As the brake actuating mechanism forces the brake shoes outward, the top of the brake shoe tends to stick or wedge to the rotating brake drum and rotates with it. This effect on brake shoes greatly reduces the amount of effort required to stop or slow down the vehicle.

With most drum brake designs, shoe energization is supplemented by servo action. When two brake shoes are linked together, as shown in figure 7-14, the application of the brakes will produce a self-energizing effect and also a servo effect. Servo action is a result of the primary (front) shoe attempting to rotate with the brake drum. Because of the fact that both shoes are linked together, the rotating force of the primary shoe applies the secondary, (rear) shoe.

In the forward position, the anchor point for both brake shoes is at the heel of the secondary, shoe. As the vehicle changes direction from forward to reverse, the toe of the primary shoe becomes the anchor point, and the direction of self-energization and servo action changes (fig. 7-14). The most popular brake drum configurations (fig. 7-15) are as follows:

Single anchor, self-energizing servo action (fig. 7-15)- In this configuration both brake shoes are self-energizing in both forward and reverse directions. The brake shoes are self-centering and provide servo action during brake application. This system is provided with one anchor pin. which is rigidly mounted to the backing plate and is nonadjustable. Both forward and reverse torque is transmitted to the backing plate through the anchor pin. One wheel cylinder with dual pistons is used in this system.

Single anchor, self-centering (fig. 7-15)- In this configuration only the primary brake shoe is self-energizing in the forward direction; therefore. it provides the majority of the brake force. This system is self-centering in that the lower brake shoe anchor does not fix the position of the brake shoes in relation to the drum. The shoes are allowed to move up and down as needed. Some systems provide eccentric cams for front and rear brake shoe adjustments. One wheel cylinder is provided in this system.

Double anchor, single cylinder (fig. 7-15)- In this arrangement, each brake shoe is anchored at the bottom by rotating eccentric-shaped anchor pins. Only the primary shoe is self-energizing. and the system does not develop servo action. Spring clips are used in the middle of the shoe to hold the shoes against the backing plate. Brake shoes are adjusted manually by rotating the anchor pins. One wheel cylinder is provided in this arrangement.

Double anchor. double cylinder (fig. 7-15)- In this system the brake shoes are provided with an anchor at each heel. The anchors are eccentric-shaped to allow for adjustment and centering. Each shoe has a single piston wheel cylinder mounted at the toe of the brake shoe. Which allows both shoes to be self-energizing in the forward direction only. Eccentrics mounted in the middle of the shoe also allow for brake adjustment.