Originally Posted by
HTupolev
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.
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A mountain 1x could certainly achieve super-low gears, but that's got little to do with it being 1x.
I'd be really curious what mods were performed to make a 41T capacity RD handle "alright" a 47T total capacity (16T on the crankset and 31T on the cassette). If the b-screw is long enough, in theory you could get the RD to clear a 42T sprocket, but the RD was designed to handle a 36T maximum low sprocket, and while it is common (I've done it myself) to run a few teeth more than spec'd, 6 teeth sounds like a lot. Regardless, the capacity of the RD is still 41T. Several years ago I set about to widen up the gear range on a bike, and ignored the capacity on the RD, and it shifted terribly, and then I got smart on why, and now won't exceed it. Maybe Shimano builds more fudge in the spec'd design now, such that when they say 41T it can reliably handle much more. This is a lot of words to say that the manufacturer specified design of the GRX400 RD precludes an 11-42 (25T spread) cassette with a 16T spread on the crankset.
If this is something the OP is planing to do, unless they've got extensive experience doing their own wrenching, it would require some serious discussions with a knowledgable and trusted mechanic. I will modify my earlier comments to say, these things may be possible, but they aren't as simple as juts putting on whatever cassette one desires to run and turning the b-screw in.
Also, one doesn't have to go to a MTB group to get the components to run 1x...but the GRX400 level won't do it. It was designed to be run in a 2x drivetrain.