A bicycle has only two wheels, but why can it keep balance? Even if riding a bicycle, the master can leave the handlebar with both hands and let the car go forward without worrying about falling (but worrying about the car in front whistling past). The physicist took out a gyro, put it on the ground and began to whip it hard. As the top turns faster and faster, it looks like a tumbler. Although only one pointed to the ground, it swayed from side to side and refused to fall. This is the gyro effect: the rotating object has inertia to keep its rotation direction (the direction of the rotation axis).

The gyro has only one direction of rotation, and it is already very stable. This bike has two wheels. Obviously, when the bicycle wheel rotates at high speed, it will make the bicycle more stable. So the cyclist won't fall when he unfolds the handlebar.

But unfortunately, this is not a reasonable explanation.

Gyro effect may play an important role in maintaining the stability of bicycles, but if bicycles are stabilized only by gyro effect, beginners should not fall down when riding at high speed. However, it seems that two gyroscopes are not enough to support the rider's tens of kilograms of body tilt. When I first learned to ride a bike, I often fell badly. On the other hand, it is difficult for acrobats riding unicycle to keep their balance by gyro effect because of their slow speed or even the complete stop of their wheels.

The balance of a bicycle comes from the muscles of the rider's waist first. Skilled cyclists have automatic conditioning of body shape. When the bicycle leans slightly downward, the human body will feel that the waist muscles will move automatically, pulling the body to the other side, and the resulting reverse torque will urge the body to lift. When we learn to ride a bike, we train our muscles to complete this conditioned reflex. Once we learn it, this control loop will remain in the cerebellum, which can be activated at any time and will not be forgotten for many years.

But when riding a bike at high speed, you will feel that the car is more stable than when you first started. Why?

The balance mechanism of the bicycle itself comes from the backward inclination of the front fork. We can observe that the handlebar axis of almost every bicycle is not completely perpendicular to the ground, but backward. Because the front wheel is fixed on the front fork of the handlebar, it is also called the front fork backward. The front fork leans backward, so that the centrifugal force generated when the vehicle turns is opposite to the direction of wheel deflection, forcing the wheel to automatically return to the original middle position after deflection. In this way, the car has the stability of automatic righting. The faster the vehicle speed, the greater the recovery torque and the more stable the rider feels. This is why when riding a bicycle at high speed, you will feel that the car is more stable than when you first started.

Generally speaking, the farther back the front fork is, the more stable the car is, but the harder it is to turn the handlebar; The rear inclination is small, which makes it easier to turn the handle, but the stability of the car is not enough. But if a bicycle has no front fork at all, riding a bicycle will be a very painful thing.

Bicycle is actually a rather complicated mechanical system, and the front wheel positioning of automobile is more complicated. There are kingpin tilt-in, kingpin tilt-back, front wheel camber and front wheel toe-in, which ensure that the car runs as smoothly as possible, but reduce tire wear.