Remember when you were a kid riding in your parents car with your hand out the window?
If you held your hand out palm first, the air would push it back. But if you turned it palm down, the resistance wasnt as great, and you could control its movement. If you tilted your hand down, the onrushing air would press it toward the pavement. If you tilted it up, the air would push it toward the sky.
That is, in perhaps its simplest form, aerodynamics at work.
The air flowing around, underneath, and over a modern Winston Cup or Busch Grand National car has the same effect on the car that it did on your hand. By controlling the airflow, a car can be made to slice through the air with less resistance and be pressed against the track to increase its ability to hold the road.
You cant see air, but you can feel the effects of it on your car almost every time you are on the highway. It is what causes the buffeting as you approach the rear of a tractor-trailer. It is what makes your car rock from side to side as a big truck goes by while you wait for a light to change.
Fifty years ago, racers taped lengths of colored yarn to their cars and drove around the track to study how the thread moved in the wind. Yarn that laid down flat meant there was smooth, undisturbed airflow in that area, while pieces of yarn that danced around erratically indicated turbulence. Today, that type of research is done via computer software and the results are confirmed in wind tunnels.
Good aerodynamics are critical on large ovals such as Daytona and Talladega and on road courses such as Sears Points or Watkins Glen, where they can really have an effect on a cars top speed and handling. Aerodynamics are just as important on short ovals, such as Martinsville, where it is more important to keep air flowing to the radiator and brake ducts than over the rear spoiler.
Historys A Breeze
The most obvious attempt at aerodynamics in NASCAR was probably the old Dodge Daytona bodies raced beginning in 1969. The Mopar designers bolted a cowcatcher nose to the front of a Dodge Charger to help the normally blunt nose slice through the air. Then they added a huge basket-handle wing to the rear deck to balance the front downforce with pressure on the rear suspension.
By todays standards the attempt was crude, but the Dodge opened eyes and dropped jaws when it was first unloaded at Daytona. And, it worked. Speeds in the 200mph range became routine, and NASCAR reacted by making the winged Mopars run smaller engines. That decision killed the effort by both Chrysler and Ford, which fielded its Talladega Torino version.
Todays chassis are far more subtle and, in some ways, less effective. Compare the shape of an airplane wing to the shape of a Winston Cup Taurus. Both the wing and the race car have flat bottoms and curved tops. As the air rushes over the top of either shape, it speeds up to create a low pressure area, which airplane designers call lift.
In an airplane, the airflow is relatively smooth and stable. On a car it twists and turns and curls around as it goes over the windshield, tops the roof and then falls back over the rear window onto the decklid, only to be met by the rear spoiler before tumbling around in the cars draft.
Engineers take advantage of the pressure areas on the car. They discovered, for example, that air pressure builds up at the point at which the hood and windshield meet. They use that area of high pressure to force-feed pressurized air into the engines carburetor intake.
The cars used in the open wheel CART series use chassis that are built like upside-down wings. The low-pressure area underneath those cars sucks them to the pavement. A 1,550-pound CART car can develop 4,000 or more pounds of downforce.
Nips And Tucks
One of the challenges with a stock car is that the rules are so tightly defined, says Dr. Joseph Katz, an expert in aerodynamics and a department head at San Diego State University. He is the author of New Directions in Race Car Aerodynamics and works as a consultant to race teams and car designers. Within the limits of the body templates, there isnt much you can do, Katz says. If you take a typical stock carsay a 2001 Taurus the body creates lift.
Right from the factory, a car such as the 2001 Taurus could never be driven at the speeds reached on superspeedways. Thats where aerodynamic engineers and NASCARs rules makers come into play. They create downforce through the devices added to the body.
As a result, Gary Eaker, former aerodynamics expert for Hendrick Motorsports, says a Winston Cup car can generate about 1,400 pounds of downforce, but that the force constantly changes as the cars go down the straights, corners, brake, or run into traffic.
Thats why part of the word is dynamic, Eaker says. It changes constantly.
