When a car goes around a corner, its weight shifts from side to side. Anyone who's ridden in a car has felt that effect, when your body is pulled in the opposite direction of the turn. It's such a common experience that most people wouldn't notice it in an average car ride -- not unless their driver is going a little too fast.
As stock cars race around oval tracks at high speeds, they deal with an extreme version of this effect. This body roll affects tire grip and performance. How well a tire grips the track depends on the weight pushing the tire into the track. When weight shifts off a tire during a turn, it has less grip and therefore less speed. And anytime parts of the tire leave the track, the driver has less control over the vehicle.
A sway bar helps prevent weight transfer. It connects the suspension of the car, the chassis and the body. Because each racetrack requires a different sway bar with varying stiffness, prepared teams often keep many sway bars in stock [source: Burt].
The stiffer the sway bar, the tighter the connection between the suspension and chassis, which minimizes body roll during a turn. But it's important to remember that sway bars that are too loose or too stiff could also present problems.
Sway bars twist when one of the tires goes over a bump. If both tires go over the bump evenly, the sway bar does nothing, but if one tire goes over a bump the sway bar counteracts the movement to keep the weight of the car from shifting [source: Diandra].
The chassis plays an important role in stock car suspension, too. Read on to learn more.