Originally Posted by
bikingshearer
Not a fix, but a survival tip - If your steed starts to shimmy at speed, clamp your knees onto the top tube for all you're worth. That should change the harmonics enought to stop or at least contain the shimmy. I had a speed wobble start at 35-40 mph once
, and I thought I was going to die - until I put a death grip on the top tube with my knees. Problem solved, and my heart rate eventually came back down.
The knee-clamp trick is is no substitute for trying the various solutions offered above, but it's a damn good thing to know about, because in my experience, speed wobbles at speed (a) are no fun at all and (b) tend not to announce themselves ahead of time.
+2 and don't forget (c) they don't go away by themselves. Living in Colorado this is something I tell my kids when we ride in the hills. I've never had this happen to me but if it does I'll know what to do.
I can't believe we've gotten to post #25 without anyone mentioning FEA (finite element analysis). Shimmy can have lots of causes but it is really just an instability in the mechanical structure that is the bike and rider. Add a disturbance (unbalanced wheels, hitting a bump, normal road vibration, etc) and the system can begin to oscillate. Like a tuning fork when struck or a crystal glass when you rub your finger around it you can excite the natural resonant frequencies of the mechanical system and cause shimmy.
I've sometimes wobbled my bike intentionally (when loaded with panniers, etc) to see how prone it was to shimmy. Do that at different wobble frequencies and magnitudes and you should have a good idea on how stable your bike is. As rules of thumb, stiffer structures will shimmy at higher frequencies and adding dampening (by putting your knee against the top tube) is the best way to stop a shimmy.