Cars are born for speed. While exerting high-speed performance, in order to ensure the safety of car driving, the moving car must be slowed down and stopped according to our wishes.
The braking system can effectively control the speed of the car and is the safety guarantee of the car. Generally, cars have driving brake system and parking brake system, which are commonly known as "foot brake" and "hand brake". The braking system is mainly composed of brake pedal, vacuum booster, brake fluid tank, brake master cylinder, brake fluid pipe, ABS pump, brake, etc.
▲ Schematic diagram of the braking system structure
The brake is a device that generates braking torque to prevent the wheel from rotating. The brakes used in general cars are friction brakes, which are divided into drum brakes and disc brakes, which are commonly known as "drum brakes" and "disc brakes".
What is "drum brake"?
The reason why it is called drum brake is because there is a brake "round drum". "Drum brake" has been used in cars for a century, and is currently mostly used on the rear wheels of cars and heavy vehicles. Drum brakes are mainly composed of brake wheel cylinders, brake shoes, friction plates, brake drums, support pins, return springs, etc.
▲ Drum brake system structure diagram
The brake drum is installed on the wheel hub. When the car is not braking, it rotates with the wheel and is shaped like a round drum.
To slow down a moving car, step on the brake pedal. After the brake wheel cylinder receives hydraulic pressure from the brake master cylinder, the two wheel cylinder pistons push the brake shoe to rotate around the support pin and press against the brake drum. The outer circle of the brake shoe is equipped with a friction plate. The brake drum is decelerated by friction and finally forces the wheel to stop rotating.
▲ Drum brake braking process
What is "disc brake"?
Disc brakes are also called "disc brakes". Like drum brakes, they are named because of their shape. Its basic principle is similar to that of "drum brakes". It is controlled by hydraulic pressure. The main components include brake fluid pipes, brake master cylinders, brake calipers, friction plates, brake discs, etc.
▲ Disc brake working principle diagram
When the car is not braking, the metal brake disc fixed on the wheel rotates with the wheel.
When the brake pedal is stepped on, the master cylinder receives hydraulic pressure from the master cylinder, and the piston pushes the brake caliper to press against the brake disc. The brake caliper is equipped with two friction plates, one on each side of the brake disc. When braking, it is like two pliers clamping the rotating disc, using friction to force the turntable to slow down or even stop rotating.
▲ Mercedes Bens black 5 spokes car wheel--disc brake
Is "drum brake" better or "disc brake" better?
The process of friction braking actually converts kinetic energy into heat energy. There is no doubt that the faster the heat dissipation speed, the faster the energy conversion process. Therefore, theoretically speaking, disc brakes are better than drum brakes.
From a structural point of view, the brake components of drum brakes are in a closed space, and their heat dissipation performance is poor. The brake disc of the disc brake is exposed, so it dissipates heat quickly, is light in weight, and is easy to adjust.
▲ Ferrari equipped with carbon fiber ceramic brake discs
"The reinforced composite ceramic synthesized by carbon fiber and silicon carbide at a high temperature of 1700 degrees, this brake disc weighs about 5 kilograms, which is about 50% lighter than the usual brake disc, and its service life is up to 300,000 kilometers under normal operation."
However, since the cost of drum brakes is low and the braking effect is not worse than that of disc brakes, under the same conditions, the braking force generated is greater than that of disc brakes, so many economical cars will use the combination of "front disc and rear drum".
Working principle of hydraulic brakes
The figure below shows the working process of the hydraulic brake system with drum brakes.
▲ Hydraulic brake system working principle diagram
When the car is not braking, the brake shoe friction plate maintains a certain gap with the brake drum, and the wheel and brake drum can rotate freely.
When the car is moving to slow down, the driver steps on the brake pedal, and pushes the oil through the oil pipe into the brake wheel cylinder through the push rod and the master cylinder piston. Under the action of hydraulic pressure, the wheel cylinder piston moves to both ends and pushes the brake shoe to rotate around the support pin, and the friction plate on the brake shoe presses against the rotating brake drum.
The friction plate generates a friction torque Mμ on the rotating brake drum, which is opposite to the direction of wheel rotation. After the torque Mμ is transmitted to the wheel, due to the adhesion between the wheel and the road surface, the wheel exerts a forward peripheral force Fa on the road surface. At the same time, the road surface exerts a backward reaction force Fb on the wheel, that is, the braking force.
