Twin-leading-shoe drum brake
The twin-leading-shoe brake (2LS) is a type of drum brake that has two leading shoes, rather than the single leading shoe and a single trailing shoe of a single-leading shoe (SLS) drum brake. A leading shoe has a self-servo effect, so an advantage of a 2LS is that it provides the maximum retardation in its intended direction of travel, i.e. forwards. 2LS brakes are fitted on the front axle of automobiles, or the front wheel of a motorcycle. A 2LS brake is more powerful than an SLS design.[1]
The self-servo effect arises because the leading shoe are dragged into the friction surface of the brake drum, and thus achieving maximum braking force.[1] 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.[1]
When the vehicle is moving in reverse, or if it is stationary but facing uphill, 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.[1] A 2LS brake is virtually useless when working in the opposite direction. For this reason, it is more usual to have a leading/trailing drum brake design on the rear of the vehicle, so that there is sufficient braking when travelling in reverse or when parked on a slope.[1]