The Drum Brake: Shoes That Push Themselves Harder

Make a drum: a plywood disc 200 mm across with a 30 mm rim of cardboard strip glued round it, on a bolt axle through a plywood backplate. Line the inside of the rim with sandpaper, rough side in, as a repeatable friction surface for the drum. Make one shoe: a plywood arc that fits inside the drum, pivoted near one end on the backplate, with a piece of brake-pad friction material glued on its face. At the free end, hang the spring scale to pull the shoe out against the drum with a fixed force. Wrap string round the drum's hub, hang a weight on it, and find the smallest weight that turns the drum against the shoe — with the drum turning towards the pivot (trailing), then away (leading). The leading direction needs far more weight.

**Louis Renault, 1902** — the internal expanding shoe brake. No patent number is asserted here. Mechanical rods and cables worked the shoes until hydraulic operation (Loughead, in this catalogue) spread in the 1920s; twin-leading-shoe drums put the self-servo on both shoes for stronger front brakes. **Honest limits.** The self-servo makes the torque sensitive to lining friction, so a hot drum fades and a wet one grabs. The drum expands away from the shoes when hot, lengthening the pedal. And the enclosed drum sheds heat poorly — the reason disc brakes (the next rung) took over at the front of cars.