if you go into a curve full speed it's noticable that the rear wheels bend outwards if you look very closely. this could also cause the flipping tendency... i thought i constrained them to not do that! do the spring/damper constants also make the rear wheels' (0,0,1) axis more loose?

<edit>no, the spring/damper constants don't seem to have anything to do with it... marco, is this the reason for suggesting to use hinge instead of wheel constraints for the rear wheels?</edit>

@phantom88: you are right! maybe the visible chassis should be different from the physics chassis so that the center of gravity is lower...