As simple as falling off a bicycle Dateline: 05/09/00 Can you remember when you first learned to ride a bicycle? If you think back to that experience there a few things that might come to mind. After those awkward memories of not being able to reach the pedals and crashing into a few bushes or fences, you might recall that it was much easier to ride once you started moving. Getting going was always the most difficult part.

There is a good reason for this but it might not be what you expect. Lots of people will tell you that a moving bicycle is more stable because the wheels act like gyroscopes-spinning wheels are more stable than stationary ones.

However, a physicist called David Jones did some experiments to check out that idea and found it was wrong! He mounted another set of wheels on the bicycle. The second set was engineered to rotate in the opposite direction to the normal wheels at the same speed. If the gyroscope theory was true, the counter-rotating wheels should have canceled out the effect and the bicycle should have been more difficult to ride. When he rode the bicycle, he found it was just as easy to ride. Theoretical calculations also said that the gyroscopic effect of the wheels should be too small to make a noticeable difference when the mass of the rider is included.

Seeing as the commonly believed idea had been disproven, it remained for physicists to show why a moving bicycle is more stable. Eventually they tracked it down to be related to the position where the tire touches the ground compared to where the steering axis hits the ground. The steering axis is the straight line that the handlebars and front wheel rotate about. The physicists defined a quantity called the trail as the distance behind the steering axis that the tire touches the ground. In the simplest terms, if the trail is positive (meaning the wheel touches behind the imaginary extension of the steering axis), the bicycle is stable, otherwise it is unstable.

The basic reason why this makes a bicycle stable is that the friction of the tire on the ground pulls the tire in line behind the steering axis. This acts to straighten up the wheel. The same principle is used in shopping trolleys to keep the wheels in the same direction as the trolley.

Have a look at the wheels on the trolley next time you shop and you'll see what I mean. The trouble with shopping trolleys is that the wheels don't always spin properly or the get stuck pointing the wrong direction and the self-correcting part of the steering design doesn't work any more. To test this out idea in more detail, the physicists built some more bicycles with strangely shaped forks. Their new forks curved to give the bicycles a negative trail. Even champion cyclists could barely stay on this new bicycle. The experiments matched the theory and now all bicycle makers know the importance of trail to bicycle stability, it's just that many cyclists haven't heard this story and still believe in gyroscopic effects of the wheels.

(Gyroscopic stabilization IS a factor, but a stronger determinant is the distance that the ground-contact point of the front wheel trails behind the intersection with the ground of the steering axis of the front wheel.)
 If you want to find out the details of the experiments done by the physicists, have a look at the following articles next time you are at the library: Jones, David E. H., "The Stability Of The Bicycle", Physics Today, April1970, 34-40
 Lowell, J. and H.D. McKell, "The Stability of Bicycles", American Journal of Physics, December 1982, 1106-1112
 A report on the stability of a particular type of bicycle that was involved in some accidents. This article contains some more detailed information about trials and bicycle stability.
 All about steering a bicycle from the Exploratorium