(1) Cotter pin
(1) tail wheel ball
(2) wheels
(2) O-ring tires
(2) screws saved when removing the servo horns
needle-nose pliers - optional, not included
CAUTION - the ends of the cotter pin may be sharp! If you have needle-nose pliers, you can bend the cotter pin ends into a circle to tuck away the points. But be aware that this makes the cotter pin more difficult to remove in the future if you ever want to do that.
The robot’s tail wheel is merely a plastic ball with a hole through the center. A cotter pin holds it to the chassis and functions as an axle for the wheel.