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.
6. A Panhard/J-bar mounted on the left side of the chassis produces a more stable dynamic condition and more consistent handling due to the limited movement of the left rear corner of the car and so too the rear moment center.
Fact: While it is true that the rear moment center stays relatively at the same height through chassis dive and roll with a left-side chassis mount, that is not what you want.
As the car dives and rolls, the center of gravity of the car moves downward because, in most cases, the entire car is lower in the turns than at ride height. At the same time, with a right side chassis mounted bar, as the right rear (RR) corner of the car is moving lower, so is the bar and with it the rear moment center. With both the CG and the rear MC moving down, the rear moment arm stays relatively the same length and the dynamics remain consistent. With the bar mounted on the left side of the chassis, the opposite would be true. The moment arm becomes shorter as the CG moves down and the rear MC stays at the same height. This loosens the car in the middle of the turns-the exact reason why this design came about and was a useful crutch for a chassis that would not turn well.
A Simple Experiment
It is difficult to understand the importance of an imaginary point and how it could possibly be integral to the dynamics of the front suspension. We generally think in terms of hard points that you 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. 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 a AA arm suspension.
A two-dimensional model of a double A-arm suspension was built on a board, with spindles and upper and lower control arms. The "chassis" portion was weighted and supported by springs. A series of holes was drilled along the centerline of the chassis between the control arm mounts to simulate the CG of the "car." Arm angles could be changed to create different locations for the moment center.
For each configuration and location of 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, the suspension locked up and would not roll. As I proceeded below the MC, the chassis rolled to the left because the moment arm was now inverted.
I changed the MC location several times. Each time 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. 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. I finally had my proof-but the research did not end there.
The Industry Begins to Understand MC Racers, as well as 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, you 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 between the moment center and the resultant force line. This is called the "Effective Moment Arm." You can see how, with the effective moment arm pointed down and to the right from the center of gravity that as the moment center is located farther to the right, the effective moment center becomes shorter.
The MC has basically two locations-static and dynamic-that can be easily calculated. 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 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 race car.
Control Arm Angles vs. MC Location
Control arm angles are measured in degrees from horizontal. Therefore, level would be 0 degrees and straight up would be 90 degrees. The lower control arm angles largely control the amount of lateral movement of the front MC and the upper control arm angles mostly control the lateral location of the two MCs. Excessive lower control arm angles cause the front MC to move a greater amount as the car goes through the turns. Increasing or decreasing the upper control arm angles moves the MC side to side. Low angles in the upper control arms causes excessive camber change in the front wheels, especially in the right front wheel where it affects the handling the most.
Rules About MC Location
As you begin to design your car's front suspension, you need to determine where the MC should be located according to your type of racing and what kind of race track you will run. Here are a few general rules you need to follow:
1. The farther left the MC is located, the longer the effective moment arm and the more efficient the front suspension will be (meaning it will want to roll more). A more left location is proper for low-banked asphalt race tracks and dry slick dirt race tracks. 2. The farther right the MC is located, the less efficient the front suspension will be (meaning it will be stiffer). An MC that is located farther to the right will be good for all tracks banked beyond 10 degrees, including dirt and asphalt tracks. The higher the banking, the farther right the MC should be. Because of the higher amount of downforce caused by the banking, the front needs to be somewhat stiffer to resist excessive dive on entry and in the middle of the turns.
3. The amount of MC movement from static to dynamic locations that the car needs depends on how the race track is constructed. A track that has consistent banking throughout the turns requires an MC design where there is very little movement from the static to the dynamic location. Tracks that are constructed where the banking going in and coming off the corners is far less than the banking in the middle of the turns require an MC design that incorporates a greater amount of movement of the MC from static to dynamic locations. As the car turns into the corners, the MC can be designed to start out statically farther to the left and that is more suited to the lower banking. As the car moves through the middle where the banking has increased, the MC should move farther to the right, and that location is better suited to the higher amount of downforce caused by the high banking.
4. The lateral location of the MC range depends on the height of the center of gravity of the race car as well as the track width of the car. A modified stock car with its low CG can tolerate an MC that is farther left than a stock class car where the ride height is above 5 inches and the CG is above 18 inches. The narrower the car, the more effect there is on lateral location of the MC. A narrow track width (distance from center to center of the tires) results in a greater tendency for the front end to want to roll. Narrow stock cars should be designed with an MC that is farther to the right than wider cars.
