DougG

I have two bikes with 105 components (ST-5600 series). One is a triple where I built up the drivetrain myself, and is all 105 pieces. The other is a Cannondale compact double with an FSA crank. The bike that I built up shifts up and down perfectly -- no problems.

The Cannondale, however, has problems with shifting in the mid-range of the cassette (it's a 50/34 crank, 12-27 cassette). If I adjust it so that I get consistent smooth downshifts (i.e., smaller to larger cog), then it has a problem upshifting and I sometimes have to click the shifter again, which then moves it down two cogs instead of one. If I then tweak the adjuster to take some tension off the cable for a better upshift, it often misses shifting to the larger cog! I just can't get it exactly "right." This really only happens between the 4th and 6th cog.

Anyway, this is not a huge deal since I can always get it to complete the shift, but I was wondering about a possible root cause. The derailleur is just a slave to the shifter, so I would guess that there could be something out of tolerance in the shifter that causes it to not exert the right pull in these shift positions.

Anyone else had experience with not being able to get things adjusted just right on a similar setup?

FBinNY 

The number one cause of this is cable friction. The wire drags a bit, just enough that the RD spring doesn't pull it all the way back. You compensate for this by biasing the trim outward a bit so it upshifts, but downshifts are sluggish. If you trim in upshifts are sluggish.

There's a very simple test for this. Shift to the middle of the cassette and reset the trim for quietest running in the middle gears and clean down downshifts. working in the problem area, downshift one gear. Pedal a few revolutions to settle everything, then upshift one gear. Odds are it'll hang (that's the problem), now pluck the inner wire hard like a guitar string. The plucking pulls out any hanging wire and when you let go the RD may move out a bit and complete the shift.

If plucking on upshifts makes a difference you've confirmed cable friction. If you're lucky, it's only in the Chainstay/RD loop and oiling that will solve it. Otherwise strip everything, clean and lube the housings with very light oil and reassemble. Make sure all the housings run naturally straight on to the cable stops since any misalignment here causes friction. Also be sure to use ferrules made for index housing, because the conical bottom of a brake ferrule pinches down the end of the housing onto the wire.

DougG

Wow -- good reply! I'll take a look at this ASAP. Thanks.

dsbrantjr

If you have eliminated cable friction as a root cause perhaps you are overshooting the proper adjustment. How much are you turning the adjuster? Sometimes a quarter-turn is too much. Try very small increments.

Retro Grouch 

If that doesn't work look into derailleur hanger alignment. 9 and 10 speed drive trains can be real picky about perfect hanger alignment.