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.
If you have both the time and filament, I recommend printing the chassis components ("Chassis Left.stl" and "Chassis Right.stl") at 100% infill. Even though I've incorporated a torsion damper at the front of the chassis, the added strength of 100% infill will assist in keeping the gear train better aligned at full spring tension.
Assembly video is here: https://www.youtube.com/watch?v=q_Jom7imnNw.
Video of prototype run is here: https://www.youtube.com/watch?v=zo1t8SiTtTM.
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.
Designed using Fusion 360, sliced using Cura 2.3.1, and printed in PLA on an Ultimaker 2+ Extended and Ultimaker 3 Extended.