Annoying shifting issue - missing one sprocket

jrab
jrab Posts: 99
edited December 2009 in Workshop
Just finished building a "new" winter bike (more of a frame-swap than many new bits....) All ready to go except for a niggle with the gear shifting.

It's 9-speed 105 mech and shifters, with 12-26 cassette and new Dura-Ace cables. Mech, chain, cassette and shifters were all previously together on one bike working fine. It shifts OK up and down the cassette, apart from the second-largest sprocket (23-tooth) - both up and down the block it basically skips this sprocket, sometimes just rolling on the edge of the 26-sprocket briefly before dropping onto it. Doesn't seem to matter what I do with the cable adjusters, any improvement on shifting to the 23 sprocket seems to mean messing up the shifting on all the rest of the block.....

Question is, I'm not sure whether this symptom means too little cable tension or too much? Which way should I go? I don't understand what is wrong to make it skip one sprocket while otherwise shifting ok?

Cheers for any help. Driving me mad!

Richard.

Comments

  • Have you tried resetting the H & L screws on the rear mech and then reindexing? I find that if the H screw is a wee bit out, you can get these anomalies with shifting.
  • John.T
    John.T Posts: 3,698
    Have you replaced the outer cable at the rear end. A worn sleeve in this can cause all sorts of problems like the one you have. I had it further down the cassette but the same symptoms.
  • jrab
    jrab Posts: 99
    Thanks for replies.

    Both gear cables, inner and outer, all sections, are brand new. Silicon-lubed outers, stainless steel inners, far-from-cheap DA ones.

    I did the old trick of working the shifters several times and plucking the cables off the downtube (bow & arrow stylee) before re-setting the cable clamps, so there shouldn't be much/any stretch in them.

    I have checked the limit-screws and tinkered with them a couple of times, but I can't see how this would help the index positions?

    I'm not sure what you mean by "re-indexing", unless you mean adjust the cable tension so that what is now the index click for sprocket 3 becomes the click for sprocket 2, and so on? Would this help - surely I'd just end up with either over-slack or over-tight cables?

    Cheers,
    Richard.
  • John.T
    John.T Posts: 3,698
    Is the cable clamped correctly at the rear mech end. IE Properly in the groove and not round the tab on the washer. This will upset the indexing.
    Indexing is set by the shifter and the only index adjustment is by cable tension. (Or as above by routeing the cable clamping incorrectly as is sometimes done when mixing Shimano and Campag components.)
  • Smokin Joe
    Smokin Joe Posts: 2,706
    John.T wrote:
    Is the cable clamped correctly at the rear mech end. IE Properly in the groove and not round the tab on the washer. This will upset the indexing.
    +1

    The cable should go through the clamp on the wheelside of the mech, the other way will give the problems described.
  • jrab
    jrab Posts: 99
    I think so, I'll have to have a look.

    I've heard of this before (and the trick of using it to make hybrid setups work), pretty sure I'd fit the cable in the groove automatically. Worth checking though, thanks.

    Richard.
  • Wappygixer
    Wappygixer Posts: 1,396
    Is your mech hanger bent?
    This can cause all sorts of shifting issues
  • jrab
    jrab Posts: 99
    10 marks to Wappygixer!

    Rang LBS earlier, he suggested this too. Took it in, he put his gauge on it, sure enough not straight. Shortly after, one straight mech-hanger and one slick-shifting transmission.

    Not sure why I didn't think of this - just assumed it would be ok.....

    Thanks for the input guys.

    Richard.