How often do drum brakes need to be replaced?
April 25, 2020
The majority of modern vehicles have disc brakes, however some vehicles use drum brakes on the rear axle. Rear drum brakes also function as parking brakes.
Rear drum brakes.
The parking brake mechanism is connected to the rear drum brakes via parking brake cables
. How long do drum brakes last? Drum brakes are known for their longevity. In some vehicles, rear drum brakes can last for up to 150,000 miles, although there are many factors that can cause brakes to wear out sooner.
When do drum brakes need to be replaced? Car manufacturers don't specify intervals for replacement, but advise having the brakes inspected regularly and replace if needed.
Typically brake inspections are done when doing the tire rotation. Since drum brakes are hidden behind drums, the drums have to be removed for the inspection. Car manufacturers provide specifications and guidelines for the mechanics on the inspection process in the service manual.
Technician measures the inner brake drum diameter.
For example, the photo shows the technician measuring the inner drum diameter.
Replacing drum brakes typically means replacing the brake shoes and drums, as well as related hardware.
If additional parts fail the inspection, they will need to be replaced too. For example, if the wheel cylinder is leaking or not working properly, it's replaced at the same time. As an option, drums can be resurfaced (machined) or reused if they pass the inspection. The maximum (discard) diameter is often stamped on the drum. Replacing drums is always a better option, since machining makes the drums thinner and more susceptible to overheating.
Drum brakes have many more parts than disc brakes and, as a result, take more time to replace. After the replacement, drum brakes need to be re-adjusted. The parking brake mechanism might need to be adjusted too.
Replacing rear drum brakes (shoes, drums and hardware) in an average car in a repair shop costs from $420 to $750 per axle. Brakes are replaced on both sides at the same time.
Drum brake in a car.
If you plan on changing your drum brakes yourself, make sure to follow manufacturer's instructions and safety precautions. It's a very precise job, as drum brakes have many parts that are assembled in a special order. Brake shoes are different and must be installed correctly; there is a primary (leading) and secondary (trailing) shoe. It's a good idea to do one side at a time, so that when assembling, you can compare to the other side.
How do drum brakes work?
The drum, bolted to the hub, rotates together with the wheel. The brake shoes, and the wheel brake cylinder are attached to the stationary backing plate made from thick metal.
Duo-servo rear drum brakes.
The backing plate is bolted to the axle or spindle. An anchor, riveted or bolted to the backing plate provides a stop for the brake shoes. In some vehicles, the single anchor is placed at the top (duo-servo), in others, at the bottom (non-servo), as in the image below. The non-servo setup is mostly used in small cars.
When the driver presses the brake pedal, hydraulic pressure created in the brake master cylinder travels through the hydraulic braking system and pushes the pistons out of the wheel brake cylinder. The pistons push the brake shoes out. As a result, the friction material (lining) comes into contact and slows the rotation of the brake drum. The drum absorbs and dissipates the generated heat.
Duo-servo drum brakes have a "self-energizing" effect, where the rotating drum drags and "wedges" both brake shoes against the anchor; as a result, brake shoes are pressed against the drum with a self-multiplied force.
Non-servo rear drum brakes.
When the brake pedal is released, return springs pull the brake shoes away from the drum. The adjuster provides the resting position for the brake shoes. It is needed to maintain proper gaps between the shoes and the drum. In modern cars, the adjuster is automatic and is designed to self-adjust as the brake shoes wear out. The most common problem with the automatic adjuster is when it seizes up and stops working. In this case, it will need to be freed and serviced or replaced.