Look at the palms of your hands ... then consider they probably have more surface than the brake pads on any one corner of a race car. Rub them together and feel the heat that even gentle friction can create. It is that heat that makes rotors glow red and light up the inside of wheels during night races.
An engine takes air and fuel and adds heat to create power. Brakes take energy and convert it to heat through friction, then disperse it back into the air. At a track like Martinsville, a driver may use those hand-sized brakes to slow a 3,400-pound stock car close to 1,000 times during a race. The brake pads are all that are between him and parking his car in the trunk of the one in front of him.
"You can win or lose on deciding which brakes to use," says Kurt Busch, who races the No. 97 Rubbermaid Ford for Roush Racing.
The concept of disc brakes has been around for more than 100 years, although the first one used a piece of wood rubbing against a hard rubber wheel. They became common in World War II on bombers and fighters and first appeared in their current form about 1949 on the tiny, low budget Crosley.
On race cars, discs replaced factory-installed, cast-iron drum brakes that overheated and were prone to failure from brake fade and blown-out slave cylinders.
Brake pad technology has come miles since disc brakes were first used to stop airplanes. They were the brake of choice in the aircraft industry because they were light and simple to service. The fact that they remain in the airstream while in use helps to keep them cool for better grip. But hidden under the bodywork of a stock car, the brakes are subject to tremendous heat, yet must perform the same way every time if the driver is to have confidence in them.
Performance Friction provides the bulk of the Winston Cup teams with their brake pads. Years ago, pads were made up mostly of asbestos and organic fibers. They wore well, but the asbestos created health problems for crews and is no longer available. The next generation was a mix of metal and organic fibers, which has been replaced with higher tech mixes.
Performance Friction offers teams six different carbon metallic brake compounds in 12 to 15 different shapes that vary depending on which caliper is being used. Why so many? The work the pads do changes with the type of track. At Martinsville, for example, the brakes never get a chance to cool down; on a superspeedway, they may seldom get up to temperature. To accommodate those conditions, some compounds give a stronger initial "bite" while others may take a bit of heat before they work best.
"A Daytona pad must have a good initial friction and a good pedal feel for the pit stop," says Denny Gaylor, of Performance Friction. "The brakes are usually cold, but they have to work 'right now' when they are applied.
"At Martinsville, the brakes can have an initial friction like the Daytona pad, but they must also have good stopping power above 1,000 degrees (F). A pad for Sears Point has to have good cold friction and good high temperature stopping capabilities."
To customize the pads, a different combination of materials is used to change the friction level. Gaylor says many drivers use the same material at every track they run. Every driver, however, likes a certain "feel" when he nails the brakes before turning into a corner. That feel can be changed through pedal stroke, brake piston size, and the characteristics of the brake pad. On a multi-car team it is possible that each driver will want a different pad based on his preference.
"We do some experimenting during testing," Roush racer Busch says. "We'll use a different combination of pads and calipers and master cylinders until we get something that works the way we want it to. The team is so knowledgeable that we usually go to a track and know just what we want, but we still do a lot of testing because the technology in pad material changes so much.