Originally Posted by
SamSam77
Despite being similar to the old derailleur, the new M592 derailleur has a slightly different geometry with how it sits on the bike. One result is that the b-screw adjustment cannot bring the derailleur pulley as close the largest cassette sprocket as ordinarily suggested (i.e., turning the screw to its limit still leaves a relatively large gap), though this is often a fairly forgiving spec. Could this explain things? I am still using the chain length (number of links) that I was using with the old derailleur. Another idea is that maybe the chain needs to be slightly longer or shorter by a link or two? Though this, too, I would not normally expect to enable shifting perfectly in one direction but cause lags going back the other way up/down the cassette.
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.