If you give a bike a good push down a hill, it can ride off into the sunset without you. Moving bikes can stay upright even without a rider. Why is this?
The folks at MinutePhysics take a look at the phenomenon:
When a moving bike begins tilting to one side, it also automatically steers in that direction, bringing it back into balance. There are several reasons for this, physically speaking, all of which come together to help keep a bike upright:
Bikes have a steering axis that’s tilted slightly backward, toward the seat. The bike wheel makes contact with the ground slightly behind that axis, so when the bike turns, the upward force of the ground turns the wheel and handlebars in the same direction.
The weight of the handlebars is positioned slightly in front of the steering axis. When the bike turns, the downward pull from the mass of the handlebars helps turn the wheel in the same direction.
Gyroscopic precession, which governs how helicopters steer, also affects bikes. When a force is applied to a spinning wheel, the movement happens 90 degrees beyond where it was applied. So force applied pushing the bike left from the ground actually turns the middle of the wheel left.
The reason a slow-moving bike doesn’t stand up is that the wheel doesn’t have enough time to turn to keep the bike in balance.
So, maybe don't push your bike down a big hill.
All images from MinutePhysics // Screenshots via YouTube