Although many frustrated but undaunted five-year-olds may disagree, it is remarkably easy for most people to keep their balance on a bike. Professional stunt performers can literally perform stunts on every part of a modified bicycle without touching the ground or even pedaling. Once a bike and rider take off from a stationary position, the bike appears to become astonishingly stable in a short amount of time.
So why isn’t it harder to balance on a bicycle than it apparently should be? The answer lies in the nature of equilibrium and some laws of physics. The hardest part of learning to ride a bicycle for many people is creating enough momentum through pedaling to allow the bicycle to stabilize. Once a cyclist learns to mount and distribute his weight, the rest is up to the natural physics of the wheels, seat, pedals, brakes and handlebars.
A bicycle contains two wheels in a straight line. At first, this would seem to make balancing much more difficult, but the physics actually make it easier. When a structure like a spinning wheel starts spinning in one direction around a central axis, it wants to keep moving in that direction. At a certain speed, centrifugal force takes over and makes it extremely difficult to push or pull that wheel in another direction. In the case of a spinning bicycle wheel, the resistance created by the centrifugal force is much stronger than the force of gravity on the ground. As long as the tires are spinning and pointing in the same direction, it’s much easier to balance on a bike than it is to use force to knock it over.
Another reason it is easy to maintain proper balance on a bicycle is the position of the seat and pedals in relation to the wheels. A bicycle’s center of gravity is very close to the seat and in line above the pedals. When a cyclist mounts a bicycle and distributes his weight across the saddle, frame and handlebars, he is right at the center of gravity. This is the same principle of balance that allows tightrope walkers to move on a thin wire with a weighted pole. When a rider becomes perfectly balanced with respect to the center of gravity, he only needs to make small adjustments to steer into corners or change direction.
If a bicycle’s tires were offset instead of perfectly aligned, or if the seat was positioned far forward or behind the CG, it would be really difficult to balance on a bicycle.
A bicycle becomes stable only when it exploits the natural laws of physics and the principles of balance.
A unicycle is considerably less stable than a bicycle, but the rider still benefits from sitting directly above the center of gravity and creating centrifugal force through pedaling.