Although the profiles of Winston Cup cars are very much alike, subtle differences make them respond differently to air flowing over them. Teams also have some latitude in shaping the car within the confines of the templates. A nip here or a tuck there can make a huge difference at 180mph.
Eaker says that if you stacked up the centerline templates for each of the Winston Cup bodies, they vary little one from the other. But thats like looking at the maximum horsepower of an engine, he explains. It doesnt tell you how much power youll get off the corners.
The same is true for airflow. It is the total shape of the panels that makes the difference, not just the centerline. Look at the roofline of the Taurus: from the rear of the car, it slopes down on the edges. On the Monte Carlo, the roof is almost flat. The way the air acts on the roof makes a big difference in pressure.
Katz says the only thing teams can do to massage the aerodynamics, while staying within the rules, is to play with the rear spoiler and tinker with the air going to cooling ducts in the front of the car.
Aerodynamics are highly influenced by whats done with the cooling ducts, Katz says. You can make very small changes there and influence drag by two or three percent. Ducts are taped over during qualifying runs to keep air from entering the engine bay and creating resistance under the car. During a race, they have to be opened in order to make the engine survive.
Eaker says that on tracks such as Daytona, the critical points are the lower front edge and the rear upper edge of the chassis.
Rule Book
NASCAR recognizes the influence of aerodynamics on both handling and speed. Late last season it changed the requirements for the Dodges, and the result is that they were immediately more competitive. While the cars were good qualifiers ever since they took the pole for the 2001 Daytona 500, they didnt seem to be able to perform in traffic. The change helped to plant the front end of the cars, making them more stable in the draft.
NASCAR fiddles with the aerodynamic packages throughout the season. It is one of the ways it can maintain parity within the field.
It also changes the rules from track to track and season to season. Last fall, NASCAR announced that when the Cup cars return to Daytona this year, they wont have the vertical strip of metal on the roof required in 2001. But this year the teams will have to bump the spoiler up to 55 degrees.
The roof strip disturbed the airflow over the roof and reduced the low- pressure area, siphoning some of the lift.
Eaker figures the 2002 package will result in slightly higher speeds at Daytona. Learning how the cars handle in heavy traffic with the 55-degree rear spoiler may not be known for sure until race day.
Draftingespecially on ovals like Daytonacreates a whole different set of aerodynamic problems. As air tumbles off the rear spoiler, it creates an area of low-pressure turbulence Katz compares to a bubble of air.
When a second car gets into that bubble, both his car and the one in front will be able to go faster, he says.
But if the second car gets too close to the lead car, the air gets disturbed and the balance of the lead car can be upset.
Drivers say the rear end gets light, but Katz contends that what happens is the weight shifts to the front of the car and the front end gets heavy, which gives the front tires more grip and suddenly increases how fast the car reacts.
Eaker contends that air trapped underneath the lead car makes it light and reduces the available traction.
Light back end? Heavy front end? It really doesnt make a lot of difference if you are the driver headed tailpipe first into the outside wall. The results are pretty much the sameheavy impact on the rear bodywork.
While designers and aerodynamics experts can massage the body to direct the air going over the car, there isnt much they can do with the air going underneath it. So they try to keep it out of there.
The idea is to remove as much air as possible from beneath the car and create an artificial vacuum that will help pull the chassis toward the pavement. The air dam pushes air to the side, skirts that run underneath the rocker panels help to keep air from seeping beneath it, and the rear spoilerwhich creates a low pressure area at the back of the carhelps draw the remaining air out from beneath the chassis. On a large oval, it doesnt take much of a dent in the bodywork to really disturb the airflow and take a good handling car and move it to the back of the field.
Katz says the bottom of the car is still relatively unregulated, so there is probably work that can be done there.
The rules basically tell us where the hardware has to be, adds Eaker. We cant deviate much in where things like the fuel cell, truck arms and axle go. Thats all dictated. We can play a bit with the rake of the car, changing its attitude on the track to alter the airflow beneath it.
But like everything else, as soon as one thing changes, you have to change something else to compensate for it.