What Keeps a Moving Bicycle Upright

Other variations on the same question can be phrased as

•How Does a Bicycle Work?

•What scientific principles keep bikes upright?

•Why is it so easy to ride a bike once you have learned?

•Is there an invisible wall, as hinted by C. S. Lewis in Prelandra, (1944, p. 68) that prevents a bike from falling over?
The answers to these and varied questions can be either short or long. In China tourists are told a joke that a bike falls over “Because it is two-tired.”
A friend who is a retired professor of physics, University of Illinois, quipped that a bike works,
“Because you pedal it.”

These are some of the short versions. A somewhat longer version is provided by visiting the various sub-headings in this “Bicycle Science” section.

As a guide to this section, please be advised that it was written almost like a manuscript. Unless you are going for a specific result, our suggestion is that you start with “Intro” (introduction), and then move on down the line of sub-heading tabs from left to right.

The focus on this “Bicycle Science” section will be to present the almost three decades of bicycle related research (1983 to present) performed by Dr. Richard Klein at the University of Illinois in Urbana-Champaign.

One thought on “What Keeps a Moving Bicycle Upright”

  1. The bicycle remains upright because of the same terrestrial force field that causes it to fall over. When a stationary bike falls over it rotates about the contact points held by a lateral force equal to product of the weight and the sine of the angle of rotation. When the bike is moving this “camber” force pushes the bike sideways toward the fall. Because the bike is moving forward and sideways, the direction vector is not aligned with the wheels and this creates slip angles at both wheels. The resulting slip angle forces oppose the camber forces so without steering the bike would fall over at low speed. The front slip angle force, however, causes the front wheel to steer until it is aligned with the direction vector and the slip angle force goes to zero. The rear slip angle force cannot go to zero and lateral motion at the rear contact is inhibited. Since there is no opposition to the camber force at the front, the bike yaws, steering into the fall, bringing the center of mass over the contact line. As speed increases the slip angles will grow smaller and the camber forces will hold the bike upright without steering. More visual aids can be found at: https://www.youtube.com/watch?v=ONZM5RnGi1Q

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