Twin leading drum brake
From Wikipedia, the free encyclopedia
The twin leading drum brake design is typically found on the front of automobiles. Its advantage is that it provides maximum possible retardation in its intended direction of travel, i.e. forwards. Everything else being equal, it is more powerful than a leading/trailing design [1].
The term "twin leading" is used because both shoes are "leading", moving with the direction of rotation of the drum and thus exhibiting a self-applying effect, i.e. being dragged into the friction surface of the drum and thus achieving greater braking force[2].
A "trailing" shoe would move against the direction of rotation, being thrown off the friction surface of the drum and would not retard the drum so effectively[3].
When the vehicle is moving in reverse, the role of the leading and trailing shoes is reversed. A twin-leading design thus becomes twin-trailing, with the rotation of the drum working against the brake shoes, trying to throw them off the drum. The exact opposite of the self-applying effect[4]. A twin leading design is consequently not very effective when working against its intended direction of travel. For this reason, it is more usual to have a leading/trailing design on the rear of a the vehicle, so that there is sufficient braking when travelling in reverse or when parked on a slope[5].
