Bicycle Mechanics - Ultegra RD6600 GS & a 28t Cog?

I know the largest cog spec on this 2009-10 10spd long cage derailleur is a paltry 27t, but does anyone know (for sure) it will handle a xx/28t cassette? I'd like to sneak in 28t, but not at the risk of shifting problems. Thanks.

I have a 11-32 on mine.

I have a 11-32 on mine.

Irrelevant. Completely and utterly.

OP: The 27t spec is based on an assumed hanger length. Your bike may have a hanger that is shorter or longer than that. You won't have shifting problems if it clears the 27t right now (assuming the b-screw is set properly to begin with). The problem you *may* run into is that the upper pulley won't clear the 28 and depending on how much, may be un-useable.

The bottom line is, try it.

OP: The 27t spec is based on an assumed hanger length. Your bike may have a hanger that is shorter or longer than that. You won't have shifting problems if it clears the 27t right now (assuming the b-screw is set properly to begin with). The problem you *may* run into is that the upper pulley won't clear the 28 and depending on how much, may be un-useable.

The bottom line is, try it.

Got it! :thumb: Any rule of thumb on mm of distance needed between pully and cog? If I wanted to push it, I suppose I could shoot for xx/30t. I canmeasure the mm distance between the 27t and the pully now.. but what's the min. spread I should be willing to chance?

Got it! :thumb: Any rule of thumb on mm of distance needed between pully and cog? If I wanted to push it, I suppose I could shoot for xx/30t. I canmeasure the mm distance between the 27t and the pully now.. but what's the min. spread I should be willing to chance?

You want the gap between the largest cog and the upper pulley to be as small as possible without the pulley hitting the cog. The smaller the gap the better in regard to shifting performance, just as long as the pulley doesn't bump into the cog. The B screw is used to adjust the size of the gap.

And to add a little more info, the fact that it's a GS derailleur has no affect on the largest cog compatibility, the SS derailleurs are the same in that regard. The cage length affects how much chain the derailleur can keep taut (i.e., chain wrap capacity).

A 28T cog will have a radius about 0.08" (2mm) larger than a 27T so if you have, or can adjust for, at least 2 mm clearance between the upper pulley and the current 27T cog, the 28 should work.

Generally, posters have reported that even a 30T cog works with most Shimano road rear derailleurs on most frames but, as operator noted, a close coupled derailleur hanger can keep it from working on some frames.

Odds are good that a 28 will work, a 30 may or may not. I have run a 28 on my wife's 6500 GS. Hanger was built into the dropout plate (Trek OCLV).

And the result:

A IRD 12-30 fits no problem. I gave it a little more room with about 2mm added from the B screw but it would have worked, shifted OK, anyway. I think I could achieve about a 6mm clearance if I cranked the B screw all the way down, so some room to spare. Thanks all.