This simple diagram shows...
This simple diagram shows the two ends of the moment arm. The front roll angle is a product of the height and magnitude of the center of gravity/centrifugal force. The moment center resists this force. The length of the moment arm influences how much the front wants to roll.
This method made a lot of practical sense because we have two axles in a stock car, each supporting the weight of one end of the car, and each resisting the lateral forces created by cornering. In physics, we are taught that everything is fluid or somewhat flexible. Dividing the car into separate halves allows us to analyze each one independently to determine its desire.
If you will remember, we recently did a story that covered an advanced seminar given by race engineer Claude Rouelle. We discovered that he teaches this very same method and, to our surprise, the use of this method has become much more universal than we could have imagined when we wrote about it in 2003.
The method helps us to determine the components we need in order to create a balanced setup. These components consist of the spring rates, the moment center locations (front and rear), and the weight distribution.
A more realistic picture of...
A more realistic picture of what goes on in 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 of the time to produce a resultant force. We use the magnitude and direction of the resultant force to determine the effective moment arm length.
There are several critical reasons that a balanced setup is essential to optimum chassis performance. First of all, we can accurately predict the weight transfer if the setup in the car is balanced. An unbalanced car redistributes the weight on the four tires in a very unpredictable way. If we cannot determine the desires of each end of the car, balanced or not, we cannot accurately predict the exact amount of weight transfer and, ultimately, how much weight ends up on each tire at mid-turn.
Unbalanced setups abuse one or more tires. It is this abuse that causes the tires to wear or heat up excessively. Balanced setups distribute the weight and workload evenly on each pair of tires.
It is important to note here that this method, although fairly new in concept, has been applied successfully to stock cars for many years. Cars representing every class of stock cars have had this method applied to them with predicted results. In other words, when the cars were balanced, they became fast and consistent, and this showed in the even tire temperatures front to rear at each side. The lap times became very consistent and the cars handled very well.
When the moment center is...
When the moment center is located to the left of the centerline between the front tires, the effective moment arm is relatively long. This promotes a larger roll angle at the front and makes the front more efficient.
The primary front suspension systems that are used today are the double A-arm (the most common), the solid axle, and more recently, the strut, with the top of the shock acting as a substitute for an upper A-arm. The solid axle system reacts just like a rear differential system, so it will be included in the rear suspension discussion.
A double A-arm suspension has a moment center that represents the bottom of the moment arm. The top of the moment arm is the center of gravity of the sprung mass of the front 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, and that force is resisted by the moment center.
If you stick a shovel blade firmly into the ground and then pull on the end of the handle, your arms represent the lateral g-force, the end of the shovel handle is the center of gravity, and the blade at the ground is the moment center. The handle of the shovel is the moment arm.
Many stock cars have a front...
Many stock cars have a front geometry design that puts the moment center to the right of the centerline between the front tires. As the MC is designed further to the right, the front suspension becomes much stiffer due to the diminishing length of the moment arm.
The most efficient way to apply a force to the shovel is to pull the handle at a right angle. In a double A-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 moment arm.
In a stock car, we really have two forces being applied to the top end of the moment arm/center of gravity. One is the lateral force of cornering known as centrifugal force, and the other is gravity. 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 the combination of the two forces react in the front suspension through the moment center. The effective moment arm is the result of the direction of the resultant force and the location of the moment center.
The rear suspension is much...
The rear suspension is much different than the front suspension. The spring base is felt at the top of the springs. The moment center resists the lateral centrifugal force that causes the suspension to roll. Changing the MC height, rear spring rates, and spring split are the primary methods used to adjust the rear roll angle.
The effective moment arm is the right-angle distance between the resultant force line and the moment center. This is very important because we can see that when the moment center is moved to the right or left, the effective moment arm length changes. 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 force to roll our stock car.
The amount of lateral force and the length of the effective moment arm are contributing factors that help us predict exactly what roll angle the front of our car desires. If the front end of our car was not rigidly connected to the rear, it would roll to a predicted angle and a predictable amount of load would transfer from the left-front tire onto the right-front tire. But that is not the case. The two are connected, and we cannot accurately predict the weight transfer at each end of the car unless we know the desires of each end.