There are some extremely important aerodynamic considerations which a driver must be aware of while racing with other cars at close quarters. If any two cars are within one or two car lengths of each other, they will have a considerable influence on each other’s drag (known as the drafting effect) and/or aerodynamic lift (known as stability effect). Drafting has been used by racers for years, but it wasn’t until a study by Chrysler in 1971 (SAE Paper No. 710213) that any real data was available to show how it worked. They also demonstrated the operation of the slingshot passing technique, in which the trailing car is always at an advantage.
The results of the Chrysler study showed that the aerodynamic drag of the trailing car was reduced by about one third as it approached the rear of another car. The drag on the leading car is reduced also, but not quite as much. Therefore, two or more cars working together can increase their top speed by a considerable amount, as long as they remain in tandem. When two cars pull abreast, the air drag increases on both. The slingshot effect comes from the fact that the trailing car can accelerate into the low drag wake created by the car ahead, and use that momentum to pull out and pass. When the pass is completed, the other car is then at the advantage, and can repass at the next straightaway. In actual practice on superspeedways, two relatively equal cars may maintain their positions until the last lap, when both try to be the trailing car to slingshot at the finish line.
But even when you know the science of drafting and slingshoting, there are some details to keep in mind. First, the trailing car can develop a serious cooling problem, because not only is the frontal airflow reduced, but it is filled with heat and exhaust from the leading car. Second, is the question of gearing, since a pair of racecars may end up running hundreds of rpm over what seemed like the correct gear ratio under normal racing conditions.
Drafting is not quite as valuable on short road racing courses because of the diminishing effect found at lower speeds and the difficulty in staying within a few car lengths of each other. If two cars are perfectly equal in acceleration and they leave a 50-mile-per-hour corner bumper to bumper, the gap will naturally increase to as much as two car lengths by the time they reach 150 miles per hour. The reason can be seen by comparing their spacing in seconds. If the lead car is 15-feet long, then it is 0.2 seconds ahead at 50 miles per hour. But 0.2 seconds at 150 miles per hour is equal to 45 feet, or a gap of 30 feet between cars. So the trailing race car must have a slight advantage in acceleration to keep closed up until aerodynamic effects rise enough to help, or else wait to draft out of a faster corner.
Another serious aerodynamic interaction to consider while drafting is the reduction in aerodynamic downforce on the trailing racecar. The Chrysler study showed the values for large sedans, while the effect on open-wheel racecars with wings was demonstrated in a report by the Jim Clark Foundation in 1969. In both cases, the results show that there are considerable changes in downforce on both cars, not only in tandem, but when they are side by side as well. What is worse, the changes vary greatly from front to rear, producing sudden changes in overall stability. For example, as the trailing car approaches, its front downforce diminishes, creating understeer, while the leading car’s rear downforce diminishes, creating oversteer. There have even been extreme cases where the front of the trailing car began lifting, until the car was flipped over on its top! As long as racecars require large amounts of downforce, there is no way to eliminate this effect. But drivers should at least be aware of the problem and possible consequences, and be prepared for the changes in handling that drafting may bring.