Editing Drum brake (section)

==Drum brake designs==

[[File:Drum brake testrender.jpg|thumb|Rendering of a drum brake]]

Drum brakes are typically described as either leading/trailing (also called "single leading") or twin leading.<ref name="aabook"/>

Rear drum brakes are typically of a leading/trailing design (for non-servo systems), or primary/secondary (for duo servo systems), the shoes being moved by a single double-acting [[hydraulic cylinder]] and hinged at the same point.<ref name="aabook"/> In this design, one of the brake shoes always experiences the self-applying effect, irrespective of whether the vehicle is moving forwards or backwards.<ref name="aabook"/> This is particularly useful on the rear brakes, where the parking brake (handbrake or footbrake) must exert enough force to stop the vehicle from traveling backwards and hold it on a slope. Provided the contact area of the brake shoes is large enough, which isn't always the case, the self-applying effect can securely hold a vehicle when the weight is transferred to the rear brakes due to the incline of a slope or the reverse direction of motion. A further advantage of using a single hydraulic cylinder on the rear is that the opposite pivot may be made in the form of a double-lobed [[Cam (mechanism)|cam]] that is rotated by the action of the [[parking brake]] system.

Front drum brakes may be of either design in practice, but the twin leading design is more effective.<ref name="aabook"/> This design uses two actuating cylinders arranged so that both shoes use the self-applying characteristic when the vehicle is moving forwards.<ref name="aabook"/> The brake shoes pivot at opposite points to each other.<ref name="aabook"/> This gives the maximum possible braking when moving forwards, but is not so effective when the vehicle is traveling in reverse.<ref name="aabook"/>

The optimum arrangement of twin leading front brakes with leading/trailing brakes on the rear allows more braking force at the front of the vehicle when it is moving forwards, with less at the rear. This helps prevent the rear wheels from locking up, but still provides adequate braking at the rear.<ref name="aabook"/>

[[Image:Batavus Fryslan hub detail.jpg|thumb|Shimano Nexus front hub with roller brake]]
Because aluminum wears more easily than iron, aluminum drums frequently have an iron or steel liner on the inner surface of the drum, bonded or riveted to the aluminum outer shell.