[Joe Johnson]

With just the middle and rear wheels in play the aspect ratio is 24 Wide by 12.5 between wheels, this is enough that the thing should turn, even with very grippy wheels (given 2 CIMS each side plus 24:1 to 8" wheels).

Either the torque isn't getting into the gearbox, it isn't getting to the wheels or it is getting to the wheels but something else above and beyond traction to the carpet (including scrubbing loads, aka thrust loads) is using up the torque.

He says that the motors are both wired right. The thing moves at about the right speed going straight. I suppose that going straight it will use the right amount of current.

To me that means something is binding when the robot tries to spin. The wheels could be hitting something, the frame could be racking and causing a bind, the wheel's may have bad thrust bearings or bad thrust load management, the chain may bind up when the wheels see the twisting loads, etc. ... we are looking for something extra that happens only (or most prominently) when the wheels see significant side loading due to being forced to scrub along the carpet.

I'd bet a Mt. Dew on it.

Joe J.