We know the roll center isn't the point about which the chassis rolls, but instead, it's a critical element of the front dynamic structure of the car.
A more appropriate name would be the "moment center." This point represents the bottom of the front moment arm. Because it is of great importance, and its design location has a huge effect on the performance of the front of the car, we now need to know where the moment center is for our cars.
You can ask someone if they know the location of our car's moment center, but the only way to be sure is to locate it yourself. Once you find the numbers, it helps to have a computer software program for the calculations. Here are the steps and methods to be used to measure the pivot points in order to determine the locations of the front moment center:
Step 1--Locate a level floor on which to measure your car. The most important area is between the front tires and extended fore and aft 10 inches. Roughly, a rectangular area that is 20 inches by 70 inches will do.
Step 2--Position the car over the level spot with all of the weights in the car including the driver. Make sure to have the correct cambers set, the tires aired up to race pressures, and put the car at normal ride height. If the engine has been removed, support the car at ride height and remove the springs and shocks. Reduce the air pressures in the front tires until the left-front (LF) lower ball joint flange drops 1/2-inch closer to the floor and the right-front (RF) ball joint flange drops 5/8-inch closer to the floor. Dropping the spindle will closely simulate the amount that the tire squashes when all of the weight is on the tires with the springs and shocks installed.
Step 3--Take a measurement at each front wheel so we can later return the wheel assemblies to their same positions in relation to the chassis. We can measure at the outside of the tires from the bottom rim of the wheel to a mark on the fender or on the inside of the tire from the top of the ball joint stud to a point on the engine hoop bar. Either way, the idea is to have a way to accurately reposition the wheel assembly later after we have raised the car.
Step 4--Jack the car and support the chassis at the four corners on jackstands. Keep the stands away from the actual area where we will be taking the measurements in order to be able to access and measure to the pickup points under the front suspension. We want to raise the car the same amount at each corner. A good distance to raise the car is 10 inches. As we measure the height of each point, the offset (amount we raised the car) will have to be subtracted from each distance and 10 is an easy number to subtract.
Start by adjusting the LF corner, then the RF, then the left-rear (LR) corner. Shim the right-rear (RR) so the chassis is fully supported at all four corners and does not rock and roll. The RR measurement may be a little off due to chassis flex. As long as the other three corners are set to ride height plus the constant offset, then we will be OK.
Step 5--Remove the front springs and shocks and jack the LF and RF wheel assemblies up until they are positioned so we have the same measurements as we took in Step 3. Put a link in place of the shock at each side so the wheel assembly is supported in the same position as if it were at normal ride height with all of the weight on the tires (Step 3 measurements used).
The link can be an old tie rod or tubular lower control arm with opposite-threaded Heim joints of a usable length. This type of temporary link is easy to install and adjust for length.
If those are not available, a piece of 1- x 1/8-inch iron or aluminum strap metal may be used. Cut the piece in two so the pieces are long enough to overlap when installed in the shock mounts. Drill a hole in one end of each piece and bolt the two in the shock mounts.
With the wheel assembly positioned correctly, align the straps and lock them together with a pair of vise grip pliers. Drill another hole through both pieces and bolt the two together. Label each set of straps (LF, RF, etc.) so you can re-use them if you need to re-measure later. Once the two wheel assemblies are secured into their proper positions, remove the tires and wheels.
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 we can mark that point on the ball joint support band on the control arms.
We can quickly locate the center of each different type of ball joint by 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 anti-dive 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.
Step 7--Take all of the height measurements first. If a direct vertical measurement cannot be taken, use a level to project the height out from each point or construct a fixture to use to represent the distance so a measurement can be taken away from the car.
When measuring the lower chassis points, use the pivot 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 front 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 anti-dive. To measure the inner 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, 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.
In the case of a strut-type upper control arm, measure to the center of the Heim joint that is along a perpendicular line between the ball joint and the centerline of the car. For standard A-arms, the "pivot" point will be at the center of the control arm shaft. This point represents the average height of the two mounting bolts (when anti-dive is present) as well as the width of the rotational axis.
Measure and record the heights to the centers of rotation and remember to subtract the offset amount that we raised the car.
We ultimately want to end up with measurements that will represent the heights and widths of the four ball joints and the four chassis pivot points as if the car were at ride height. All height measurements must be made with the car above the usual ride height so we cannot forget to subtract the offset that was used to raise the car.
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 halfway between the front tire contact patches.
With the wheels and tires on the car, mark a point on the floor at each inside edge of the front tires. 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 inside edge of the RR tire to mark a rear centerline point.
The right side tires are supposed to be inline or very close to it, so our points will be parallel to the right-side tire patches and centered between the front contact patches.
Place a string over these two center points, pull tight, and hold each end with blocks of lead or a concrete block if no lead is available.
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 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, plumb down, and mark an offset point on the floor. Write the offset amount (use 20 or 30 inches) on the masking tape beside the point.
Measure from each point to the centerline and don't forget to subtract the offsets. Once all measurements have been taken and recorded, enter all of the height and width measurements into a chassis software program of your choice.
We are interested in the location of the moment center in both static and dynamic positions. Static represents where the moment center is located when the car is at static ride height, and the dynamic position is where the moment center migrates as the car dives and rolls in the turns.
The correct design of the dynamic location of the moment center is critical to how your front end will work and to the overall balance of the setup in your car.
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The best way to support the spindle while taking measurements is by using a spare tie rod
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For standard A-arms, the "pivot" point will be at the center of the control arm shaft. Thi
We ultimately want to end up with measurements that will represent the heights and widths