How Do Bicycles Stay Upright?

When a moving bike starts to tip to one side, turning the front wheel in that direction moves the wheels back underneath the rider, restoring balance. Riders do this automatically, but a bike's design also helps: the fork's angle creates 'trail', so the front wheel tends to steer into a lean by itself. Gyroscopic effects from the spinning wheels contribute a little but are not essential. A 2011 study showed that a specially built bike with no trail and no gyroscopic effect could still be self-stable, proving the physics is more complex than once thought. Speed matters, which is why a moving bike is far easier to balance than a stationary one.