"The 33"-Road Bike Racing - high speed wobble

I experienced this for the first time last week decending a mountan at 70K/h. Ahh that was scary. I got out of trouble by not panicing, siting up a bit, bleading off speed with the rear brake which did not do anything to stop the wobble so I pressed my knees into the top tube. Bingo, presto, wobble gone. I have done these high speed decents plenty and since and it has only happened the once.

What is the cause?

sometimes a harmonic feedback frequency can be set up in your fork, frame, or handlebars. Other times it can be equipment failure (out of true wheel, broken skewer, cracked frame or fork). Finally it can be due to extreme changes in gravitational center. If you were in a normal position, that is not it. I would inspect everything VERY carefully.

Putting more weight on the front wheel cures this for me, ive had it happen quite a bit decending at 45k/hr with my hands off the bars.

also throw in the possibility of a loose headset into the mix.

sometimes a harmonic feedback frequency can be set up in your fork, frame, or handlebars.



I used to have this occur when I first got my Giant. It stopped when I changed my seat height.

Yeah, I'd check everything over really carefully. I've never had this happen with my hands on the bars at speeds as high as 60mph.

I'll add to the mix to check that your front spoke tension is still high enough, in addition to DRW's trueness check.

My current bike will occasionally shimmy over 30mph with hands off the bars, but what can you expect from a 62cm frame?

Yeah, I'd check everything over really carefully. I've never had this happen with my hands on the bars at speeds as high as 60mph.

I'll add to the mix to check that your front spoke tension is still high enough, in addition to DRW's trueness check.

My current bike will occasionally shimmy over 30mph with hands off the bars, but what can you expect from a 62cm frame?

a jungle gym on wheels?

I experienced this for the first time last week decending a mountan at 70K/h. Ahh that was scary. I got out of trouble by not panicing, siting up a bit, bleading off speed with the rear brake which did not do anything to stop the wobble so I pressed my knees into the top tube. Bingo, presto, wobble gone. I have done these high speed decents plenty and since and it has only happened the once.

What is the cause?

3 of my bikes have high speed wobble. They are all big frames and I believe that has something to do with it. Two are lugged steel and the other is AL. My TI compact frame rides straight and true at any speed. Knee on TT is all I need to do so when ever I descend I clench the TT (or just one knee touching) which is a good aero position anyway.

a jungle gym on wheels?

:roflmao: That makes the price justification that much easier :)

My current bike will occasionally shimmy over 30mph with hands off the bars, but what can you expect from a 62cm frame?

For it's owner to not be so frigging tall, duh.

Tire pressure correct front and rear? Headset? Too many peripherals/electronics attached to handlebars?

Sheldon Brown has this article from Jobst Brandt regarding Shimmy or Speed Wobble:

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

Sheldon Brown has this article from Jobst Brandt regarding Shimmy or Speed Wobble:

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

Thanks for the link. Thanks all for the responces.

I did a thorough check on the bike and found the rear wheel to be a little more out of tru than I like. The wobble initiated in a corner and I think the freqency of the rear tire wobble (more pronounced in a corner yet undetectable by me), + feedback with the frame built up the resonance. My knee pressing the frame broke the cycle.

Another satisfied customer.

Glad you found the problem. Yeah, I've found that slightly out-of-true wheels, either at the front or rear and start a sympathetic vibration in the frame. Clamping your knees on the frame will dampen the vibrations.

A long-term fix is truing the wheels and stiffening up some part of the frame/fork will raise the oscillation-frequency to higher speeds than you'd ever be able to accomplish. On my bike, going from an alloy fork to a stiffer carbon one moved the speed where the wobble can barely be felt from 55mph to 72mph (which I'd hit maybe once a year if I can find just the right mini-van coming down San Marcos Pass :) ).

Putting more weight on the front wheel cures this for me, ive had it happen quite a bit decending at 45k/hr with my hands off the bars.

Why do you decend with your hands off the bars http://fairwheelbikes.com/forum/images/smiles/Smiley_confused.gif

if you see recneps on a downhill http://fairwheelbikes.com/forum/images/smiles/Smiley_abscond.gif

It could also be caused by a wheel that appears true but is supported by a loose hub which aggravates the problem.

Why do you decend with your hands off the bars http://fairwheelbikes.com/forum/images/smiles/Smiley_confused.gif

if you see recneps on a downhill http://fairwheelbikes.com/forum/images/smiles/Smiley_abscond.gif

I've ridden downhill on short descents no handed, I'd just got my CAAD9 and I was just blown away by how straight it tracked. I don't make a habit out of it, especially now...