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High Speed Shimmy

Description:¹

You are out for a spin on your new lightweight road bike. On a steep descent you reach a speed of maybe 45, even 50 mph when all of a sudden your front wheel begins to flutter back and forth and the whole bike shakes uncontrollably. You manage to bring the bike to a stop but you have just been scared out of your wits; you have experienced “shimmy.”

Discussion: You can minimize the tendency of a bike to flutter or shimmy by bracing your knee against the bike’s top tube during descents, this action tends to dampen the harmonics. Finally, if you should get into a high speed shimmy; try not to panic, grip the top tube between your knees, and apply the rear brake first, very gently (right brake lever) and only apply the front brake after you have come out of the shimmy.

per Dave Moulton, frame designer:
A bike with a shimmy problem usually has a design flaw in the frame and there is little you can do to correct it short of changing the frame. However do check that the head bearings are not loose. Also fitting a slightly heavier tire to the front wheel may increase the gyroscopic action of the spinning wheel enough to correct or lessen the problem. The design flaw will still be there but you have added and element that may alleviate the tendency to shimmy.

Shimmy is usually caused by not having enough trail. To explain trail for those who don’t know: If you draw a line through the center of your head tube and therefore the steering column, that straight line will reach the ground at a point (Point B.) ahead of the point where the wheel contacts the ground (Point A.)I always built my bikes with at least 2 ½ inches of trail. Trail is common to all wheeled vehicles, cars and even a shopping cart will have it.

If you make the head angle steeper it means less trail because you move point B closer to point A. Also if you increase the fork rake (Fork offset.) you make for less trail; in this case point A moves closer to point B. The worst scenario is a bike with a steep head angle and a long fork rake; trail can be reduced to almost zero. Trail keeps the bike going in a straight line, and also assists a two wheeled vehicle in its self steering abilities.

As you lean to the left, point B moves to the left and the wheel Pivoting on point A will turn to the left. The gyroscopic action of the spinning wheel also plays a big role in self steering, but this is another subject and I only mention it because the heavier the spinning wheel, the more it keeps straight. Road bikes with ultra light wheels and tires are more sensitive to small changes in the amount of trail.

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¹ excerpt from Dave Moulton’s Bike Tech http://www.prodigalchild.net/Bicycle6.htm#Shimmy (↑)