It's to get the chain takeup of a long-cage derailleur, without the long cage. A shorter cage is less likely to get bent, or to get banged against rocks, roots and other debris.
Suntour did a full model range of 3-pulley rears as a special arrangement for Nishiki (early 80s?). My LBS (the late lamented Missing Link of Berkeley CA) had a display in the window of their repair shop with about six different models of 3-pulley derailleurs mounted on it: LePree, Honor, Cyclone, XC, Mountech etc. I had a 3-pulley Mountech sitting in my parts box for about a decade, waiting for a project. When I suddenly had to build up a Trek 720 to replace the Raleigh International pack mule I wrecked just before Christmas 2021, I was about to transfer the Huret Duopar Eco over from the Raleigh when I remembered the 3-pulley, and decided to try it for awhile. I've been riding it for about 18 months, and it's been perfectly fine. And I've never pretzeled the pulley cage by accidentally bumping a barcon and getting the derailleur in the wrong gear, which is more than I can say for the Duopar Ecos; I've twisted two Huret cages that way, which is why I've collected four or five of them. In operation, the "third" pulley at the lower front of the Suntour is a secondary tension pulley; it only pulls into contact with the chain when the chain is especially slack. When the chain is running loose, the second pulley (primary tension pulley) at the lower rear swings backwards, bringing the third pulley into contact with the chain and supporting the run to the chainring better. When the chain's tension increases, the cage rotates forward, disengaging the third pulley from the chain. Peter Adler Berkeley, CA/USA On Sunday, August 6, 2023 at 5:48:25 PM UTC-7 alan lavine wrote: I have one of those 3 pulley derailers also but haven't used it. What's the purpose? -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bunch+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/rbw-owners-bunch/0947de94-83ae-4ac3-878a-a3d7dce5a29an%40googlegroups.com.
