Originally Posted by
Antifriction
Duragrouch: you can get 15-84" on a folder, but not with the Super Short - it has 39t tensioning capacity, and the configuration you mention needs 47t. I don't know of an RD usable on folders with that rated capacity, but 43t can be had with Shimano's Smart Cage siblings, Tourney TX-800 and Altus M310. That's enough for this:
Allowing rattle, their capacity is 47t, but unfortunately their max cog is 34, & I tested that on another pseudo-cassette, & wasn't able to shift onto the 37t, even with B-screw maxed.
Smart Cage is a clever trick that permits that whopping 43t capacity with reasonable ground clearance. Here's BF's take on it:
What is shimano smart cage on a rear derailleur?. The Altus is the better of the two - metal body instead of plastic, and due to an exceptional 15t lower pulley - you can see how huge that looks on a derailleur as it's fondled on this 48-second video
48-second video
even better clearance than the Tourney; it's
Grant Peterson's all-time favorite RD, shipping on lots of Rivendells.
Regarding 20t gap on the front, it can be done; I ran 28-48 on my Dahon for 2 years, using a Tourney FD I hacked up (because clamp was too small for Dahon tube) & attached with hose clamps, and Sheldon Brown used to rock a 51/26:
Regarding conservative ratings & the hidden powers of cheap derailleurs, you might enjoy a deep dive thread, entitled
What Shimano doesn’t want you to know about the $12 Tourney TY-300. Eating this 11-40 1x for breakfast, in the XBiking subforum of a site whose name I believe we're not allowed to speak here.
Lots in common! When I went 2X on my Dahon, I used a Tourney with GS cage, all of $13 at the LBS. It was the only one that had a claw mount plus worked fabulously, I think 43 or 45 cap, has a bit larger pulleys than older RDs. (I had used this once before on a Dahon Speed (stock 1X) that had instantly shredded the plastic Dahon forward-mounted RD on test ride because of a rusted chain link, both Dahons of that era had no standard RD hanger, plus Dahon wanted $80 for replacement of their cheap RD. And, the Tourney reversed shifting back to "normal" (from rapid Rise), using same gripshift, just indicator numbers wrong.)
My new Dahon (not yet unpacked) has a replaceable RD hanger so can use any standard RD.
The Dahon Curl D9 has a 3rd pulley (with no teeth, just a roller I think) forward 90 degrees to the idler puller, for more chain takeup on the highest cogs, the bike is 349 wheels so very limited clearance for a conventional RD cage.
The M310 in the riv link looks nice, I can see the appeal, but it won't work on a 42, just up to 34, which is what my Tourney GS does. But for a huge drop at the chainrings it would work. I personally think that unless you are a road racer, go for as wide a gear spacing as you can get on the front, having 2 rings for only 1 additional gear is a waste for me.
Yeah I hate that it's impossible to find a clamp FD for the Dahon seat tube OD, you would think by now that ONE supplier on amazon would have one. My old steel Dahon is 40mm I think, I got an old Litepro FD adapter for it. The new aluminum frame I am guessing will be 41 or 42mm, would take a different adapter, used to be sold by same bike shop but now out of business; Weirdly, the one for aluminum Dahons has the FD interface much further aft, perhaps needed for FD cage clearance with the seat tube (I had to grind a bit of the cage lip off of mine, unusually wide and thick, for FD to go far enough in on the needed standard chainline of 43.5mm).
I can't go smaller inner 'ring than 34 on my 110 BCD. I'd need a double or triple with a 74mm BCD inner. Thing is, I LOVE my hollow spindle crank with external bearings; Ages ago, Shimano made hollowtech 2 road triples, but not in a long time. And I want the low Q-factor of a roadie.