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Kevin ClemensNotes From the Road

Take a Brake
By Kevin Clemens

There is no doubt about it. Engines are sexy and brakes are not. Engines get all of the attention and brakes get covered in dust. Yet if you want to drive at today's highway speeds, or just want to drive safely, brakes are what it's all about.

What Do They Do?
Most people would say that brakes stop the car. Wrong! The friction of the tires against the roadway is what slows down and stops a car; the brakes actually stop the wheels from turning. Because most modern cars use disc brakes exclusively, we will ignore drum-type brakes for this article. To simplify things, we won't get into anti-lock brakes either.
Disc brakes consist of:
  • Brake discs (or rotors)
  • Calipers
  • Brake pads
  • Brake lines
  • Brake master cylinder

A disc brake has a plate-like disc (also called a rotor) attached to the wheel. This plate is squeezed by the brake pads within a caliper, much the same way that you might squeeze a spinning plate between your thumb and fingers to slow it down. The brake pads consist of two layers - a metal backing plate and a high-friction material that actually does the work by pressing against the brake disc. In addition to serving as a "platform" for the friction material, the backing plate also protects the brake caliper from excessive heat buildup.
The brake caliper that wraps around part of each disc contains small pistons. These pistons push the brake pads against the disc. The pistons are pushed outward by the hydraulic action of brake fluid that comes from the brake master cylinder. The caliper often contains two or more such pistons so that the brake pads will evenly distribute pressure against the brake disc.
Generally speaking, the more pistons in a caliper, the more evenly the pressure on the pads is spread onto the brake disc. That's why high-performance cars have multiple-piston brake calipers. The hydraulic brake pressure is built up in the master cylinder by the action of your foot on the brake pedal. The pressure travels through the brake lines from the master cylinder to the calipers. Most brake systems have power assistance that helps increase the action of your foot on the brake pedal, reducing the braking effort.
Heat is the Enemy
Now that we know the basics of how brakes work, it would be good to know what causes problems. Heat is the number-one enemy of good braking. As the brake pads press against the brake disc, enormous amounts of heat are generated. To remove this heat, especially from the heavily loaded front brakes, the disc is usually ventilated, with internal vanes that pull cooling air from the center of the disc to the outer edges. By "pumping" hot air away from the discs, the brakes remain cooler and less prone to "fading."
Fading occurs when the brakes are overused and overheated. In the worst cases, the brake fluid can actually boil in the calipers, causing a loss of braking ability. When brakes fade:
  • The pedal travels further, closer to the floor.
  • Badly faded brakes can result in the pedal going all of the way to the floor without any braking action occurring.
  • Braking distances increase dramatically.
  • The brakes will emit a burning odor.
  • In the most extreme cases, the brakes can actually smoke or catch on fire.
When brake fade happens, the only thing you can do is pull over and allow your brakes to cool.
Old Tech, New Tech
Not too long ago, brake pads were made from asbestos materials that were very effective in resisting heat, but produced a dust that was a health hazard. Asbestos pads also produced significant amounts of gasses during hard braking. To prevent the gasses from interfering with the contact of the brake pads with the brake disc, holes were often cross-drilled into the discs of extremely high-performance vehicles.
Because newer brake pads don't use asbestos, not as much gas forms and the latest technology uses slots in the brake discs to promote cooling and disrupt gas buildup. The slots are also much better at preventing stress cracking, something that drilled rotors often suffer from. The new materials used in today's brake pads are safer for the environment, but, unfortunately, they can cause brake discs to warp prematurely, causing a pulsing feel in the brake pedal.

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