okay, so, I’ll probably ramble a bit here, but this crab module interests me, and I didn’t get a chance to go over and see how it worked while in Houston (mostly because I’d forgotten about its existence.)
If I understand how this works correctly, the motors used to power the module are ‘off-module’ and this allows you to input them into a shifting transmission before they’re connected to the crab module. This also allows for other applications of the motors in a drivetrain including powering other modules and other wheelsets.
So, presumably, the gold bevel gear is the input gear from the motors (transmissions, etc.)? The red bevel gears then turn, turning the two sprockets on the left, turning the wheel?
The module is itself turned by the two sprockets just below the gold bevel gear? These sprockets are concentric to the shaft the bevel gears are fixed to, but they’re not attached. Instead, they’re attached to the part of the module that houses the wheel.
Assuming all that’s correct, there are some things I can’t get my head around, exactly, and I was wondering what solutions y’all found to them, or if they were a problem at all.
Primarily, wouldn’t driving the wheel while turning the module create the potential for each operation to interfere with the other?
That is, if the wheel wasn’t being driven and you rotated the crab module, the wheels would presumably roll on their own due to friction? If that were the case, wouldn’t this drive everything backward until you reach the motor? …and if that happened, wouldn’t the motor resist the motion, thus resisting turning the crab module at all?
Or, because it appears you’re rotating the wheel about a single point of contact on the carpet, this wasn’t a problem? That is, any other arrangement but what you have here would work as I described above?
…hrm, maybe I just answered my own question. 
Still, if I’m misinterpreting something, I’d appreciate it if someone could set me straight.
M. Krass,
Everything you said is correct. The only reason we had 2 sprockets on the crab module was for spacing. One crab module only had the lower sprocket. The Van door motor did the steering of both steering wheels. The “torque steer” you describe is real, but we only used 4" wheels and it was very minimal. We did not see any reverse motion when steering and not driving. Last year we did not use the sprockets near the wheel and used two wheels with the drive gears in the middle. The torque steer was pretty noticeable, but it didn’t really hinder us all that much.
One note on your observation about the reasons for relocating the transmission using bevel gears. The main reasons we do it is to save space and to be able to shift rather easily. A shifting transmission on turning wheels makes them a pretty large thing to package.