Front-running teams in every series tend to have developed methods of finding the perfect balanced setup. While trial-and-error has worked fine for years, we have now defined what constitutes a balanced setup and the method for predicting our cars' handling balance. Courtesy of NASCAR
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.
Not Entirely New
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?
This simple diagram shows the two ends of the moment arm. The front roll angle is a product of the height of the center of gravity and magnitude of the centrifugal force. The moment center (MC) resists this force. The length of the moment arm influences the amount the front wants to roll.
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.
The AA-Arm Suspension
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."
A more realistic picture of what goes on in the front of a stock car as we negotiate a turn takes into account the banking angle of the racetrack as well as the vertical and lateral location of the moment center. The centrifugal force combines with the gravitational force, that exists all the time, to produce a resultant force. It is the magnitude and direction of the resultant force that we use to determine the effective moment arm length.
Two Forces at Work
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.
Effective Moment Arm
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.
When the moment center is located farther to the left of the centerline, the effective moment arm is relatively long. This promotes a larger roll angle at the front and makes the front more efficient or soft. | 
Many stock cars have a front geometry design that puts the moment center to the right of the center line. As the MC is designed further to the right, the front suspension becomes much stiffer due to the diminishing length of the moment arm. | 
If we can accurately predict the roll angles of both the front and rear suspension systems as the car turns, then we can change components so that the two systems are balanced, creating the correct amount of load transfer from left to right. This balance is good for our car and exactly what we attempt to do when we apply trial-and-error methods of chassis setup. |