Answers provided by Senior Tech Editor Bob Bolles (unless otherwise noted)
Q When making measurements for designing the roll centers, is the "centerline" the chassis centerline, or the centerline measured from the center of the left front wheel to the center of the right front wheel? Also, when designing the upper control arms, should the same length be used for both sides? I'm using the same size now for each side, but didn't know if there was a benefit from running two different sized arms. I thought they would change angles at different rates, therefore shifting the dynamic roll center left or right. Am I correct in figuring this?
A For dynamic analysis, we always want to know where the roll (moment being a better description) center is located in relation to the two front tire contact patches. That is where the car "feels" the moment center. I always find the distance between the front tire contact patches and use half that distance, measured in from the center of the right front and the right rear tire contact patches, to establish a centerline of the car. It is always suggested that you line up the right side tires' contact patches when you are doing your basic chassis alignment so the centerline you are establishing is parallel to the right side tire contact patches, as well as being in the center of the front tire contact patches.
One of the primary reasons we use different length upper control arms is so we can adjust the angles of the arms to locate the moment center. Some cars have no means to adjust the angle of the upper control arms and so length changes, which change the arm angles, help accomplish this. The moment center will almost always move to the right (when turning left) after the car dives and rolls in the turns and whether the arms are equal or not does not necessarily affect that.
The two critical front-end design goals are:
1) location of the moment center after the car dives and rolls as it is in the middle of the turns. This dictates the stiffness of the front end and is a factor in balancing the setup.
2) establish a right upper control arm angle that will yield zero camber change after dive and roll. This means that the right front tire will keep the same camber angle relative to the racing surface throughout the entire lap. This has been found to be very beneficial after analyzing many different front-end designs. We know that if the camber continues to change on entry and through the middle, the right front tire will not grip as well and the car may develop a push regardless of how well-balanced the rest of the setup is.
Forward Bite For '80s Monte Carlo
Q I run a mid-'80s Monte Carlo (Hobby Stock with no weight jacks or racing shocks) and I have a big problem getting forward bite. It wasn't as big a problem with a sway bar and 31/44-inch of preload but the car handles much better through the corners without it. The car weighs 3,200 pounds with driver, with 43 percent rear weight, and I'm guessing 55 percent crossweight now without the sway bar. My springs are as follows: RF 1,300, LF 1,000, RR 200, and LR 250. The car is awesome in the middle of the corner and pretty good going in. What do I need to do? I've been told to reduce the split in the rear, and someone else has told me I need softer springs.