Heat is a blessing and a curse....
Heat is a blessing and a curse. Heat, to a point, increases stopping power. Go over that point and the brakes fade away.
Heat in the braking system is a necessary evil. In order to stop a race car, the brakes must turn the car's kinetic energy into heat energy. This is done by creating friction between the brake pad and the rotor. The more heat the pad can generate, the quicker the car will stop. If you build excessive heat during the race, the brake pads will fade or the brake fluid will boil. In either case, you will lose your brakes and the race.
The key is to know how to maximize braking without producing excessive heat. The proper choice of brake components will allow you to go deeper into corners, and run longer and harder without crashing.
Let There Be Light
The first thing every racer thinks about is getting the lightest components possible. It is true that lighter-weight brake parts will lower the car's rotational weight. This makes it easier to slow down and accelerate the car. It will also lower the unsprung weight on each corner of the car. However, there is a limit on how light brake components can be.
An extremely lightweight aluminum caliper can lose clamping force, because the hydraulic pressure that is pushing against the caliper is trying to spread it apart. Too much deflection in the caliper will result in lost braking force. Instead of squeezing the pads tighter on the rotor, the caliber will bow out when you push on the pedal.
The other detrimental aspect of owning the lightest calipers is reduced heat dissipation. There is simply less material to remove the heat. This causes the brake fluid to boil quicker, which loses braking force. The excessive heat will also cause metal fatigue in the caliper. The front caliper will deteriorate to the point that it will be marginal within two seasons, and will need replacing.
The same thing will happen with lightweight rotors. Less material is available to control the heat, therefore, the pads heat up and glaze over. The extra heat transfers to the caliper and boils the brake fluid. The extreme heat will crack the rotor, forcing its replacement within one season. Lighter is not always better.
The heart of any brake system...
The heart of any brake system is the caliper, the rotor, and the brake pads. Their size is dependent on the amount of heat generated and the length of the race.
Pick up any brake manufacturer's catalog and you'll find a choice of 10 to 20 different calipers. How do you choose the right one for your application? Start with the type of caliper, then look at the piston size, and then the pad size. The last option is the weight.
There are two types of caliper: billet and cast. Which one is best depends on its design. A billet caliper tends to be stronger, but the open-bridge design has a tendency to flex more than cast calipers. On the other hand, some cast calipers are a little more superior, because of their design, than an open-bridge, lightweight caliper. It comes down to the caliper's size, clamping force, and pad size.
Its rigidity and the overall piston area determines a caliper's clamping force. If the caliper flexes, some of the effort that goes into the pedal and down the line as pressure is lost, and not available to squeeze the rotor adequately. Part of the effort is going into expanding the caliper.
The caliper's stopping power is determined by how tightly it grips the rotor. Stopping power is a function of piston area, not pad size. The piston area in the caliper determines its clamping force. To ascertain if a piston area is right for your application, calculate the area of all the pistons on one side of the caliper. The formula is: Area = (piston diameter) 2 x 0.785. (See chart.)
For instance, a caliper with two 1 3/4-inch pistons has an area of 4.8 square inches. The same caliper with two 1 3/8-inch pistons has an area of 3 square inches. The caliper with the larger pistons has 50-percent-more clamping force--even though the caliper is the same size.
There is a long-standing debate...
There is a long-standing debate as to which type of caliper is best: cast or billet. It all comes down to the design. The elimination of heat is one of the major goals of the caliper. The open-bridge design allows the heat to radiate from the top of the caliper. Some of the manufacturers are using stainless steel pistons and heat shields, because they do not transfer heat as readily.
The new pad materials are not as detrimental to the rotors. The pad manufacturers are creating friction materials that transfer a little of the material to the rotor. This way, the pad is working against itself and not biting into the rotor. As the materials become better at stopping the car, the pad generates more heat. This is causing the brake pad to develop a taper. As the pad taper increases, less of the pad surface is making contact with the rotor. When this happens, you lose stopping power.
To counter this, the brake manufacturers developed differential-bore calipers. They use a smaller piston at the front of the caliper to decrease the clamping force, and a bigger piston at the rear to increase clamping force. This compensates for irregular pad wear. With a differential-bore caliper, the pads wear evenly and live longer. Obviously, the braking force increases, because the entire pad surface is in contact with the rotor.
