Reactor

Hello all,

    I'm trying to simulate a car going of a ramp.  I grouped the car and all
parts except the wheels.  I then grouped each wheel and tire.  So I have a car
group and for wheel groups.  The problem is that when I apply the wheel
constraint I can't pick the car group as the parent and the tires as the child. 
Can anyone tell me what I'm doing wrong.  Should I not group parts, should I
attach them to each other?


Thanks,