It sort of looks like this may be because the car is floating slightly above the ground. Have you tried dropping the car down slightly and seeing if this helps?

Thanks you were right!

Hi Irish - I see you got my other message. Guess what - the artifact you posted here is ALSO from the bias. I guess you solved the problem for this picture by lowering the car, which is fine. But generally, the AO bias is a cranky thing. If you set it too HIGH, the shadow breaks away from the object as you see here. If you set it too LOW, you get black patches as in your previous post. Face_off has pointed out the using AO "nodes" in the material room seems to be less cranky and more controllable than the general Light based AO. However, I had a character that showed BOTH artifacts on the thigh SIMULTANEOUSLY. This was both with light-based AO and material-based AO. Since one artifact requires raising the bias, and the other requires lowering the bias, if you get both at the same time on the same part, there is no solution. In that case, I had to morph the part just slightly to change the geometry enough so that I only had one kind of AO artifact to deal with.

hey thanks for the info!