Brake Drums

Drum brakes were standard equipment on cars and trucks for decades, then in the 1970s they were relegated to the rear axle by front disc brakes. Today most vehicles have four wheel disc brakes, but drum brakes are still used at the rear on many trucks and non-performance cars. Although disc brakes have many technological advantages, drum brakes generally provide noise free operation and simplify parking brake design.

A typical drum brake assembly has two “shoes” which provide the metal frame to which the friction lining is bonded or riveted. The shoes are held to a backing plate by hold-down springs and actuated by pistons in a hydraulic wheel cylinder. When the brakes are applied, the friction linings contact the friction surface of the brake drum, which encloses the brake assembly and turns with the wheel. Return springs retract the shoes after brake application.

The brake drum mounts on the axle or wheel hub, and some drums incorporate the hub. Most brake drums are made of solid cast iron, but there are also steel and aluminum drums with cast iron liners. The machined friction surface on all drums is cast iron. Both steel and aluminum drums are lighter than one-piece cast iron, and aluminum drums run cooler. Some drums have cooling fins cast into the drum and some drums have a coil spring around the circumference of the drum to dampen noise and vibration. Brake drums come in various diameters and depths (width) according to application.

The friction surface of a brake drum naturally wears with use, which enlarges the inside diameter of the drum. All drums have a maximum, or discard diameter that is usually cast into or stamped on the outside of the drum. It is unsafe to use a drum with a diameter larger than this specification, because it will be too thin to properly dissipate heat and will not have the structural strength to withstand braking force without possibly distorting or cracking. Drums can also be scored from dirt and extremely worn shoes, and become bell-mouthed and develop heat checks, hard spots, and cracks from excessive heat and stress.

Machining a worn drum may not remove friction surface defects within diameter specifications, and some defects can’t be eliminated by refinishing. Instead of finding out you need new drums while you’re in the middle of the repair, why not be proactive and order a set of our replacement drums so you have everything needed to complete the job? Our replacement drums are made to OE specs, with the proper metallurgy for long life. They’re precision machined for smooth brake operation, and balanced to ensure vibration free performance. We also offer complete drum kits that include drums, shoes, hardware, and wheel cylinders.