datlas 

This is now ancient history, but the original 10 speed Ultegra 6700 chain (cn6700) had a major defect that caused breakage (it happened to me) and that’s why they stopped making that version and quickly changed to the cn6701. So even 10s chains can break if defective.

SpedFast

Yeah, that'll make them too soft to even use. What were you thinking?

SoSmellyAir

It was (and still is) pretty hard to get OMS in SoCal.

puma1552

OK, what the utter hell did I do wrong here?

-I replaced the chain with the exact same model of Shimano chain.
-I sized it to match the prior chain, which was sized to match the original Shimano chain as delivered from Colnago.
-Text on the chain faces outward.
-Quick link is fully seated on both sides.
-I routed it properly through the jockey wheels; I took pictures before I removed the old chain and verified everything is routed properly through the tabs etc. near the jockey wheels. I was careful to never really touch the rear derailleur in any forceful way. Derailleur hanger is not bent and is flush to the frame as always.
-Chain runs beautifully. No hiccups, no stiff links, no nothing. Runs flawlessly forwards and backwards in any gear or combination of gears.

EXCEPT -

-As I've now been sitting here running through the gears over and over and over for almost two hours now, I can't for the ******g life of me get the goddamn chain to move off the smallest cog to the next one with a single shift. This was NOT a problem prior to replacing the chain, and I did not mess with the derailleur at all. And here's the kicker - when I first started testing it after the replacement, it wouldn't shift. Then suddenly it was shifting off the smallest cog perfectly fine. Then as I was about to make one last run through the cassette up and down, it quit moving off the small cog again and now I'm sitting here absolutely pissed this isn't working and this is turning into an all damn day chain replacement.

Ideas? Entire drivetrain is 5800 series 105 including chain and cassette, so it is NOT a compatibility issue. Only thing I can think of is this was like this before the chain replacement and/or the forces on the chain/bike/derailleur when out on the road are different than when holding the bike up by the saddle and shifting with no load on the bike such that it's creating unfavorable dynamics that don't exist when the bike is out on the road under load - I had no such issues shifting recently when out on the road just prior to this chain replacement.

SoSmellyAir

Mis-sizing the chain by a link (either too short or too long) should not affect shifting between the two (physically) smallest cogs in the cassette if you are on the big chain ring. Assuming that the RD cable housing(s) is/are seated properly at the RD and at all entries and exits through the frame, or at all stops on the frame if externally routed, your symptoms sound like a severely frayed RD cable that is about to snap; when was the last time you replaced the RD cable and housing?

puma1552

You were on to something with the cable - it turns out I guess I was just overdue for a half turn counterclockwise on the rear derailleur barrel adjuster. Did that and we are butter smooth throughout the entire cassette now. I had a feeling prior that I was probably getting to the point where an adjustment was probably going to be needed since it's been awhile, but I was reluctant to do that while doing the chain replacement since I didn't want to start messing with multiple variables while doing a like for like chain replacement. But sure enough, as soon as I went ahead and turned the barrel adjuster a half turn, everything became gravy all the way up and down the cassette including the multigear change friction sweeps. I guess I was just in the back of my mind knowing I probably needed to do that but not maybe realizing how bad it needed to be done until I sat down to replace the chain; I guess replacing the chain forced me to acknowledge I was also just plain overdue for that tension adjustment even if I felt like it was shifting fine out on the road - maybe it actually wasn't, I am rarely on the highest gear on the bike so I guess it must've been that way before the chain replacement and I just didn't notice it the time or two recently that I may have been in that highest gear.

Thanks all, looks like we are good to go now. Got pretty pissed for a while there, lol.

SoSmellyAir

Glad that a simple RD cable tension adjustment solved the problem. I hope the solution holds. Do note that a frequent need for tension adjustment for an RD cable that has been working just fine for a while may reflect fraying and the imminent demise of the cable.

puma1552

Thanks, it's only the second adjustment I've ever made on this cable so hopefully we're good. Guess it just shows that I should've tested the shifting off the road right before the chain replacement to surely have an apples to apples before and after comparison.

EDIT: nevermind, that's the first time I've ever adjusted that cable. Was mixing up my bikes.

DOUBLE EDIT: Did a 30 mile ride and everything is perfect with both the chain and shifting. Not one single skip or bad shift.