Now that the VEX line includes a differential, tank style steering gets much more interesting. When I can afford two differential kits, I will try out the double differential steering technique.
I first read about this in Popular Science (July 1985 issue, pages 60-61). {I would post the scan but want to avoid copyright issues.} By using two differentials, one motor “drives” the two tank treads in the same directions while the other motor drives the tank treads in opposite directions. As a picture is worth a thousand words - and others have described it - and I don’t want to retype it, check out the following two links. Apparently, this method has been around for decades.
http://www.gizmology.net/tracked.htm
[http://www.scmec.us/diffs3.html
I want to start with the two-differential steering because I can use gear reduction on the steering motor to gain more “precision” steering by reducing steering “speed”. I don’t necessarily need the steering motor to deliver the same RPMs as the forward/backward drive motor. Thus, the gear reduction for steering.