Step 6

Establish and mark the center of rotation of each ball joint. It is important to know the exact location of the pivot point of the ball joint so that we can mark that point on the ball joint support band on the control arms. To find the center of each different type of ball joint, we can quickly locate it using a simple method of placing the ball joint in a vise with the stud pointed up and then lining up the shaft as it moves from side to side. Now that we know where the center of rotation is located in relation to the band, we can mark a point on each ball joint on the car. Remember to allow for antidive angle in the control arms as well as the control arm angle. These angles will affect where you place the point to measure to. Use 3/4- or 1-inch wide masking tape and place a piece over the ball joint band. Clean the surface first to remove all grease and dirt. Use a fine tip black marker and a small straight-edge to make a cross on the tape to represent the center of the ball joint for height and width measurements. Do this for the upper and lower ball joints.

Step 7

Take all of the height measurements first. If a direct vertical measurement cannot be taken, then use a level to project the height out from each point or construct a fixture to use to represent the distance so that a measurement can be taken away from the car. When measuring the lower chassis points, use the pivot that is closest to a line lying at right angles from the ball joint to the centerline of the car. For the stock GM lower control arms, use the center of the front bushing. For a strut type of lower control arm, use the center of the bushing or center of the Heim joint that is on a line lying 90 degrees off the centerline of the car projected through the center of the ball joint. The upper chassis mounts often have antidive. To measure the chassis pivot point, average the heights of the centers of the two mounting bolts and measure the width to the center of the control arm shaft. In the case of an upper control arm that uses Heim joints and links, measure to the pivot that is closest to a line that would be lying perpendicular to the centerline of the car from the center of the ball joint.

Step 8

Establish a centerline for the purpose of moment center location. This will not be the true centerline of the chassis, but rather a point half way between the front tire contact patches. Note that the car “feels” the MC in relation to the tire contact patch points and therefore we need to know the relationship of its location to those tire patches. After placing the wheels and tires back on the car, mark a point on the floor at each outside edge of the front tires. I often hang a plumb bob over the bulge in the tire to the floor. Measure between these points and divide that measurement by two. Place a mark on the floor between the front tires that represents half the distance between the tires. Using that same half-distance measurement, measure from the outside of the RR tire to mark a rear centerline point. The right side tires are supposed to be inline or very close to it, and so our points will be parallel to the right side tire patches and centered between the front contact patches. If you know your RR tire offset, you can add or subtract from the front half distance for the rear measurement. Stretch a string over these two centerline points, pull tight and hold each end with blocks of lead or a concrete block if no lead is available. We will be measuring from this line to each of our eight pivot points.

Step 9

Now it’s time to measure the widths. Drop a plumb line down from each center of rotation for the four ball joints and the four chassis pickup points and place a mark on the floor. The framerail and lower control arms may prevent you from dropping straight down from some of these points. In that case, measure out beyond the spindle at a right angle to the centerline and plumb down and mark an offset point on the floor. Write the offset amount (use 20, 30, and so on inches) on the masking tape beside the point. The point we will measure from at each ball joint must be either at the front or rear of the BJ so the measurement will be an accurate width dimension. When marking these points on the side of the ball joint, make sure you are looking either directly to the front or rear of the car.

Measure from each point to the centerline and don’t forget to subtract the offsets.

Step 10

Once all measurements have been taken and recorded, enter all of the height and width measurements into a racing geometry software program. Check to make sure that each measurement is entered correctly. Try to think out the difference in the numbers so that they make sense. If the inner points of the upper control arms are higher than the ball joint measurements, and you know the physical angle shows the ball joint to be higher, re-check your measurements. We are interested in finding the location of the MC in both static and dynamic positions. Static represents where the MC is located when the car is at static ride height and the dynamic position is where the MC migrates to as the car dives and rolls in the turns. The dive and roll numbers you will enter into the program are in direct relation to the type of car, track banking angle, and setup stiffness. Try to use dive numbers that represent what the center of the car is doing at mid-turn and the roll angle is how many degrees the chassis is rolling relative to the track surface.

Low banked tracks will produce more roll angle and less dive than a high banked track. If you’re running on bump devices or into coil bind, then simulate where the chassis would be relative to that attitude and put those numbers in dive and roll in the program. These numbers must make sense and simulate the attitude of the car at mid-turn. Shock travels may include additional travel distance as a result of hard braking on entry into the turns and may not accurately represent the mid-turn attitude. Review the past Circle Track articles on moment center design to determine if your moment center location is correct for your application. The design of the location of the moment center is critical to how your front end will work and is an important ingredient in determining the overall balance of the setup in your car.