Why are drum brakes used on large trucks and disc brakes used on cars?

Drum brakes and disc brakes each have their own advantages and disadvantages, and they coexist in parallel. No one can completely replace them. Most cars use disc brakes, while most large trucks use drum brakes. A drum brake is a circular brake drum that contains two semi-circular brake shoes. A brake cylinder is installed between the two shoes. When the brake is applied, the cylinder pushes the two brake shoes outward, generating braking force in contact with the brake drum. The brake drum is equivalent to the brake disc of a disc brake, and the brake shoes are the brake pads. The advantage of drum brakes is that they have a simple structure and a large contact area between the brake shoes and the brake drum. Basically, the contact area is determined by the size of the brake drum, resulting in a relatively large braking force.

A disc brake is a circular brake disc with one brake pad on each side. When the brake is applied, the brake cylinder will push the two brake pads to clamp the brake disc, generating friction. Compared to drum brakes, it has a smaller contact area, and the contact area is determined by the size of the brake pads. The disc brake method is a clamp type, just like two fingers gripping something, while the drum brake is pushed outward. Therefore, the braking force generated by disc brakes is far less than that of drum brakes. Of course, due to the small weight of the car and the limitations of the tires, the braking force of disc brakes is already sufficient to quickly lock up the brake disc, so there is no need for greater force. Disc brakes have two advantages: quick response, good heat dissipation, and generally do not experience brake failure due to overheating. Since the braking force is sufficient, the car is agile in movement, has high driving precision, and also needs to use the advantages of rapid braking response, naturally the car needs to use disc brakes.

There are other low-end vehicles that also use drum brakes on the rear wheels, commonly known as front disc and rear drum brakes, and there should be no problems. Because the price of the vehicle itself is not high, cost saving is important. Drum brakes are lower than disc brakes, so drum brakes are used for the rear wheels. During the braking process, 70% of the braking force is provided by the front wheels, which use disc brakes, so there are no issues with safety and responsiveness.

Why can't a large truck use a disc type? Firstly, there is the issue of cost. With lower costs, greater braking force can be generated. Why do we need to use higher costs? Secondly, the inertia force of the truck is large, requiring a large amount of braking force. The braking force of the disc brake is insufficient, and it cannot have good braking performance. If the brake discs are made large and the brake pads are also made large, it can generate a large braking force, but there is not enough space at the wheels to install. In terms of durability, drum brakes are also more durable. Taking a sedan as an example, when drum brakes are used on the rear wheels, the front brake pads need to be replaced three or four times before the rear wheels need to be replaced. Some cars have never been replaced in their lifetime and are still durable. Drum brakes have only one drawback, which is poor heat dissipation. When driving downhill on long slopes, if the operation is improper and the brakes are used too much, the probability of brake pads overheating and a decrease in braking force is much higher.

From：WONDEE Auto parts

2024-6-28