Car owners who have run through transportation understand that drum braking is the most commonly used brake on trailers, but when buying a car, they can always hear the salesperson introduce the design of "front disc and rear drum".

The so-called "front disc and rear drum" refers to that the front wheel of the vehicle uses disc brake and the rear wheel uses drum brake.

In fact, in 2019, the Ministry of transport issued the relevant standards for tractors and trailers, and made clear instructions on the restrictions on the installation of braking devices for tractors and trailers. It is required that all steering wheels of the steering axle of traction vehicles with a maximum speed greater than or equal to 90km / h should be equipped with disc brakes. In other words, most new models need to be equipped with at least one axle disc brake.

Why? In fact, this also starts from the different characteristics of drum brake and disc brake.

1. Drum brake

The drum brake is composed of brake drum, brake shoe, brake wheel cylinder, friction plate and return spring. During braking, the brake shoe rotates outward under the action of the actuator, and the friction plate on the outer surface is pressed against the inner cylindrical surface of the brake drum to produce friction torque to the brake drum.

The most obvious advantage of drum brake is strong braking force. It is braked by friction between the friction plate and the inner wall of the brake drum. The larger the friction area, the greater the friction and the greater the braking force.

However, due to the closed and wrapped brake drum, the heat dissipation of the drum brake is poor, and it is easy to accumulate a large amount of heat energy in the braking process, resulting in the thermal deformation of the friction plate and brake drum, causing braking recession and jitter, and affecting the braking efficiency.

In addition, the structure of drum brake is more complex than that of disc brake. When braking, multiple components such as wheel cylinder and adjusting arm need to respond one by one, so the braking response speed is slow.

2. Disc brake.

The disc brake is hydraulically controlled. The main spare parts include the brake disc, wheel cylinder, brake caliper, oil pipe, etc. the brake disc is fixed on the wheel. With the wheel rotating, the wheel cylinder is fixed on the base plate of the brake, and the two friction plates on the brake caliper are installed on both sides of the brake disc respectively.

Its working principle is that when stepping on the brake pedal, the slave cylinder piston receives the liquid pressure transmitted by the oil pipe, presses the brake caliper to clamp the brake disc and brake the brake disc. When the brake pedal is released, the pressure is released and the piston returns to its original position.

The biggest advantage of disc brake is fast response speed. According to the relevant experimental data, when the three-axle tractor selects full disc brake and emergency braking at the speed of 60km / h, the braking distance can be shortened by about 10 meters compared with drum brake. Secondly, it also plays a good role in reducing the dead weight of the vehicle.

In addition, the relevant data show that the braking force difference between the left and right wheels of the vehicle using drum brake is about 30%. Because of the self-adjusting function of the vehicle using disc brake, the difference is less than 10%, which can improve the comfort of the vehicle when driving.

Compared with drum brake, the brake disc and brake caliper of disc brake are fully exposed in the air and have excellent heat dissipation performance. Especially when the wheel rotates, it can speed up the air circulation around the disc brake and is not easy to affect the braking effect due to overheating.

Even after the vehicle wades, the brake disc is shaped like a vertical dish, which is not easy to accumulate water. When the brake disc rotates, there is a large centrifugal force, and the water between the brake disc and the brake caliper is easy to be thrown out. Therefore, the reliability of the disc brake in water is relatively high.

Of course, the disc brake is not completely flawless. Due to its structural problems, the disc brake has weak strength, which limits its service environment and transportation conditions, and is not suitable for extreme transportation environments such as heavy load. At the same time, compared with the brake drum, the brake disc area is smaller and the heat capacity rate is worse. Under the same braking force, the heat generated by the disc brake will be greater than that of the drum brake.

It can be seen that both disc brake and drum brake have their own advantages and disadvantages. If they are applied properly, they can play a better effect. Therefore, even if upgrading to disc brake will indeed increase the investment cost of vehicle enterprises and owners, the installation of disc brake on the steering shaft of tractor may also be a choice under the combination of safety and economy.