Drum servo relates to drum brakes but more specifically the leading and trailing edges of the shoes. Drum brakes can have effective self-servo characteristics based upon which way the shoes are hinged, this has the effect of increasing stopping power without any additional effort by the driver because the rotation of the drum. The shoes within the drum have a leading edge and a trailing edge and are designed so that the leading edge comes into contact with the drum before the trailing edge. The rotating force within the drum picks up this leading edge and wraps/pushes it hard into the drum which magnifies the braking force; In conventional rear brake design only one shoe will have this characteristic. Also, the effect on the leading shoe is
