praise dakinis

I recently got a Shimano GRX RX400 10-speed rear derailleur which I’d like to set up 1x for my fantabulous and slightly wacky city commuter (think Spinergy Rev X wheels, late ‘80s steel frame in neon colors, mustache handlebars). It has a capacity of 41t and max/min top sprocket of 11t, but I’m just noticing on the Shimano site that it says “Low sprocket: Max. 36T/Min. 32T”. The Ultegra cassette I’ve got is 11-25t. Will that cassette work with the derailleur even though its low sprocket is just 25t? (FYI: I’m thinking to run a fairly large chainring on the front, but haven’t settled on a size yet.)
Thanks!

Noonievut

Short answer - I'm guessing it will work fine. That being based on how flexible the GRX components are with other similar speed components (e.g., Tiagra 10sp). I thought I read a post about someone using a chainring with a large sprocket of 46T.

Might be a good question for the bike mechanic sub-forum, from the perspective of understanding how a RD works with cassettes and why it has a stated range, how people get around it, and what is the absolute min/max (and why). I would be curious for one...maybe we'll get the answer here.

Elvo

The cage might be too long for such a small sprocket. Also keep in mind that you need to use 10 speed tiagra or 10 speed grx shifters with a rx400 or r4700 derailer.

dsaul

It will work, but the upper jockey wheel will be too far away from the large cog for ideal shifting. The bigger issue is that the GRX 10 speed derailleur is designed with 11 speed cable pull, so (as Elvo said above) you need to use a GRX 10 speed shifter, Tiagra 4700 10 speed shifter or switch to 11 speed cassette and shifter. It won't work with any other 10 speed Shimano shifter.