Roll Center Discussion - Moment Center Technology

At the beginning of every article, I try to lay out the reasoning behind the technical piece and entice the reader to continue to read because most of all, there may be one thing on those pages, or hopefully more, that will strike a chord and cause the person to think. It's when we mull over the performance possibilities of our race cars that we are enjoying the sport the most for most racers.

It's no different for the subject at hand. When I first discovered the "power" of this technology, it was a revelation. Defining how something works and how it affects other parts of the puzzle is a very rewarding endeavor. I wanted to get the message out right away. Now, some 13 years later, I still feel the necessity of explaining the value of roll centers and the name I coined for that geometric point which better defines it, moment center.

The reason I come back to delve into this subject more deeply every two to three years is because I'm constantly taking calls from racers of every division about their MC. They want to know, "Where should it be located for my type of race car?," "How do I measure it?," "How do I change the location?," and inevitably how refining this point's design made the race car a whole new and wonderful piece of machinery that is now competing for wins.

Still, like happened a few days ago, a racer will email me with something like this (actual wording): I'm having trouble with our race team especially the owner believing in roll centers..., and no one wants to help me. I had to work four 12 hr shifts this week only to show up at the race shop with none of the measurements done that we need... I can do most the measurements by myself but...

After all I've written about this important subject, I still get reports like this. Oh well. For those of you who care about performance and want your car to be the best it can be, please read on.

Definition of the moment center The moment center, and there are two of them in a race car, is the bottom of the moment arm for a suspension system. The moment arm is like a leverage bar that creates the rolling force in a double A-arm suspension. The top of the moment arm is the center of gravity of the sprung mass (the weight of the front of the car minus the unsprung components such as the wheels, tires, brakes, rearend, and more) and the bottom is the moment center.

Myths About MCs Here are some popular myths concerning the moment centers that have now been disproved:

1. The front moment center is at the center of the car.

Wrong. The front moment center is rarely at the centerline of the race car. It also moves laterally as the car dives and rolls, some designs moving to the left and some to the right.

2. The front moment center location is not important to the dynamics of the chassis.

Wrong. The front moment center location is critical to the dynamics as we will show. Because it's the bottom of the moment arm, its location dictates the length of the moment arm and therefore the amount of force that will initiate roll in the chassis.

3. When we change front control arm angles, we're really affecting the camber change characteristics and/or the jacking forces on the instant centers and the changes to the handling of the car are related to that, not the new moment center location.

Wrong. We can move the moment center with little effect on the camber change characteristics and/or the "jacking effects" and see a drastic change in the handling. This serves to disprove those notions.

4. I can draw out my moment center on the garage floor or on paper to find its location.

Wrong. The reason why the static location, the one you draw out, is not really important is because the MC moves as the chassis dives and rolls going into and through the turns. Where it ends up is the most important aspect of MC design because it affects the turn dynamics where we desire our performance. It's very difficult to draw this dynamic location without a seriously complicated drafting software program.

Explanation of moment center Here's an explanation of how the front MC location is derived and what affects its location. The sketches are showing both the static and dynamic locations. Moment center geometry software will help you determine the dynamic location for your MC.

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. 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 its 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's 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 isn't directly connected to the chassis and has no bolt through it. So, some years ago while I was trying to understand how the moment center really worked, I decided to build a model to find out exactly what influence the MC had on an AA 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 moment center.

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 wouldn't 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 MCs Over the past 10 years, hundreds of racers, as well as numerous race car 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.