SELF-ENERGIZING BRAKES

• All modern hydraulic wheel brakes of the drum type have a 'self-energizing" or 'servo' feature in which the force of the rotating drum is utilized to increase the brake pressure.

• In Figure when the vehicle is traveling forward, the drum is rotating in a counterclockwise direction.

• When the brakes are applied, the primary shoe at the left tends to move in the direc tion of the drum rotation, because of the friction of the rotating drum.

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• Since the primary shoe is linked to the secondary shoe at the bottom the secondary shoe is forced around against the anchor pin at the top.

• The result of this wrapping action is that both shoes are forced into tighter contact with the drum and the braking pressure is more uniformly applied.

• When the brakes are applied while the car is in reverse, the secondary shoe tends to move in a clockwise direction against the primary shoe, forcing the latter against the anchor pin.