1x4459 Technic, Pin WITH Friction
1x3708 Technic, Axle 12
1x6582 Wheel 20x30 Balloon Medium
1x x89 Rubber Belt Large (Round Cross Section)
Step 2: Prep the Wheel
Insert the technic pin into one of the outside holes of the wheel. Push the rubber belt through the center hole and loop one end over the technic pin. This will allow the belt to wind up.
Step 3: Wind Up
Push the axle through the loose end of the rubber belt (opposite side of the technic pin). Hold the wheel and spin the axle around to wind up the rubber belt.
Step 4: And Let Go
Place the wheel on a smooth surface and let the axle go. The rubber band will unwind and the axle will push against the surface moving the wheel forward. If it wants to go in the wrong direction, flip the wheel over so the axle is on the otherside.