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I have a MTB which I can't adjust its shifting properly. I replaced the rear derailleur and shifting cable with new ones but it still doesn't shift properly. I suspect the problem might be derailleur hanger. It seems straight to me but could it be bent milimetrically? Is there a quick and easy method to understand that?

Edit: Maybe I'm wrong to blame derailleur hanger. Actual problem is that my rear derailleur can't go up to the biggest cog atm (I have a 9sp cassette). This is not due to limit screw because I can move the derailleur by hand. Somehow I can't have enough tension on the cable even though I open the tension barrel on the shifter all the way to its limit.

Btw my shifter is an Altus while the derailleur is a Deore. And the cassette is an Alivio HG400.

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A bent hanger usually manifests as issue mid cassette. If that's the issue (i.e. limits are set correctly and first and last gear work fine) then it's a safe bet the hanger is at issue. If you can't tell by looking then the only real way to verify it is either replacing the hanger with a known straight hanger or using a derailleur hanger alignment gauge (e.g. Park Tools DAG).

If you have to re-adjust your limits due to the chain going over the cassette, then your derailleur is most definitely bent.

As a MTBer running a 1x12 drivetrain a derailleur hanger alignment gauge was a necessary investment for me to keep my drivetrain running smoothly.

One other possible issue is cable housing. If the cable was replaced but the housing was reused, then there can be friction that causes issues. My experience with that has been mainly delay downshifts (the derailleur is slow to release).

If you are using a Shimano clutched derailleur, make sure you turned the clutch back on. Forgetting to turn it back on can cause shifting issues which the chain bounces around on trail.

Update (after more info from Ender):

If you are unable to shift to the largest gear on the cassette and your barrel adjuster doesn't have enough range, then it's a cable tension issue. It could be the new cable stretched. In any case you can fix it as follows:

  • Shift to the smalls gear on the cassette (i.e. relieve all the tension from the shift cable by shifting down all the way).
  • Turn your barrel connector all the was in, then back off a little (to give yourself some adjustment range to loosen slightly if need be)
  • Unscrew the bolt holding the shift cable to the derailleur and loosen it so it is putting no tension on the derailleur (it won't be in your case since it's too loose, but I've had a bike come from the factory too tight so as a general approach it's a good idea)
  • Verify your limit screw is still set ok (visually the derailleur should still line up with the small cog and you should be able to pedal the drivetrain without the chain dropping or shifting up)
  • pull the shift cable taut and tighten down the bolt on the derailleur. When you pull, pull so it's taut and not causing tension, you don't want to to be pulling the derailleur up to the next gear. If you left a bit of back adjustment in the barrel connector, it's not that big a deal if screwing the cable down makes it a bit too taut (you can just loosen at the barrel)
  • Carefully shift to the largest cog (incase it goes over), tightening the barrel connector if need be to get it to go all the way up. Then adjust that limit screw.
  • Shift through the entire range to make sure every gear is working properly.

Don't worry about messing anything up. If you haven't done derailleur adjustments it can be a bit intimidating but anything you do can be undone. A 9-speed drivetrain should be fairly easy to get dialed in. 12-speed can be an act of frustration as the tolerances are much tighter.

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    Your statement about the requirement for hangar alignment gauges is in line with what Dave Rome, formerly at Cyclingtips, wrote about the topic.
    – Weiwen Ng
    Commented May 4, 2023 at 21:21
  • It can also cause issues on of the ends of the cassettes. You index the gears on the lower and end the upper end is off while the middle is so so. Then you try to index it at the upper end end the lower end is off.
    – Vladimir F Героям слава
    Commented May 5, 2023 at 8:32
  • Maybe I'm wrong to blame derailleur hanger. Problem is my rear derailleur can't go up to the biggest cog (I have a 9sp cassette). This is not due to limit screw because I can move the derailleur by hand. Somehow I can't have enough tension on the cable even if I adjust the tension barrel on the shifter to its limit.
    – Ender
    Commented May 5, 2023 at 15:30
  • Ahhhh... ok I'll edit my answer
    – shox
    Commented May 5, 2023 at 16:53
  • Ok... I followed your advises but not much changed. I am able to gear up a cog from smallest cog by opening the barrel but no matter how far I open it more it doesn't go up to 3rd gear. Problem begins here. So it doesn't go up to the biggest cog as well. Also it doesn't go back to the smalles cog when I shift back down.
    – Ender
    Commented May 6, 2023 at 7:29
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There is such a tool as a hanger alignment gauge. This is a long rod that screws into the hanger, with a perpendicular feeler that you position against your rim at a few points (see instructions--note that I am linking to Park Tools' guide, but there are other companies that make these). If the feeler hits the rim in the same way at multiple positions, your hanger is true. The rod also gives you leverage for bending the hanger back into alignment.

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    One pointer on using a DAG: always use the same point on the rim, e.g. where the valve is. This eliminates error due to the wheel being out of true.
    – shox
    Commented May 4, 2023 at 20:06

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