It has been said that drag races are won or lost at the line. While the massive horsepower of many modern engines can help to offset some losses at the line, the fact is that milliseconds lost at the start line can translate into tenths of a second lost through the traps. Good suspension setup is critical to a hard launch, and all good suspensions are built around the tires.
Torque can do one of two things: it can either launch your car or break the tires' mechanical and molecular adhesion to the ground. Theoretically, enough rubber on the road can turn any supercharged monster into a stop-light hero, but more often than not it just spins the tires. This is one area in which boost-controlled turbo engines have an advantage: the driver or mechanic can set the boost controller to progressively increase boost with vehicle ground speed. This takes a little punch out of the launch, but makes the car, as a whole, much more predictable and forgiving at the limit.
The tire's contact patch (the point at which it touches the ground) is roughly rectangular in shape. As you apply torque to the tire, the back of that rectangle "digs in" and pulls the tire to the ground. When combined with weight transfer on acceleration, this pulling makes the tire flatter on the bottom than the top. Tires with a very tall sidewall permit more of this traction-enhancing deformation. While such deformation is a boon for acceleration, it also allows the tire to deform sideways under cornering. This is why very low-profile tires do well on road courses but often spin on the drag strip.
If you watch a top fuel dragsters' tires in slow motion, you'll notice that the tire tread doesn't move precisely with the rims. When the engine applies torque to the wheels, the tires' soft and flexible sidewalls allow the rims to rotate a little bit before the tread does. This wrapping, or "wrinkling," does two things: it stores potential energy in the tires' sidewall (kind of like a stretched rubber band) and makes the tires smaller in diameter. The smaller diameter tires raise the car's effective gear ratio (transmission gear ratio times final drive multiplied by the difference in tire circumference) to a more acceleration-oriented profile, and the rubber-band effect helps to release more energy on the launch. This stretching-and-snapping back can put more of the engine's power to the ground at launch without tripping the staging lights.
Believe it or not, most drag slicks are (in effect) a sort of continuously variable transmission. While sitting still, the tires' outside circumference is fairly small but grows larger as centripetal force pushes outward on the tread. A top fuel drag slick can grow as much as 25 percent in size from the head of the track through the traps, which has a huge impact on the car's effective gear ratio. This fact alone is good reason to account for the tires' dynamic characteristics when setting up the car.
