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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.
Rear drum brakes are typically of a leading design, or primary/secondary, the shoes being moved by a single double-acting hydraulic cylinder and hinged at the same point. In this design, one of the brake shoes always experiences the self-applying effect, irrespective of whether the vehicle is moving forwards or backwards. This is particularly useful on the rear brakes, where the parking brake must exert enough force to stop the vehicle from traveling backwards and hold it on a slope. Provided the contact area of the brake shoes is large enough, which isn’t always the case; the self-applying effect can securely hold a vehicle when the weight is transferred to the rear brakes due to the incline of a slope or the reverse direction of motion. A further advantage of using a single hydraulic cylinder on the rear is that the opposite pivot may be made in the form of a double-lobed cam that is rotated by the action of the parking brake system.