If you do not know the exact location of the center of rotation of your ball joints, this
Measure and record the heights to the centers of rotation and remember to subtract the offset amount that we raised the car. Make sure that the tape is vertical for all measurements. It's easy to concentrate on the numbers and forget to line up the tape.
Step 8 Establish a centerline for the purpose of MC location. This will not be the true centerline of the chassis, but rather a point half way between the front tire contact patches. The reason for this is that we want to know where the MC is located in relation to the tires because that is what is important to the car. The car "feels" the effect of MC location in relation to the two contact patches regardless of where the frame rails or other components are located.
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, measure from the outside of the RR tire in towards the middle of the car and place a mark for the 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. Pull a string over these two centerline points, pull tight and hold each end with blocks of lead or a concrete block.
Step 9 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 frame rail 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 etc., inches) on the masking tape beside the point.
Mark the center of rotation of the ball joint on a piece of masking tape affixed to the co
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.
Step 10 Measure from each point on the floor to the centerline string and don't forget to subtract the offsets if you have used them. Once all measurements have been taken and recorded, enter all of the height and width measurements into a racing geometry software program.
We are interested in 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 very dependant on the type of car, track banking angle, and setup stiffness. Try to use numbers that make sense and simulate the attitude of the car at mid-turn. Shock travels may include additional shock travel from hard braking on entry into the turns and may not accurately represent the mid-turn attitude.
Review the CIRCLE TRACK articles on moment center design to determine if your MC location is correct for your application. The design of the location of the MC is critical to how your front end will work and is a critical ingredient in determining the overall balance of the setup in your car.
For standard A-arms, the "pivot" point will be at the center of the control arm shaft. Thi
In the case of a strut-type of upper control arm, measure to the center of the Heim joint
We ultimately want to end up with measurements that will represent the heights and widths