However, a shimmy can strike without all of these conditions being met you can be pedalling along with both hands on the bars, for example. You’re tense and/or cold, when a shiver might initiate it.In our experience, a speed wobble is most likely to occur when:
Notice how it starts at the handlebar and then affects the whole bike. The video above shows someone purposely inducing speed wobble (don't try this at home, kids!).Īnd here’s a typical example (below) of it happening spontaneously out on road. That occurs because the center of mass of the front-end assembly is ahead of the steering axis.” If you hold a bicycle over your shoulder and swing the front and from side to side, you can see how the wheel steers the opposite way. “As Brandt says, the flex of the frame in torsion brings the contact back to centre – but at the same time also causing it to oversteer to the other side. In shimmy, the forward motion of the bicycle propels the sideways motion of the front wheel and fork blades. “The sideways motion of the fish's tail at the back end of the fish propels the fish. “Gyroscopic forces may play a part, but the mechanism is the inverse of how a fish propels itself through the water,” says Allen. Trail of the fork acts on the wheel to limit these excursions and return them toward centre.”įellow bicycle expert John Allen doesn’t completely agree with Brandt’s views.
This steering action twists the top tube and down tube, storing energy that both limits travel and causes a return swing. “Shimmy is caused by the gyroscopic force of the front wheel whose tilt is roughly at right angles to the steering axis, making the wheel steer to the left when it leans to the left. “Bicycle shimmy is the lateral oscillation of the head tube about the road contact point of the front wheel and depends largely on frame geometry and the elasticity of the top and down tubes,” according to the late US mechanical engineer Jobst Brandt. There’s a lot of debate out there about what exactly is happening when you experience a speed wobble. Sometimes the movement is so bad that you’ll end up coming off the bike and that’s always bad news.
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. 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 (the video above is from a YouTube user called Dean Magnusons).
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