Look, Ma! No hands!
April 27, 2011 § Leave a comment
I’m always interested in the “things we know that ain’t so”, and here’s one that popped up in Nobel Intent at Ars Technica, reporting on a recent paper in Science. The issue addressed in this post is that bicycles are unreasonably stable. Even while learning to ride one, and falling off it fairly frequently, you probably noticed that a funny kind of stability existed, if you could only learn to access it. I remember wondering about this, and accepting without much question the idea that the gyroscopic effect of the spinning bicycle wheels was responsible for keeping the whole bicycle upright and moving in a straight line. But apparently that’s not true. The authors of the Science paper built a computer model of a bicycle and realized that in the model they could remove the gyroscopic effect and still get stability. Puzzled by this, they then built a bike with extra counter-rotating wheels, which isn’t significantly stabilized by the gyroscopic effect, and found that this test bike is still able to steer itself to recover from falls, even in the absence of a rider. Another explanation for bicycle stability that has been bandied about is that there is a gap between the axis of steering and the point where the front wheel touches the ground (not, I think, a notable feature of penny-farthings), and that this may help explain stability of steering. But no, this doesn’t seem to be the explanation either; a bike lacking both this feature and the gyroscopic effect still steers into falls, and thus fails (to a large degree) to fall over when you would think it should.
Having demolished the two commonly accepted explanations for bicycle stability, the authors admit that they haven’t found much of a substitute: though their model accurately reproduces reality, they can’t extract a simple physical explanation from the mathematics that underlies their model. They also suggest that there may be more stable points in bicycle phase-space than have yet been explored; an interesting idea if you’re looking to create new amphibious, portable, or just plain weird bicycle designs.
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