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.