The last laps of a race are often the best. When the checkered flag is waved partnerships are dissolved, heated competition ratchets up to the nuclear level and drafting becomes less of a strategy and more of a weapon.
The Archer brothers, Tommy and Bobby, made their mark in the Sports Car Club of America (SCCA) when they used a technique called bump drafting, also referred to as impulse momentum in engineering circles, to steal the lead from the competition. The technique involves the trailing car driving up and actually hitting the lead car's bumper to shoot it ahead while pulling the trailing car behind it.
Dr. Jerre Hill said he was skeptical about the process, and that the math and physics didn't quite mesh with the reality. But whether it works or not, the technique is impressive -- and dangerous. The trailing driver needs to hit the lead car in precisely the right spot and at precisely the right angle. Failure to do so can lead to disaster. Hill said as the trailing car comes closer to the lead car the air stream under the car, the downforce, is disrupted. As this happens the nose of the lead car begins to raise a little adding an upforce to the equation. The lead car's tires have less contact with the track and can often slip entirely even before the bump is made. Drivers will often take advantage of this in the last laps by getting close to a lead competitor in exactly the wrong spot and unsticking their tires by manipulating their downforce stream. They may also add a small bump and just that fast, a potential first place finisher moves to the back of the field.
While the bump draft often affects the lead car, the trailing car is taking some risks as well. NASCAR race cars, in some ways, are sensitive machines. The same draft that pulls them along can also rob them of the air they need to cool their superheated engines. Several races have been won by a savvy lead driver that allowed a trailing car to creep up from behind, stay there just a little too long, and leave the field with a heat-blown engine.
The slingshot is a classic move -- a crowd-pleaser, the gold standard of drafting strategy. Simply put, the last laps of a race are coming and a trailing driver needs to steal the lead. The trailing car drafts the lead car, uses the pressure reduction to ease its movement through the air, waits for the right curve, mashes the gas and uses the extra power to slingshot ahead for the race victory.
If you're interested in a few drafting success stories, take a look at the next page.