Originally Posted by
SoSmellyAir
No, a RD-5800-GS has a
minimum large cog size of 28T.
RD-5800-GS (shimano.com). But I am using a 12-25 cassette, forcing me to have the B screw all the way out. I can get the two RD pulleys pretty close to vertical but not perfectly vertical. It shifts fine so I am OK with that. Next year, before I replace the RD cable, I am also installing a RD-6800-SS and a 11-28 cassette, so maybe then I can get the two RD pulleys perfectly vertical then.
Minimum large cog?? There’s such a thing?
Well, the manual you linked to, for a 2x, on page 8, specifies “Max 27T
or less.”