It’s no secret to short-track racers that the best—and often only—place to pass on a small track is on turn exit. It’s the place where the car’s overall package comes into play, and, if you are missing a single component, you’re probably a sitting duck. You’ve got to have the right brake package to run the car into the turn aggressively and still be able to bring it around for the proper exit angle. You’ve also got to have the low-end torque to win the drag race from the turn exit. Finally, you have to have the suspension dialed in to properly handle the transition from maximum deceleration to maximum acceleration. Because you’ve got to whoa before you can go, let’s take a look at maxing out your braking system to get those faster lap times.
Friction Factors
“Your brake pads, or friction material, and your brake disc, or rotor, are probably the two things on your car that you can change quickly and cheaply that can make a big impact on your lap times,” says Don Burgoon, president of Performance Friction, a manufacturer of racing discs and brake pads. “We’ve helped some teams find the right combination with their brakes and seen them pick up two- to four-tenths on a ½-mile track.”
When it comes to stopping, Burgoon knows what he’s talking about. His company supplies brake rotors and pads to many of the top teams in racing, including Hendrick Motorsports and Joe Gibbs Racing in Winston Cup, PPC Racing in the Busch Series and Steve Francis in Dirt Late Models.
The first thing to realize, Burgoon says, is that getting the right brake package on your car is about more than slapping on the most aggressive brake pads you can find. Instead, take advantage of your options when it comes to pads and rotors to help tune the car in concert with your suspension and make it handle under deceleration.
“Friction is never perfect,” Burgoon explains, meaning that a constant force applied at the brake pedal does not mean a constant braking force at the wheel. “Different pads, rotors and other components all affect your stopping power, but a big factor is the pads. You can’t just look at one number and say ‘This is how much braking force I’ve got.’ It’s more complex than that. You’ve got to look at your initial bite, how it carries through the brake zone and how much torque rise or fade you get through the stop.”
For example, if you are a driver who prefers a car slightly loose on entry (to help get the car to turn) and tighter on exit (when the power is on to help keep the car from spinning), you can put a set of pads on the back, which bite quickly and maintain a fairly constant friction throughout the turn. On the front, use a set that increases in friction (otherwise known as Mu) as pressure is applied. So what does this mean on the track? As force is first applied to the brake pedal, the rear brakes are more effective than the front, making the car oversteer, or loose, going into the turn. Meanwhile, the brakes on the front are quickly gaining in stopping ability and surpass the brakes on the rear somewhere around the apex of the turn. This makes the car understeer, or more resistant to spinning out, in the transition from brakes to power.
Many racers try to achieve this with the balance bar, but while the balance bar can bias pressure front to back, it is static once that bias has been set and cannot vary over the course of a turn. Someone with an understanding of brake pad “personalities” can tailor the way their car handles at different parts of the corner. To find out more about specific pads, Burgoon recommends calling your manufacturer.
Disc Drive
The key to brake pads is all about friction. But, when you turn your attention to rotors, think speed. Too often brake drag is caused by rotors that have developed runout, which can be anything from high spots to outright warping. A rotor that isn’t straight will make contact with the pads inside the caliper even when the brakes aren’t engaged—that’s unnecessary drag and a drain on horsepower. Runout is a product of heat, and heat, as we all know, is the energy generated by the process of stopping the car. There is no way in a conventional braking system to slow a car without generating heat, so we are left with two options to reduce runout: vent the discs to remove heat from the area, or use rotors that can withstand prolonged high temperatures. The best solution, of course, is a combination of both.
So how can you tell if you have runout, and how much can you live with? Simple: Jack your race car up and spin the wheel. If there is uneven resistance from the brakes, you have runout. How much you can live with depends on how badly you want to win.
Keeping your brakes cool is the first step to making them last throughout the race, and it will also add to the lifespan of the disc. You can do this by ducting cool air through the nose of the car to the front brakes and simply by being careful not to drag your brakes. Generally, excessive heat is only a problem with asphalt racers, but you can monitor your temperatures by sight (Are your rotors glowing just in the turns or all the way around the track?) and by using temperature decals on your calipers and temperature paint on the rotors. As a guideline, cast-iron rotors should never exceed 1,400 degrees Fahrenheit.
Unfortunately, there are limits on how much cooling you can provide your rotors. Racers often try to use rotors that are too lightweight (read flimsy) in order to reduce rotating weight. But the lighter the rotor, the less heat and pressure it can withstand without deforming in some manner. It’s the same dilemma racers face on every part of the car: How much durability can I sacrifice in order to shave weight? The reality is an extra pound of rotating weight in the rotor isn’t that big a deal (see accompanying sidebar), while the performance detriment of a rotor with even moderate runout can be almost impossible to overcome. A rotor that’s making contact with the caliper because of runout is a double whammy. It’s robbing horsepower down the straights because of drag, and it’s not doing its job of effectively slowing the car in the turns because the pad cannot make solid contact with all of its surface area against an uneven rotor. You will feel it as a brake pulsation.
Dirt Late Model driver Steve Francis has seen it all when it comes to brakes. The lightweight dirt cars are not as hard on brakes as asphalt stock cars, so many teams have tried tricks such as carbon fiber and titanium rotors. But Francis has found that old-fashioned cast iron works best for him.
“I can remember a few years ago everybody was on carbon-fiber stuff and all that,” Francis says. “I’ve tried all that lightweight stuff, but I believe, in general, you are better off to stay basic with the brakes. When we tried the lightweight stuff to reduce rotating weight we really couldn’t see much in terms of lap times. In fact, whatever we might have gained by reducing rotating weight, we ended up losing by the end of the race with being able to stop the car and making it do what we want with the brake pedal. Plus, they are not as dependable. I may run a little heavier setup, but I like knowing at the end of 100 laps my brakes are still going to be good and I’m not going to have any problems.”
Harold Holly, crew chief for defending NASCAR Busch Series champion Jeff Green, agrees when it comes to racing on asphalt.
“You have to use rotors that are strong enough to stay true,” he says. “A lot of times a driver may not even be aware he has a vibration in the brakes. At the big braking tracks you’ll really wear the pads down, and if the rotors start to vibrate and shake, they push the pistons back. Then the driver has to start pumping the brake to get his pads pushed back out to the rotor.”
