[jskene]

With 4-wheel, tank-drive, as you are using, your robot must be wider than it is long, or you will have trouble turning. Make sure your robot is 36" wide and 30" long, not 30" wide and 36" long. This will make a huge difference in the ability of your robot to turn.

If you cannot do this, then ensure that the lateral friction of your wheels is less than the longitudinal friction. That is, make sure that it is easier to push your wheels sideways on the carpet than it is to push them forward or backward. You may be able to do this by modifying the tread pattern on the tire, as mentioned in an earlier post.