Note the difference in each...
Note the difference in each car's left front wheel. Photo by Jeff Huneycutt
Well it has finally happened. The stock car racing world has embraced the importance of front moment center location and design. Everywhere I turn racers, car builders, engineers, and racing schools are talking about moment centers (also referred to as roll center) and where it should be located. This is fantastic news because it means we can now talk the same language.
The growth of information about the front moment center during the last 10 years has helped us to understand the way in which this invisible point controls the front dynamics of our racecar. We first published information about the front moment center in the May 1998 issue of CIRCLE TRACK. At that time, much of the industry did not understand exactly how the moment center (MC) worked and many thought that it was just not important.
The front moment center location...
The front moment center location is the result using the intersections of lines extended from the centers of rotation of the control arms to form the instant centers. The MC is at the intersection of lines drawn from the left and right ICs to the centers of the contact patches of the tires.
Since then, we have defined exactly what the MC does and where it should be located based on research and input from racers across the globe. We have also coined the word for that point that better defines its role in the cars dynamics-moment center. There are several roll centers in our racecars which includes the kinematic roll center, which is based on the motion of the chassis, and the dynamic roll center which is the bottom of the moment arm in every double A-arm suspension. Because the moment center is seldom the point about which the chassis rolls, we should stop referring to it that way.
Definition Of The Moment Center The MC in the front suspension is the bottom of the moment arm for that 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 (CG) of the sprung mass (the weight of the front of the car minus the unsprung components such as the wheels, tires, brakes, rearend, etc.) and the bottom is the MC.
Myths About Moment Centers
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 racecar. It also moves laterally as the car dives and rolls, some designs moving to the left and some to the right.
When considering the angle...
When considering the angle of the control arms, never rely on the angle of the tubing that connects the ball joint and the chassis mount. The center of rotation of the ball joint is almost never directly in line with the center of the tubing. If we lay an angle finder on the tubing, the angle we read will not represent the true "arm angle" which is the angle of the line between the centers of rotation.
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 is 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 are 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 MC while affecting the camber change characteristics and the "jacking effects" very little 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 is very difficult to draw this dynamic location without a seriously complicated drafting software program.
Explanation Of Moment Center
Here is 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.