pic: Crab Drive

[RogerR]

judging from the picture it looks like the middle of your wheel and the center of rotation of your module (the vertical rod, i'm assuming) are offset from each other. depending on the wheel traction and strength of your drive and rotation motors, this could cause it to behave oddly. also, if you were to get into a pushing match with this set-up, your modules could get pushed out of alignment as well. this is not really a major problem; all you need to do is shift the module's center of rotation so that its centered over the wheel.