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.
"We made a mistake at Sears," Busch adds. "At the end of the race we had the car to beat, but I lost the brakes because we didn't use a large enough (fluid) reservoir and we boiled the fluid away."
Some teams even change the type of material from the front and rear of the car.
"Most teams use the same friction front and rear on short tracks, but on a speedway they will use a less aggressive pad in the rear to prevent a 'loose in' condition," Gaylor says.
The less aggressive pad reduces some of the braking at the rear end, which helps prevent the rear of the car from locking up and sliding when the weight shifts to the front of the car and unloads weight from the rear tires.
"The biggest thing is to have drivers- even guys running in local short-track series-experiment with different compounds until they find something that works for them," Gaylor says. "Don't just run something because someone else does. They are like a pair of shoes, they have to feel good to the driver."
One driver may get on the brakes slightly earlier than others, but not brake as hard. Another may wait until the last moment and mash the pedal. Each type may want a different feel from the brakes.
Short Track Advice
If you are racing at Daytona in a Winston Cup car, you should be stopping with Winston Cup brakes. But if you are on a short track in the middle of Kansas, racing in a Street Stock division, heavy-duty street pads are probably just what you need. That's the advice of Charles Darsey, test manager for Dana Brake Parts, which builds Raybestos brand brakes and friction material sold under other brand names.
"One of the biggest mistakes you can make as a Street Stock driver is to go to someone's parts catalog and order what Rusty Wallace is using," says Darsey. "If your car is set up right, a heavy-duty, semi-metallic pad is probably all you need. If you are in a Street Stock and lighting up the rotors like Rusty Wallace at Bristol, you probably have other problems you need to tackle before changing brakes."
Here's some other advice Darsey offers:* Bigger is better: If the rules allow it, go to the biggest brakes you can fit on a car. "Sure, it will increase unsprung weight, and that's not good, but what it will do to the braking will probably offset that." The more surface area you have, the better the brakes will work, he says. They will heat slower and have more surface to release heat.
* Keep cool: "Air to the brakes is never a bad thing."
* Drilling vs. slotting: "It will reduce the weight a bit, but on a Street Stock car, it probably isn't worth the time and effort. The gains are marginal at best."
* Master cylinders: Darsey prefers using a single, double chamber master cylinder with a proportioning valve rather than twin master cylinders. Two cylinders share the pressure you put on the system while a single master cylinder uses all the pressure you apply everywhere, he explains.
* Brake bleeding: "It should be done at least once a month."
Cracking The Code
Performance Friction's code for labeling its racing brake pads is based on the year each pad was introduced to the racing market. The company has developed pads with increasingly greater frictional coefficients; that means that the higher number usually means more stopping power. Of course, there are subtleties. Here's a more complete breakdown to give you a better idea of how to break the code.
|Pad Number ||Description |
|80 ||P.F.'s lowest-friction pad. Not really aggressive when cold, but it does generate more stopping torque as it heats up. |
|83 ||Fair-to-moderate friction when cold. Aggressive friction ramp once it heats up. Can get so strong as to upset the chassis. Basically, the friction ramp never levels out. |
|90 ||Good initial bite, somewhere between 80 and 97. Slow ramp (less than 97) that trails off as it gets hotter. |
|93 ||Somewhat high bite. Ramps very hard, but between 1,200 and 1,400 degrees the friction ramp starts to fall off similar to 90. |
|97 ||Slightly higher cold friction than 80. Frictional coefficient ramps up even higher as it heats up. |
|01 ||High bite. Consistent frictional coefficient with no appreciable temperature ramp. |