A wind tunnel is basically a large tube with a powerful fan at one end. The fan re-creates the effect of air currents over an object. Wind tunnels aren’t new—the Wright brothers built a rudimentary one of their own to test wing shapes back in 1901.
These days, wind-tunnel testing is what increasingly separates front-running race teams from the backmarkers because so much of a modern race car—the engine, the tires, and so forth—is tightly regulated by racing rules. In many series, the greatest gains are to be made in aerodynamic adjustments, the last frontier where creativity still pays dividends.
One place where race teams go for this testing is the Auto Research Center (ARC) in Indianapolis, Indiana. The ARC tunnel is a continuous loop, like a tube bent in a circle and joined at the ends. A 450-hp electric motor drives a fan that is 12.5 feet in diameter and produces blowing air at more than 100 mph. This tunnel is what’s known as a half-scale tunnel because the test subjects are painstakingly accurate half-scale models used to simulate the real thing.
Although the models don’t have working engines, they’re perfect replicas in every other detail and require a team of machinists four months to construct. Much of a model—the brake rotors, the wheels—is whittled down from aluminum blocks, and the body is made from carbon fiber. The level of detail is astounding—the model I saw of a 2002 Champ Car had tiny pin-sized shafts for suspension pushrods, miniature radiators, and perfect mini-replicas of the brake ducts. The end products are striking, and for what they cost—$250,000—they should be.
The ARC tunnel has an uncommon feature called a rolling road—a five-foot-wide belt that runs at the same speed as the air—that is critical to the accuracy of the tests. In the real world, as it travels down the road, the car is in motion while the air and the road are still. In the wind tunnel, however, the model is stationary, so an accurate simulation requires that the air and the road move. The rolling road is also important because it spins the wheels, creating, among other things, invisible vortexes that greatly affect the airflow over the car.
"Precision is vital," explains Bruce Ashmore, one of ARC’s owners, "The tests have to be repeatable, and the data have to correlate to the real world. So in addition to the rolling road, the airflow in the tunnel is carefully managed with several sets of vanes that smooth out the turbulence created by the fan, and the temperature, which tends to rise from the frictional effects between the air and the tunnel walls, is cooled to 72 degrees."
Ashmore, 47, is no stranger to wind-tunnel testing. In the late ’70s, he apprenticed at Lola Cars, the well-known British company that designed a variety of successful racing cars, including the first Ford GT40, and he worked there designing race cars until 1993. He’s since held a variety of posts in the U.S. racing community and now splits his time between the ARC operation and freelance design work.
The main goal of aerodynamic development hasn’t changed. Automakers visit ARC to exploit that rolling road, aiming to reduce drag and increase fuel economy, which is ever more important. Race teams want to know which shapes produce the most downforce with the least amount of drag. But the accuracy of the testing—thanks to features like the rolling road and vastly improved data-gathering equipment—is better. Like every part of the car, aerodynamics is a compromise. More downforce allows higher cornering speeds at the expense of straight-line velocities. But race teams also want to know the effect different setups have on the aerodynamic balance. For example, does lowering the nose take away too much rear downforce? And what happens to the airflow when the car gets slightly sideways? They also check airflow through the radiators.
It’s an iterative process: Move the model, measure the effects, and on and on, at a cost of $1100 per hour. Teams test in wind tunnels constantly because that old adage about vehicle shapes—"If it looks right, it probably is"— simply isn’t true. Ashmore says there could be two cars that appear identical to the eyes, but one will have 20-percent-more downforce than the other. The details count, and racing, as we know, is a game of inches, or in the case of these models, a case of fractions of inches.
So, one might wonder, if correlating the model data to the real thing is so important, why not use a real car? The main reason is simply the cost difference. A half-scale tunnel is roughly one-eighth the cost of a full-size unit but provides 90 percent of the needed data. Also, until a full-scale, rolling-road tunnel that’s under construction in Concord, North Carolina, opens sometime this year, there simply isn’t a facility available where U.S.-based race teams can test their actual cars. Even well-funded teams have to carefully choose where to spend money, and model testing is more cost effective (top-level formula and prototype race cars run at least twice the price of the half-scale models). Plus, the models are easier to work with and can be run simultaneously with the race car, gathering data that are fed to the teams between races.
Soon, however, models and wind tunnels may become obsolete, thanks to ever more powerful computers and accurate simulation programs. Right now, says Ashmore, even the most powerful computers require more than a month to do the calculations that spit out a day’s worth of wind-tunnel data. But we all know how quickly computing power improves, which is good because someday one of those very trick models may end up on eBay.