Recently renewed the drivetrain to Shimano 600 Ultegra 8-speed roadbike. All
is peachy except for shifting DOWN to one particular sprocket at the rear.
This is an STI equipped bike - when shifting up through the gears - each
gear change requires only a small movement on the bar lever to switch
sprockets and it does this as slickly as any bike I have ever seen...
however, when moving DOWN from the 5th cog to the 6th cog it will nearly
always fail to engage and just chatter away unless I make that particular
change much more positive at the lever end (i.e. requires a bit more of a
"knack" - the lever needs to be snapped across a little bit quicker and a
bit further to make the same slick change).
I originally thought the sprocket in question may have damage or a fault but
looks fine (it was a completely new cassette after all). Have tried all
variations of cable tension and it seems to make little difference - the
tension can only be adjusted to make this particular shift work at the
sacrifice of UP-shifting so does not seem to be cable tension related. The
other idea I had was maybe dirt/grit in the derailleur spring restricting
the hanger movement at a particular cable position but I have given it
several thorough cleans and no difference. Could it be at the STI lever end?
I would be very surprised of wear there - I assume each gear position has a
ratchet/notch affair within the lever - is there a separate "notch" for each
gear position and if so is it likely that the one position in question is a
bit ropey? (Have to say it does not "feel" any different when operating the
lever). Any ideas?
The STI system and bike as a whole has covered less than 2500 miles since
new so would be very surprised if wear is behind the problem.