Originally Posted by
shasta
Have you ever tried spinning a wheel while you hold it by the axle? If you try to change its axis of rotation from vertical to horizontal it will fight you. The faster the wheel spin the wheel the more it fights you. When you are on the rollers this force helps the bike stay vertical.
I'm sure there is a fancy name for this principle but I don't know what it is. Gyroscope?
It's called precession (often called gyroscopic force or gyroscopic stability). Precession actually isn't necessary to keep a bike upright on the road but on rollers I imagine it is.