Downhill speed shimmy

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Thursday, August 25, 2005
By Yokota Fritz


If you ride on the road competitively, you might have experienced the wobble or shimmy that occurs at high speed. My old steel bike I rode in the 80s would shimmy at about 42 mph. Brushing the top tube with my knees dampened the shimmy enough for me to get the bike under control. The Trek I ride now has never shimmied, although I've taken it up to 54 mph.

There are varying opinions about the cause of high speed shimmy. Some cyclists claim shimmy has something to do with frame alignment or bearings. I'm of the opinion that the bike has a resonating "sweet spot" when the wheel rotation amplifies the natural frequency of the bike. If you've experienced this shimmy, you'll know that the bike feels weirdly out of control as it forcefully jostles you from side to side.

Dana Albert writes of watching a near-crash when his friend's bike started to shimmy badly on a steep downhill at more than 58 mph.
"What John had, though, was far more severe. His bike developed a simple harmonic motion with an amplitude of at least a foot and a half. He was slaloming down the road with a violence that was bound to crash him. A person couldn’t direct this motion if he tried: it was sickeningly precise, as geometric as a sine wave, indicating that physics had taken over. John had become a helpless passenger, a captive of malfunction, like a pilot whose plane has lost its flaps and has become a missile."
The article is perhaps overly philosophical, but if you've experienced the speed shimmy you'll know what he's writing about.



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