A photograph of glowing orange brake rotors blazing away through fast-spinning spokes is truly an awesome sight. That is, unless the car in the photo is yours, because brakes lose their ability to slow the car when they are too hot, and because you will have to pay for replacement pads and rotors far too frequently. If overheating brakes are on your list of problems, it’s time to add a brake cooling system to your racer or improve what you have. The materials and fabrication work required to do the job can be cheap, simple, and light, if the cooling air is ducted from the most effective source to the most effective destination. This article will show you how to make an effective brake cooling system, and describe where that heat comes from and some of the dynamics of brake fade.
When your car is traveling at high speed on a racetrack, it has a lot of kinetic energy. To slow down to make it around a corner, some of that kinetic energy has to be converted to a different type of energy. Automotive brakes work by converting kinetic energy into thermal energy. A non-geeky way to say the same thing is that brakes convert speed into heat. Iron rotors glow orange when their surface temperature exceeds 750°F at night and 975°F on a sunny day. Those are alarmingly high temperatures, yet glowing rotors are a relatively common sight.
There is no stock road car braking system can withstand the severe demands of race track driving, so a race car that is based on a road car needs more braking system capacity than it had in stock form. Because every brake pad compound is designed to be a good thermal insulator, most of the heat that is generated goes into the rotors. There are three factors that determine the sizing and cooling requirements of a braking system.
First, the rate of heat input determines how much rotor swept area is needed. Swept area is the shiny portions of the rotor faces. The rate of heat input is determined by how grippy the tires are, how much aerodynamic downforce is applied to the tires, and how hard the driver is applying the brakes. The surface area requirement is determined by the rate that iron can transfer heat from the swept surfaces into the volume of the rotor material. If your rotor faces are covered in small cracks and pits after one track session, you need more rotor swept area.
Second, the total energy that the brakes have to absorb in the worst-case braking zone determines how much rotor material is required. The total energy is determined by how much the car’s speed changes from the beginning to the end of the longest braking zone, and by how much the car weighs. The amount of rotor material that is required depends on the maximum temperature that the rotor material can tolerate without cracking.
And third, the total braking energy input during a full lap determines the required air cooling rate. All of the heat that goes into the brakes has to be dissipated into the air stream to keep the rotor and pad temperatures below their structural limits. If your rotors develop a slightly bluish tint or wear rapidly, they have been overheated.
If your car doesn’t have enough airflow cooling the brakes, you may experience pad fade, brake fluid boil, cracked rotors, high pad and rotor wear rates, or any combination of those problems.
Brake Cooling System Design
A cheap, light, and effective brake cooling system can be made by routing air from a high pressure area on the body into the center of each rotor. The goal is to increase the air flow rate through the brake rotor vanes, which will cool the rotors faster.
Inlets
The front face of the car is always the best place for brake cooling air inlets because the pressure there is higher than anywhere else on the car when it is moving. The inlet duct does not need to blend into the bodywork. A sharp edged inlet will work just as well as a smoothly blended transition. Each inlet should have a coarse stainless steel wire mesh screen over it to block debris. Inlets for rear brakes should be higher than the top of the front fender well edge because the air flowing along the side of the car is much less turbulent above that height. For rain or cold weather, start with about half of each inlet taped over and adjust as needed from there.
Brake Ducts
A wide variety of flexible duct hoses that are rated for high temperature can be used to direct air from the inlet to the rotor eye. The hose should have at least a 3-inch inside diameter, preferably larger. Route each hose through the car with large, smooth bends. You will probably need to cut a hole in each fender well and relocate a few items to make a smooth path for each hose. The hose path has to clear the wheel and tire at full steering lock in both directions, and full bump and droop if you want the hose to last very long.
