BMW Coupes and Sedans 1970-1988 Repair Guide

Types of Brakes

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DISC BRAKES



See Figures 1 and 2


CAUTION
Brake linings may contain asbestos. Asbestos is a known cancer-causing agent. When working on brakes, remember that the dust which accumulates on the brake parts and/or in the drum contains asbestos. Always wear a protective face covering, such as a painter's mask, when working on the brakes. NEVER blow the dust from the brakes or drum! There are solvents made for the purpose of cleaning brake parts. Use them!

Disc brake systems utilize a disc (rotor) with brake pads positioned on either side of it. Braking effect is achieved in a squeezing manner. The disc (rotor) is a metal casting, sometimes with cooling fins between the braking surfaces to enable air to circulate between the braking surfaces making them less sensitive to heat buildup and more resistant to fade.

Dirt and water do not affect braking action since contaminants are thrown off by the centrifugal action of the rotor or scraped off the by the pads. Also, the equal clamping action of the two brake pads tends to ensure uniform, straight-line stops. Most disc brakes are inherently self-adjusting.

There are three general types of disc brake:

  1. A fixed caliper.
  2.  
  3. A floating caliper.
  4.  
  5. A sliding caliper.
  6.  

The fixed caliper design uses two pistons mounted on either side of the rotor (in each side of the caliper). The caliper is mounted rigidly and does not move.



Click image to see an enlarged view

Fig. Fig. 1: An example of a disc front brake assembly



Click image to see an enlarged view

Fig. Fig. 2: Although this looks like a disc brake assembly, it also includes a hand drum brake inside the rotor cap

Sliding and floating designs are quite similar. In fact, these two types are often lumped together. In these designs, the pad on the inside of the rotor is moved into contact with the rotor by hydraulic force. The caliper, which is not held in a fixed position, moves slightly, bringing the outside pad into contact with the rotor. There are various methods of attaching floating calipers. Some pivot at the bottom or top, whereas others slide on mounting bolts. In any event, the end result is the same.

DRUM BRAKES




CAUTION
Brake linings may contain asbestos. Asbestos is a known cancer-causing agent. When working on brakes, remember that the dust which accumulates on the brake parts and/or in the drum contains asbestos. Always wear a protective face covering, such as a painter's mask, when working on the brakes. NEVER blow the dust from the brakes or drum! There are solvents made for the purpose of cleaning brake parts. Use them!

Drum brakes employ two brake shoes mounted on a stationary backing plate. These shoes are positioned inside a circular drum which rotates with the wheel assembly. The shoes are held in place by springs which allow them to slide toward the drums (when they are applied) while keeping the linings and drums in alignment. The shoes are actuated by a wheel cylinder which is mounted at the top of the backing plate. When the brakes are applied, hydraulic pressure forces the wheel cylinder's actuating links outward. Since these links bear directly against the top of the brake shoes, the tops of the shoes are then forced against the inner side of the drum. This action forces the shoes to contact the brake drum by rotating the entire assembly slightly (known as servo action). When pressure within the wheel cylinder is relaxed, return springs pull the shoes back away from the drum.

Most modern drum brakes are designed to self-adjust themselves during application when the vehicle is moving in reverse. This motion causes both shoes to rotate very slightly with the drum, rocking an adjusting lever, thereby causing rotation of the adjusting screw.

 
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