brake shoe

A brake shoe is a curved metal backing plate, typically made from cast iron or steel, that carries friction lining and presses against the inside surface of a rotating drum to create braking force. The shoe itself is non-load-bearing; it exists only to position and support the actual friction material, which wears down with use and must be replaced periodically.

Drum brakes typically use two shoes per wheel, mounted on a stationary backing plate inside the drum. When hydraulic or mechanical pressure is applied, the shoes spread outward and the friction lining contacts the drum's inner surface. The curved geometry of the shoe must match the drum's radius closely, usually within 0.5 mm, to ensure even contact and predictable braking torque. Shoes are held in position by anchor pins and return springs that pull them back when brake pressure is released.

The friction lining bonded or riveted to the shoe facing is the wear surface; typical materials include organic, semi-metallic, or ceramic compounds with coefficients of friction between 0.3 and 0.5 depending on temperature and application. Lining thickness on a new shoe ranges from 4 to 6 mm for automotive brakes, though heavy machinery may use thicker material. As lining wears, the shoe backing itself is exposed and can score the drum, causing vibration and reduced braking performance.

Variants and failure modes

Shoes are manufactured in pairs, one leading and one trailing, sized to match specific drum diameters and vehicle or equipment specifications. They cannot be interchanged between different brake systems without loss of performance. Common failures include uneven lining wear caused by contamination or drum runout, delamination of lining from the backing plate due to moisture ingress, and heat cracking of the friction material under sustained heavy braking. Glazed linings, which develop a hard shiny surface and lose friction grip, result from overheating and require replacement.

Drum brakes remain standard on heavy trucks, trailers, and industrial machinery where their simplicity, durability, and resistance to water ingress outweigh the slight heat dissipation disadvantage compared to disc brakes. Passenger cars largely switched to discs from the 1970s onward, but drum brakes persist on rear axles in budget vehicles and are still found in older equipment across construction and mining operations.