block of wood, 4 nails for axles, 4 plastic tires, and material to add weight such that it is no heavier than 5 ounces. That's it - nothing else is allowed.
We had a father shave the tires such that there was a small bead of "tread" in the middle of the tire for the car to ride on; he was told to replace the wheels or his son's car would be disqualified. Pack master said that it would never be allowed at the district races, so he couldn't allow it at the pack level.