What is a Drum Brake
Drum brakes are not used on many cars today, but you can still find them used in the rear wheels of some vehicles. Drum brakes were phased out in the 1970's, replaced by the more effective disc brake system. The most common use of a drum brake today is as a parking brake. This is because it is very difficult to engineer a disc brake that operates when the vehicle is not in use, and because it is relatively easy to design a parking brake to fit within the hub of a rear disc brake.
A drum brake is a braking system that operates by causing friction on the inner surface of a spinning drum.
Brake drums spin because they are attached to the axle through a hub and the friction is created by a set of pads, commonly referred to as brake shoes, rubbing up against the brake drum. The brake shoes are pushed into the drum by a hydraulic piston that extends when the brake pedal is pressed. As the brake shoes are pressed into the drum, the shoes catch and wedge themselves into the drum, causing additional braking power. Because the brake shoes are wedged into the drum, they need to be mechanically retracted. For this action, a number of springs are used to pull the shoes off the drum and reset the brake piston for the next use.
Components of a Drum Brake
Here is a quick list of common components found in a Drum Brake system.
- Drum
- Brake Cylinder and Pistons
- Primary and Secondary Brake Shoes
- Brake Adjuster
- Brake Adjuster Mechanism
- Adjusting Lever
- Pivot Nut
- Star Wheel Adjuster
- Brake Cylinder
- Parking Brake Link
- Parking Brake Lever
Servicing a Drum Brake
The most common part that needs replacement in a drum braking system are the brake shoes. The shoes are attached to the wheel cylinder, which is basically a metal backing plate used to support the shoe when it's under compression. As the brake shoes wear down, there will be an increase in clearance between the shoe and the drum. This causes the drum brake to be less effective at stopping the vehicle. The shoes don't need to be replaced until they are worn down to 1/32", or .8 mm. If your drum brakes aren't working as well as they used to, but still have plenty of pad life, they probably just need to be adjusted. Very old drum brake systems require manual adjustment, but most newer systems have self adjusting mechanisms built in.
The mechanism only works when the brakes are applied while driving in reverse. The brake adjuster has three primary components; the adjusting lever, the wheel adjuster and the adjusting screw. In essence, if there is too much room between the shoes and the drum, the adjusting lever will rock back far enough to catch a cog the star-shaped wheel adjuster, causing the adjusting screw to extend, which moves the wheel cylinder and shoes closer to the brake drum.
Common Automotive Brake Components
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