Leading/trailing Drum Brake

The single-leading-shoe drum brake (SLS), a.k.a. "leading/trailing drum brake", is a basic type of drum brake design. It is typically found on the rear wheels of cars and motorcycles. An SLS front brake is not uncommon on smaller motorcycles and scooters; and SLS front brakes used to be standard on older cars, such as the Bedford CA. An advantage of an SLS brake is that is equally effective whether the vehicle is travelling forwards or in reverse. An SLS brake is less powerful as a twin-leading-shoe drum brake, but that is not critical on the rear where excessive braking force can cause the rear roadwheels to lock.

The term "leading/trailing" is used because there is one shoe that is "leading", moving with the direction of rotation of the drum and thus exhibiting a self-applying, or self-servo effect, i.e. being 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, being thrown off the friction surface of the drum and not retarding the drum quite so effectively.

When the vehicle is moving in reverse, the role of the leading and trailing shoes is reversed. What would be the leading shoe when the vehicle is travelling forwards becomes the trailing shoe, and vice-versa.

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