Originally Posted by
AlanKHG
Hmm, a little more experimentation and it seems to me the crux of the issue is that there's a lot of slack in the chain when I have a lot to wrap, and that the parallelogram pivot seems less springy than it ought to be. This derailleur is on a bike I found abandoned, so the elements may have caused that pivot to not be quite as springy?
There is no spring on the Duopar pivot. In use, the derailleur body is pulled to the fully forward position (as in Pastorbob's pictures) by chain tension. The derailleur body can pivot back to facilitate wheel changes.
Huret used a couple different mounting bolts as well. Most of the Eco models I've seen have a simple bolt with or without a locknut on the back of the bolt inboard of the hanger. Some of the titanium ones have a split mounting bolt with internal threads and a small allen head bolt installed from the inboard side of the dropout to lock it in place.