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A car is built out of graphics & physics. It can be quite difficult to see how things are interrelated. The later Racer versions introduced the concept of a (virtual) nullpoint, around which the car is built.
The nullpoint is the center of modeling the car (not just graphically). Most things are specified relative to the nullpoint. In the past, the CG actually was the same as the nullpoint. So it makes sense to start out with a CG at (0,0,0) and try to keep it that way.
To find the height of your nullpoint, set (in the car's car.ini) car.cg.y to 0, then run Racer with the car sitting still on a perfectly flat track. Press Ctrl-9 to see the CG height; that is the height of your nullpoint as well then. You may want to record this value as a comment in your car.ini file.
In Racer, enter the console command 'show carpoints' to see the nullpoint. In the image below, it's the whitish point in the center of the car. The green point just next to it is the CG location. Other green points in front and rear of the car are light flare location.
The CG (Center of Gravity)
Try to start out with a CG near or exactly at (0,0,0). It will simplify things to have offsets being positive to one side, and negative to the other side. The CG is specified relative to the nullpoint; starting from v0.8.27 this CG is correctly transformed so that cars really rotate around the CG. Before that, it actually rotated around the nullpoint, which was not that bad when the NP was close to the CG, but got progressively worse when the CG moved further away.
(last updated November 13, 2012 )