How Backing Plates Work

One of the most common maintenance jobs performed on any vehicle is replacing the brakes. When they start to feel a bit squishy or begin to squeal, something must be done. Maybe you can swap out brake shoes yourself, or maybe you need to take it to the shop to get the job done. But what's holding all those brake parts together? If you're talking about drum brakes, the unsung heroes are the brake backing plates.

Brake plates are, according to the automotive glossary at AutoZone.com, "Stamped steel plates upon which the wheel cylinder is mounted and the brake shoes are attached; metal plates that serve as the foundation for the brake shoes and other drum brake hardware" [source: AutoZone].

Which means that without the backing plates, the brakes would have nothing to hold on to. In order for friction to stop the vehicle, the brake shoes must be firmly fixed -- that's where this particular brake part comes in.

There can be some confusion around the term "backing plate," as it can also refer to a part used in disc brake systems. The brake shoes in those systems have a backing plate to which the friction surface is glued or riveted. They are shaped like commas, whereas the brake backing plates found in drum brakes look more like shallow dishes with holes used for fasteners.

Now that we know where the backing plate is located and what it looks like, let's look a little further at the function of backing plates.

Function of Backing Plates

The function of backing plates is to hold the braking system together so it can work properly. With drum brake systems, each wheel has two curved brake shoes mounted on the stationary backing plate. The brake shoes are shaped like the letter C with friction material on the outside curve. They're attached to either side of the brake plate. This assembly is, in turn, mounted inside a cast-iron drum that rotates along with the wheel. Springs hold the brake shoes in place so they don't contact the drum when the brakes aren't active.

When the driver pushes down on the brake pedal, this places pressure on the hydraulic fluid in the brake lines. At the top of the backing plate is a wheel cylinder containing two pistons, one at either end. The pressure from the brake fluid enters the wheel cylinder and forces the spring-loaded pistons to move outward from the wheel cylinder, pushing the brake shoes against the inside of the brake drum.

The friction material on the outside edge of the brake shoes coming into contact with the inner surface of the brake drum is what stops the car. But without the stability and function of brake plates keeping all of these brake parts in place and taking the brunt of the braking force, none of it would work.

If you want some more information about backing plates and the other forces that bring your car to a halt, stop by the next page.

How Backing Plates Work

Function of Backing Plates

Lots More Information

Related HowStuffWorks Articles

More Great Links