The size of the caliper determines the size of the brake pad. Pads that are longer, wider, and thicker dissipate the heat better, and you will get longer pad life. The front brakes on a pavement car do 75- to 80-percent of the braking. The newest brake pads have a higher coefficient of friction. They generate so much heat that the caliper manufac-turers have developed different approaches to keep the heat from transferring through the piston to the brake fluid. Some use stainless steel heat shields to act as thermal blocks. Others ceramic-coat the inside of the caliper and the piston, to insulate the brake fluid from the heat.
If the racer uses a quality competition brake fluid that is good up to 570 degrees F, it should not boil under normal racing conditions. The bigger problem is the piston O-rings. Although these are high-temperature O-rings, they are only good for around 300 degrees F. If you have a brake rotor that is running 1,000 degrees, it is very easy to have the caliper run 300 to 400 degrees.
That heat is transferring from the brake pad, through the piston, into the caliper, and is deteriorating the O-rings. It used to be you could run 10 to 15 races without replacing the O-rings. Now with the severe heat caused by the better brake pads, the O-rings start to degrade, break down, and stick. This causes the pistons to not retract properly and generate more heat. It is a vicious cycle. Some racers replace O-rings before every race.
For racing, a vented iron...
For racing, a vented iron rotor is best. The vanes are either curved or straight. The curved vanes achieve more surface area than the straight vanes, so the rotor will cool better. A curved-vane rotor will also pump more air through the rotor. The number of vanes, and their thickness, length, and shape vary according to the racer's braking requirements.
When purchasing a rotor, you must consider the temperature range you will be running, and how often the rotor will see heat spikes.
"Rotors are made from three mate-rials. Iron is the most-used rotor material in racing. It is very stable," says Wilwood's Doug Burke. "It operates over an impressive temperature range. We make recommendations based on the operating range, the length of time spent at a continuous temperature, and thermal cycling."
The rotor provides a surface for the pad to work against and to remove heat. For racing, a vented iron rotor is best. You want to have the largest rotor you can fit inside the wheel, because it has the greatest amount of mechanical leverage.
There are two types of vented rotors: straight vane and curved vane. A curved-vane rotor is structurally stronger than a straight-vane rotor. It can achieve more surface area than a straight-vane rotor, so it will cool better. A curved-vane rotor will also pump more air. The number of vanes and their thickness, length, and shape vary according to the racer's braking requirements.
The size of the master cylinders...
The size of the master cylinders is used to fine-tune the brake balance in the car. For asphalt racing, most drivers use a smaller bore cylinder on the front brakes and a bore one size larger on the rear brakes. Because the front brakes do 75 percent of the braking, this setup gives the front brakes more pressure. Some racers are installing the same size master cylinders and using the balance bar to fine-tune the brakes.
Some racers will drill their rotors, in an attempt to make them lighter. This will only work if you race a light car over a short distance, and your track does not require hard braking. For most Saturday-night racers, drilling the rotor is not advantageous, because the surface temperature of the rotor is hotter than the inte-rior temperature. Every hole has a high thermal gradient. This causes uneven expansion and contraction, which causes cracks. When a rotor cracks, it becomes a boat anchor.
The racer who seeks a lightweight rotor would be better off with a smaller-diameter, thinner-wall rotor. Rotors are designed in different thicknesses to handle a variety of temperature ranges.
Mark Wood at Outlaw Brakes explained the rotor further. "The surface area is the key to the rotor. In curved-vane rotors, the vanes are longer than in a straight-vane rotor. It increases surface area. It offers more area to dissipate the heat. It will be a faster-cooling rotor. The more vanes a rotor has, the more cooling can be done. The downside is that it will be heavier, with more rotating weight. You must determine if the driver is hard on the brakes and needs the extra vanes. If the driver is easy on brakes and doesn't get them as hot, you can go with fewer vanes. The cheapest, lightest-weight rotor is not necessarily the best. If a rotor gets too hot, it radiates heat back into the caliper and boils the fluid. A rotor's diameter and width are determined by your cooling needs. A rule of thumb: You want the biggest-diameter rotor that can fit inside the wheel. Bigger rotors dissipate heat better."
