the treads would stay on with a groove in the middle but the modules would only turn when the robot was moving so this would hopefully not be much of a problem. I would be using a fisher price to turn the module through a 42mm 256:1 banebot gearbox.
The reason that I don't want to use a third roller to reduce turning force is because the module passively rotates on one of two axes. If there was a third wheel then it wouldn't be stable and there wouldn't be as much surface area on the ground and the point force is higher. Since this would be for an all terrain robot this would be detrimental to moving through things like mud. I know that the gears need more protection for things like that.
Thanks for the thoughts and that lego bot is sweet
