Problem: Inadequate rear braking on drum brake equipped vehicles
Cause: poor shoe to drum contact
Solution: There is a quick and effective method of determining brake shoe to drum contact. Using a ball point pen make a crosshatch pattern across the face of both brake shoes as shown in Figure 64.1. Install the brake drum and road test the vehicle. Remove the brake drum and check to see how much of the crosshatching has been “erased”. Proper contact will result in most if not all or the pen marks being smudged or worn off. If only a small portion of pen marks remains it indicates near full contact and no service should be necessary. If only a small portion of the pen marks have been effected by the test drive a review of the causes listed below is in order.
More Info: Efficient rear drum brake operation relies on the correct mating of the brake shoe’s friction surface to the friction surface of the brake drum. If the brake shoes and drum do not mate correctly the rear brake will not operate at 100% efficiency. Incorrect shoe to drum contact can commonly affected by four things.
1. The brake shoe’s friction surface should be parallel to the friction surface of the drum. The shoe lands on the backing plate are responsible for determining the brake shoe’s position in relationship to the brake drum’s friction surface. If the shoe lands are worn as shown in Figure 64.2 it can change the position of the brake shoe resulting in the brake shoe no longer being parallel with the drum’s friction surface. The long term result of this is shown in Figure 64.3. This shoe is tapered from the inside edge to the outside edge due to the worn shoe lands. Always check the shoe lands for excessive wear during an inspection. On duo-servo drum brakes (adjusters at the bottom) the lower 2 shoe lands usually experience the most wear.
Figure 64.2 Figure 64.3
2. The brake drum’s friction surface should be perpendicular to the drum’s mounting surface. If the drum is what is known as tapered or bell mouthed it will reduce the contact area between the brake shoe and drum.
3. Proper shoe to drum contact relies on the brake shoe arc closely matching the inner diameter of the brake drum. The brake shoe friction material is ground at the factory to a specified thickness and to a specified arc. Due to wear and service the brake drum diameter can range from new to discard. the brake shoe to drum contact may be effected due to the different brake shoe manufacturing tolerances and variances in brake drum diameter. This can result in less than 100% contact between the brake shoe and the brake drum. Minor differences will be compensated for once the brake shoes wear but major differences will result in only a small portion of the brake shoe contacting the brake drum.