I think it makes sense to create a post for this. This is not something we typically do and it is not incredibly hard, just needs a little explanation. The center of gravity plays an very important role in determining how much weight you will shift, laterally and longitudinally, as well as how much body roll you will have.

The things we most concern ourselves with is where are we shifting the weight and how much. We typically want to shift the least amount of weight possible and balance out the weight shift between front and rear to be fast, but not too fast for track conditions and the driver's experience level to be in the wall. Knowing where your center of gravity (CG) height sits and the height of your front and rear roll centers, which determine your roll axis, will determine how much and where that weight will shift. There are other factors, of course, but for the purposes here that is good enough.

Keep in mind here too, once you calculate your CG for a car, you don't have to keep doing it. You can always copy that from one car setup to the next. If you feel your setup has changed enough to change your CG, then recalculate. Changing corner weights by adjusting the shock collars, weight jacking, does not change the CG. Minor changes to ride heights will not change the CG much; not sure if it is worth the work.

Where does the CG sit?

Where does our CG sit? As it turns out, which makes sense logically, it is the weighted average of the masses of the car; this includes the car itself, passengers, and other loads. So, if we were to put a 50lb weight in the car, this will to shift the CG in that direction. Here is a visual and explanation on that, taken from Fundamentals of Vehicle Dynamics (Gillespie, 2021).

So, when trying to calculate you CG height, which can be touchy, it helps to understand where you CG should about be sitting.

Important Stuff:

What we are really doing is raising the back end of the car, with the scales, to measure the change in weight in the front. It is highly recommended to take at least two measurements and average them out. It really is not a lot of weight that shifts and can be sensitive to kids who have a hard time sitting still and other. If you want to take the "wiggle" factor out, just mimic their weight in the seat.

Get your scales into fractional lbs. I use Longacre scales and if you contact their support they can guide you on how to do that.

You want to a least raise you rear tires around 10% of the wheelbase, so at least 4.5 inches; which is three 2x's. I've gotten good measurements off of that. If you are using some construction lumber, then just make sure they are straight and level. I have used a 2x4 and a 4x4 screwed together. I have also used 3 2x6's. I would recommend making sure they are long enough to extend a little out past the scales.

You need to use understand that you CG height should come in somewhere around 7-12 inches from the ground where your driver sits in the car. If you are not somewhere close to those numbers, double check what you are doing. Think about where you loads are sitting in the car and how much they weight. If you CG height doesn't look about the weighted average of those masses, then you need to redo things. These are very sensitive calculations. Always reach out to your car manufacturer if things get questionable.

Let's talk about springs, this is a little bit of work. You can't have your shocks and springs on the car when you do this. My suggestion is to make four radius rods, one for each corner of the car, which mimic the ride height and replace your shocks with the radius rods. The advantage of the radius rods is that you can also adjust them to get your ride height exactly and, if down the road, you change it a little or want to use them on another car, you can adjust them. Whatever you do, it has to be rigid and maintain ride height.

Play with entering the numbers on the Setup and see where it calculates you center of gravity. Find numbers you are comfortable with and are kind of where you expect them to be. This takes time and patience.

Procedure:

Once you get your car to a point where you feel you have a "race ready" setup in terms of weight distribution, not including weight jacking, then you can calculate your CG. Here are the steps ...

Have one person lift up the back end of the car and have another put down the boards, or whatever you are using, and scales on top. Put the back end of the car back down on the scales. Make sure to set your tire pressure as normal.
Measure the height of the board and record it
Calculate your front weight by adding the front left and front right corner weights and record it.
Repeat steps one through three with another lift height, typically the board flipped or whatever you are doing.
Average the lift heights and that will be what you enter in the Setup for Lifted Height
Press the Calc CG button to calculate the CG location, assuming you have entered in all the other needed information. Click on the ? in the corner for more information on the Center of Gravity tab
Keep entering numbers and find where you think your average, or good numbers, are sitting. Use that as your final entry into the Setup.

Calculating Your Center of Gravity

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