In Part One of this series on racecar dynamics we introduced a new way of looking at the vehicle's suspension systems. We learned that it is important to match the desires of both ends of the car in order to have a truly balanced setup. This balance is what we have always desired when applying trial-and-error methods to tune our setups.

It is important to note here that the methodology, although fairly new in concept, has been applied successfully to stock cars for over 12 years now. Cars representing every class of stock car have had this method applied to them with predicted results. In other words, when the cars were balanced, they became fast and consistent, which showed in the tire temperatures being even front to rear at each side. The lap times became very consistent and the cars handled very well.

Much has been written about how the systems in stock cars work. There are many theories out there, some valid, some not so valid. The great equalizer and final judge is competition. It doesn't take long to discover the validity of a certain theory. A quick trip to the racetrack tells us all we need to know. The problem is that most theories have never been tested. How are we to buy into a particular train of thought if it has never been proven to be correct?

The primary front suspensions used today in stock car racing are the double A-arm type, the strut-type with the top of the shock acting in place of a top A-arm, and the straight-axle, used mostly in modified and sprint cars. This last system reacts just like a rear differential system, so we will cover that with the rear suspension explanation.

A double A-arm suspension has a moment (roll) center that represents the bottom of the moment arm at the front end. The top of the moment arm is the center of gravity of the sprung mass of the car. As the center of gravity/center of mass tries to continue in a straight line as we turn the corner, a lateral force (centrifugal force) is exerted on the chassis at the center of gravity and that force is resisted by the moment center.

If you stick a shovel blade firmly into the ground, then pull on the end of the handle, your arms represent the lateral g-force, the upper end of the shovel handle is the center of gravity, and the blade at the ground is the moment center. The most efficient way to apply a force to the shovel is when we pull at right angles to the handle. In a AA-arm suspension, this is almost never the case. Usually the resultant force (a combination of the lateral force and gravity) is not perpendicular to the "shovel handle."

In a stock car, we really have two forces at work being applied to the top end of the moment arm, the center of gravity. One is the lateral force of cornering known as centrifugal force, and the other is gravity. What many engineers fail to recognize is that the force of gravity is always present, even in a static state. What we have learned in studying the principles of statics and dynamics is that these two forces combine into one resultant force for which we can calculate a magnitude and direction.

By looking at the direction of the resultant force, we can see the true picture of how these forces react in the front suspension through the moment center. The effective moment arm is the result of the direction of the resistant force in relation to the location of the moment center.

The effective moment arm is the right-angle distance between the resultant force line and the moment center. This is a very important concept because we can see that when the moment center is moved to the right or left, the effective moment arm length changes significantly. As in the shovel handle analogy, a longer handle can apply more force to the end, just as a longer effective moment arm applies more roll moment to our stock car.