Calling a "sway bar" by any one of its proper names will give you some idea of its function. Also known as anti-sway, anti-roll and stabilizer bars, these important components supplement your car's main suspension to keep everything where the factory put it. Left unchecked, a non-stabilized suspension will try almost as hard to flip your car over, as it will to keep the tires planted.
There are three basic types of springs: leaf, torsion and coil. Leaf springs attach at either end to a car's chassis, and resist movement by bending. Torsion springs are long bars that work by twisting, and coil springs are essentially re-shaped torsion bars that work by both bending and twisting. Most cars use coil springs that attach at either end of the axle and connect to the car's chassis near its corners. A sway bar is essentially another type of spring, a torsion bar that spans from one axle hub to the other beneath the car. The middle of the sway bar runs just under the frame; a set of keyed brackets keep it secured tightly to the frame.
Coil springs act as the chassis's primary means of suspension, working as a cushion to allow the wheels to move up and down in their respective wheel wells. When a car leans into a turn, it compresses the springs on the outside of the turn -- the right side, if you're turning left. But all of the car's springs maintain some kind of pre-load, meaning that they'll provide some upward push if allowed to extend. So, as the car's body leans to the outside, the inside springs provide a bit more push, tilting the car's body even more. Since traction depends completely on load transfer, the outer wheels end up doing all the work while the inner ones are just along for the ride.
While the coil spring's main job is to control body movement relative to the wheels, the sway bar's job is to control the movement of the wheels relative to each other. Think of the sway bar as a second spring deprived of pre-load, one that will always try to keep the wheels where they started out instead of just pushing them downward. When a wheel goes up into the chassis, the sway bar tries to push the chassis back up and off the wheel to transfer weight back to the other tire. A sway bar's job is to balance out the coil spring, allowing the suspension to stay level to the body instead of just pushing the wheels down toward the ground.
Like any other aspect of a suspension, you can have too much of a good thing as far as sway bars are concerned. Since sway bars always try to keep the wheels from moving relative to each other, a thick sway bar will force an independent suspension to act like a solid axle. A solid axle does exactly the opposite of what you want, lifting up the tire on the inside of a turn instead of allowing it to drop down and provide traction.
The coil springs are your suspension's first line of defense where body roll is concerned; sway bars are just supplementary devices used to fine-tune a car's weight transfer and handling balance. Slightly thicker front bars will enhance steering response, but too-thick bars will quickly transition from snappy steering to deadly understeer with little to no warning. The same holds true for rear bars; a good set of bars will enhance rear-end stability during quick left-right-left transitions, while a too-thick bar will make your car prone to dangerous snap oversteer. Snap oversteer is what happens when a car seems to have plenty of grip while turning, only to break away like glass and spin with little to no warning.
