Drum Brakes: Understanding its Components and Operating Principle

The modern drum brake system can be traced back to the early 1900s, when it was first adopted by a German automobile manufacturer. An improved version of the design was patented by another European OEM two years later. Early drum brake linings were made from asbestos, a naturally occurring fibrous material valued for its resistance to heat, electricity, and chemical corrosion.

In its earliest form, the drum brake relied on cable rods and levers working together to mechanically actuate the brake shoes. By the 1930s, this mechanical system had evolved into a hydraulic design, incorporating a wheel cylinder and piston to regulate braking force through oil pressure.

Components of Drum Brakes

A drum brake system consists of several components that work together to ensure effective and reliable braking. The main parts include the following:

Brake Shoe

Brake shoes are among the most critical components of a drum brake system. Each brake shoe is typically made of steel and lined with friction material. Every wheel assembly contains two brake shoes: a primary shoe and a secondary shoe. Shaped like a semi-circle, the brake shoes do not remain in constant contact with the brake drum but move outward to create friction when braking force is applied.

Backing Plate

The backing plate is a thin, circular metal plate that serves as the foundation for the drum brake assembly. It supports and protects internal components while providing mounting points for the brake shoes, wheel cylinder, and springs. The multiple holes in the backing plate are designed to align with standard drum brake components.

Wheel Cylinder

The wheel cylinder is a small but essential part of the drum brake system. Its primary function is to convert hydraulic fluid pressure into mechanical force, pushing the brake shoes outward against the brake drum. A typical wheel cylinder consists of a bleeder screw, piston boots, pistons, and a cylinder housing. The performance of the wheel cylinder directly affects braking efficiency and responsiveness.

Return Spring

Return springs play a crucial role in restoring the brake shoes to their original position after braking. A drum brake system typically uses two return springs: an upper spring and a lower spring.
The upper spring, located beneath the wheel cylinder, pulls the brake shoes back once hydraulic pressure is released. The lower spring helps maintain proper alignment of the brake shoes and ensures consistent contact with the adjuster mechanism.

How Do Drum Brakes Work?

When the rider presses the brake lever or pedal, the applied force is amplified by a brake booster and converted into hydraulic pressure by the master cylinder. This pressure is then transmitted to the wheels through a network of tubes filled with brake fluid.

Once the brake fluid reaches the required pressure, it pushes the pistons inside the wheel cylinder, forcing the brake shoes outward. The shoes press against the inner surface of the brake drum, which rotates in sync with the vehicle’s speed. The resulting friction slows the rotation of the drum, causing the vehicle to decelerate and eventually stop.

The two brake shoes operate in a leading and trailing configuration, depending on the direction of rotation. This design allows the drum brake to generate consistent braking force whether the vehicle is moving forward or in reverse.

Drum brakes are also commonly equipped with integrated parking brake mechanisms. Because the braking surface is located around the circumference of the drum, drum brakes can generate strong braking force and typically offer longer service life. Although they tend to retain heat and have a more complex internal structure, drum brakes remain a cost-effective and powerful solution for rear brake applications, particularly in certain vehicle platforms such as those from EAN.