On each side of the car we have upper and lower control arms, in a double A-arm suspension system, and those have pivot points at each end being a ball joint at the spindle and bushings or heim joints at the chassis mounts. If we draw a line through the upper control arm pivot points and again through the lower control arm pivots, these two lines will intersect at a point called an instant center (IC). For most stock cars, these IC intersections lie to the chassis side of the spindle. Each set of control arms on each side of the car has their own IC. If we also draw a line from each IC to the corresponding center of contact patch of the tire on the same side as the control arms we used to create the IC, then the intersection of the two lines from the left and right ICs defines the moment center location.

A Simple Experiment
It is very difficult for us to understand the importance of an invisible point and how it could possibly be important to the dynamics of the front suspension. We generally think in terms of hard points that we can put a bolt through and that are attached to the chassis. The MC is not directly connected to the chassis and has no bolt through it. So, some years ago while I was trying to understand how the MC really worked, I decided to build a model to find out exactly what influence the MC had on a double A-arm suspension.

I built a 2-D model of a double A-arm suspension on a board, with spindles, upper and lower control arms, and the "chassis" portion was weighted and supported by springs. I drilled a series of holes vertically along the centerline of the chassis between the control arm mounts to simulate several locations of the CG of the "car." I also had the ability to change the arm angles so that I could create different locations for the MC.

I began at the top CG hole and attached a string and pulled laterally to the right on the string. The "chassis" would roll to the right each time as I moved down from hole to hole. When I reached the hole that represented the MC, my suspension would lock up and would not roll. As I proceeded down below the MC, the "chassis" would then roll to the left because the moment arm was now inverted.

I changed the MC location several times, and each time when I put the CG nail in the hole that represented the MC, the suspension locked up. That told me that the MC was indeed the bottom of the moment arm and when the MC is in the same location as the center of gravity, there is no moment arm and therefore no lever arm to roll the chassis.

The Industry Begins To Understand Moment Centers
Over the past few years, hundreds of racers, as well as numerous racecar builders around the country have experimented with MC location and design and found the correlation between the MC location and front end dynamics to be significant. Front suspension efficiency, as well as camber change characteristics, all depend on the MC height and width. To understand how the lateral MC location affects the dynamics of the car, we need to understand how the forces are applied to the center of gravity and the MC.

Forces And Moment Arms
The true moment arm that tries to roll the suspension is represented as a line lying at a right angle between the MC and the resultant force line. This is called the effective moment arm. We can see how, with the resultant force pointed down and to the right from the CG that as the MC is located farther to the right, the effective moment arm becomes shorter.

The MC has basically two locations that we can easily calculate, static and dynamic. Static is where the MC is located when the car is at ride height and standing still. The dynamic location is where the MC moves to as the car dives and rolls through the turns. Because the control arms move as the car rolls and dives, the instant centers also move. This causes the MC to move along with the instant centers and in most cases, the MC moves to the right in a left turning circle track racecar.