[spacepenguine]

Our standard 6 wheel base has all 6 wheels powered. As long as you drop the center wheel, you shouldn't have a problem with turning and it will react nearly the same whether it's rocked forwards or backwards as long as the weight is distributed evenly. We also direct drive our middle wheels off of the gearbox shaft and then use belts to drive the back/front wheels. That way if all the belts break, we'll still be moving because the middle wheel should always be on the ground.

Alternatively, you can change the center of rotation on a tank drive by driving say the back 4 wheels and putting omnis on the front. This would be good if you had a rear mounted tower and wanted to rotate around the base of the tower, for instance.