During the last five years, the growth of information about the front and rear roll centers has helped us to understand how these unseen points control the dynamics of our race cars. When we first published information about the front roll center in the May 1998 issue of Circle Track, much of the industry did not understand exactly how the RC worked and most thought it was not important at all. Since that time, we have defined exactly what the RC does and where it should be located. We have also coined a new term for this point-"moment center"-that better defines its role. Because the "roll" center is not the point around which the chassis rolls, we should stop referring to it that way.
What is the moment center The moment center (MC) in both the front and rear suspensions is the bottom of the moment arm for each system. The moment arm is like a pry bar that controls the rolling forces for each end of the car. Its top is the center of gravity (CG) of the sprung mass (the whole weight of the car minus the unsprung components such as the wheels, tires, brakes, rearend, and so on) and the bottom is the moment center. That is the simple explanation, but the true definition is a little more involved.
Explanation of moment center There are upper and lower control arms on each side of the car (in a double A-arm suspension system) and those have pivot points at each end, a ball joint at the spindle, and bushings or Heim joints at the chassis mounts. If you draw a line through the upper control arm pivot points and again through the lower control arm pivots, these lines will intersect at a point called an instant center (IC). Each set of control arms on each side of the car has its own IC. If you also draw a line from the IC to the corresponding center of contact patch of the tire on the same side as the control arms that were used to create the IC, the intersection of the two lines from the left and right ICs creates the moment center.
Myths About MC Here are some popular myths concerning the moment centers that have now been disproved.
1. The front roll/moment center is at the center of the car.
Fact: The front moment center is rarely at the centerline of the race car.
2. The front moment center location is not important to the dynamics of the chassis.
Fact: The front moment center dynamic location is critical to the dynamics, as we will show.
3. When changing front control-arm angles, you are really affecting the camber change characteristics-and the changes to the handling of the car are related to that, not the new moment center location.
Fact: You can move the moment center without affecting the camber change characteristics of the car and see a drastic change in the handling and how the front end works.
4. The rear moment center is located where a line through the ends of the Panhard/J-bar intersects the centerline of the car.
Fact: The rear moment center height is the average height of the two ends of the locating device. It is "felt" halfway between the tops of the two rear springs. This definition can be found in older automotive dynamics books dealing with the subject of the straight axle suspension and has since proven to be accurate. The angle of the bar and its lateral location has an effect that can redistribute the weight on the four tires and that effect is different than the dynamic affect that the rear moment center has related to chassis roll.
5. The rear moment center moves laterally as the car dives and rolls.
Fact: The lateral location felt by the chassis remains midway between the top of the spring mounts. The rear moment center height is affected by the vertical movement of the ends of the locating device (being the Panhard or J-bar) due to its height being determined by the heights of the ends of the bar. As the ends move vertically, so too does the height of the moment center.