I probably should have said, an existing physics engine isn't possible, because I'm working in AS3.

Obviously in C++ you have the luxury of engines like Bullet, that apparently perform very well.

The only existing physics engine is HIDEOUSLY slow, so I really have no choice but to do all my own physics, which so far, work pretty well.

I have a highly efficient broadphase and sphere to triangle routine, for general object to environment collisions, but obviously vehicle physics are a fair bit more complex than that.

Also as it happens, the handling and collision detection on my soft body car is absolutely fine, the only bit that isn't working is the orientation (when introducing roll), which should be significantly easier than the handling and collisions. I have a collision routine for all 4 wheels and it works perfectly. My car is essentially a 3D rectangle tied together with constraints.

So really, orienting it to the terrain shouldn't be difficult at all, I have 4 points that define a plane (the wheels), maybe my best bet would be to simply orient the vehicle to the normal generated by these plane points.

Thanks for your help anyway jyk.

Because I'm working on so many things at once I tend to overlook the obvious.

For some reason I was incrementing the rotation of each axis on the car from the angles defined by the front/back of the car, and the left/right wheels.

When of course, the best, simplest and most accurate thing to do is simply define the matrix from scratch for the car's orientation.

So obviously the car's z axis is the vector from the front and back of the car, the car's y axis is the plane normal defined by the 4 wheels, pointing up, and the x axis is the cross product between the z axis and normal.

It works perfectly now.