We all know that the brakes on your race car-and even your passenger car, for that matter-work by converting kinetic energy into heat. In layman's terms, that means heat from friction created by the brake pads clamping against the spinning rotor.
The downside, of course, is that excess heat can damage many components in the braking system. Too much heat causes brake fluid to boil. When this happens, pockets of air are introduced into the brake lines, causing a mushy pedal and reduced braking performance. Other penalties include warped and cracked rotors, leaking oil seals in the calipers, and glazed brake pads. Finally, excess heat from brakes can also cause handling problems that often baffle beginning racers. A cherry-red rotor can heat the air inside the surrounding tire and cause greater-than-expected air pressure. Usually, the overworked brakes are on the front of the car, so the car mysteriously gains a push about halfway through a race that wasn't there during practice (when the driver wasn't pushing the car as hard or as long).
An effective brake cooling system isn't difficult to fabricate or install. Shaun Mangum of Mangum Motorsports fabricated everything on this Late Model Stock except the duct hose.
The easy answer is simply to make sure you keep your brakes cool. But how is that accomplished? What is the best method for keeping your brakes within the proper operating range? And what is the proper operating range, anyway?
"Your brake temperatures really need to stay below 400 degrees Fahrenheit," says Derek Spencer of Performance Friction, who works with race teams on their brake packages. "And 400 is kind of pushing it. If you get above that, the seals in the calipers start to crack and deteriorate. If you can keep your maximum temps below 350, you are doing good. Anytime you find you have exceeded 410 degrees, it's a good idea to go ahead and rebuild your calipers."
Determining how hot your brakes are getting is relatively simple. You can apply special heat-sensitive paint to your brake rotors that will fade when a specific temperature has been reached. Most brake manufacturers sell the paint in sets, with different colors for specific temperatures. Simply apply the special paint to small sections of the outside edge of the disc (not the disc face). After a run, pull the wheel and inspect the rotor. The hottest section that has faded paint shows the range you've exceeded, and the next section in the range that has paint that isn't faded shows you what you are still below.
This driver may have all the cooling he needs for his driving style on this track, but don't make the mistake of thinking that the inlet in the front valence should only be as large as the diameter of the duct hose.
You can also check maximum temps on the caliper, but the recommended method is to use temp stickers instead of paint. The stickers use the same principle as the paint. A row of different colors fade depending on the maximum temperature reached. Caliper temp stickers can be easily removed after a run and replaced with fresh stickers for multiple tests.
Directing air to the rotors by installing air ducts is the simplest and most efficient method for cooling your brakes. Most speed shops sell high-temp flexible hose that is made for the application. There are a variety of sizes, but hoses with a 3-inch diameter seem to work best for most applications. They provide adequate airflow and are small enough to be routed through the car. On almost all car setups-asphalt and dirt-the brakes are biased toward the front, so the front requires the most attention. Simply connect the hose to a hole in the front valence or bumper with a piece of hard duct flange and route the hose to the center of the rotor. The straightest route allows the most airflow.
Mangum helps cool the rear brakes by running a duct along the truck arm and directing the outlet as close to the center of the rotor as possible.
Wind tunnel testing has proven that the area of highest air pressure on a race car is the center of the valence below the bumper. This, of course, is the location for the radiator inlet, so your best location for brake cooling inlet holes is on either side of the radiator box. For an air inlet, cut a hole in the front valence that's the maximum size allowed by your rule book. For just about any Saturday night racetrack, you need all the airflow to the brakes you can get. For example, many racers cut a round hole in the valence the same diameter of the cooling duct, even though the rules allow a larger opening (often 4 by 3 inches or 12 square inches). You can increase airflow to the brakes by making the opening as large as possible and using a section of solid duct to funnel the incoming air into the flexible duct. You can fabricate it from aluminum or purchase one of the many options available in speed shops or online.
When routing the flexible duct, try to keep it as straight as possible. Just like the intake port in your engine, whenever air is forced around a curve in your ducting, it slows down. It's impossible to get it perfectly straight, but small, gradual bends are better than short, sharp ones. You should also consider the moving components on your race car when securing the hose. Leave room for tire and suspension movement (including control arms and the sway bar) so that the duct doesn't rub against these components when the car is on the racetrack and become damaged.
This shot is from a wind tunnel test in which airflow through the brake ducts was one of the things being tested.
Finally, you should point the business end of the duct directly toward the center of the rotor. Don't make the mistake of pointing it at the caliper or the side of the rotor, as this can cause more harm than good. A brake disc is designed to be cooled outward from the center. When the car is rolling, the directional cooling vanes inside the rotor pull air from the center of the disc and force it out. So providing a steady stream of cool air to the center of the rotor cools the entire disc. A cooler rotor also keeps the caliper cooler. Spencer warns against blowing air directly against the side of the disc because this only cools that side while the opposite side stays hot. The difference in temperature between one side of the rotor and the other can cause warping and cracking. Also, fabricating or purchasing rotor caps or covers to force air into the center of the rotor and hold it there will greatly increase the system's cooling efficiency.
It isn't always necessary to cool the rear brakes because so much line pressure is normally biased to the front. But when it is necessary, it can be difficult. The solid rear axle makes it impossible to blow air into the center of the rotor, and the movement of the suspension can make routing the ductwork a headache. Most racers who use a truck arm suspension secure the duct to the suspension arm and route it so that it blows on the rear disc. Since the truck arm usually has an I-beam design, it is relatively easy to zip-tie the duct against the arm to stabilize it. This can be more difficult in other types of rear suspensions because the arms are round and much shorter. Regardless, try to get the duct inlet as low as possible to capture as much of the limited airflow as you can underneath the car.
Many racers absolutely hate to use larger brake rotors because it adds unsprung weight. But it can help maintain good and consistent lap times during a race because the additional thermal capacity helps limit heat-induced brake fade.
If that isn't enough, consider using a small electric brake fan to push more air into the rotor. These are inexpensive and becoming increasingly popular on asphalt cars of all types. Spencer cautions, however, that you can harm your overall brake cooling system if you don't install these fans strategically. "Those fans are powered by an electric motor and spin at a constant speed," he explains. "Usually, they move air at about 35 mph. Going down the straights, you can get much more flow than the fans will allow through your ducts, and the fans will actually cause the air to back up and not allow it to get to the rotor. You can keep this from happening by not installing the fans in line with the duct that leads the inlet air to the rotor. Instead, install it in its own path and have the duct from the fan "Y" into the main line. This way you get the best of both worlds."
Finally, if you are still having cooling problems, reconsider the braking components you are using. Signs of overheating include boiled brake fluid (it will turn a nasty brown color), rotors that are prematurely cracking or warping, stuck pistons in the calipers, and glazed or cracking brake pads. The trick is to use the rotor to absorb the heat from the friction produced by rubbing against the brake pads and retain it until it can be radiated into the air. It only makes sense that a larger rotor has a larger thermal capacity. It can hold more heat before radiating it into the caliper and brake fluid or becoming damaged. A larger rotor also has more surface area and radiates more heat during the brief respite the brake system gets during turn exit and on the straights. So if you are continually having brake problems or experiencing stubborn fade in the middle of a race, try going larger. Yes, bigger brake components weigh more, but they may help you get around the track faster.