A speeding stock car pierces the air as it travels. Air swooshes over the top of the car and is deflected by the spoiler attached to the rear deck. If another car trails immediately behind, nose to tail, it continuously enters airspace affected by the car in front.
The trailing vehicle, if it tailgates at a certain proximity, can take advantage of the lead car's aerodynamic force. The air behaves as if the two cars are one. The displaced air behind the lead car creates a partial vacuum that sucks the trailing car ahead at an increased speed, or at the same speed with reduced engine effort and lower fuel consumption. This is called drafting. Both cars can travel faster than either car can go by itself [source: Turner].
Drafting can be a very powerful racing technique, but it has a serious liability. The trailing car suffers a reduction of downforce on its front tires, resulting in a loss of stability and handling coming out of turns. This is aero push, also called a "tight" condition, requiring the trailing driver to ease off the accelerator to regain traction [source: ESPN].
Aero push forces drivers to make careful calculations. On the one hand, many drivers can remain competitive in a close race by piggybacking on the lead vehicle, taking advantage of the increased force and decreased engine strain. The effects are especially beneficial on straightaways. At the corners, though, the dangers come into play in reduced maneuverability and greater likelihood of losing control.
Aero push has become almost the dominant feature of NASCAR races. Fans have complained that racing has lost some of its appeal, as riders remain in fixed positions for long stretches at a time. Drivers challenge each other less, riding in tight single-file rather than side by side. On the positive side, a greater number of vehicles can stay near the leader of the pack.
Aero push -- and all of racing aerodynamics, for that matter -- is all about the downforce.