Dave, this is the best way to do a tank track that i've ever found
It works for tank tracks, Chains on gears, Chain saw blades, anything of that nature.
ITs the way i would set up a system like this that was necessary for animation.
allows for geometry constraint to the ground plane, and easy setup of SDK linked and driven y the Tanks group node's translate and rotate so that all you ever have to do after initial setup is to just grab the tank group in outliner and translate or rotate it as you please, everything else will then be automated.
hope this helps,