Chassis Tuning With Brakes

For years brakes received only minimal attention in stock car racing. Teams simply bolted on the biggest disc brake system they could find and used it everywhere they competed. If the car stopped or slowed down when the brakes were called on to do their job, then fine and dandy.

Times have changed, however, and the technology behind brake systems has evolved along with the rest of stock car technology. Teams today have several options available when choosing a brake system, including, among other things, different pad compounds, rotors of various sizes and weights, calipers with different piston sizes, and the ability to change the bore size of a master cylinder.

To appreciate how brakes have become crucial to a car’s setup, consider this: Teams will often use separate brake packages for qualifying and racing. “Subtle changes in the system can be a major advantage when qualifying positions and victories are measured in hundredths of a second,” says Carl Bush of Wilwood Engineering, a brake manufacturer for stock car racing.

Bush says that with so many variables involved with proper brake tuning, teams have begun to realize that many spring, shock, sway bar, and chassis weight adjustments can be eliminated with a small adjustment to the brake system.

Changes brought to a car when brakes are applied include deceleration drag on the driveline from the engine, weight transfer from rear to front and left to right, and stress on tires. It is imperative, given those variables, that a brake system be properly balanced, able to minimize the shock that is delivered to a chassis. If not, corner handling will be lost and the driver’s comfort and concentration will be compromised.

“Brakes have a major impact on handling and overall performance,” says Bush. “There are a lot of things happening when the driver hits the brake pedal. If the driver is fighting the brake system, he will not be racing the other cars. The driver will begin lifting off the throttle early, and lap times will suffer.”

To avoid that, Bush says the number one objective when chassis tuning with brakes is having the right amount of front-to-rear brake bias in the car. A 70-30 split from front to rear is a general guideline for racing on asphalt. Bret Gipson, motorsports engineer for Performance Friction, a major Winston Cup participant, says the range can vary from as high as 75-25 down to 58-42, with 70-30 being a baseline figure.

Solving Problems

Gipson and Bush caution that improperly tuned brakes can create handling problems that leave racers in a head-scratching mode. Heat, in particular, can cause problems, according to Gipson. On short tracks, or other places where braking is critical, heat can boost front tire pressures and create handling woes.

“What that can do, and I’ve seen it happen, is give as much as a 10-pound split from right front to right rear, just from the brake system,” Gipson says. “We obtained that with a Winston Cup car at Phoenix where they were actually running happy hour without any of their cooling ducts installed. So they were trying to dial the chassis in for a pushing condition, and they were having a hard time getting the car to where it would turn in the center of the corner.

“That was because they didn’t have the final cooling package on the car like they should have; that way you end up chasing what you think is a suspension push but it turns out to actually be a brake push. That’s pretty common. There are quite a few teams that don’t realize they’re doing it.” Such problems can be sorted out during testing, which today involves some teams looking at different brake combinations with nearly the same respect afforded shock packages.

Competition within the industry has forced companies to improve their brake components, particularly pad compounds. Asbestos was the material of choice for years, but companies struggled in recent years to find replacement materials that would offer stopping power, long wear, and high fade resistance against heat.

“Today, there are friction compounds that far surpass the limitations of asbestos-based brake pads,” says Bush. “They stop better, last longer and withstand far greater heat before fading. The last part about fade has become a ‘Catch 22.’ Now that brake pads are available that won’t even fade when you are melting the rotors, racers drive in harder, use more brake, and stay on them longer. This generates more overall system heat.

“These higher sustained heat levels have caused problems with other parts that never existed before. Heavier rotors, insulated pistons in brake calipers, higher grades of brake fluid, and brake cooling ducts that sometimes include electric cooling fans are now necessary to finish races on extreme braking tracks.”

Gipson says Performance Friction has upgraded its product offerings to meet today’s more sophisticated demands.

“From 1988 until, I’ll say, ’98 we were probably the majority pad in Winston Cup racing, with the same compound over a 10-year span,” says Gipson. “We’ve actually had two new materials released in the last year to keep up with the competition.”

As the components and technology found in brake systems have evolved in recent years, the list of brake system tips has grown. With the help of Gipson and Bush, Stock Car Racing has compiled the following list:

Do ...

• Bleed your brakes before every race and make sure there is fresh fluid in the complete system. The fluid in the reservoir should be clear and full.

• If you have a fluid failure due to excessive heat, flush all the old fluid out of the system and bleed the system with new fluid before you try to race again.

• Always try to achieve the proper front-to-rear balance with the hydraulic components, because the size of a caliper piston or master cylinder doesn’t change with heat. Only use pads to tune brake bias when there are no other options.

• Know what your brakes are doing. Drivers should know what brakes feel like cold, warm, and excessively hot. This includes knowing whether you can modulate them when hot or whether they go straight into lockup. Having good feedback to the crew chief makes a big difference on what he can do when tuning.

• Know what pad compound you are running. All compounds have different friction values at different temperatures. It is very important to match the pads’ friction and heat range to your race car. If you have a firm pedal, but the car isn’t stopping as it should, you may need a higher friction pad. If the wheels lock every time you touch the pedal, you may need a lower friction pad. If the pads lose friction as the race goes on, you may need pads with a higher heat range.

• Try to run equal master cylinder sizes and equal brake pad compounds front and rear. Try to get the overall bias split with larger front caliper piston sizes and/or larger diameter front rotors.

• Check caliper and pad alignment over the rotor. Welded brackets aren’t always in the same location. The caliper should be centered laterally over the disc within the range of a .015 inch shim. Check the pad contact relative to the outside diameter of the rotor. They should be flush. If the pad hangs off the edge of the rotor, it may be the wrong bracket, or it may have been incorrectly installed.

Don’t ...

• Don’t rely on different brake pad compounds to tune front-to-rear bias. Most brake pads change friction as they heat up, and the front brakes and rear brakes do not heat up at the same rate. Consequently, the amount of brake bias also changes as the temperature becomes further apart front to rear. So if your car’s handling changes as the race progresses, a change in front-to-rear bias could be to blame if you used two different pad compounds.

• Don’t wait until you experience pedal fade during a race to do fluid maintenance.

• Don’t try running the same fluid after a fluid failure due to heat, even if the pedal comes back after it cools down.

• Don’t throw on the brake package someone else is running and hope it works for you. First you should analyze the specific differences within the system.

Common Misconceptions

• A lighter rotor is always going to make your car faster. The rotor is the radiator of the brake system. It has to have enough mass to absorb and dissipate heat. A lighter rotor will get hot faster and actually distort under temperature. This will lead to increased lap times because the driver has to stay on the brake pedal longer to get the car slowed down to turn.

• A larger caliper gives more braking power. A larger caliper carries a larger pad that will last longer and sustain more heat. However, braking power is mostly affected by piston area within the caliper.