Few things on a race car are as regularly underappreciated as the brake system. Yes, stock car racing is about going fast, but that's all the way around the track, not just the straightaway. The best braking system for your race car is one that allows you to slow the car quickly without upsetting the car so you can roll through the turn smoothly and get back on the gas as soon as possible.
Pleasing the Driver
Of course, when it comes to brakes, it's as much about finding the pedal feel the driver is looking for as it is pure stopping power. How firm do you like the pedal? How much pedal travel do you prefer? Do you like the front brakes to be significantly stronger than the rear so that the brakes will not cause the car to be loose on turn entry, or do you prefer a more balanced feel? These questions are all about making the driver comfortable behind the wheel, but they are important.
Finding the right brake balance...
Finding the right brake balance means optimizing everything from the brake pedal to the pads. Fortunately, many companies have already done the grunt work for you. Performance Friction, for instance, regularly tests all of its pad and rotor combinations with every caliper they're designed to fit.
Try to think of your brakes as a comprehensive system, from the pedal all the way to the rotors. In fact, let's go further than that: Continue your consideration all the way to the cooling duct inlets in the front bumper cover. After all, brakes that receive a steady flow of cool air will last longer and feel better late in the race than brakes that have boiled the fluid in the calipers and are only a little more effective than dragging your feet on the asphalt.
When it comes to pedal feel, the master cylinders are an important consideration. All things being equal, a master cylinder piston that is smaller will push less fluid for a given amount of pedal travel, but will create a higher line pressure. A larger master cylinder piston will have less line pressure but will push a greater volume of brake fluid, which means less pedal travel.
"The bigger the bore size you have, the more volume it pushes and less line pressure it creates," explains Derek Spencer of Perform- ance Friction. "You can use this to dial in a pressure split [balance] between your front and rear brakes. The most common setup with dual master cylinders for a typical short-track asphalt car is a 71/48-inch bore master cylinder for the front brakes and 1-inch bore master cylinder on the rear brakes.
A brake bias bar and adjuster...
A brake bias bar and adjuster assembly can provide for easy and quick tuning as to how much pressure is applied to the front brakes versus the rear. This can be a valuable tool, as conditions change a car's handling during a race.
If the driver wants more feel, you can drop the bore size-perhaps to a 131/416 in the front and a 151/416 in the rear. That will give you more feel and also a softer pedal, or more travel. That's because the smaller pistons are pushing less fluid, and it takes more pedal movement to push the pistons out of the brake calipers. If you go the other way with larger master cylinder bore, it will give you a stiffer, harder pedal. The greater volume of fluid moved by the larger pistons will require less pedal movement to move the pistons in the calipers, but because it produces less line pressure you have to push harder to get the same amount of braking forces.
"The opposite is true if you are working with the piston area in the brake calipers. A larger piston requires more fluid to move it, so it makes the pedal feel softer and requires more pedal travel. Racers will run into this when they change racing classes and run a different style of calipers. For example, I have witnessed a lot of racers have trouble when they move from Late Model Stock with the GM single-piston slider caliper to a series where four-piston racing calipers are allowed-like Super Late Model. They are used to the single, huge piston in the calipers that can contribute to a softer pedal, and when they go to a four-piston caliper, which is really better for racing, they have a hard time getting used to the stiffer pedal it produces."
A bigger rotor may come with...
A bigger rotor may come with a weight penalty, but its ability to absorb and vent heat without warping can be a distinct advantage over a smaller rotor that warps under heat. This slotted rotor from Performance Friction utilizes a floating hat assembly designed to reduce warping that can come where the hot rotor face meets the cooler hat.
A Matter of Balance
A brake bias adjuster can be an incredible tool for a knowledgeable racer. A bias adjuster can easily change the amount of braking pressure sent to the front brakes versus the rear. As fuel burns off or the track changes with the setting sun, you may prefer to change the brake bias more to the front or to the rear. A bias adjuster within the driver's reach is perfect for that, but it is often too tempting to use the bias adjuster to cover other handling sins.
