tank tracks issue with chain elements

Hi guys

here is a file where I set up a simple animation following some tutorials
available online.

the problem is it only works i a left or right animation along the x axis.
everything else makes it explore.

any tip?

http://dl.getdropbox.com/u/239525/Tank_Track.blend

You already started a new thread for this? Why?

http://blenderartists.org/forum/showthread.php?t=170284