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When I am pedaling hard and shift to a larger sprocket, the shift is not smooth or sometimes doesn't happen at all. Sometimes even double up-shifts don't work. There is no problem shifting when I test for it without riding. What could be the issue?

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Generally, when shifting to a larger sprocket, you should ease off on the pedal force somewhat. (Prior to indexed shifting you couldn't shift a larger sprocket under load at all.) –  Daniel R Hicks Feb 18 '13 at 11:37
    
I do reduce my pedal force to allow the chain to shift, but the problem presents itself only when I am need of a shift. I would have to check what is indexed shifting. Thanks :). –  Dronacharya Feb 18 '13 at 11:45
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Indexed shifting is where you shift with something that 'clicks' from one gear to the next moving the deR one gear each time. Prior to that you basically had a lever that moved the cable and you choose your gear by how far you moved it. If things started making a lot of noise, you would wiggle the shifter a bit. Good fun. –  Ken Hiatt Feb 18 '13 at 15:54
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It is possible that your derailer is in need of adjustment, or that some drivetrain components are worn. Hard to say without actually seeing/testing the bike, though. –  Daniel R Hicks Feb 18 '13 at 16:25
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3 Answers

That is perfectly normal because when you are pedaling hard the chain is under tension and resists being moved by the derailleur. Before shifting to a higher gear slow your pedaling slightly.

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But the derailleur works on the slack side of the chain where there is no tension. Considering that it should work irrespective of how hard I am pedaling. Today I noticed it doesn't work even when I am pedaling very lightly. It usually shifts after 5-6 rotations of the pedal. –  Dronacharya Feb 19 '13 at 6:26
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On ground, after shifting, look at your bike from the back. The sprocket into which you have attempted to shift, and the pulley in the derailleur should be aligned. If the pulley is more to the right than sprocket, you need to get it aligned to the sprocket, by tightening in the screw on shifter or on the rear derailleur. As you tighten the screw, you should see pulley moving horizontally.

If that does not help, other problem might be that your chain hanger is bent, but that happens much less often, and typically after some crash or hard hit on the rear derailleur. If that is the case, taking your bike to LBS sounds like a good idea.

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I will try this out. Thanks. –  Dronacharya Feb 22 '13 at 7:05
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up vote 0 down vote accepted

After a lot of tinkering and research I found the derailleur was in the need of indexing adjustment. Just twisted the barrel adjuster anticlockwise a little and the gears are shifting promptly now. The above link advises that indexing adjustment is the most frequently required adjustment for rear derailleur.

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Please consider accepting Davorin's answer since this is what they suggested. –  amcnabb Apr 21 '13 at 20:23
    
This is not what Davorin suggested. –  Dronacharya Jan 21 at 8:59
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