Originally Posted by
hokiefyd
The M591 is of a traditional derailer design where the upper pulley rotates up as the cage rotates rearward...to keep the upper pulley tracking close to the sprockets as it shifts down the cassette. The M592 is a "Shadow" derailer and, as you're seeing, has a different geometry. It's designed for fairly large cassettes, so it may very well be that you can't get the upper pulley very close to the cassette, even on the largest sprocket. Making this situation worse is the geometry of the upper pulley -- its axis is
on the rotation axis of the cage, so that upper wheel does NOT rotate up as the cage rotates rearward. So the gap you already have at the largest sprocket becomes even wider as you shift down the cassette, resulting in poor shifting behavior.
I've had a few of these types of derailers on several bikes and I just don't prefer them for this very reason. I find them very difficult to tune correctly because of that gap. Note that some of Shimano's newer derailers of this style (like the
Deore M5100) have gone back to a more traditional cage design where the upper pulley rotates up as the cage rotates rearward. I think one of these would work better...but they're not available in 9-speed, unfortunately.
I bet you can put your old M591 back on the bike and get it shifting perfectly pretty easily. If so, I'd humbly recommend you buy a new M591 (or other derailer of similar design, like an RD-T4000) if you need to replace your current one. Another option might be switching to a Microshift Advent and accompanying shifter...these usually work very well.
Thank you, hokiefyd, for the very helpful reply. My original intent was to do a like-for-like
derailleur replacement, but inadvertently got the 592 instead of the 591. Since I thought that, in principle, both should work, I just went with it. And it nearly does work, it does shift the chain between sprockets, it just has this very annoying lag when reversing directions. I replaced the old, 591
derailleur since its jockey wheels were extremely worn and, more importantly, the derailleur frame seemed to be a bit bent/twisted from an accident, which is why I don't simply swap back to the old one. If I understand you correctly, the different geometry and movement style of the 592
derailleur results in a larger pulley wheel / cassette gap than typical (and larger than what I had before with the 591), which means there is a little extra slack in the chain at this critical point. So when the
derailleur tries to pull the chain to change gears, it cannot get quite enough movement to make the change cleanly/completely. I probably compensate for this by adjusting (biasing) the cable tension so that at least one shifting direction works alright, but this results in the other direction being worse off and thus producing the shifting lag going to the other way.
Regarding
Kontact and KCT1986's comment on the side that the fixing both that clamps the cable, could you elaborate on how that could possibly affect the shifting issue that I am describing? At the end of the day, despite the cable itself perhaps being a bit more deformed, that end of the cable is secure and transfers its tension to nearly the same location on the derailleur.