# Why is it easier to balance on a moving bike than a non-moving one?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

Dear Straight Dope:

Dear Straight Dope:

Why is balancing on an unmoving bicycle so much harder than balancing on a moving bicycle?

SDStaff Karen replies:

Because modern bicycles are equipped with a pair of  gyroscopic stabilization devices that require the motion of the bike in order to operate. These devices are known as “wheels.”

What is a gyroscope and how does its stabilizing power work? A gyroscope is just something spinning. A spinning object has angular momentum, whose magnitude is dependent on the speed of rotation, the mass of the object, and the distribution of that mass with respect to the axis of rotation. Angular momentum, like its homely cousin linear momentum, is conserved. For our purposes this means that once a gyroscope gets lined up in a certain way, it wants to stay lined up. That, in short, is how gyroscopic stabilization works.

Angular momentum is a vector quantity — it points in a definite direction. For example, a rolling coin has a different direction of angular momentum than a coin spinning like a top. The trouble with angular momentum is that, since it involves something that’s turning, often it’s not obvious what that direction is.

Fortunately, physicists have come up with a convention for the direction of angular momentum that makes angular momentum physics easy. This convention is known as the Right Hand Rule. Using your right hand, curl your fingers in the direction an object is spinning. Your thumb points in the direction of the angular momentum vector. (There are cross products and moments of inertia and other fancy physics stuff involved — if you’re sufficiently fascinated, get a book or take a physics course.)