Quote Originally Posted by lunacycles View Post
If your rear derailleur hanger is designed and aligned correctly for the gears and components you are using (e.g., "road") you shouldn't have the rear derailleur guide pulley bumping up against any cog with the b-tension screw adjusted correctly...just hasn't been my experience. Even if this were to be the case, I'd rather live with a little chain noise in the small/large combo than risk ripping my rear derailleur/derailleur hanger off due to a too short chain...

I determine chain length by "dummying it up" in the small/small combo, and ensuring the chain is just short enough that there is no chain rub against the bottom part of the rear derailleur cage in this combo. I then shift through all gears to ensure it can handle them, keeping in mind stated derailleur capacities as stated by the manufacturer. This has never failed me.

And I am trying to understand: what does chainstay length have to do with it? Thanks.
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.


MDHillSlug, I guess you can try loosening the H limit now some, but do it slowly. Like 1/8 of a turn at a time, then re-check the shifting. You might want to put up with a little bit of rubbing if you don't use the small cog a lot, if that helps you keep the chain on.