Tech Center: Brakes on Short tracks

August 19, 2013, Ron Lemasters Jr.,

NASCAR Sprint Cup Series race cars feature three pedals, and all are important

The most important pedal, for reasons that really need no description, is the throttle. Without it, the car is a really slick static advertising panel worth a couple hundred thousand dollars. The clutch is important because it links up the 800-plus-horsepower engine to the transmission and turns it from slick advert panel to moving billboard.

The third pedal, while not as sexy as the other two, is absolutely essential: the brake pedal.

With Bristol Motor Speedway on the horizon, the brake pedal is going to be even more important than it already is. Richmond in a few weeks is the same way. Any short track is going to put a premium on braking, and even 1-mile tracks like New Hampshire are a brake tech’s worst nightmare.

Bristol is a place where beating and banging is an art form and the nose of the car is for something besides holding the brake ducts in place. The brake ducts at a short track are very important, because if there’s no air, there’s too much heat, and if there’s too much heat, bad things happen.

Modern brake packages are such an improvement over the old-style systems. Four- and six-piston calipers, rotors engineered with all the care of the space shuttle, brake fans and the whole nine yards make it so.

Bristol is a rhythm track as much as anything. Hard on the gas, off and onto the brakes hard, float through the center with the car rotating beneath you and then hammer down off the corner. You do it all again in about five seconds, so on and so forth.

Bristol is easier on brakes than Martinsville because of the banking and the speed. Average speed at Bristol is in the 120-mph range, while Martinsville is much slower. The banking also slows the car a touch, because you’re transferring weight into the right side of the car.

With the changes to the banking at Bristol, it’s a little easier on brakes than it was in the past. If you got too far into the corner and went up the track, you were either going to spin out or you were going to get rooted off the bottom.

If you got rooted off the bottom, it was like getting on an express elevator to the back of the pack. With the progressive banking (minus the top part, which was shaved off to make it more like the old track), you can run the top groove.

That makes it easier on the brakes, because you can drag them a little more to set the car and then float, prior to putting the hammer down.

The G forces at Bristol are already incredibly high due to the banks, and heavy braking makes it worse. At Martinsville, for instance, drivers are at 3.6 Gs under straight-line braking and under 2 Gs of force when standing on the loud pedal.

As you know, the inside of a NASCAR stock car is very toasty. Depending on the weather, it can reach 130 degrees inside the cockpit. When you’re bundled up in the latest Nomex gear, it feels like the surface of the sun.

The heat generated by the engine is one thing. There are shields and such to deflect the heat away from components critical to operation. The headers especially are nearly molten, and that heat has to go somewhere.

With the heat shields in place, there’s still a lot of ambient heat to deal with, and the teams use bead blowers to keep them away from the tires. Tires are made out of rubber, and rubber, like most other compounds, has a specific melting point.

This is especially true at the bead of the tire. The bead is the part that tucks up inside the rim and holds the tire on the wheel. It can melt easily, and when you break the seal, the air (nitrogen, actually) goes where the leak is. That means the tire loses pressure, the driver loses control and the team usually loses all or part of a race car.

There’s a handling aspect to keeping the heat of the brakes away from the bead as well. The hotter the tires get, the more pressure builds inside the tire. Under-inflated tires are bad in the sense that they tend to wear the shoulders and roll over the sidewall. Over-inflation is a bad thing too, because it means the wear is in the center of the tire and the spring rate changes. That means the car plows, further wearing the tire and eventually causing it to lose pressure…often explosively.

Staying cool is important in a late-summer race like Bristol, and keeping the brakes cool is even more important on a fast, tight track like Bristol.