Drum brakes are one of two broad categories of brakes on modern vehicles. They are most often seen on the rear of the vehicle while disc brakes operate the front of the vehicle. They are similar to disc brakes in the fact that they are designed to help the vehicle stop quickly and safely. They have several features that make them unique.
The Components of a Drum Braking System
Drum brakes are made up of several components that include the backing plate, the drum itself, the brake shoe, the wheel cylinder as well as several pins and springs.
Backing Plate: This is the base for the rest of the components, which is attached to the axle sleeve. The wheel cylinder and brake shoes attach to the surface of the backing plate.
Brake Drum: This component is often made of cast iron that is resistant to wear and can conduct heat. It has the job of rotating the wheel and the axle.
Wheel Cylinder: One wheel cylinder is needed for each brake. One piston is connected to each end of the cylinder to operate the brake shoes. The cylinder applies pressure to the pistons to push the brake shoes towards the drum for stopping.
Brake Shoe: Often made of steel, the shoe consists of two pieces that are welded together with a lining attached either by rivets or adhesive. Three V-shaped notches are located on each side and are often known as nibs. They rest against the support pads that are on the backing plate. The shoes are installed to these pads. One shoe is placed closer to the front of the vehicle and is called the primary shoe. The secondary shoe is just a little further back. Many times, the two shoes are interchangeable.
Automatic Adjuster: This component fills the space between the drum and brake shoes as the shoes wear down with use.
In addition to the parts that make up the drum brake, the system also includes a mechanism to operate the emergency brake.
How the Parts Work Together
The driver applies pressure to the brake pedal inside the vehicle, which sends hydraulic pressure from the master cylinder to the pistons. The master cylinder is the main component of the braking system and is different from the wheel cylinder which operates only one brake. The pistons are pushed towards the shoes, which presses them against the drum. This forces the vehicle to stop.
When the driver removes his or her foot from the brake pedal, the springs cause the shoes to go back into place. They do not move very far away unless the components are worn. When this happens, the driver will have to press the brakes down farther to release enough fluid to travel to the pistons.
Using the Emergency Brake
The emergency brake is connected with a cable that pulls on a lever that is attached to the shoes. When the emergency brake is applied, the lever forces the shoes apart. The lever provides direct control rather than going through the wheel cylinder, which allows the vehicle to stop even if the regular brakes have failed.
Wear on the Drum Brake
Because the shoes are doing the most work in this system, they wear down over time and require replacing. If they are left on for too long, the drum may be damaged. In many cases, the drum can be salvaged and sanded down to fit new brake shoes.
Symptoms of issues with Drum Brakes
- Sponginess in the brake pedals
- Noise when pressing the brakes coming from the rear brakes
- Shuddering in the vehicle when pressing the brakes
- Excessive rust or corrosion on the brake drum
These symptoms indicate that one or more components of the drum braking system needs to be repaired or replaced by a professional.
This article originally appeared on YourMechanic.com as How Drum Brakes Work and was authored by Joyce Morse.