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I've got a Shimano Sora on a 8-gear cassette, and have odd behavior and skipping, shifting under stress.

This started with a trip to the mechanic for a tune up which replaced the chain. I had some skips so I replaced the cassette. Still more skipping, a second trip to the mechanic didn't do much good.

I tried adjusting it myself and found some odd behavior:

  • Had to adjust the H screw way out, and it doesn't seem to reliably return to the same spot while pedaling (while working on it).
  • When I start at the smallest gear and shift to the largest, everything looks perfectly aligned.
  • When I start at the largest gear and shift to the smallest, the first click does almost nothing. The second goes to the 7th gear, third click to the 6th... and the seventh click skips from the 3rd gear to the first.
  • After all this, it doesn't seem to move back into the original position without a little push.

Any ideas on what's going wrong here? It seems like tension pulling the chain out is not sufficient, but I don't see any adjustment for that.

Am I barking up the wrong tree?

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Is the derailleur cleaned and oiled? Is the cable kinked, torn or rusty? "Doesn't return to original position" sounds like dirt in a joint or a cable not moving as it's supposed to. Also: The mechanic didn't really check his work, it seems. Maybe look for someone more thorough? –  arne Oct 16 '13 at 7:28
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Check this out. –  Vorac Oct 16 '13 at 9:38
    
Maybe cable tension. After checking up all arne's concerns and making sure they are OK: 1) loosen both H and L a lot 2) adjust cable tension 3) test shifting 4) adjust H and L. –  Vorac Oct 16 '13 at 9:41
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4 Answers

  • First disconnect the cable to rear derailleur and using your hand push the derailleur towards the spokes and then release it. Do you feel any binding? If so, something is wrong with your derailleur.
  • With the rear derailleur cable still disconnected, grab hold of the cable and slide it through the housings. Is it catching on anything? Some resistance is normal but it should move easily.
  • Before reconnecting the cable, check the stroke on the rear derailleur. At the bottom, the guide pulley should line up with the smallest cog. Push the derailleur towards to the spokes until it stops. The guide pully should lineup with your largest cog. If either of these aren't true, use the H and L screws until they are.
  • Verify that your cassette lockring is properly tightened. A loose lockring will cause imprecise shifting as the cassette can move on the free hub body
  • Press the release lever on your shifter until it's full released and then reattach your rear derailleur cable. Press the upshift lever and adjust the barrel adjuster on the derailleur until chain moves up on cog. Do this a few times to make sure you've got just enough tension in the cable.

At this point the rear derailleur and shifter should be able to shift up and down your cogset. If you're still having issues, you might want to check the shifters themselves.

Some of the older 8 speed levers used to have a problem with releasing. The grease in the mechanism would dry out and cause the release mechanism to function poorly. You would mostly see this when releasing when from your biggest cog. You can spray some lube into the shifter body while working the two shifter leve

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I've experienced similar setup difficulties with my rear derailleurs and my advice is to check your cable tension. There a few ways to do this (not all may apply):

  • Barrel adjuster at the shifter
  • Barrel adjuster at the derailleur (less common)
  • Manual: release the clamp on the cable and pull it tight again, making sure there's no slack

Some other spots to check:

  • Beneath the bottom bracket where the cable guide sits
  • Ends of cable housing: are they clogged?
  • Any crimps in the cable/housing?

Other than this, you might just have to take it in and let the shop do all the work/adjustment.

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It could be a number of things.

The most likely it sounds as if your cable is sticking/jamming. Try replacing your cable and cable housing. This shouldnt cost much for DIY (read up on how to do it, you dont want to crush the housing when cutting), but would cost more from a mechanic. I would firstly check that your deraileur isnt out of alignment and/or loose or bent first. Another thing you can do is check that you have the right chain for your cassette and that the chain isnt worn (a cheap tool will check this for you).

If your mechanic has not solved this, I suggest trying a different one.

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A tune-up fundamental SHOULD be the proper alignment of the rear derailleur hanger. If the shop doesn't use a Park Tool DAG-1 [derailleur hanger alignment gauge], or some other manufacturer's, to inspect, verify, and correct the relation of the hanger to 3 different points on rim of rear wheel at 180 and 45 degrees apart it is throwing parts at a bike. As the hanger should hold the rear derailleur in direct line with each cassette chain ring, any deviation -- inward or away from the cassette -- induces erratic shifting and skipping that mimics cassette and chain wear. (Misalignment is difficult to "see" without this tool but may be suspected if the bike has been bumped in a rack, dropped, or fallen, on the drive side.) The tool at $65 to $75 will be used for diagnosis by a careful shop but is unlikely to be worthwhile for an occasional home wrench.

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