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Stability of a bicycle

Question: We can keep our balance on a moving bicycle, but then why it is nearly
impossible to balance on a stationary bicycle?


This one is a case of the conservation of angular momentum.

Any rotating object, in this case your bicycle wheels, has a property called angular momentum in just the same way that an object moving along has linear momentum. Just as a moving body requires a force to change its linear momentum so a rotating body requires a turning force (or torque) to change its rotation. You will know that linear momentum is a vector, well angular momentum is also a vector and in this case it is the direction of the axis of rotation that requires the force to change it. (The rate of rotation will also require a force but that is irrelevant in your question).

Now a stationary bike wheel has no angular momentum and so does not need a force to change the direction of the axle in other words the bike can easily fall over.

However the rotating bike wheel has angular momentum and so requires a force, in some cases a considerable one, to make the axle of the wheel change direction, and so the wheel stays upright.

If you can get a bike wheel separate from a bike try tying a piece of string to one end of the axle, spinning the wheel and then seeing if you can hold the wheel up by this string it should work. This is also related to the properties of the gyroscope.
© Keith Gibbs 2007