subject: The Sway Bar Knowledge [print this page] The sway bar isn't really a sway bar-at least not in a race car. It doesn't control sway like it might have on your grandmother's old Essex.
The first thing to understand about a sway bar is that it is actually a torsion bar laid across the car. It connects the suspensions on each side so that when one side acts, the other side reacts. It can be found at either end of the car, although the sway bar is usually seen only on the front of short-track stock cars.
While the sway bar doesn't have anything to do with sway, it is one of several items that provide some control over weight shift when the car tilts to the right in a left turn.
The function of a sway bar is usually to prevent weight transfer from one side to the other in cornering situations. By preventing weight transfer, the inside wheel in a corner maintains more of its static weight.
This very attribute of the sway bar often limits its use to paved track racing. On dirt tracks, the stock cars usually need to transfer weight to the right front to load the right-front tire enough to make it grip the loose surface. Therefore, a sway bar would detract from the handling.
A sway bar is often used on the front of a paved track race car. In this situation, the available traction favors both left and right tires being equally loaded. The sway bar provides an automatic adjustment of weight.
As the chassis rolls to the right, the sway bar tries to lift the left wheel, thus somewhat equalizing the weight between the two.
Here is what happens. The sway bar connects the suspension on both sides. This is not a rigid connection. The sway bar floats in bushings mounted to the chassis. Then, each end is connected to a suspension member, usually a lower control arm. The control arm connection is accomplished with links that allow the sway bar to adapt to the changing angles as the suspension moves.
Now, in a left turn, the chassis rolls (or leans) to the right. The right wheel goes up in relation to the chassis. As the right wheel goes up, it tries to lift the left wheel through the action of the sway bar. Therefore, the weight that the sway bar lifts on the left wheel is weight that doesn't transfer to the right side. This aids in controlling the weight shift during cornering.
With a sway bar able to control the weight shift, it would be easy to come to the conclusion
that we could make a rigid bar and bolt it to the lower control arms and stop all the weight shift. That won't work. The bar could never be totally rigid, and neither could the chassis. Furthermore, we don't want to stop the entire weight shift, but we want to control it. Control in this case means allowing only the weight shift that balances the right side tire traction against the left.
A certain amount of weight must be transferred to the right front to give it the traction to get the car into a turn. It is this balance that lets the left side contribute to the turning force, too. This balance often is a critical factor in handling.
Notice that the ends of the...read full caption
A difference of only a few thousandths of an inch in the diameter can make a significant difference in its spring rate. Remember, the sway bar is a torsion bar, which is a spring. As such, it will have a spring rate. The working length of the bar and the diameter are the factors that determine its spring rate.
The next question: How much force do we want? The simple answer is . . . the right amount! Unfortunately, this answer is not found in a book in exact numbers. There are a number of ways to look for the answer. Driver feedback, observation of the car during cornering, the careful use of a tire temp gauge, and learning to use wheel scales are some of the ways. Through all this you must remember that a fast race car is the best combination of all the factors in the car. The sway bar is just one of these tools.
