martha walsh

Two more cents on this thread: High-speed wobble is

about the scariest experience I've had (twice) on a

bike. I ride a 54 cm bike and low-profile wheels but

was able to determine the cause in both cases. In the

first event, one of my wheels was out of dish (not

something you usually check for). The second event

was caused by my tail light, which was fastened to the

seatpost and touching the top of the rear stays where

they attach to the seatpost tube. The very slight

pressure of the light was enough to dampen the natural

vibration of the frame, or at least to set it off into

wildly unnatural vibration. Pressing your knees

against the top tube does help, but braking seems

perilous. There is scads of reading material on the

web on this subject. -Martha Walsh





--- Mike Murray <mike.m-@obra.org> wrote:

 Steve Getsiv [mailto:ste-@ocfp.com] wrote:

"I think we could all benefit from a discussion of

what can cause the

wobbles at high speed and the basics of checking

your equipment thoroughly

prior to a 50mph+ descent."



Doug Evans [mailto:Dou-@sprbrk.com] wrote:

"Does anyone knows what causes the speed wobble

phenomena? I haven't

experienced it myself, but I know someone who had to

get off his bike to

remove and replace his wheels during a race, as his

bike would get the

wobbles even at low speeds. Does it have something

to do with natural

frequency?"



Adams, Mike [mailto:mad-@huttig.com]wrote:

"Check headset, wheel true, and frame alignment."



The causes of speed wobbles are discussed in this

note:



http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html



Bad frame alignment, headset and wheel bearing

adjustment do not cause

shimmy. Heavier or more high profile wheels can

make shimmy more likely.

Longer, more flexible frames are more likely to

shimmy (which is why I don't

need to worry about it). Braking does not stop it

per se, however since it

is speed dependant and slowing will take you out of

the range of speed that

causes shimmy. Simply putting a leg against the top

tube will generally

stop it.



Mike Murray



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