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TBLS
03-27-2016, 10:50 AM
Considering changing inner ring on Ultegra 6800 11 speed compact crank from 34 to 42 (shimano 9000 ring)...Will front derailleur (also shimano 6800) work or will it need adjustment? No change in outer ring


Assume no but often incorrect!

oldpotatoe
03-27-2016, 11:13 AM
Considering changing inner ring on Ultegra 6800 11 speed compact crank from 34 to 42 (shimano 9000 ring)...Will front derailleur (also shimano 6800) work or will it need adjustment? No change in outer ring


Assume no but often incorrect!

Should be fine w/o doing anything...42/50? Thinking the inner cage won't hit the inner ring when in the big ring...but not sure if it's a 50/42. And for right below..the pairing gig is vastly overplayed. It really means nada.

RowanB
03-27-2016, 11:14 AM
Not 100% on the adjustment, but are you aware the chainrings are paired? I.e. the 53 is made to shift best with the 39.

11.4
03-27-2016, 12:32 PM
This has come up several times before. Ring matching isn't that big a deal -- Shimano shifting is so good anyway. Where you may run into problems is that the big ring is hollow and tapers up towards the teeth. The spacing between the wall of the big ring and the teeth of the small one is the correct distance with, say, a 34/50. When you go to a 42/50, you've moved the teeth up to where the big ring is tapering down to the teeth. Most of this taper is on the outside, but there's some on the inside. It can mean, depending on your frame geometry, how well you adjust your front derailleur, your chain, whether you are cross-chaining, and so on, whether the increased gap between small and big ring will cause the chain to float on top of the small ring teeth when downshifting. I ran into this as a fairly common problem. Shifting up is no problem, but shifting down the chain seems to have shifted but then can skate unexpectedly across the top of the small chainring teeth. Nice way to crash. If your bike lets it shift fine, then no problem. Just watch out for this complication. It doesn't always happen, but it's happened enough on bikes we've worked with (and owned) that I'm cautious to recommend it without precautionary comments.