The Master Cylinder
The master cylinder should be matched to the piston area of the front and rear calipers. A master cylinder transforms the mechanical pressure from the pedal into hydraulic pressure, which activates the piston in the caliper. The master cylinder affects both pressure and volume. In general, a smaller master cylinder will provide more brake-line pressure or more braking force. It will give a softer pedal feel because the pedal will travel farther. A larger master cylinder will make less brake-line pressure and less brake force. The pedal feels stiffer because there is less pedal travel.
To determine if your brake...
To determine if your brake system is right for your application, use temperature paint or stickers to record the maximum heat gene-rated by the brakes. Rotors can see 1,000 degrees F. The caliper probably should not be above 400 degrees F. The piston O-rings are a high-temperature material, but can only sustain a temperature of about 300 degrees F.
Most asphalt racers want more front brake than rear brake. If the front and rear calipers are the same size and have the same size pistons, then different size master cylinders can be used to provide more braking for the front. Generally, the front master cylinder is one size smaller than that used for the rear brakes. For instance, use a 7/8-inch on the front and a 1-inch on the rear.
Using separate master cylinders for the front and rear brake systems is beneficial for several reasons. The first is safety. If one system fails, the other system will safely stop the car. Second, different-sized master cylinders for the front and rear brakes allow for tuning the brake balance in the car. The third reason is the ability to fine-tune the brakes, using a balance bar that ties both cylinders to the brake pedal.
Racers must view their brakes as a total system. The ultimate stopping power results from properly selecting the components of the system. It begins with the brake pedal ratio, then goes to the master-cylinder sizing and the proper balance of calipers. It includes the correct size and type of rotor, and concludes with the selection of the brake pad material. All the pieces must work together, or the only way you will stop is to run into something.
Making blind recommendations is dangerous. The choice of calipers depends on the weight and speed of the car, the type and size of track you are running on, the length of the race, and the size and thickness of the rotor. A rule of thumb for asphalt racers would be to use a caliper with bigger pistons in the front of the car and smaller pistons in the rear. This will give around 50-percent-more braking force in the front.
DPI has a new rotor design--it...
DPI has a new rotor design--it has scalloped the rotor. This reduces the weight and some of the moment of inertia. This manufacturer has taken a 9-pound rotor and made it 5.5 pounds.
Dirt racers should use smaller piston calipers on the front and calipers with bigger pistons on the back. Some dirt chassis builders run the same size caliper on each corner, but use big pistons on three corners and smaller pistons on the right front. Before buying any brake components, check with the brake manufacturers for their latest recommendations.
Getting air to the rotor is critical to extending pad wear. The hotter the rotor gets, the faster the pads wear. The racer wants to get as much air as possible to the rotor. Run the biggest duct possible without interfering with the suspension travel. It needs to be as straight as possible and run into the plenum, or center of the rotor. Rotors pump air from the inside out. A second duct can dump air on the surface of the rotor.
DPI's Dan Press says, "We have worked with racers who were losing their pedals at the end of a race. For the next race, the only change they made was to add a brake duct. That solved their brake problem. The key is to get as much air blowing through the rotor as possible to cool it off."
Use a high dry boiling point...
Use a high dry boiling point racing-brake fluid. Change it before every race. Once the can is opened, the fluid starts attracting water and begins to lose its boiling point. The lower the boiling point of the brake fluid, the quicker your brakes will fade.
Change your brake fluid before every race! In order for the brake system to work its best, you must have a brake fluid with the highest boiling point possible. Begin with a racing brake fluid like AP 600, AP 550, or Wilwood 570. They all have very high dry boiling points--around 570 degrees F. The dry boiling point is the minimum temperature at which the fluid in a sealed bottle, in its most uncontaminated state, will boil. The racers should be concerned with the dry boiling point--not the wet boiling point mandated by DOT rules. The wet boiling point is when the fluid cannot absorb any more moisture. At that point, the fluid cannot boil at a temperature lower than 284 degrees. DOT 3 fluid has a dry boiling point of only 401 degrees F.
All brake fluids absorb water. Water gets into brake fluid from humidity, washing your car, and from natural condensation when the fluid gets hot and then cools. As water accumulates in brake fluid, it slowly deteriorates the performance of the fluid. It lowers its boiling point. When the fluid heats up, it expands. This gives you a spongy pedal and brake fade.
