center of mass and complex shapes

I was wondering how to get accurate center of mass, my vehicle is very jittery is as are some of my other bodies, i think it is because of the center of mass , is there any way to find accurate center of mass, also how can i make a complex shape like a car chassis from three boxes, do i attach them with bones?

The center of mass, by default, is in object space (0,0,0). You can change it if you wish though.

And to connect three boxes into a single solid shape, use a compoundcollision.

Maybe this help you: http://newtondynamics.com/forum/viewtopic.php?t=1199.

thanx for the help