Carl Bush of Wilwood Engineering agrees that the balance bar should only be used for fine adjustments to compensate for tire wear, fuel burn-off, or changing track conditions. The key is finding the car's balance, the correct front/rear bias with the balance bar centered, or neutral. "Typical asphalt stock cars run on average at about 70 percent front brake bias," Bush says. "A big-block dirt Modified will use nearly the exact opposite. Dirt Late Models fall somewhere in the middle. If a racer is always adjusting the balance bar completely to one end of the car or the other in an attempt to correct the handling during braking, you probably need to re-evaluate your component mix."
Your options for changing what's called the static bias, or brake bias, without the use of the balance bar are numerous. They include rotor diameter, caliper piston size, master cylinder piston size, and even brake pad compounds. Increasing rotor diameter increases the braking power, or bias, at that tire.
You should try to have your brakes in this range with the bias bar centered. As conditions change at the track, the bias bar is available to you to make changes. If the bias bar is cranked all the way to the front or to the back to make up for a poor setup at the drop of the green flag, you've lost it as a tuning tool.
Caliper technology is constantly...
Caliper technology is constantly advancing. One of the biggest advancements for the guy racing with real-world money is Wilwood's new forged Superlite and Dynalite calipers. They feature greater rigidity of cast calipers without the cost associated with billet units to produce a combination that is both effective and affordable. Also notice the lack of any hard edges or ridges-points that are most likely to become the origins of stress cracks.
Spencer recommends spending a little testing time at the track to find a well-balanced braking setup. Tracking brake temperatures just as you track tire temperatures can tell you a lot about what your car is doing. On asphalt, Performance Friction representatives say they generally like to see front brake temperatures 150-300 degrees higher than the rear. If it's greater, then you are probably not getting the full potential from your rear brakes. This will result in the front brakes being overworked, and the fluid in the calipers will boil, which will seriously degrade your ability to slow down in the turns. If the split is less than 150 degrees, you probably lose on entry under hard braking.
Choosing the Right Combination
Finding the best combination of parts for your braking system can be daunting. If you are racing a series that does not place many restrictions on what you can run, your options are nearly unlimited. One of your best options is to choose a manufacturer or dealer you trust, preferably one that will work with you closely and not just from behind the parts counter, and start with their recommendations.
Bush assists racers with their brake setups at every level and recommends relying on a manufacturer's experience. "Wilwood has been deeply involved with racing for 27 years," he says. "In many cases, not only do we have a recommended baseline for a particular type of car or series, we also have a baseline for the specific track you plan to run. That information can be used as a starting point, but car style, driver preference, and chassis setup can all require adjustments."
As we mentioned earlier, a larger-diameter rotor will increase braking force. This is because the pads are moved farther from the axle centerline, and that essentially increases the leverage the brakes have on the wheel. There are several other advantages of a tall rotor, but racers by and large tend to avoid putting large rotors on their race cars because of the additional weight.
One option for a larger, yet...
One option for a larger, yet lightweight, rotor is a scalloped piece, like this cast-iron unit from Wilwood.
Any weight added at the wheels can be difficult to swallow because it is rotating weight. Not only does it add to the overall weight of the car, but it must also be spun, significantly increasing its inertia. Also, given the same amount of weight, the farther it is from the axle centerline, the more difficult it is to spin. That's why many racers have difficulty with the idea of adding taller rotors that are also heavier. But if you consider that the entire brake and caliper assembly fits inside the wheel and tire, which can be just as heavy, you realize that an incremental increase in rotor weight and diameter does not make a significant change in the rotational inertia at each wheel.
"When considering rotors, you have to consider what they actually do," Bush adds. "One advantage of the rotor diameter is to provide leverage to the caliper to slow the wheel. A bigger rotor will give more leverage and help with braking power. The trade-off here becomes higher rotating inertia from the larger diameter and added weight. But the extra weight can be a secondary advantage. Another function of the rotor is to absorb and dissipate heat. Overall weight and cooling vane areas are the primary considerations here. For example, a 48-vane, 12-pound rotor will cool and heat cycle far better than a 32-vane, 9-pound rotor. There is a never-ending battle between trying to run the lightest possible part and having it be heavy enough to endure the event. If the rotor is too light, it may crack or become heavily grooved. Inadequate cooling capability can also allow the pads and calipers to become overheated and possibly fade. However, if a rotor is too heavy, it will certainly cool and last longer, but it can also cost a little in terms of lap times.
