Our team has done tank treads for the past 6 years, so we have done quite a bit of research in the topic. If you are going to use treads, consider them just like wheels: Use 6 rollers on the bottom and drop the middle one (3 on each side). We used an inverted fan belt as our tread; you have the sproket from the gearbox pushing down on the top and driving it. If my explanation wasn't very good, the full CAD of our latest robot is on
FRCDesigns . Team 639.