5. The circle track stock car wants zero camber change-relative to the racing surface-in the right front wheel after the car dives and rolls through the turns. The right upper control arm angle mostly affects the right front (RF) camber change characteristics. When designing the MC location, you need to find the correct right upper control arm angle that will result in zero camber change after the car dives and rolls in the turns, and then keep that angle. The left front wheel will always lose +1/4- two degrees of camber regardless of the upper control arm angle, so changes to the left upper control arm angle can be used to locate the MC range laterally.
6. The entire dynamic MC range for dirt and asphalt stock cars lies between 10 inches to the left of the centerline of the car and 20 inches to the right of the centerline of the car. Moment centers that lie outside that range will be detrimental to the performance of the car. Shallow upper and excessive lower control arm angles (inside points lower than the ball joints) contribute to MC locations that are outside of the optimum range.
The Rear Moment Center
The rear moment center design is fairly simple. Mount a Panhard or J-bar lateral locating device to the rear axle housing and to the chassis, and adjust the height of the ends to raise or lower the rear MC. That's all there is to how the rear MC relates to the roll dynamics of the rear suspension system. As previously stated, the lateral locating device should almost always be mounted to the right side of the chassis. There are exceptions to this rule.
Left Side Bar Mounting
One example where it might be an advantage to mount the bar to the left side of the chassis involves dirt track stock cars, where it is desirable to angle the bar to point it at the RR tire contact patch. This exerts a lot of force and most of the rear weight on the RR tire during cornering. This is sometimes needed for dry slick conditions in order to cut through the dry material so the tire can grip the track surface.
The RR tire will be the dominant tire driving the car off the corners. Mounting the bar on the left side of the chassis can cause both a dirt car and an asphalt car to be inconsistent if there is a lot of traction in the racing surface. This is always true with asphalt stock cars and asphalt race tracks and sometimes true with dirt cars running on a racing surface that has a lot of grip. It would be ideal for dirt cars to be able to quickly switch the mount from side to side in order to be able to adjust to changing track conditions. Most tracks start out the day moist with lots of traction and then either go black slick or dry slick. The left side mount would usually only be necessary for the dry slick condition.
Bar Angle Influence
Angle in the Panhard/J-bar will cause a certain amount of weight jacking as the car rounds the turns, and this can be mistaken for changes in MC location. The rear MC height is always the average height of the two ends of the locating device. During cornering, the bar will always want to straighten out and be parallel to the track surface if it is mounted to the chassis on the right side. If you angle the bar where one side is at a different height than the other, weight will be redistributed to some degree by this effect.
By mounting the bar at an excessive angle for left side chassis mounting, extreme differences in weight distribution can be achieved, if that is your desire. As the car corners, the left end (higher end) of the bar will try to ride up and over the right (and lower) end, and this creates a serious jacking effect. Weight is transferred off the springs and redistributed onto the right rear tire. The left rear tire retains little weight. Most of the weight of the rear of the car is now on the RR tire.
At this point in the setup, moment center dynamics are of little or no use. The redistribution of weight that goes with the angled/left chassis-mounted bar negates any positive effects you could have achieved by dynamically balancing the chassis using moment center location as one of the tools to accomplish that.
For all asphalt applications and most dirt racing situations, a balanced setup using optimum MC locations makes the car faster and much more consistent. Around 90 percent of handling problems associated with stock cars are where the car is tight and will not turn well. The front MC location, as well as the balance of the setup you choose, are major contributors to this condition.
Differences in Handling Caused By MC Location
The primary reason two seemingly identical cars will handle differently can usually be traced to a front MC in a different location from car to car. It doesn't take much of a difference in arm lengths or chassis mounting point heights to cause a pair of cars to experience different handling characteristics. Just a few years ago, many professional stock car teams experimented with different upper control arm lengths to tune the handling while holding to preselected spring packages and weight distribution numbers. They were essentially moving the MC left and right in a trial and error way to try to find the ultimate handling balance for a particular race track.
Today, we know better than to waste time doing that. There is a combination of spring rates, moment centers, and weight distribution for each car at each race track that will make the car balanced and consistent. Knowing the role of the MCs and being willing to make changes so that the MCs are in the right position is one of the most important steps to take to achieve the total handling package.