Drum brake is also called block braking, which is realized by pressing the brake block on the brake wheel. Drum brake is the early design of the brake system, the design of its brake drum in 1902 has been used in the carriage, until about 1920 began to be widely used in the automotive industry. Now the mainstream of the drum brakes is the internal tension, its brake block (brake shoe) is located on the inside of the brake wheel, and the brake block opens outwards when it is on the brakes, and friction the inside of the brake wheel to achieve the purpose of braking. Compared with disc brake, the brake efficiency and heat dissipation of drum brakes are much worse. The brake force stability of drum brakes is poor, and the braking force on different roads varies greatly and is not easy to control. Due to the poor heat dissipation, a lot of heat will be gathered during the braking process. Brake blocks and wheel drums are prone to extremely complex deformation under high temperature, which are likely to cause brake degradation and trembling, resulting in a reduction in braking efficiency. In addition, after a period of time, the drum brake should be regularly adjusted to the clearance of the brake shoe, and even the whole brake drum should be removed and the brake powder accumulated and accumulated. Of course, drum brakes are not useless. They are cheap and conform to traditional designs. The four wheel car in the braking process, because of the inertia, the load of the front wheel usually accounts for the 70%-80% of the full load of the car, the brake force of the front wheel is larger than the rear wheel and the rear wheel is assisted by the drum brake. So the car manufacturer adopts the brake mode of the front disc and the rear drum in order to save the cost. However, for heavy vehicles, because the speed is generally not very high, the brake shoe is more durable than the disc brake, so many heavy cars still use the design of the four wheel drum type.

　　The rotary element of the drum brake is the brake drum, the fixed element is the brake shoe, and the brake shoe rotates outward under the action of the actuating device. The friction plate on the outer surface is pressed against the inner cylinder of the brake drum and produces the brake friction torque to the drum.

　　Any device which adds force to the hoof end to turn the hoof is commonly referred to as the brake shoe actuating device, and the brake shoe actuating device has a wheel cylinder, a cam and a wedge.

　　The brake of the hydraulic brake wheel cylinder is called the wheel cylinder brake, the brake with the cam as the actuating device is called the convex wheel brake, and the brake with the wedge as the actuating device is called the wedge type brake.

　　Advantages and disadvantages

　　The brake efficiency and heat dissipation of the drum brake are much worse than that of the drum brakes. The braking force stability of the drum brakes is poor, and the braking force on different roads varies greatly, and it is not easy to control. Due to the poor heat dissipation, a lot of heat will be gathered during the braking process. Brake blocks and wheel drums are prone to extremely complex deformation under high temperature, which are likely to cause brake degradation and trembling, resulting in a reduction in braking efficiency. In addition, after a period of time, the drum brake should be regularly adjusted to the clearance of the brake shoe, and even the whole brake drum should be removed and the brake powder accumulated and accumulated. Of course, drum brakes are not useless. They are cheap and conform to traditional designs. The four wheel car in the braking process, because of the inertia, the load of the front wheel usually accounts for the 70%-80% of the full load of the car, the front wheel brake force is bigger than the rear wheel, the rear wheel is auxiliary braking, so the car manufacturer uses the brake square of the front disc and rear drum in order to save the cost.

　　Working principle

　　In a car brake drum, there is usually only one cylinder. When the cylinder is subjected to the hydraulic pressure from the total pump when the cylinder is brake, the piston at both ends of the cylinder head to the hoof end of the left and right brake shoes at the same time. The force is equal. However, because the wheel is rotating, the brake drum acts on the brake shoe pressure asymmetry or asymmetry, resulting in self boosting or self reducing force. Therefore, the one side brake shoe in the industry is called the collar, and the one side of the brake shoe is called from the hoof, the friction moment of the collar is 2 to 2.5 times from the hoof, and the wear degree of the two brake shoes is not the same.

　　In order to maintain good braking efficiency, there should be an optimal gap between the brake shoe and the brake drum. With the wear of friction lining, the gap between brake shoe and brake drum increases, so there is a mechanism for adjusting clearance. Drum brake in the past need to manually adjust, adjust the gap with a feeler. Now the car drum brakes are automatically adjusted, and the friction liner will automatically adjust the gap between the brake drum and the drum after wearing. When the clearance increases, when the brake shoe is released more than a certain range, the adjusting clearance mechanism will pull the adjusting rod (pawl) to the position of adjusting a tooth under the tooth, thereby increasing the length of the connecting rod, making the position of the brake shoe and restoring the normal gap.

　　Car drum brakes are usually used for rear wheels (front wheel disc brake). In addition to the lower cost of the drum brake, there is an advantage that it is easy to combine with the car (parking) brake combination, the rear wheel is a drum brake, and its parking brake is also combined on the rear wheel brake. This is a mechanical system, which is completely separated from the brake hydraulic system on the car: using the hand lever or the treadle (American car) to tighten the steel cable, manipulate the rod of the drum brake to expand the brake shoe, play the parking brake function, make the car do not move, unlock the steel cable, and return the spring to restore the original brake shoe. Position, the motive power disappears.