Disappearing model parts


I am stuck with the following problem:
I load a model file, a car, containing two groups. One group is for the car itself, the other is for the wheels (I think). Loading this model works fine, but as soon as I make a collision on the main group, the wheels disappear. I tried creating a collision on the wheels as well, which results in a totally screwed up model.

Any suggestions?

Could you package up the whole thing in a little zipfile, and send it to me? jyelon@gmail.com