Chain sizing/shifting issues question

[RobJ]

Large chainring, large cog, with the chain not threaded through the RD is the way I have been sizing chains for my bikes. Add two extra rivets using a master link chain. Never an issue.

I am finishing up a build and adjusting the derailleurs and the chain won't shift into the last two large cogs when in the large front ring. Realize that brings up cross-chaining and not a way you would normally ride, but still should shift into this combo. It's a 11-28 cassette and Ultegra 6870 reg cage RD. Spec'd at 28t max. When trying to shift up the rear the chain totally locks up. The RD pulleys are pretty much horizontal which makes me think its a chain length issue. If the FD isn't aligned or adjusted enough outward, I see where that could be an issue, but then it wouldn't shift properly in other cogs/combos. Any thoughts?

[rustychisel]

Originally Posted by RobJ

The RD pulleys are pretty much horizontal

As in pulled forward such that the derailleur cage is almost parallel to chainstay? Eek. Add two links to chain and try again.

Chain gets jammed between cassette and upper pulley at that angle, most likely.

[oldpotatoe]

Originally Posted by RobJ

Large chainring, large cog, with the chain not threaded through the RD is the way I have been sizing chains for my bikes. Add two extra rivets using a master link chain. Never an issue.

I am finishing up a build and adjusting the derailleurs and the chain won't shift into the last two large cogs when in the large front ring. Realize that brings up cross-chaining and not a way you would normally ride, but still should shift into this combo. It's a 11-28 cassette and Ultegra 6870 reg cage RD. Spec'd at 28t max. When trying to shift up the rear the chain totally locks up. The RD pulleys are pretty much horizontal which makes me think its a chain length issue. If the FD isn't aligned or adjusted enough outward, I see where that could be an issue, but then it wouldn't shift properly in other cogs/combos. Any thoughts?

Chain(obviously) too short..try small-small and chain just off the bottom of the rear der...spring just starting to be pulled..

[mt2u77]

Originally Posted by oldpotatoe

Chain(obviously) too short..try small-small and chain just off the bottom of the rear der...spring just starting to be pulled..

Yep. That's the way I've always done it. Gives you the max length chain, which is nice if you ever want to throw a bigger cassette on for a hilly ride. Plus, it's always easier/cheaper to take another link out rather than add them back in.

[FlashUNC]

Chain wayyyy too short.

Reality is all modern stuff is designed outta the factory with cross chaining in mind. Manufacturers know we do it, might as well build that in.

[dddd]

6870 RD should easily shift to/from the largest cogs IF the chain is in fact two links longer than the full-tight condition on biggest sprockets.

Maybe post a photo of the derailer trying, should provide clues as to what's going on with your particular setup.

Chain length on bikes with vertical dropouts needs to be from 2-4 links longer than full-tight, determined by the exact "chainstay length".

Also, in bicycle lingo, "one link" equals 1/2".

Oh, one last thing I would do first, I would shift to the biggest cog/smallest chainring, then back out the B-tension screw until the pulley teeth are nearly rumbling against the tips of the largest cog's teeth, since the pulley will only move further from the cogs when the large ring is selected.

I had someone come by just yesterday with a Shimano 10s road setup that shifted with great hesitancy, despite having fresh cabling that was working well. I determined that their asymmetrical Shimano chain was installed backwards by a local shop. I myself had a Shimano 10 road bike that despite good cables and adjustments shifted just as poorly as that one after I installed a new SRAM 10s chain, and which subsequently worked perfectly with a same-length Shimano 10s chain.

[Bwana]

Are you sure your cable tension is right? I had trouble setting up a Rival 22 RD the other day, despite all of the limit screw adjustments and stuff being right, it would not shift up the last two cogs.

Finally figured out I hadn't pulled it hard enough to seat a cable stop and when I shifted the first few times it seated the stop, but now the cable tension was way too low. Gave me fits for a good 10-15 minutes

[loxx0050]

Originally Posted by Bwana

Are you sure your cable tension is right? I had trouble setting up a Rival 22 RD the other day, despite all of the limit screw adjustments and stuff being right, it would not shift up the last two cogs.

Finally figured out I hadn't pulled it hard enough to seat a cable stop and when I shifted the first few times it seated the stop, but now the cable tension was way too low. Gave me fits for a good 10-15 minutes

Not possible for the OP since he is setting this up with Di2 (6870 or electronic).

[dddd]

One thing about Di2 is that the derailer moves exactly at it's own pre-set speed while shifting, so perhaps the abrupt movement is causing the chain to lock up as the cage jams it against the larger cogs.
So in this case, I would try tightening the B-tension screw to gain clearance, as a diagnostic to see if this allows the shift to occur freely to those largest cogs.

[RobJ]

Thanks everyone for the helpful insight.

OP, not me the OP, OP as in OldPotatoe had it right. Had another chain sitting around and used his sizing method and bingo! Actually surprised how much a difference in length the two different methods produce. Park, GCN and a large number of sites/videos all show the method I was using. Think I will stick with this one...

[oldpotatoe]

Originally Posted by RobJ

Thanks everyone for the helpful insight.

OP, not me the OP, OP as in OldPotatoe had it right. Had another chain sitting around and used his sizing method and bingo! Actually surprised how much a difference in length the two different methods produce. Park, GCN and a large number of sites/videos all show the method I was using. Think I will stick with this one...

Huzzah..works everytime, even with some 'odd' setups like 12-3010s/12-29/11s and short cage rear der..

[Bwana]

Originally Posted by loxx0050

Not possible for the OP since he is setting this up with Di2 (6870 or electronic).

Oh whoops. Another detail I missed. I'm on a roll lately.

[dddd]

Now I'm curious as to what length of chain is working now.

Starting at full-tight on the largest sprockets, are you now adding 4+ links to that? As I mentioned, the normal range of chain length is from 2-4 links based on the exact chainstay length.
Perhaps here, something like 4.0 links is better than 2.0 links(?), at such exact chainstay length you would have to make that choice.

With the 11-28t and 50-34t together putting the largest-allowable tax on your particular derailer's chain take-up capacity, might the chain now sag when using the smallest sprockets?

And just to clarify, I never ride with the chain on the smallest sprockets, but do quite frequently engage the big-big crossover, usually as I crest a hill. I expect a road bike to shift snappily and quietly to-from the largest cogs while in the big ring, better yet if no trim action is needed.

[Matthew]

I've always done it like oldpotatoe too. Works like a champ.

[tombtfslpk]

I agree, small small is my preferred, but you do have to run it through the derailleur. Sometimes that takes three hands.

OldPotatoe's picture might have blocked the tool he uses to qualify length.
Take an old spoke, I use welding wire, or wire coat hanger (do they even have these any more?). Bend each end just past 90 degrees. Hook the ends through the chain links and move one end a link at a time to get that "just right" spot on the rear derailleur. Hold the dangling chain ends together to determine where to open the chain for a quick link or pin.