Why is it easier to ride a bicycle than to balance it when stationary?

Answer 1

Because its wheels can move in the direction it is leaningThere are two ways to think about this: statically and dynamically. From the static point of view, moving the wheels in the direction that a bike is leaning gets the contact patches back under the center of mass. From the dynamic point of view, moving the wheels in the direction that a bike is leaning generates a righting moment about the center of mass. In either case, it is the lateral movement of the wheels that does the job, and forward (or backwards in some cases) motion along with a non-zero steer angle is necessary for wheels to move to the side.

Once a bike is moving forward, there are at least four known reasons that it will steer in the necessary direction, toward the lean:

1. The rider applies a torque to the handlebars in the correct direction. It is easy to demonstrate, however, that many bicycle are self-stable without a rider when moving forward at the right speed.

2. The gyroscopic effect on the front wheel. No, the wheels don't simply resist tipping. Instead, the front wheel precesses by steering toward the direction it tips. It has been demonstrated, however, in a famous paper by Dr. David Jones in Physics Today that gyroscopic effects are not necessary for a bike to be easily ridden.

3. A geometric parameter called trail causes the front wheel to turn in the direction that the bike leans. It has been demonstrated, however, in a paper published in Science that a bike with no trail and with no gyroscopic effect can be self-stable.

4. The mass distribution of the bike. The authors of that same paper in Science conclude that the only thing left in their bike that could make the front wheel steer automatically in the direction that the bike leans is mass distribution.

So, for any particular bike, one or more of these four effects in combination can steer the front wheel toward the direction that the bike is leaning. Then the forward motion can move the wheels in that direction which arrests and corrects the lean. None of this can happen when a bike is stationary, and that is whey it is easier to ride a bicycle when it is moving than to balance it when it is stationary.

Answer 2

It is easier to balance a bicycle rolling forward than a bicycle at rest because when rolling forward you can move the support points, the tire contact patches from side to side merely by steering the front wheel and thus keep them under the center of mass.

Answer 3

When bike tires spin, they have a gyroscopic effect, causing them to stay upright.