Derailleur adjustment with compact crank

[lunacycles]

Chain wrap length has to do with the size of the large ring, the size of the large cog, and the length of the chainstay. It needs to be long enough to reach from the large ring to the large cog (along the length of the chainstay) without pulling the rear derailleur too far forward.

I can tell you that with a 34T inner ring, a 25T large cog and a smaller, compact race geometry rear triangle, that being one link too long will cause the rear derailleur pulleys to rub the rear cog when in that combo even with the B-screw tightened all the way (how my newest bike was built). There's nothing wrong with the derailleur hangar. I don't have this problem on a bike with longer chainstays or a bike with a 36T inner ring. With a compact, you still need a lot of chain wrap to get into the larger cogs while in the big ring, but then there's even more chain slack when in a 34T ring compared to standard cranksets. Of course being one link too long also causes a little bit of rub on the bottom part of the pulleys sometimes, but this isn't a problem functionally. Not like having the derailleur body bumping against the cog. I used a chain length calculator and verified that the chain was one link too long. Removing it still requires the B screw to be all the way tight, but there's no more bumping. There is more noise in the big/big combo, and I'm sure there's rougher tension on the chain when in the big ring by looking at the RD pulley angles, but it was necessary.

Howdy aicabsolut, thanks for your reply and for the opportunity to respond again.
Over 20 years of wrenching I have experienced my share of b tension screw problems (i.e., derailleur guide pulley bumps against largest rear cog in small/large combos), but in my experience I haven't found that removing a chain link is the best answer, particularly if it creates a situation in which the chain is potentially too short and can potentially cause permanent damage to the components or frame. I hear you had this problem on your current bike and you solved it by removing a link. That's impressive troubleshooting. But to say this is a general tendency on bikes with short chainstays and a compact crank is a bit misleading/generalizing, imo, particularly in recommending that one should consider running a chain that may be too short in an effort to solve it (per your post previous to this one). Given the bikes I set up (frequently with the drivetrain set up you describe) I personally believe your situation is more the exception than the rule.

As a manufacturer, I believe what is the more likely culprit in your case is that your derailleur hanger is either not correctly aligned in the fore/aft sense or your hanger is too short for your app. That is: the threaded portion of the hanger needs to be the correct distance behind the dropout or shifting issues like this can arise. And/or, if the vertical distance between the dropout and derailleur mounting hole is not long enough, you can have the problem of the derailleur pulley being too close to the cog when running a larger rear cog, regardless of b-tension screw adjustment (and one reason I tend to put longer derailleur hangers on the majority of my frames). A lot of road frames come with short hangers, and maybe the one on your frame just isn't designed for a 34/28 (although that would seem odd given the current road offerings). Anyway, big old thread drift putting readers to sleep is going on here, but I did want to clarify my explanation a bit.