Originally Posted by
Badger6
Better than fair. Max low sprocket size (as spec'd by Shimano) is 36T. You will see where folks might get an additional 2T out their RD, but let's be realistic, at that point you're running the RD beyond spec and performance MAY be affected, as in won't shift reliably onto the low (biggest) sprocket.
The way to get a derailleur to handle huge cogs is to get the jockey way to sit farther from the cogs (through b-screw and/or hanger extenders), so poor shifting is more likely to happen in the high gears.
I have one friend using a RX400 derailleur with an 11-42 cassette and a 48-32 crankset, and it seems to work mostly alright. I don't remember if he's got a hanger extender, but they're easy to add if needed.
IMO, without scrapping the existing drivetrain and converting to 11 speed (or even 12 speed) and 1x up front
I don't see what that solves.
A mountain 1x could certainly achieve super-low gears, but that's got little to do with it being 1x.