[JesseK]

Thanks for posting this. I came up with a 10" wide version of this for a single side and couldn't figure out how to get it down to 9". Going length-wise just might be the key depending on the game and the robot's necessary manipulator clearance. Unfortunately with this design, it's not very plausible to add another 1-2 motors to each side without adding an entire new shifting mechanism.

If I ever get my current concept idea done I'll rebuild the old (10") design and show you what I mean -- 3 motors per side and it could push 40lbs of books across my carpet if I pushed down on the back of the bot with my finger for traction. (5:3 High and 1:6 Low)