Mechanics

Brake Drum

Rear Brake Drum
Brake Drums

Brake drums play a very important role in the overall performance of your vehicles entire braking system. In this article we will explain what a brake drum is, what it does and hopefully everything else you will need to know about them.

Brake drums are usually found on the rear of vehicles, but prior to the late 60’s and early 70’s they were standard equipment as the front brakes as well. Brake drums are the friction point where the brake shoes contact to slow a vehicle. They vary in size depending on the application: large work trucks will use a larger drum brake, while passenger cars use a much smaller brake. Brake drums can be made of different materials such as cast iron, on some vehicles they have used aluminum to save weight and improve the cooling of the brakes.

Brake drums are usually stamped with a “maximum diameter” measurement somewhere on the drum for safety and performance purposes. This must be checked when replacing the shoes and the drum requires machining. If the drum diameter gets too large, the friction surface becomes too thin and can distort or become overheated very easily which can cause brake “fade” or brake loss. Another issue that may arise if the drum diameter gets too large is rear brake lock up. This will occur when the diameter of the drum is so large that too much of the brake shoe is contacting the friction surface, causing them to grab or lock up when the brakes are applied.

Brake drums are also the emergency brake on most vehicles (with rear drum brakes). When the emergency brake is pulled (lever) or pushed (pedal), it uses a mechanical linkage and cable system to force the brake shoes out against the brake drums, holding the vehicle in place. Most vehicles with rear disc brakes have the inside of the rear rotor machined like a small brake drum, with a small set of shoes for the emergency brake.

Brake drums also come in a wide variety of price ranges. Less expensive drums tend to be made of lower quality materials and may not perform as well. When the material is too soft, it can cause the drum to become oblong (egg-shaped) which will cause a pulsation in the brake pedal. Lower quality material may also not work as well with the brake shoe material causing excessive pedal effort and possibly noise from the brakes. It is always a good idea to use high quality OEM (Original Equipment Manufacturer) parts to get the most out of your brakes and avoid problems later down the road. As with any automotive repair, proper protective clothing and equipment should be used at all times.

When replacing a brake drum, it must be cleaned very thoroughly of all foreign coatings. Brake drums are coated after they are manufactured to prevent rust on the mating surface, this coating must be removed before installation or it will contaminate the brake shoes and cause performance issues or noise in the brakes. To clean any brake parts or mating surfaces, you can use an aerosol brake parts cleaner and a clean rag. Wear eye protection when using spray cleaners, they can spray back into your eyes and cause damage.

If further assistance is needed, our certified car repair technicians are ready to answer your car questions.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-08-18)