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I have a Shimano Tiagra derailer on my 9 spd cassette and it's been behaving really strangely.

On the road, when I attempt to shift to bigger cogs, the chain will struggle to move up, sits on the bigger cog for a moment, and then jumps all the way back down to the smallest cog (sometimes just 2nd smallest).

There is definitely a feeling of difficulty of movement coming through the lever when I try to shift.

However I have it in a stand right now and it shifts up and down, albeit awkwardly and noisily. I'm not sure how to make the proper adjustments if the issue isn't happening while the bike is stationary. It's working somewhat fine now, but once I get on the road, it's unhappy again.

I can identify the different parts of my derailer and have a feeling it's a problem with the cable tension, but I'm clueless as to how to address the issue.

Thanks, any advice will be a big help!

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    If it's jumping all the way to the highest cog, my guess would be there is a problem in the shifter, that the ratchet mechanism is either broken or gunked up with old grease. – Noah Sutherland Sep 28 '17 at 19:07
  • How many miles on the chain and rear cluster? – Daniel R Hicks Oct 29 '17 at 19:35
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If you feel clueless about bike maintenance or troubleshooting, the good news is that there is quite a lot of good content on this subject available, which makes it easy to learn how various bike systems work, how to perform regular maintenance and adjustments and figure out problems. My favorite is Park Tool Company's site and YouTube channel. They have a series of videos that deal with derailleur systems:

. There are other guides available. These are easy enough to find by googling.

To address your specific problem: because you say you feel excessive resistance at the shift lever, I would initially check that there is no impediment to the free movement of the shift cable and derailleur. There may be a problem in the cabling, at the rear derailleur, or in the shifter itself.

The derailleur auto upshifting (to smaller sprockets) after downshifting (to larger sprockets) may indicate a bigger problem with the shifter. If the chain is only upshifting one sprocket it may be that derailleur is not moving the chain far enough and the chain is falling down to the next smaller sprocket. If the derailleur auto-shifts several sprockets I would suspect that there is a problem with the shifter and it is not holding on to the shift cable properly.

Things you can do to diagnose the problem:

  1. Get the rear wheel out, observer the derailer and run the shifter through its range properly. Does it move at each shift and through the full range? If not it points to a problem with the shifter.

  2. Manually inspect the cable run to make sure they are no snags, twists or kinks and that the housing is properly in each cable stop.

  3. If you have exposed cable rather than full run housing: shift to the highest gear/smallest sprocket, grab the shift cable in different places on its run and pull outwards from the frame to actuate the derailleur to its innermost (low gear) position. If you feel resistance or roughness there is an impediment 'downstream' from where you pulled the cable.

  4. Disconnect the cable from the derailleur. Manually check the derailleur will smoothly sweep through it's full range, outer limit to inner limit, you should of course feel spring tension but no roughness.

  5. With the derailleur still disconnected, manually put a little tension on the cable at the derailleur end (you may have to use pliers to grip the cable, but don't crush it), run the shifter through it's range. Do you still get resistance at the shifter? Does the cable move at the derailleur end at each shift?

  6. If the above does not show up a problem with the derailleur or cable run, you will want to look at the shifter itself. I'd want to try to see if the shifter is slipping back to higher gear positions after being shifted to low gear positions. If you can, get hold of the cable as close to the shifter as possible in it's run, put tension on it and run the shifter from high to low gears, it should pull cable the cable in in increments. if you can pull the cable back out out, the shifter is probably work out or broken and will need to be replaced I'm afraid.

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I think your indexing might be off. Before trying Argentis answer, try adjusting the derailur's barrel adjuster to get the jockey wheels in line with the cogs.

Watch this for reference

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How old is the drivetrain? If it's more than a few years old and being ridden daily treating yourself to some new cables will make your bike feel like new. Check for wear of chain, cassette and chainring

You will never achieve perfection if your derailleur hanger is bent. Use the park tools DAG-2 to get it straight. Once straight, the rest can generally be adjusted with simple tools like mentioned in the answers above.

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