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Although my rear derailleur is properly indexed it's suddenly started to double or even triple shift on an upshift - i.e. when moving to harder, smaller cogs on the cassette.

This happens whether I'm riding or even if the bike is completely stationary and I'm only moving the shifter in isolation - I can feel and see the display on the hood jump 2-3 positions. It only happens in what would be the mid positions of the cassette - the very bottom gear and the top few gears aren't affected when I shift up from them.

Downshifts to larger, easier cogs are completely unaffected.

I've tried peeling back the brake hood and giving the mechanism a good blast of penetrating lubricant but to no avail.

Is this likely to be a shifter only problem and what if anything can I do to rectify it?

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Check the following:

  • If you remove the cable tension, do you get the right number of clicks for the shifter? If not, you may have a broken pawl or gummed up pawl. If blasting with wd-40 doesn't work, you need to get a new shifter, cause taking apart a brifter and putting it back again is hard.
  • Is the derailleur bent? It isn't possible to adjust a bent derailleur satisfactorially. To unbend it properly, you need a special tool.
  • Is the chain worn? If so, get a new chain. Check with a gauge.
  • Is the cassette worn? If so, get a new cassette. And probably a new chain, for good measure. See this link on chain + cassette wear.
  • Is the derailleur adjusted properly? The cables do slip over time, and you may need to readjust them. Follow the directions here.
  • Is the cable slipping or have excess friction or under too much or too little tension? You can clamp down the cable and/or replace it. You can also get weird things if your frame flexes enough to pull the cable, e.g. if you have under-bottom bracket shift cable routing.
  • When I remove the cable tension I do get the right number of clicks, so I guess the shifter is actually okay, many thanks. I've checked the chain and cassette and neither are worn. I suspect there is some friction in the cable that is affecting the upshifts only - when I unclamped the cable there is evidence of fraying where it's clamped at the rear derailleur, could this be the problem? – Kiver Aug 1 '15 at 10:21
  • Did you check if the derailleur is bent? That's the source of about 95% of shifting issues. Normally cable issues are the housing developing friction, but depending on how frayed it is, it can cause problems. – Batman Aug 1 '15 at 13:00
  • It doesn't look bent and I can't recall an impact but I know it's not something you can be 100% sure about by eye so I've booked it in at a LBS to be checked with the proper tool and otherwise troubleshooted in general. I'll report back, many thanks for the suggestions, at least I know the shifter's okay :) – Kiver Aug 1 '15 at 13:29
  • Well, probably okay. it is possible but unlikely that the pawls are damaged in a way that the thing works without tension but wont work with tension. – Batman Aug 1 '15 at 13:51
  • That last comment was unfortunately correct. Diagnosis from the LBS is that alignment is fine but that the shifter has a defective pawl which isn't reliably working when the cable is under tension. Therefore a new shifter is needed. Nonetheless thanks for all the help :) – Kiver Aug 5 '15 at 12:02
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The shifter has a broken pawl mechanism inside that prevents it from shifting up (to smaller, harder cogs) reliably, when the rear derailleur cable is under tension. This is causing the rear derailleur to shift up 2-3 gears at a time for every use of the shifter. Given the difficulty in fixing shifters a replacement one is the course of action that has been recommended to me.

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