An updated version of my earlier design (http://www.thingiverse.com/thing:430050), this version includes stronger spring and gears, weaker pawl, and o-ring tires for traction. The first photograph is of the actual prototype. The second photograph is a screen grab of the cad version. You will need to purchase two #18 o-rings (1 3/16" O.D., 15/16" I.D., 1/8" diameter) and two #111 o-rings (2 1/2" O.D., 2 1/8" I.D., 3/16" diameter), which are available on line, and at plumbing supply / box stores. Video of assembly: https://www.youtube.com/watch?v=q_Jom7imnNw. Video of prototype run: https://www.youtube.com/watch?v=zo1t8SiTtTM Designed using Fusion 360, sliced using Cura 2.3.1, and printed in PLA on an Ultimaker 2+ Extended and Ultimaker 3 Extended.