Science to Watch For: Martinsville

I’ve had a number of requests to identify science people should look for at the track before the race weekend starts, so here are few things to watch for this weekend at Martinsville.

Martinsville was the second race I spent with the No. 19 team last year while I was researching my book, The Physics of NASCAR. Martinsville was a stark contrast to the first track I visited, Atlanta. Martinsville is a short track and, in addition to having a slight preference for short track racing, I really like the intimacy of the track. The garage is really more of a lean-to. There are no fancy windows for fans to watch the teams through. The National Anthem is usually performed by local people and they play it the way it was written. Even the drivers have to park their motorhomes outside the track. My favorite memory of Martinsville was the Sunday morning I spent sitting in my car waiting for the garage to open. There was a preacher singing hymns outside Jeff Burton’s hauler. The Sun hadn’t risen when the service started, but as it did, I watched the mist in the valleys lift. It was a wonderfully peaceful moment. Then the garage opened.

The most important thing on a car at Martinsville are brakes. Martinsville is 0.526 miles (2777 ft) in length. The straightaways are 800 feet. That means that about 57% of the track length is straight and 43% are turns. I’ve sketched out Bristol and Martinsville in the figure below. (And yes, Bristol is asymmetric. That’s not just my crummy drawing.) In addition to being asymmetric, Bristol is rounder. Drivers spend more time turning there. Martinsville is long and narrow. You’ll hear announcers call it ‘turns connected by dragstrips’, and that’s pretty much the way the drivers drive it. They get on the gas as soon as possible coming out the turn, get as much speed as they can down the straightaway, and then brake hard to enter the next turn. Keep an eye on the on-screen displays of brake and throttle during practices. The turns are tighter, so you need more grip to get around them, which means that the speeds at Martinsville tend to be slower. That doesn’t mean the racing is any less interesting or the cars are any easier to set up.

The huge demands Martinsville places on brakes means that teams use larger brakes. That means larger brake calipers (larger diameter calipers and often six calipers instead of four), plus the brake pads have a larger area and are thicker. It also means glowing rotors. A moving car has kinetic (motion) energy. When the car slows down, according to the law of conservation of energy, the kinetic energy has to be transformed into other kinds of energy. Kinetic energy is transformed mostly into heat at the brakes because brakes work using friction. If you rub your hands together for a few seconds and put them to your cheeks, you’ll find that your hands are warm. Where there is friction, there is also heat. When the brake pads drag against the brake rotor, the pads and the rotor get hot. At tracks where the driver doesn’t have to brake as hard, the rotors may heat up a little and for a short time. At Martinsville, you’ll see the rotors turn red for a much longer time. SPEED often has a camera on the brake rotors during practice and that gives you a great view of how hot the rotors get and how long they stay that way.

Just as friction between the tires and the track abrades the tire surface, friction between the brake rotor and the brake pad abrades the brake pad. During the race, look for close-ups of the pit stops. When the front tire changer removes the tire, look for black brake dust. That dust used to be brake pad. Sometimes there is so much dust that the tire changes have a hard time seeing until the dust settles.

The interaction between the brake pads and the rotor is very similar to the interaction between the tires and the track. Friction is one of those phenomena that scientists do not entirely understand, especially when you start dealing with materials like rubber that stretch and come apart. Friction originates from forces between molecules in the two things that are rubbing together–in this case, brake pad and rotor. There are two types of friction at work. Abrasive friction is the type of friction at work when a piece of sandpaper rubs on wood. The energy needed to remove the molecules in the brake pad comes from the kinetic energy of the car, so the kinetic energy of the car decreases and brake pad gets thinner.

The second type of friction is adhesive friction. When the molecules from the brake pad come off, some form the dust I mentioned earlier. Other molecules transfer to the brake rotor, forming a thin film of brake pad molecules. The friction between the brake pad and the film of brake pad material on the rotor is different than the friction between the brake pad and the bare rotor. The best analogy is to imagine that you have a piece of gum stuck to your shoe. If you step on the sidewalk, you’ll get one type of resistance. If you step on another piece of gum, the two pieces will stick together for awhile until you pull your foot away. The latter is adhesive friction. Brakes rely on both types of friction to decrease the motion energy of the car, and both types of friction produce heat.

The second thing to look for at Martinsville is load transfer. I’ll dive into that in more detail for the second Martinsville race, but here’s the brief summary. The grip each tire has is proportional to the force pushing down on the tire. When the car is sitting still, each tire has roughly the same fraction of the cars’ weight pushing down on it. When the driver brakes, the front tires have more force pushing down on them than the back. You’ll see the splitter go from a few inches off the track while the car is accelerating down the straightaway to almost riding right on the track when the driver brakes. That means that the front tires have more grip than the rear. The reverse happens on acceleration: More weight is on the rear wheels than on the front. When the car corners, the outside wheels support more load than the inside.

Coming out of the corners, you’ve got a combination of the shifts from accelerating and turning. The right rear gets the most load and the left front the least. If the driver is, for example, accelerating too much coming out of the corners, you will see the left front tire spin or actually leave the ground for a few seconds. The SPEED TV guys usually do a great job focusing on this issue when they cover practice. They’ll often highlight one or two cars that are having this problem, which is more of a problem in the new car because of the higher center of gravity. If your favorite car is one of the ones being highlighted, that’s generally not a good thing. I remember at the spring Martinsville race last year that Elliott was having a very difficult time getting on the throttle coming out of turn 2. They could actually superpose the qualifying laps of the cars and you could see that he was as good as the pole sitter into turn 1, but lost precious fractions of seconds coming out of turn 2.

For those of you in the Charlotte area, I’ll be explaining load transfer in greater depth, including some video from last year’s Martinsville race, during a talk I’ll be giving April 7th at the University of North Carolina–Charlotte. The talk (which is free to attend) will be at 7:00 p.m. in room 281 of the College of Health and Human Services Building on the UNC Charlotte Campus. Parking is available at the new Union Deck and you can find maps of the campus here.