Seal Plates
Seal plates are the hardest parts of the system to make because they require good fabrication skills and robust attachment. Just aiming the hose at the middle of the rotor will not accomplish much. The purpose of a seal plate is to force the air flow in the hose to go only through the brake rotor vanes and nowhere else. Seal plates can be made from either steel or aluminum sheet that is at least .040 inch thick. Each seal plate should cover the whole inside diameter of the brake rotor and minimize air leakage around the spindle or axle. It should clear the brake pad and its backing plate by at least 1/8” all around. The backing plate should only have about a 1/16” gap to the inside face of the brake rotor to minimize air leakage. The seal plate needs an oval-shaped hole in it with a flattened metal tube welded on. The tube should transition from its flattened shape to round and mate up with the flexible duct hose. The transition tube has to clear everything on the car through the full bump, droop and steer ranges. A hose clamp will secure the hose to the tube.
Pad and Caliper Cooling
Forced air cooling for the rotors usually fixes any issues with overheated pads as well. If fluid boil is still a problem with good rotor cooling in place, a separate air cooling system can be made to direct cool, high pressure air through the gap between each brake pad backing plate and the caliper. Really elaborate duct work is required to make that effective, but it can be done by a skilled fabricator.
A race car with overheated brakes is dangerous and slow. Because overheated brakes is such a common problem, preventing it should be at the top of the prep list to modify a car for racing or track day duty. If you can’t stop, don’t go. Fortunately, making an effective air cooling system for your brakes is an achievable do-it-yourself project for a reasonably skilled fabricator. If not, there may be manufactured goods on the market for your particular application.
Driving Techniques and Maintenance for Marginal Brakes
If you are having severe braking problems, you are done for the weekend. Don’t drive it on any track until you have modified the car enough to make it safe. However, there are some simple things that you can do to get through a day at the track if the brakes on your car are close, but not quite up to the job.
Warm up the brakes gradually during your out lap and your first complete lap. This will reduce thermal stress by heating up the center of the brake rotor so that each hard braking zone will produce a smaller temperature differential between the face and the center of the rotor. It is possible to warp a brake rotor in a single braking zone if the brakes are stone cold initially. The first stop is a lot harder on the rotors than the second one. That’s why drag racing can easily lead to warped brake rotors.
Brake earlier and less forcefully than your tires are capable of. This will help reduce the rate of temperature change at the rotor faces. Your lap times will suffer, but your brakes won’t.
Drive a cool-down lap before you stop, while being very easy on the brakes. During this lap, you are trying to get rid of as much brake heat as possible. When the car stops, so does the flow of cooling air. As the remaining heat spreads through the braking system, the brake fluid may boil in the calipers.
Install new brake pads and bed them in before the event. The rate of brake pad wear will be very high on the track. Freshly bedded new pads will most likely last all day. Thin pads probably won’t. Also, the additional mass and insulation of full-thickness pads will help reduce the brake fluid temperature slightly.
Use a racing-specific brake fluid with a high boiling temperature. Of course, even the best fluids will boil when they get too hot. Bring extra fluid and your brake bleed kit to the track. You may need to bleed the brakes after every track session.
There are two types of brake fade, and each exhibits a different symptom. See if either of these has happened to you before:
Pad Fade
If the car does not slow down as much as usual for a given amount of brake pedal force, the brake pads have faded. What is happening is that the more volatile ingredients in the pad compound have become hot enough to emit gases. These gases have nowhere else to go but between the pad and the rotor, reducing the effective contact pressure between them. That is why slotted and drilled rotors exist. Usually, you can smell the gases that are produced by pad fade.
Fluid Boil
If the brake pedal travels farther than usual and feels rubbery, the brake fluid has boiled. Pressing the brake pedal will feel more like stepping on a soccer ball than stepping on a bowling ball. Brake fluid is nearly incompressible, but when it is heated beyond its performance limit, gas bubbles form in the fluid. The gas bubbles are compressible, which causes the spongy pedal feel. Brake fluid will not reabsorb all of the gas bubbles when it cools down. The only way to get rid of the gas bubbles is to bleed the calipers. Before every track session, it is a very good idea to hop in the car and press the brake pedal to find out if the brakes need to be bled.
We found this video on YouTube posted by a fellow driving at Sydney Motorsport Park, formerly Eastern Creek Intl. Raceway, and it demonstrates a couple of things. First, it shows how handy a GoPro camera is. Second, it shows rotors glowing orange on a sunny day, which means the rotor is hitting upward of 975 degrees Fahrenheit. Seems like a brake cooling system might be in order.
