
>
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