Derailleur adjustment with compact crank

[HillSlugger]

My FD has been well behaved on my last two rides but yesterday the RD decided that it wanted to skip past one of the cogs towards the middle of the set. Everything else seemed OK. What would cause this all of a sudden and what do I do to fix it?

[aicabsolut]

Did it do it repeatedly?

Sometimes, we just get the "throw" wrong of the lever. Other than excessive wear to the cassette or chain, that's a fairly good guess when you have skipping problems on one cog and towards the middle. Did it matter which side you were shifting from? (Did it skip only when shifting easier or harder into that cog?)

[HillSlugger]

Did it do it repeatedly?

Sometimes, we just get the "throw" wrong of the lever. Other than excessive wear to the cassette or chain, that's a fairly good guess when you have skipping problems on one cog and towards the middle. Did it matter which side you were shifting from? (Did it skip only when shifting easier or harder into that cog?)

Yes, repeatedly. Yes, in both directions. The chain, cogs, and cranks are all new, with less than 250 miles on them.

[aicabsolut]

Only in the middle on one cog? You might want to check that the cassette is still on there tightly.

Hmm... the only time I had a similar experience it turned out (weirdly) that the rear cable was installed too tightly. I thought maybe tightening it would help because it was pretty new, so when that didn't work, I loosened it and the problem was solved. I'm not sure that's typical, though.

[HillSlugger]

Yeesh, I just realized that I rode this week on my race wheels, the first time in a while. I guess the cogs aren't lining up quite the same as the other wheel?

[aicabsolut]

Yeah, the spacing can be different between wheels meaning you might need some derailleur adjustments to make shifting optimal when switching. Are you running the same cassette spread on the two sets? That's another factor, though mostly has to do with the B-screw.

[HillSlugger]

Yeah, the spacing can be different between wheels meaning you might need some derailleur adjustments to make shifting optimal when switching. Are you running the same cassette spread on the two sets? That's another factor, though mostly has to do with the B-screw.

Yes, same cassette on both (12-27). Didn't have this problem on my road bike with the triple/long rear cage.

I have discovered that my chain length *will* allow me to get onto the big-big combination. At the other extreme on the small-small (or next to smallest cog) the jockey wheels start to rub on each other.

I suppose I'd be much better off if I could HTFU and learn to ride a 12-25!

btw I made some adjustments to the tension using the barrel adjuster and the shifting was better behaved today, but not perfect. Instead of skipping past the 3rd cog (or was it 4th?) it was reluctant to get up onto it, but only when I was in the small chain ring. I suppose I should be switching to the large chain ring instead of going to this combo.