Because we have presented the fact that the front roll center, or moment center (MC) as we like to call it, is so important, it would be nice if we explained how to measure your car for MC location. These measurements are best put into a geometry software program. It doesn't take too long and the results could be substantial.
Taking the measurements to determine the MC location is not that difficult. Racers in general will always do what is needed to perform, from making last minute engine changes to staying up all night to prepare for an important race. We are showing the process for taking measurements for a two dimensional software program. The process of measuring the front end takes from two to four hours, depending on how organized you are and how much help you have. This is considering that you will take your time and do it right by checking every measurement and verifying the data. If you want to use a three dimensional program, you will need to take fore and aft measurements from a datum line, and this will add about an hour or two to your process.
Check the floor where you...
Check the floor where you will be taking measurements. A good way to check for flatness if you don't have a builders level is to pull a string over 1-inch (or whatever works) blocks and measure from the string to the floor at the points where the height measurements for the pivot points will be taken. If the gap between the string and the floor varies more than 1/16-inch, then find a new spot, or note the difference at each point and adjust the height measurement as you go along.
The following are the steps and methods you can use to measure the eight pivot points used in a two dimensional software program in order to determine the static and dynamic locations of the front moment center. For most types of stock cars, the information we want to know about the front moment center location can be found accurately enough by taking two dimensional measurements and using a two dimensional geometry software program.
Step 1 One of the most important steps is to locate a level portion of the garage floor on which to measure your car. The most important area that should be smooth and level is that area in between the front tires and extended fore and aft 10 inches plus or minus from the "axle" line. So, roughly a rectangular area that is about 20 inches by 70 inches will do.
Pull a string over 1-inch blocks set at the outside and under where each tire will sit and across the area to be used and measure along the string to see if the floor varies more than 1/16-inch in the exact spots you will be measuring to. If it does, move and find another area. Get this part right and you will be able to trust your numbers.
Step 2 Remove the string and blocks and 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, and the tires aired up to race pressures.
If the engine has been removed, support the car under the frame rails at ride height and remove the springs and shocks. Reduce the air pressures in the front tires until the LF lower ball joint flange drops 3/8-inch closer to the floor and then the RF ball joint flange drops 1/2-inch closer to the floor. Dropping the spindles this way will closely simulate the amount that the tire squashes when all of the weight is on the tires with the engine, springs, and shocks installed.
To record the spindle position...
To record the spindle position you can measure from a point on the lower control arm (such as the top of the lower ball joint stud) to a place on the rollbar engine hoop. Record these measurements.
Step 3 Take a measurement at each front wheel so that 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. Some teams will measure the length of the shock. Whichever way you choose, the idea is to have a way to accurately reposition the wheel assembly later on after we have raised the car and removed the wheels.
Step 4 Jack the car up and support the chassis at the four corners on jack stands. 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 without the stands being in the way. We want to raise the car the exact same amount at each corner. A good distance to raise the car is 10 inches, but any dimension will do. As we eventually measure the height of each chassis pivot point, the offset (amount we raised the car) will have to be subtracted from each distance and 10 is an easy number to subtract.
Here is a sample offset chart....
Here is a sample offset chart. The offset that the car was raised must be the same at all four corners of the chassis. Add the constant offset number to each of the four ride heights and make sure that each corner is at the proper offset height.
Start by measuring the LF corner, then move around the car clockwise to adjust the other three corners to a dimension equal to their ride height plus the offset amount. Then make sure that the chassis is fully supported at all four corners and does not rock or roll. You can shim one of the rear corners to stabilize the car, usually the RR. As long as the two front corners are set to ride height plus the constant offset and tight on the jack stands, then you 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 that you have the same measurements as we took in Step 3. Install a link in place of the shock at each side so that 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 a strut rod or tubular 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, then a piece of 1-inch by 1/8-inch iron or aluminum strap metal can be used. Drill a one-piece link the exact length of the installed height of the shocks for each side. Or, you can cut a piece in two so that they are long enough to overlap when installed in the shock mounts, drill a hole in one end of each piece and bolt the two to the shock mounts.
With the wheel assembly positioned correctly, overlap the straps and lock them together with a pair of vice grip pliers. Drill another hole through both pieces and bolt the two together. Label each set of straps, i.e. LF, RF, so that you can reuse them if you need to remeasure later on. Once the two wheel assemblies are secured in their proper positions, remove the tires and wheels.
The best way to support the...
The best way to support the spindle while taking MC measurements is by using a spare control rod or lower strut rod. Both of these usually have opposite rotational threads and therefore are easy to use to fine tune the spindle height. The spindle must be positioned in relation to the inner pivot points exactly as it would with the spring installed and the car at ride height with all of the weight in it. Shown is a new unit from Coleman Racing Products.
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 and measure to it.
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 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. 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 laying 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, measure to the pivot that is closest to a line that would be laying perpendicular to the centerline of the car from the center of the ball joint. If the two mounts are equal-distant from the perpendicular line, measure to both and average the measurements for height and width.
If you do not know the exact...
If you do not know the exact location of the center of rotation of your ball joints, this simple method will allow you to establish the true center. Place the BJ in a vise with the shaft pointing up. Rotate the shaft all of the way to the left and mark the centerline on the housing over a piece of masking tape. Then rotate the shaft all the way over to the other side of the housing and mark the centerline again. The intersection is the true center of rotation.
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...
Mark the center of rotation of the ball joint on a piece of masking tape affixed to the control arm band. You will need to "project" the center point out to the tape taking into account the control arm angle and direction of the ball joint shaft (due to king pin inclination in the spindle).
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"...
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 antidive is present) as well as the width of the rotational axis.
In the case of a strut-type...
In the case of a strut-type of 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.
We ultimately want to end...
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. We cannot forget to subtract the offset that was used when raising the car.