Originally Posted by
Andrew R Stewart
Things that effect auto shifting is first the lever's friction. Enough friction and the cable will not move, of course you won't be able to shift either. The der's return spring is what's pulling the cable as the cable goes through the tug/relax cycle.
This is the issue that Simplex "retrofriction" levers and SunTour "Power" levers were designed to address. In the Simplex lever, a spring in the lever serves to counteract the derailleur's return spring so shifting against the derailleur's spring takes much less effort. The SunTour levers use a ratchet wheel in a similar manner. Either design is a superior replacement to plain friction levers.