hey everyone, i've got a bit of a problem here.  I'm supposed to model a variator.  The problem is this, how do i model a belt/chain loop where the two radii of the loops are meant to change? I am reasonably certain it can be done because of this video:

http://www.youtube.com/watch?v=38TscMR687Q&feature=related

If anyone could help me out i would greatly appreciate it.

many thanks

ben