[pwnageNick]

I love this design for what it's goals are (low weight, single speed drive with minimal parts).

I don't think most of the issues raised above are too big of an issue. I'm not sure I buy the thin wall being an issue for mounting the CIMs, and if the wall of the tube really did flex from the weight of the CIMs, some simple part (3D printed would be a good source) that goes between your belly pan and motors would solve that problem easily.

As far as pressing the bearings into the tube for the drive shaft, this is the only thing I could see being an issue. A simple fix would be some small 2D bearing blocks that mount to the tube for the bearings to be pressed into.

I would definitely just incorporate this into the drive tube. I don't see the benefit of it being a bolt on gearbox; at that point you might as well go with some bolt on COTS gearbox.

Maybe I missed it somewhere in the thread, but what gear is on the drive shaft? I think I saw someone say 64T. I think if you switched that up to 72T, that would probably be a more reasonable ratio for a single speed drive, and it would also easily solve any clearance issues with the CIM mounts and allow you to stick with 2x1 (which I think is necessary for it to be a viable choice to incorporate into the drive tube). A larger drive gear would push the CIMs out, allowing you to rotate the mounting holes a bit, keeping your 1/8" clearance from the gear while gaining some clearance from the top wall of the tube.

Great design, I really love it. I had worked on a similar idea before, but somehow that design ended up migrating away to having the gears on the outside of the tube (in the chassis) with a chain-in-tube setup.

EDIT: You may have had hesitation about a 72T gear on a 4" wheel; that would explain the 64T gear.

EDIT AGAIN: Disregard the part about the larger gear buying you more clearance space, I misunderstood how you had the mounting set up. I sketched everything you have out and see your dilemma.