Of course, a lighter caliper can only shave tenths off of your lap times if it is straight. Spencer points out that another advantage of beefier rotors isn't just longer life span, but also an increased ability to stay flat. "The inertia you get rid of by going with smaller, lighter rotors can result in slower lap times," he says. "Because when the rotors become warped, they drag. This leads to loss of control, inconsistency, overheating, and modulation problems, all of which ultimately affect lap time.
"When rotors get hot, they grow," he continues. "When they cool, there is contraction. This is a constant dynamic event. If you have rotors that are too light, the lower thermal capacity will cause it to heat up much faster. That means it's going to grow and distort, or warp, much faster than a larger rotor. When that happens, the rotor will get out of round, and part of it is going to be touching the pads as it spins. The drag this produces robs horsepower and slows you down all the way around the track. You think you may be cutting your lap times with the smaller rotors, but if they're causing more drag, that can just kill your performance.
"It's easy to tell if you have this problem. Just spin the wheel when the brakes are hot in pit lane. If the calipers are true to the disc face, that wheel should spin free. But if it makes half a turn and then slows dramatically or stops, then you know you have brake drag or something else wrong. Check your rotors. If they are out of round, meaning the rotor face isn't flat, you might want to try a bigger rotor that can handle more heat."
There are also several important considerations to make when choosing brake calipers. "Caliper selection is based on two main criteria," Bush says. "One is the required pad volume, and the other is matching the piston area to both the master cylinder volume or pressure output and the overall bias requirement. Heavier, faster cars running longer races in sustained heat will wear pads far more quickly than a light car running short sprints on a light-breaking track. The pad wear rate dictates pad size requirements and therefore the overall caliper body size. But it's the actual piston area size that must be considered when trying to set up bias or affect clamping power. Going to a larger-caliper body doesn't necessarily give you more stopping power unless it also has increased piston area or is capable of effectively running at higher line pressures. But, it will certainly take longer to wear through the larger pad."
You religiously bleed your brakes and even flush the fluid at scheduled intervals. You keep a sharp eye on your rotors for cracks or signs of warping. Pads are properly bedded. Lines are checked for leaks. You've got it all covered, right? But how often do you rebuild your calipers? Do you even have a caliper maintenance program? If not, you need one. Carl Bush of Wilwood Engineering has specific tips for keeping your calipers in prime condition:
"A good way to judge caliper maintenance intervals is based on pad wear. After a caliper has been used long enough to wear through a set of pads, some basic routine maintenance should be performed. Please note that it's not necessary and usually not recommended that the caliper halves be separated during a rebuild. As the pads wear, the pistons become extended from the bores.
"Before reseating the pistons into the bores to accommodate the installation of the new pad set, the pistons should be removed from the body, fully cleaned, and inspected for wear. Never use anything coarser than fine steel wool to clean the pistons. The pistons should be clean and completely smooth on the outer surfaces. They should slide freely in and out of the caliper bores when the seals are not present. Damaged or worn pistons should be replaced. New O-ring bore seals should be installed and the pistons then fully seated into the caliper body. The caliper should also be cleaned inside and out and blown completely dry before reinstalling them on the car.
"This is also the time to completely flush the system with fresh fluid. Any brake maintenance should also include a routine inspection of the entire caliper body, including the areas around the mounting lugs, the bridges or bridge plates, bleed screws, and fluid tubes-if so equipped. Worn, damaged, or suspect parts should be replaced. The system should also be flushed and the calipers equipped with new seals at the beginning of each new season. If a person does not possess the equipment or knowledge to confidently and correctly carry out the maintenance procedures, they should seek the assistance of an experienced and capable technician."