Only buy brake fluid in small sealed cans. If any fluid remains after topping off the system, toss it. Do not try to save the small amount left in the car. It will become contaminated while sitting on the shelf. The most common complaint is "My pedal is spongy." This problem can be remedied by changing the brake fluid before every race.
Wilwood's Doug Burke cautions racers, "DOT 5, silicone brake fluid should not be used in racing. It is highly compressible, due to the fact that aeration occurs very easily. It is not hygroscopic, so water will accumulate into little drops, and at high temperatures will turn to steam and expand. You can actually get a little brake drag because the fluid is expanding and putting pressure in the pads. At the high temperatures in racing, you will get a spongy pedal."
Because racers use the biggest-diameter...
Because racers use the biggest-diameter rotors possible for more brake leverage, very little air comes through the wheel to cool the brakes. Ducting cool air from the front of the car is mandatory. One hose should go to the plenum, or inside center of the rotor. Rotors pump air from the inside out through the vanes. Additional ducts can direct air to the surface of the rotor to cool it.
How do you know when it is time to replace the rotor? Dan Press at DPI and Sierra Brakes answered, "As soon as you start feeling some ridges on the surface, take them off. You will see a wear pattern developing. If you catch it early, the rotor can be blanchard-ground five to ten thousands. This will make the rotor as good as new.
"I don't like turning the rotors. A blanchard grind takes the surface molecules in different directions. It is not going in just one, as on the brake lathe. With a lathe, you are already putting grooves on the rotor. The pads will follow that groove when breaking it in. When you grind them, the pad will wear more evenly.
"You can have them ground for around $15 per rotor. Then the rotor is a good piece again. If you let it go too long, you start removing too much material. The more material you remove, the less heat dissipation the rotor will have, and the quicker the heat will build up. If you buy a good rotor to begin with, you can grind them two or three times before they are too thin. When it gets too thin, it will crack."
How do you tell when the O-rings in the caliper are worn? Press shares this tip: "You can tell by a quick check. Put the car on jackstands. Push on the brake pedal and release it. Rotate a tire by hand. You should be able to turn the wheel freely. If the wheel doesn't roll freely, the piston is sticking because of the O-rings. This is a good indication that you need to pop the piston out and look at the O-rings. The fronts are doing 75 or 80 percent of the braking. That is where you see all the heat. If you are running any 100-lap races on a short track, you can flatten the O-rings after one event. On the rear calipers, you probably will not put in a set of O-rings all year.
"Also, look at the grease in the wheel bearings. The same heat that is cooking the O-rings is going to the hubs, bearings, and spindles. If you are running more than 30-lap races, you need to look a lot closer than we have ever done before. When heat builds up, it is hard to dissipate. When the race gets longer and longer, the heat dissipates slower and slower.
"If you have a brake fluid recirculator, it makes your system like a self-bleeding system. We used to just take the old fluid out of the reservoir and pour in fresh fluid. With a recirculator, it self-bleeds the system. You don't have to totally flush out the lines. It cuts down on the maintenance work."
Dan Press Industriesbr>1397 Hwy. 506
Vader, WA 98593
Rebco Racing Enterprises
P.O. Box 150
Cumming, GA 30040
P.O. Box 1787
Buelton, CA 93427
Wilwood Racing Products
4700 Calle Bolero
Camarillo, CA 93012
A brake recirculator is a...
A brake recirculator is a smart choice for most brake applications. Just as the name implies, it circulates the brake fluid out of the caliper and back to the master cylinder. This helps to reduce the temperature of the brake fluid. Some of the new recirculators are self-bleeding. This makes replacing the brake fluid much easier.
It is important to know the...
It is important to know the caliper piston area to determine what size caliper is best for your application. This information is also used to balance the brakes, front to rear, on the race car. The formula to calculate the piston area is: Area = (piston diameter) 2 x 0.785. Use only one side of the caliper in the formula. For differential pistons, use the total of both piston areas.
There is a correlation between...
There is a correlation between the piston area of the caliper and the proper master cylinder size. This chart is a starting point to determine which master cylinder size is best for your application. In general, asphalt drivers use a master cylinder one size smaller on the front brakes. Dirt racers reverse that and use a one-size-smaller master cylinder on the rear brakes.