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shimmy at high speed, frame , wheel or fork ?(7 posts)

shimmy at high speed, frame , wheel or fork ?eayste
Aug 7, 2003 6:43 PM
the owner of the LBS I work at part time asked me a question after talking to a customer.
He asked me because road bikes are a "grey" area for him.
He's a great mechanic and good to work with but he lacks what I call ride expirience.
The problem was the customer complained that at speeds greater than 35 mph the bike would start to shimmy.
Other riders told the customer to clamp the top tube with her legs to help stablize it .
The owner asked me about this and I told him that that does work alittle depending on the speed and rider.
Curious about the shimmy after the customer left he asked me what my expirience with this was.
I informed him that with my personal expirience , I think it was the fork that was causing it.
I have owned a 1997 C'dale R300 that I bought with a cro-mo fork . At high speeds there was no shimmy.
I put a Carbon Fork on it with bladed legs and it started to shimmy at around 40 mph.
With a newer bike , a 2002 C'dale R400 , that came with a Cro mo fork . With speeds over 40 mph . No shimmy.
Installed a non bladed fork with a relaxed rake and there was shimmy around 40mph.
Now having sold the R400 , I now have a R1000 with a fork with more stout legs , it starts to shimmy around 50 mph.
So I am asking what have others of you had expirience with shimmy at higher speeds.
Frame ? Fork ? low spoke tension on front wheels ?
could be a number of thingscyclist of all trades
Aug 7, 2003 7:30 PM
but it probably has to do with how the riders weight is distributed between the wheels. i'd bet the bike is set up w/ a short stem and just by lengthing it, you may see the shimmy disappear. try moving the saddle on the rails fore or aft as well. of course this will mess up the fit, and the rider may find that they just will not fit the frame w/o it having that shimmy.
re: shimmy at high speed, frame , wheel or fork ?eddie m
Aug 7, 2003 8:02 PM
If the resonant frequency of the frame matches the resonant frequency of the wheel, the bike will shimmy. Typically, lightweight frames shimmy more than heavy frames, and light wheels shimmy less than heavy wheels.
Clamping the headtube between the knees sometimes helps, as does keeping more weight on the handlebars. the best solution is to put one pedal down, and get out of the saddle. That changes the way the rider's weight is carried by the frame, which changes the resonant frequency, similarly to the way changing the length of a pendulum changes its frequency.
Another solution might be to use a front wheel with a very light sew-up rim, which might have a somewhat higher resonant frequency. When sew-up tires were common, I don't recall shimmy ever being a problem.
re: shimmy at high speed, frame , wheel or fork ?yellowspox
Aug 8, 2003 2:50 AM
I had a couple of "bib filling" shimmy problems a few weeks ago on my R1000 and asked the same question here on the board. I run Spinergy Spox on this bike. A couple of suggestions seemed to have helped although I haven't hit the same speeds that caused the major problems. The knee on the top bar, weight slightly forward, and a relaxed grip on the bars all work....so far.
slightly lose front hub?marcoxxx
Aug 8, 2003 4:37 AM
same problem here last month and we found my ft ksyrium hub to be a little lose, tighten and I was shimmy free.

m
Defective Tire?boyd2
Aug 8, 2003 5:39 AM
I once had this problem and found that it went away when I changed tires. I always figured the tire was thick in one spot.
have you checked headset?cyclopathic
Aug 8, 2003 9:13 AM
loose headset is the most common problem.