The single-leading-shoe drum brake (SLS), a.k.a. "leading/trailing drum brake", is a basic type of drum brake design.
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The term "leading/trailing" means that only one shoe is "leading", moving into the rotation of the drum and thus exhibiting a self-servo (or self-applying) effect. The leading shoe is "dragged" into the friction surface of the drum and thus achieving greater braking force. The other shoe is "trailing", moving against the direction of rotation, is thrown away from the friction surface of the drum and is far less effective. An advantage of an SLS brake is that is equally effective whether the vehicle is travelling forwards or in reverse. When the vehicle is moving in reverse, the role of the leading and trailing shoes is switched. What would be the leading shoe when the vehicle is travelling forwards becomes the trailing shoe, and vice versa.
SLS brakes on cars
Although SLS front brakes once used to be standard on early cars such as the Bedford CA, SLS brakes are now found only on the rear wheels of cars. The car's handbrake typically operates only the rear wheels, and when the vehicle is stationary on a slope prior to a hill start, the rear SLS brakes are essential in arresting any rearward motion.
Classic Mini's made by Austin and Morris were fitted with single leading shoe drum brakes from 1959-1964.
SLS brakes on motorcycles
SLS (single leading shoe) brakes are still used for the rear wheel of many motorcycles, and for the front wheel of smaller bikes and scooters. An SLS brake is less powerful than a TLS, but that is not an issue for a motorcycle's rear brake. Compared to cars, bikes are higher and have a shorter wheelbase, so weight-transference under braking is much more pronounced. Excessive braking force can cause the rear wheel to lock, so it is normal for a bike's rear brake to be much less powerful than the front. Since 1969, modern motorcycles tend to have disc brake(s) on the front wheel, and sometimes a smaller disc brake on the rear wheel. Before disc brakes became commonplace on bikes, performance machines tended to have twin-leading-shoe drum brakes, and some racing machines even had four-leading shoe drum brakes.