[Deke]

I have seen the 11t, 13t and 14t cim gears, and it does seem like a nice way to get a quick reduction. I believe the 13t and 14t have a different c-c distance than the 11t or 12t. The 30t gear can also be reduced for slower speeds, the 50t gear is pretty much set for a 3 inch tall tube and the cim motors. Any larger (unless it is external) it will rub the walls, any smaller than 50t, there isnt access to the backside of the shaft between the 2.5" cim motors. As Pault suggested, the tubes could be cut for more clearance.

I like Nemo's suggestion of larger tubes, versus an external gear. Protection the gears from potential damage during a season seems valuable. It appears 3x2" tubing is available in 6061, whereas 3x1.5" is only available in 6063. Staying away from 6063 would be nice, and the 3x2 gives more room for bearings and a larger mounting surface for mechanisms.

I agree Nemo that chain will package nicer, but all chain I have dealt with requires tensioner's because of wear. Although belts are wider and don't package as nice, they should require much less maintenance.