Brake & Wheel Parts

Drum Brake systems

Drum brakes are an older style of brake system that is most commonly found on the rear of light and medium duty vehicles with discs on the front. Drum brakes may be found at both the front and back of older vehicles and most Heavy-duty trucks and trailers although, disc brakes are quickly becoming the standard on new HD trucks and trailers.

Brake Drums – Brake drums are bowl-shaped, and the friction surface is on the inside of the drum. From the outside, the drum will be the only visible part of the brake system, even with the wheel off. When the vehicle is moving the drum spins around the brake shoes, and when the brake is applied the brake shoes are forced outward into the friction surface of the drum. Properly inspecting drum brakes required the removal of the drum.

Brake Shoes – Brake shoes are steel with a friction material bonded or riveted to the surface. Brake shoes will be curved to match the inside diameter of the drum. Brake shoes have a larger surface when compared to brake pads, so they will tend to run cooler as less force is required to generate the same stopping power. Drum brakes have the disadvantage of requiring return springs to pull the shoes back into a released position when brake pressure is removed.

Hydraulic drum brake wheel cylinders – Wheel cylinders use the hydraulic pressure to push the brake shoes apart and into the brake drum. Return springs in the wheel end pull the shoes back to a released position and return the pistons in the wheel cylinder.

Air brake camshafts – Brake cams are used to actuate the brake shoes on an air brake system. Brake cams are generally referred to as “S Cams” as the end has an “S” shape used to push the shoes farther as the cam is twisted more.

Air brake slack adjusters – Slack adjusters are used on air brake systems to compensate for wear of the brake shoes and drums. Hydraulic systems will not use a slack adjuster as the fluid automatically compensates for wear.