One of our teammates brought this issue to my attention. He experienced a severe “shimmy” while descending the big hill on Route 42. His bike just started shaking and he couldn’t control it. Fortunately he was not close to his fellow riders and was able to slow down without crashing. Very scary.
What is a Speed Wobble?
Essentially, what happens in a speed wobble – sometimes called a shimmy – is that the front end of the bike oscillates, moving quickly one way and then the other, several times per second. Your instinct in this situation is probably to grip the handlebar tighter to regain control, but the harder you grip the bar the worse it gets.
When shimmy does occur, it’s a case of the bike-and-rider system oscillating at its “resonant frequency” — the system’s natural frequency of oscillation, as determined by the physical parameters of that system. Think of the sound a crystal glass makes when you run a wet finger around its rim — this is another example of a system vibrating at its resonant frequency.
What causes it?
• You’re traveling fast. (I’ve read above 31 mph)
• You’re tense and/or cold, when a shiver might initiate it.
• You’re not pedaling.
• You’re riding no handed.
• The saddle is set high.
• The frame is long.
However, a shimmy can strike without all of these conditions being met; you can be pedaling along with both hands on the bars, for example.
How to stop a Speed Wobble
• Try to stay calm; tensing up exacerbates the problem. Deep breaths.
• Grip the top tube with your knees.
• Lift your weight from the saddle very slightly, but don’t stand up.
• Reduce the strength of your grip on the handlebar. Keep your arms bent.
• Slow down. If you’re going downhill and this requires braking, gently squeeze the levers, don’t lock up the front wheel. (I’ve also read where you should only apply the rear brake)
If you are having similar issues, short of changing the frame, you could try to change the dynamics of your bike. Perhaps different tires, wheels, stem or handle bars.
Quote of the week: “I relax by taking my bike apart and putting it back together again.” – Michelle Pfeiffer, actress
Here is a short article with some videos showing this issue.
Here is a longer, more in depth article.