Sway Bar Rating Techniques - Chassis

It is important to know your sway bar rates in order to relate that to the setup you desire. There are problems associated with looking up the rate in a table or other publications because of several factors that affect the working rate of the bar. We learned of a method that takes into account all of the variables to provide the true working rate of any sway bar.

This all started when, in my spare time, I was helping a friend who owns and manages a Grand Am DP team. The sway bars on those cars can be quite complicated to calculate the rate for because of some unknown factors such as the exact hardness of the metal used, the effectiveness of the blades on the ends of the bar, motion ratios that are non-linear (change with motion), and more.

I did some research and came across a method described by James Hakewill in a July 4, 2006, publication on his website. He is a driver who races formula cars. I convinced the team to go through the process of measuring the rates of all of the combinations of sway bars. We modified the method to suit our car and were able to identify several problems with the current bars we had been using in combination with the blades.

I'd made a significant discovery here, and I immediately related this experience to stock cars. We have the very same situation with not really knowing our true bar working rate. So, I took the method over to a local race shop and proceeded to rate their sway bars on a typical Late Model Asphalt Touring car. What follows is a detailed account of how I did that and, of less importance, the results.

What Affects The Rate of The Sway Bar? There are several things that affect the rate of a sway bar, or more accurately said, the spring rate at the end of the arms related to how that affects the overall wheel rate. The sway bar resists chassis roll and with some of the more popular setups, minimizing and/or eliminating chassis roll is a desired effect helped along by using large-diameter sway bars. Here is a list of what can affect the rate:

1. The length of the portion of the bar that will twist with chassis roll affects its rate.

2. The outer diameter of the bar affects its rate. The larger, the stiffer it will be.

3. Hollow bars are less stiff than solid bars and the wall thickness affects the rate.

4. The arm length affects the rate, the longer the arm, the softer the bar rate.

5. The material the arm is made of ultimately affects the rate if it istoo soft and bends.

6 The material the bar is made of, or the hardness of the steel, affects the rate. There is a modulus of elasticity for each type of steel that depends on the mixture of metals and the hardening process used in the manufacturing process.

7. The installation ratio affects how the bar rate is translated to the wheel rate.

These are the seven variables that affect the amount of work the bar does and there could be more in some applications. What this means for us is that trying to calculate the rate of the bar and how much work it does is very subjective at best.

The method we used here necessarily takes into account all of the variables and any we did not mention. It is fairly simple to do and is very accurate. Here is how it works.

Overall Principle of the Method What we are going to do is cause the sway bar assembly to support the weight of one corner of the car. In our Grand Am car, we had sway bars in the front and rear, so we had to go through this process for both ends of the car. On a stock car, we usually only have one bar up front. We should run through this method for each bar we will possibly use.

We chose to use the Right Front corner to rate the bar. Sitting at ride height, the RF wheel supports X amount of sprung weight of the car. If we can cause the sway bar to act as the spring holding up that corner, then there will be a deflection of the wheel and we can measure the amount of deflection and convert that to a spring rate in pounds per inch, much like the normal spring rate at the wheel.