Drum Brake System

There are two types of braking systems used in today’s vehicles, drum brakes and disc brakes. Both systems use hydraulics to press a brake pad or shoe to the rotating surface attached to the axle. Though very common on older vehicles, today drum brakes are primarily found on the rear axle of lighter vehicles such as the Honda Civic, the Toyota Corolla, and the Ford Ranger, or larger work vehicles such as box trucks.

Drum brakes were first patented in 1902 by Louis Renault. They were used at all four wheel positions on most vehicles throughout the first half of the 20th century. Disc brake systems were developed around the same time, but were not adopted into widespread use until after World War II. In 1953, Jaguar won the 24 Hour Le Mans race in large part because the disc braking system it used had superior stopping power to the drum brake. From that point on there was a large-scale shift from drum brakes to disc brakes. Drum brakes are still made today, but they are much less prevalent than they were from 1900 to the 1980’s.

Drum Brake Systems have 4 primary components, the brake drum, the brake shoes, the wheel cylinder, and a variety of metal springs and clips usually referred to as the hardware kit.

Brake Drums

Brake Drums are the visible part of the rear brake system and rotate with the tire and wheel assembly. When the driver presses the brake pedal, the brake shoes are forced to press against the inside of the brake drum causing the vehicle to stop. The brake drum is usually made of a special cast iron that is resistant to wear and heat, but it will eventually wear down. Once the brake drum becomes too thin to dissipate heat properly it will warp, increasing braking distances, and causing a pulsation in the brake pedal.

Brake Shoes

At each wheel position there are two brake shoes, and each brake shoe is made up of the brake shoe web and the brake shoe lining. The brake shoe lining is part of the brake shoe that grinds against the brake drum to cause it to stop spinning. Brake shoe linings can be made from a variety of components, but for most of today’s vehicles they are made from a combination of organic, metallic, and ceramic materials.

Organic brake shoes (sometimes referred to as NAO, or Non-Asbestos Organic brake shoes) are manufactured using a combination of natural and synthetic substances such as glass, rubber, resins, and Kevlar®. These types of materials wear out quickly and produce lots of braking dust, but they cause little wear on the brake drum due to the softness of the material.

Semi-metallic brake shoes are manufactured using a combination of synthetic substances similar to those used in organic linings, but steel fibers are added to the mix to increase the longevity of the brake friction material. Semi-metallic brake shoes last longer than organic brake shoes, but they increase wear on the brake drum, and they create a substantial amount of brake dust that can cause a fancy wheel to look dirty.

Ceramic brake shoes, as their name implies, are made from ceramic materials combined with copper fibers. They can handle high braking temperatures and provide a good combination of longevity, braking power, quietness, and low brake dust. However, ceramic brake shoes do not dissipate heat as well as the other types of brake shoes, and therefore may warp the brake drum if not broken in properly.

Wheel Cylinders

The Wheel cylinders is the device that pushes the brake shoes against the brake drum. When the brake pedal is depressed, brake fluid is forced into the wheel cylinder chamber, causing pistons on either side of the wheel cylinder to press the brake shoe against the brake drum as shown in the diagram below.

Over time the internal seals of the wheel cylinder will wear out, allowing hydraulic brake fluid to leak as out. At first, the brake fluid from a leaking wheel cylinder will dampen the brake dust as it weeps out of the wheel cylinder, but as the leak gets worse, the hydraulic brake fluid will begin to coat the brake shoes, ruining the brake shoes. Eventually enough brake fluid will leak out that it will coat the entire brake system and things get very messy. When this occurs the drums, shoes, hardware, and wheel cylinders will all need to be replaced.

Drum Brake Hardware Kits

Hardware kits for drum brake systems include a variety of springs and pins that are needed to hold the system together and make the system function properly. The hardware kit also usually includes a few pieces for the parking brake, and some adjusting screws that allow one to adjust the distance from the brake shoe to the brake drum. Though some of these pieces are simply chunks of steel, the springs and retaining clips should be replaced each time the rear brake shoes are replaced to ensure proper braking function and to prevent the brake shoes from wearing out prematurely.

Backing Plate

The backing plate is a thin piece of metal that the brake shoes, the wheel cylinder, and the brake hardware all reside upon. It acts as the backbone for the whole system. Though the backing plate is only just a thin piece of metal, sometimes it will get bent and rub against the brake drum making an unusual noise. Usually the backing plate can be bent back into place and all will be fine, but in older cars the backing plate may end up rusting to the point where it can no longer act as the backbone of the brake system, at which point it should be replaced.

Typical drum brake system problems

The most common problem with drum brakes is that the brake shoe linings will run out of friction material used for stopping. When the shoe linings wear out the drum will begin grinding against the brake shoe webbing, causing a grinding noise to occur. The solution is to replace the brake shoes, and if needed the drums as well. When the shoes are replaced it is also recommended to change out the brake hardware kit as the springs will have lost their elasticity. When replacing the brake shoes, we’ll also inspect the condition of the wheel cylinders to make sure they aren’t leaking, and the backing plate to make sure it’s still up to the job.