This is an excerpt from Mike Dicely’s
“Rethink Dirt” article named “Advanced Dirt Track Theory”
Roll CentersÂ
The roll center of your chassis is the pivot point around which your chassis rolls. The roll center is controlled by the lateral linkage; this linkage controls the location of the axles under the chassis in the side to side or lateral direction. The amount of chassis roll is a function of the distance between the roll center axis height and the center of gravity height. The roll center axis is an imaginary line drawn from the front RC (roll center) to the rear RC. The longer this measurement, the more roll, shorter measurement or higher RC the less weight roll of the chassis.
Although roll centers play an important part in how your car handles, it does not control how much total weight transfers, only where and how it transfers. We can control if the weight transfers to the front or to the rear through the difference in RC heights between the front and the rear. We can control if it transfers through the springs or the lateral linkage through controlling the height of the roll centers. As we raise the RC more of the weight is transferred through the linkage, as we lower the RC the weight is transferred through the springs. If we made the RC axis higher than the center of gravity of the car, the car would actually roll to the inside of the turn, like a boat, but the same amount of weight would be transferred to the right side of the car.
The two ways we control roll centers on our dirt chassis is through the panhard bar or a Jacob’s ladder. The Jacob’s ladder’s RC can be found by intersecting the center lines of the two straps. When you change the holes where the straps mount, it changes the RC height and/or the RC side to side location. The panhard bar’s RC is located in the middle of the panhard bar. One major difference between the two designs, the Jacob’s ladder’s RC goes up when the car rolls right, the panhard bar’s RC moves down when the car rolls right.
Effects of roll center height: A higher RC resists roll, we learned in Rethink Dirt that resisting roll actually increases weight transfer; increasing weight transfer on the rear will make the car looser. Lowering the rear RC will prevent weight from transferring making the car tighter. To the masses, again, just like soft vs. stiff right rear spring rate, they had the affect of the adjustment correct, but the reasoning completely backwards.
Effects of roll center side to side location: As we move the RC to the right (depending on the exact layout and design of the chassis) generally we will make the distance between the RC axis and the CGH longer. If we move the rear RC to the right, it will make the car tighter because it will resist the roll less.
Bottom line: Lower your rear roll center to make the car tighter. Lower your front RC to make the car looser. Now all this explanation of course if for lateral traction considerations only (or side bite, I hate that word) For longitudinal traction (forward bite, I still like that word) the higher RC or a movement of the RC to the right, will keep the car sitting higher (to a point) which will increase forward bite.
Black Hills Race Talk 