Sorry I forgot to reply, but yeah, what Madhed said ^^^
I use a trimesh for the race track, even though curves would be ideal, just because the art packages and the physics middleware support them. In areas where the slope of the track changes, if you use a decent number of polygons then it feels smooth.
The body of the car itself is a regular rigid body as supported by the physics middleware. The results of the raycasting are used to do all the wheel/tyre/suspension/transmission/engine/etc calculations, which then apply forces to the body. Soft tyres and suspension will absorb the small bumps from the trimesh, just like in real life If your tri-mesh is too bumpy though, it also affects the handling/traction of the car though, also just like real life.