I tried making loops-the-loops in 3ds max and importing into rFactor and they don't work once the tires touch the upside down part.. But rfactor has too much collision issues even for a halfpipe (tried that too). It works, but just after you realize you can't make objects thinner than about half a meter for collision in rFactor. More correctly, opposing faces of an object must not be on the distance less than about half a meter, cause then the collision is either weird such as cars tend to jump unrealistically, or they fall through entirely..
As for the cars colliding when touching the splitter on big jumps, that's because car's collision model follows the splitter probably, it could be made further above it (which I always recommend), so that this strange digging the ground - sudden stop doesn't happen.