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Drum brakes - single leading edge
The next, more complicated type of brake is a drum brake. The concept here is simple. Two semicircular brake shoes sit inside a spinning drum which is attached to the wheel. When you apply the brakes, the shoes are expanded outwards to press against the inside of the drum. This creates friction, which creates heat, which transfers kinetic energy, which slows you down. The example below shows a simple model. The actuator in this case is the blue elliptical object. As that is twisted, it forces against the brake shoes and in turn forces them to expand outwards. The return spring is what pulls the shoes back away from the surface of the brake drum when the brakes are released. See the later section for more information on actuator types.
The "single leading edge" refers to the number of parts of the brake shoe which actually contact the spinning drum. Because the brake shoe pivots at one end, simple geometry means that the entire brake pad cannot contact the brake drum. The leading edge is the term given to the part of the brake pad which does contact the drum, and in the case of a single leading edge system, it's the part of the pad closest to the actuator. The diagram below shows what happens as the brakes are applied. The shoes are pressed outwards and the part of the brake pad which first contacts the drum is the leading edge. The action of the drum spinning actually helps to draw the brake pad outwards because of friction, which causes the brakes to "bite". The trailing edge of the brake shoe makes virtually no contact with the drum at all. This simple geometry explains why it's really difficult to stop a vehicle rolling backwards if it's equipped only with single leading edge drum brakes. As the drum spins backwards, the leading edge of the shoe becomes the trailing edge and thus doesn't bite.
