Hey cool, one of my Crabs got mentioned! To be totally honest, that was a previous iteration. I hadn't published this until now, but here's the latest version (assuming we're allowed to use our own wheels again).

http://picasaweb.google.com/ironspork/CrabDrive#

If you're going to do a crab, you're really going to have to commit. Get your machining resources in line, and make sure you can do it before you try. A crab frame doesn't convert to a 6 wheel/4wheel if you mess up, not very well anyway.

Good luck!