Onimpact function problem

I am just doing the vehicle collision detection for the terrain, so using the onimpact function it works not bad.
but when the car runs over a ramp, jumps and hit the ground, the function also consider that as a collision, my initial idea is changing the collision box of the tire which actually touch the ground. am I right?

ps: a small problem, when I open the tire model in 3d max 7, why I can not select the col box in the "select by name" dialog box, I could only see it in the Schematic View option?

hope someone could help me to solve the problem, thanks a lot!