There are no bad race cars, just bad geometry. Any car can be made to work by making small changes to critical points in the suspension. That is why we present articles like this one--to show you how to improve the performance of your car by doing a little shop work and applying some cutting and welding skills to perfect your car’s design. It costs very little, but the rewards will be significant. The importance of the front moment center has not diminished even with newer setups that utilize bump devices and softer springs. The forces are still there that are influenced by the MC. We often write about the importance of the front MC location for stock cars. Now, we will again explain how to measure your car to find the exact MC location. 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. This 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.
The following are the steps and methods you can use to measure the eight pivot points in order to determine the locations of the front moment center. This represents a two dimensional view of the front geometry and is much quicker and easier than taking three dimensional measurements. If you desire to go the 3-D route, you will need to add measurements in a fore-aft direction, as well as the height and width, from a baseline that is perpendicular to the centerline. For most types of stock cars, the information we desire 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. That is because the range of motion involved with negotiating turns on a short track add up to relatively small angular changes to the control arms.
One of the most important steps is to locate a level portion of the 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-plus inches. So, roughly a rectangular area that is about 20x70 inches and level 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. Measure along the string to see if the floor varies more than 1/8-inch in the exact spots you will be measuring to. If it does, move and find another area. Some teams will use a horizontal laser that can be purchased at any hardware store for this instead of a string. Get this part right and you will be able to trust your numbers.
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 car at ride height, and the tires aired up to race pressures. If the engine has been removed, support the car under the framerails at normal 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 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.
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, the idea is to have a way to accurately reposition the wheel assembly later on after we have raised the car and removed the springs and shocks.
Jack the car up 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 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 vertical measurement and 10 is an easy number to subtract. Start by adjusting the lowest corner to the new offset distance, then move clockwise around the car to raise the other three corners to their ride heights plus the offset amount. Then make sure that the chassis is fully supported at all fours corners and does not rock and roll. You can shim the left rear corner last to stabilize the car and if the height is not perfect, we just leave that corner what it is. As long as the two front corners are set to ride height plus the constant offset and the chassis is tight on the jackstands, then you will be OK.
1 Check the floor where you will be taking measurements. A good way to check for flatness
2 To record the spindle position you can measure from the wheel rim to a mark on the fend
3 The offset that the car was raised must be the same at all four corners of the chassis.
Remove the front springs, shocks, and wheels and jack the LF and RF wheel assemblies up until they are positioned so that we have the same measurements as we took in step No. 3. Place 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 No. 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, then a piece of 1x1/8-inch iron or aluminum metal strap can be used. Cut the 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 Vise-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 re-measure later on.