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I checked,

  • When the shifters are such that the chain should be on the smallest cog, the cable still has a slack, which I understand then it means is not a cable adjustment thing,
  • If a very gently push the derailleur there is a bit more room for it to move and the chain goes smoothly to the smallest cog and it remains there without any strange noise. Then I understand it is not screw limiter related: there is still room thee.
  • The derailleur seems to be perfectly parallel to the wheel, no bended.

It seems that maybe the spring inside the derailleur is not strong enough to make the last push. Is there a way to adjust this?

I also saw someone with the same problem but didn't find the answer: Rear derailleur doesn't go to smallest cog

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I think it’s most likely that your cable has excessive friction, especially in the last part where it makes a tight bend from the frame into the rear derailleur.

If you disconnect the cable from the derailleur (i.e. open the clamping screw) I assume it works as expected?

Does this also occur on the big chainring? Otherwise it might also be a too long chain.

Edit to elaborate on cable friction: First of all make sure the cable housing isn’t damaged and check that it’s seated properly in all the cable stops and end caps.

If it’s not the housing it can be the cable is starting to fray and break. Usually this happens inside the shifters after a few thousand kilometers and gradually makes it harder and harder to shift to the smaller sprockets. Sometimes it’s possible to see inside the shifter (while shifting). Otherwise you can unclamp the cable at the derailleur, pull out the first few centimeters of cable from the shifter and check them.

If there is no damage to the housing or cable it can also just be wear and dirt in the cables. Lube or grease can help for a time, but it also attracts dirt, so should only be applied as a stop-gap solution after problems have already developed. Cables+housing are wear items, it’s normal to have to replace them after a couple thousand kilometers. It heavily depends on circumstances.

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  • This is on the big chain ring. I should check in the small one. I can also check what happens if I disconnect the cable indeed, thats the first thing I should check. If that is the case (cable friction) what is the solution?
    – myradio
    Jul 24 at 8:38
  • I’ve edit the answer accordingly
    – Michael
    Jul 24 at 14:28
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I suspect you have cables and/or housings that are either old or off-brand. The friction in these can be surprisingly large. Thus the issue can be cable friction like Michael suggests.

Cable checklist:

  • Buy new Shimano 1.2mm stainless steel inner cable. If you have a recent system with a ridiculously high sprocket count, you may benefit from an Optislick coated cable. Otherwise (on 8-speed systems) a regular stainless steel cable works just fine.
  • Buy Shimano OT-SP41 / SIS-SP41 cable housing. This is very important to select correctly: I have found off-brand (non-Shimano) housings to be pure crap even when new, and even worse if worn.
  • Buy Shimano plastic shifting ferrules / end caps. Note shifting end caps are different from brake end caps. Some derailleurs and/or shifters need a ferrule with long tongue. If you don't have such a derailleur and/or shifter, then the choice is between ferrule with O-ring (sealed) and without O-ring (non-sealed). You will want to use O-ring sealed ferrules at least on the rear derailleur housing loop which is prone to dirt intrusion and very sensitive to friction due to 180 degree bend. The other parts of the system if riding in non-dirty/muddy conditions can have non-sealed ferrules but for mountaing bike use all ferrules should be sealed (with O-ring).

Then you need to cut the housing to length. I suggest not using the existing housing pieces as templates because it's possible your issues are caused by improper length housing pieces.

The four commandments of cable routing are:

  1. Handlebars must be able to be turned fully to either side
  2. No wrong direction bends
  3. All bends should be as gradual as possible
  4. Housing pieces should be as short as possible without violating the above rules

You need to use special cable cutters to cut the housing. You also should use a sharp tipped tool to pry open the plastic housing liner flattened by the cable cutters.

After installing the cable, you must eliminate initial slack from the housing. You can do so by shifting to the lowest gear (biggest sprocket) and by grabbing a location of exposed inner cable and pulling it hard away from the frame tube, against the rear derailleur "L" limit screw. If using trigger shifters, you will somewhat need to limit the force so that you don't break the shifter. If using thumb / bar-end / downtube shifters, you will be able to pull very hard if resisting the motion at the shifter end so that it won't auto-shift at the shifter.

With a new SIS-SP41 housing cut to optimal length pieces, new 1.2mm Shimano inner cable and new ferrules, and after eliminating initial housing slack, you will minimize any cable friction issues.

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