You are out for a spin on your new lightweight road bike. On a steep descent you reach a speed of maybe 45 even 50mph 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.”
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.What happens in a high speed downhill shimmy the wheel is turned one way or the other by a bump in the road or a gust of wind. (Like when swinging out of a pace line.) The caster action of the trail corrects this, but if there is not enough trail it will over correct and then correct again starting the wheel fluttering back and forth. You can see exactly the same thing on a shopping cart if you run with it across the parking lot the caster wheels will flutter back and forth in the same way.Large frames are more prone to shimmy for two reasons. Large frames are taller and also should be proportionately longer, but there is a school of thought that believes a race bike should have a short wheelbase, so the builder makes the head angle steeper to shorten the wheelbase, but in doing so lessens the amount of trail. Large frames are more flexible because the tubes are longer, also they tend to have shallower seat angles to accommodate the rider’s longer legs therefore the riders weight is more over the rear wheel.Any vehicle that has its weight towards the rear is less stable; ask anyone who has driven an old VW bus in a cross wind. So if you are a tall person with a large bike frame, try to keep your weight forward when descending. Also keep your body in a low aerodynamic tuck; if you sit up wind pressure on you chest will push more weight towards the back wheel. 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 and only apply the front brake after you have come out of the shimmy.
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 to maybe alleviate the tendency to shimmy.
On August 18th, 2006 I wrote a second, more detailed article on the subject "Shimmy Re-visited."
Sat, November 12, 2005