Some wind tunnels in Europe, on the other hand, contain rolling ground planes that more closely simulate conditions found on a racetrack. Some NASCAR teams (those associated with Ford and Dodge in particular) have tested in Europe to take advantage of that. Lacking that advantage here and unwilling to drain resources by flying to Europe, teams head to the Arizona desert, where all of the Big Three automakers-Ford, General Motors, and DaimlerChrysler-maintain facilities to test production cars, prototypes, concept cars, and the like.
In general a proving grounds site will cover several thousand secured acres, with multiple asphalt layouts and test tracks. The GM Desert Proving Grounds, for example, covers nearly 5,000 acres, while the DaimlerChrysler Arizona Proving Grounds is approximately 5,400 acres.
The GM and Ford facilities both contain 5-mile, banked ovals with long straightaways and wide, sweeping turns. DaimlerChrysler's site has 10 major facilities, including a 4.2-mile oval, a 2.4-mile straightaway, and an 18-acre skid pad at the site's Vehicle Dynamics Facility (VDF).
Teams typically use the straightaways to perform straight-line tests, which are either coast-down tests or downforce tests. In a coast-down test, the test driver will get the car to a certain speed-200 mph, for example-then clutch the car or take it out of gear. With the car coasting, engineers will measure the time and distance it takes for the car to get to another predetermined speed-50 mph, for example. Drag is then calculated using the measurements obtained. In a downforce test, the test driver will simply maintain a consistent speed so the amount of downforce can be determined.
"The straight allows for better repeatability over an oval race track," says Kent Lassen, test and development supervisor for Dodge. "We can also do repeatable acceleration tests on the straightaway. On the VDF we can explore the handling limits of the car without worrying about hitting a wall. Drivers have fun when there's no wall."
Drivers for the tests vary, from engineers employed by the manufacturers at the test facility to drivers who compete at racing's smaller levels, such as Hooters ProCup or ASA. NASCAR teams typically send a proving grounds test crew including fabricators, one or two mechanics, an engineer or two, and a driver. Tests normally range from two to three days, and some teams visit the proving grounds as often as five times a year.
Desert testing is not without its pitfalls, however, as nothing truly simulates actual race conditions.
"There are advantages and disadvantages with testing out there," says Rex Stump, a chassis engineer with Hendrick Motorsports. "It's a real-world situation. In a wind tunnel you don't have a road ripping the wind out from underneath the car, and that can be a big factor. Other issues play into it, such as when you've got pumping rotors and wheels moving during testing at the proving grounds. All those have an effect as to how the aero goes around and underneath the car. So if you're going to the proving grounds, it's more of a real-world situation.
"Conversely, the drawbacks are you are in a real-world situation, so a puff of wind, a change in ambient conditions, cold versus hot-all those things can change the numbers."
Teams rent wind tunnels for about $2,000 per hour, so tunnel testing is not a minor expense, particularly when you factor in transportation and lodging expenses. Whenever a team visits a proving grounds facility, there are additional costs, however.
"The downside of testing in the desert is it's very expensive," says Nick Ollila, chief engineer for Roush Racing. "That's not always obvious to people, but when you factor in the price of tires and the wear and tear on the motor and transmission and the rear end assembly-all of which have to be rebuilt-and then add a fire and safety crew, you've exceeded the cost of a wind tunnel test."
Ollila says the Roush teams usually take one car to the proving grounds. If the goal is to do quality control, or validation runs, they'll take several cars and compare them. "Most of the time we're doing research and we'll just take one particular vehicle," says Ollila. "We'll test various shapes on it and bring that data back and incorporate it into the rest of our cars."