In Part One of this series, we learned about the early research in chassis dynamics and something about the individuals and companies that were involved. In this final part, we will take a look at modern-day setups and see where we can apply the technology we have learned.
We have presented all of the elements that influence the chassis setup as far as balancing the two suspension systems. We discussed the moment center (previously called the roll center), how the moment arm works, and what the differences are between the front and rear suspension systems.
We explained how to make the car balanced by predicting what the front and rear suspension systems desire to do and then changing components to allow them to work together. A balanced setup-one in which both ends are working together-is what we have always tried to find by using trial-and-error setup techniques.
Let's now take a look at four types of commonly raced stock cars and see how we can produce a more equally balanced setup for each one. The primary influences on the base setup of each car are the front moment center location, the rear moment center height, and the four spring rates. The modern trend for asphalt racing is the big bar and soft spring setups. This brings the larger sway bars into the picture, and we now have one more primary influence on the setups of asphalt cars.
As we discuss the setups in these cars, follow the reasoning and methodology so that when you think about making changes to your car, you can move in a direction that makes sense and causes a more positive result. This is important because every race car is built a little differently than another within the same class or division.
Setup information in the charts will include the four spring rates (LF is the left-front spring, shown in the left upper corner of each chart, RF in the right upper corner, LR in the left lower corner, and RR in the right lower corner). The front moment center is shown first as height off the ground and then as width from a centerline that is halfway between the centers of the two tire contact patches (a negative number represents an MC that is to the left of the centerline).
The Panhard bar is listed as left/right mounting heights off the ground in inches. For the metric four-link, we show the moment center height. The sway bar diameter (if shown) is in inches, and we will assume an average wall thickness for a hollow bar. The banking angle is shown in degrees, and the last number shown is the estimated g-force.
The top numbers represent common but not necessarily balanced setups. The bottom numbers represent changes we have made to the car to make it more balanced.
Many of these cars are the metric type with a four-link rear suspension. The rear moment center in these cars is very high, usually 14 inches or higher. The original front MC is too far to the right, the spring base is narrow, the front springs are too stiff, the rear springs are too soft, and the rear roll desire will be much more than the front.
This car would be very tight and would require a lower amount of cross weight in order to make the car neutral. The balance problem would still exist, though. The two ends of the car are very different in what they desire to do. The difference in roll angle is approximately 2.2 degrees.
To balance this car, we could do several things:1. Soften the front springs and reverse split the rates to: LF= 50,RF= 002. Decrease the rear spring split to: LR = 185, RR = 1753. Make changes to the rear ride height or trailing link mounting points to lower the rear MC to 13 inches or so4. Move the dynamic width of the moment center 2 inches to the left of the centerline by increasing the upper control arm angles (inner mounts lower than the ball joints), which will also raise the MC. Installing taller ball joints is one way to do this.
With these changes, the two suspension systems are now much closer to being balanced. The car will turn much better, and we can use more crossweight to load the LR tire and promote better bite off the corners. Our roll angle desires are now within less than 1 1/2 degrees.