I have a car resting still on a flat terrain with a collider on both. When I check the convex settings the car does not fall, but splits up into the chassis, wheels and body (repelling each other). When I do not check convex, the car stays in one piece but falls through the ground. Help?
I would suggest that your car is made of a series of separate meshes, each mesh has a convex mesh collider on it and each mesh object has its own rigidbody.
I would suggest that you look at the Rigidbody Component docs and look at the section on child colliders. I would suggest that you only want one rigidbody on the root GameObject, keep the meshes to themselves and have a series of child colliders.
Regardless of what you choose to do with your colliders, you should only have one rigidbody for the car and it should go on the root.
Testing concave meshcolliders vs meshcollider is not supported. (and would probably not be a good idea performance wise). “Mesh Colliders that are marked as Convex can collide with other Mesh Colliders” I don’t know if you can test mesh collider vs terrain, but either way I’d think you’re probably better off simplifying the collisionmesh of your car into a few boxes.
If youve got something like a bowl its a concave mesh. a ball would be a convex one. concave meshes are way more difficult to compute than convex ones. When you force a mesh to be convex unity fills the gaps, so the bowl will spill its contents.
So in summary: You probably can’t test a concave collider against a terrain collider (and shouldn’t). What I think you should be doing is to use a compound collider.
read about this and the other stuff here:
http://docs.unity3d.com/Documentation/Manual/Physics.html