Before getting started, the point of most suspension mods are to increase grip. The only connection between the road and the vehicle are the tires. Each tire has a footprint, and it's a lot smaller than most people think.
This is a photo of a tire sitting on a glass plate in water with green dye.
The tire is on a car, so the footprint appears at it normally would. As you can see, the footprint is rather small.
This photo of a tire footprint uses color to show the weight distribution. If the vehicle is traveling straight, all four tires will have similar profiles. When turning, the colors will change position and each tire will have different amounts of pressure applied to it.
The colors can be thought of as the amount of work the tire is doing and where most of the work is being done. Using this photo, we can see that in addition to the footprint being small, much of the work is performed in a subset of the footprint.
Overall grip is increased when the work is more evenly distributed across all four tires. In a turn, the outer tires always do more work. The job of a sway bar is to help control body roll, which redistributes work from the outer tires, to the inner tires.
A sway bar is attached to the left and right sides of the suspension and chassis. There is typically one bar in the front, and one in the back. It controls body roll by resisting twist. It does this similarly to how a torsion bar works. The more the body wants to lean in a corner, the more resistance the bar provides.
From side to side, sway bars effectively increase the spring rates on whatever spring is being compressed the most. If both springs on a single axle are being compressed, such as when driving over a speed bump, the sway bar has no effect. In a turn, the springs on the outward side of the turn compress more than the springs on the inside. Effectively increasing the rate of those outer springs help flatten out the car. The flatter car has more weight on the inner tires. This increases their grip, and spreads the overall work. One disadvantage to a sway bar is when a single tire hits a bump, the shock from the bump is increased.
There was a time when many people felt a car only needed a front sway bar. Bars are typically used at both ends because neutral balance is more easily dialed in that way. Those who forego a rear bar try to compensate by using high spring rates. The problem with high spring rates is the car will be very stiff, may have less traction, and may rattle. Adding a rear bar allows lower spring rates. This will allow the springs to do what they do best, leaving body roll control to the sway bar, which can do a better job.
Some people just look at the sway bars and say
That's a little overly simplistic. The same problems can occur from spring and/or tire problems. That, and simply swapping bigger or smaller bars in can introduce other problems. Personally, I think it's best to find someone whose car handles well and ask them what they use, and why. I have a friend who bought and installed four sets of springs before finding some he could live with. For him, letting someone else do the leg work would have been a much easier option.
This is where I think sway bar tuning truly comes into play - Assuming you have a rear wheel drive car (front wheel drive is different, trust me), and it handles well, but whenever you're accelerating out of a corner, the rear inside tire loses traction and spins (see, I told you FWD would be different). You're being passed on the straight track because the guy behind you left the turn 5 MPH faster - even though you handle better and have more power.
The reason the inside rear looses traction is because the rear bar is too big. As the outside rear compresses, the bar makes the inside rear compress. It does this because the bar links both sides of the car. Because they're linked, the inside rear compresses like the outside rear. However, the body is leaning somewhat, so the amount weight at the inside corner is reduced. This causes a loss of traction. With no traction, guess who can't accelerate?
IF you go with a smaller rear bar, the inside rear wont compress as much, giving you more traction.
But wait, there's more. Suppose decreasing the size of the rear bar doesn't do the trick. In that case, you should go with a bigger front bar. A bigger front bar causes the inside front to compress when the outside front does. In some cases, the inside front will compress enough to lift the tire off the ground. Now that the car is sitting on three tires, guess where the weight goes? Some of it goes to the outside front, but much of it goes to the inside rear, right where the traction is needed.
© 2011 Marcus Blair Fitzhugh