A riding bicycle is stable without rider input (steering impulses and weight displacements). To stress this fact, we also say that a riding bicycle is intrinsically stable: it doesn’t need a rider to stay upright. A stationary bicycle is clearly unstable: when you don’t hold it, it falls over. To demonstrate the intrinsic stability of a riding bicycle, one must bring it up to speed while keeping it upright. This is shown in the first 15 seconds of this video::

 

 

A riding bicycle is stable thanks to the fact that the steer can turn from left to right. As a result of this, the bicycle corrects itself. The researcher in the video above (Andy Ruina) explains this, and in the video below it is nicely demonstrated:

 

 

Not all bicycles are equally stable, and this is because the degree of stability depends on different factors, such as the geometry of the front fork and the the weight distribution over the different parts of the bicycle.

The rider also has an influence on the stability of the whole (bicycle and rider together). By means of steering impulses and body weight displacements, the rider can distort the stability of the bicycle, but can also improve it. For practice, the following two rules are important:

  1. The more the bicycle stays upright, the less steering impulses and body weight displacements are required to keep stability. Swinging from left to right is bad: the bicycle is often in an unstable position, and the rider is continuously involved with restoring stability.
  2. The influence of the rider on the stability of the whole depends on the speed and the weight of the bicycle: the higher the speed and the heavier the bicycle, the smaller the influence of the rider.