backplate
A rigid metal plate inside the brake drum, which supports the wheel cylinder, brake shoes, and other parts of a drum brake.
backplate: the skeleton of a drum brake assembly
The backplate is a stamped or cast iron plate bolted to the axle housing or wheel spindle, forming the stationary foundation of a drum brake system. Typically 3 to 5 millimeters thick, it serves as the mounting surface for the wheel cylinder, brake shoes, return springs, and self-adjuster mechanism. As the brake drum rotates around it, the backplate stays fixed and holds all friction and hydraulic components in precise alignment.
Backplates are usually made from mild steel or ductile iron and are shaped with formed bosses and pads to locate the wheel cylinder and anchor pins. The plate has a lip or rim that sits against the axle flange, and mounting holes sized to match the axle studs or a backing plate retention ring. Some designs include a dust shield, a thin stamped steel cover that reduces brake dust ingestion into the wheel bearing cavity.
Load paths and failure modes
During braking, the brake shoes press outward against the drum while the backplate absorbs the reaction force through the anchor pin. Cracks commonly initiate at sharp corners where the wheel cylinder bore meets the main plate body, or where return springs attach. Corrosion from road salt can thin the metal, reducing stiffness and causing excessive shoe clearance. A weak or bent backplate allows the drum to rub intermittently and prevents proper shoe retraction, generating heat and uneven wear.
Backplates must be removed and replaced if cracked, excessively rusted, or if their bolt holes become enlarged. The plate itself is not adjustable or serviceable; replacement is the only remedy. When a complete drum brake overhaul is performed, the backplate is inspected for flatness and corrosion, cleaned with a wire brush, and sometimes treated with rust inhibitor before reassembly.
The term 'backplate' reflects its position as the back or inboard side of the brake assembly, behind the drum. It is sometimes called a 'brake backing plate' or simply a 'brake plate' in service literature, though 'backing plate' more commonly refers to the plate on disc brakes behind the pads.