By using this type of sway...
By using this type of sway bar rating fixture, we learned that some sway bar arms flex excessively, thereby reducing the rate of the bar. If the arm flexes 11/44 inch for each inch of movement in the rater, the rate indicated will be about 31/44 the amount that it would be with an arm that did not flex. The lighter-weight aluminum arms do flex quite a bit, whereas a steel arm flexes very little.
The rate of the sway bar is affected by the construction of the arm and how much that arm will bend when under a load. Aluminum arms will bend quite a bit, especially with larger-diameter bars. This affects the net rate of the bar as felt by the car. Although steel arms are heavier, they are much better to use with a large sway bar than aluminum when bending is a problem.
The resistance to twisting in a bar is the greatest at the outer portion if we view the bar in a cross section. The middle of the bar offers very little resistance, just as a small bar is easy to twist. If we take a 111/44-inch bar and gun-drill out the middle, we will have a much lighter bar with almost as much resistance to twisting as the original solid bar. If we drill out a 31/44-inch hole, the thickness of the wall of the sway bar is now 11/44 inch. That outer 11/44 inch of the bar has much more resistance than the inner 31/44 inch.
Using an example of a bar with 32 inches of effective length with 12-inch arms, a standard metal 111/44-inch sway bar has a rate of 597 pounds per inch of arm movement. In contrast, a 31/44-inch sway bar has a rate, using the same length and arms, of only 77 pounds. The rate of sway bars increases in an algebraic fashion. While a 1-inch bar is only rated at 244 pounds per inch, a 2-inch bar is rated at an incredible 3,911 pounds per inch.
Here are three different types...
Here are three different types of sway bar arms that attach to the straight NASCAR-style sway bars. The front two are made of aluminum and will bend under a heavy load such as when used with a large-diameter sway bar. Note the black steel insert in the front arm for attaching to the sway bar. This reduces the chances of the arm stripping its spline grooves, as can happen with aluminum. The one attached to the sway bar is made of steel and flexes very little under a load. It is recommended when using a 111/42-inch or larger sway bar.
The length of the sway bar arms does affect the rate of the bar. A shorter arm results in a stiffer overall rate at the end of the arm than a longer arm. For example, a 10-inch arm on a 1-inch sway bar yields a rate of 293 pounds per inch. If we increase the length of the arm to 12 inches, we see a reduced rate of 244 pounds per inch.
How to Mount Sway Bars
The sway bar should be free to twist with no binding. The attachment where the sway bar is connected to the chassis should be free of any restrictions so that bar can twist freely as the chassis rolls. The NASCAR-type sway bar is mounted in a tube with bearings at each end so the bar is free to rotate without restriction.
The arms should be clamped tight to the bar if using a straight-style sway bar. Where the bar is attached to the lower control arm, the connection should be positioned at right angles to the sway bar arm as well as the lower control arm. Any angle in the link can greatly increase the "felt" rate of the bar.
Sway Bar Preload
One benefit of using a sway bar is that it helps us get more bite off the corners. Sway bar preload, actually pre-twisting the bar, increases this bite effect. It also causes an increase in the crossweight percentage. It is best to put in preload and then set the correct crossweight in the car. That is opposite the way teams have usually done their sway bar preload, but we need to know exactly how much crossweight is in the car when we race it. If we set the crossweight and then preload the bar, we don't know where our crossweight is.
When attaching a sway bar...
When attaching a sway bar arm to the lower control arm, always make sure the link is at right angles to the arm and the lower control arm. This relationship is important as viewed from both the front and the side. If we have angle in the link, a bind will occur that will tend to increase the rate felt by the suspension.
One of the drawbacks of using a large-diameter sway bar is that we cannot preload the bar. If we try, the whole chassis will rotate, or roll left, and the ride heights will go crazy. I have watched a team fight forever to get the proper ride heights while preloading the large bar they had installed. The bottom line is that we need no preload on a large-diameter bar. The larger size provides the effect without using preload.
How Sway Bars Affect Our Setups
In past articles on chassis setup, we have stressed that our two suspension systems must be balanced and working together. We design our suspensions for spring rate and moment center location so that both ends of the car are working in harmony. The end result of a balanced chassis is that each end of the car tends to roll to the very same angle in the turns. Since the sway bar adds resistance to chassis roll, it has an effect on the balance of the car.