Mouse Trap vehicle Compnents

I think some of the most important things needed for a mouse trap vehicle are the wheels, the axle and what you have for friction on your wheels. I think the wheels must be bigger than bottle caps but smaller than a dinner plate. The size of a CD would be fine. The axle cant be to long or to short. The axle must be round so it can move in a circular movement. I think that using balloons on your wheels or rubber bands will work to create friction so the vehicle moves.