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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”.
In other words, 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.
When the driver pushes down on the brake pedal, due to which 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.
Such kinds of backing plate are found in drum brakes. Front drum brakes may be of either design in practice, but the twin leading design is more effective. This design uses two actuating cylinders arranged so that both shoes use the self-applying characteristic when the vehicle is moving forwards. The brake shoes pivot at opposite points to each other. This gives the maximum possible braking when moving forwards, but is not so effective when the vehicle is traveling in reverse.