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Recently decided to replace the rear derailleur on my 2020 Kona Honzo. I bought a 12 speed Shimano Deore M6100 SGS. Also got a new shifter to replace the stock Sram one.

Been trying to set it up for days now, but really can't get it to work perfectly. I've watched so many videos on how to set up a derailleur (Including the Park Tool one which was amazing), and I think I got the limit screws and barrel adjuster set pretty good now, because shifting works alright on all of the other cogs, other than the second largest (second smallest gear). When shifting down from third largest cog to the second largest, the derailleur cage will rub against the second largest cog. When shifting down multiple gears with one press of the lever, it will violently get stuck there.

B-limit screw is almost maxed out and it won't fix the problem still. I'm starting to think that maybe the chain is too long? Is it possible that if I change only the derailleur it would require a shorter chain? The chain itself is pretty taut, but if I pull it, the cage obviously extends and gets farther away from the cassette, which is what would solve my problem. Can I try this and see if it works? I haven't worked with chains myself that much, so could I take a couple links out and then put them back if it wasn't the chain?

Other possible culprits in cases like these could be too much housing, ie. a huge loop just before the derailleur, but my housing should be fine since I cut it like for like with the older housing. Other one I've heard is mismatching derailleur cage, but the one I got is a long cage version (SGS) designed to handle up to 51 teeth cassettes, and the one I have is a 50 tooth. This is driving me crazy, what can I do?

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    I think a few photos would help, just to make sure there is no obvious mistake with the installation. Regarding chain length: The rear derailleur should be almost fully extended on the big sprocket, you only need some slack to be able to shift from the second biggest sprocket to the biggest sprocket. But if there is still tension on the chain in the smallest sprocket I don’t think chain length is the culprit here.
    – Michael
    Aug 14, 2021 at 7:28
  • Shimano 12 speed uses a slightly longer chain than usual: dansbikeblog.com/article/…
    – juhist
    Aug 14, 2021 at 7:53
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    Why did you change the RD? Was it broken in an accident, could the hanger be bent, even so slightly?
    – Carel
    Aug 14, 2021 at 7:59
  • What was the old RD?
    – MaplePanda
    Aug 14, 2021 at 15:33
  • What new shifter model did you get?
    – Armand
    Aug 14, 2021 at 22:15

2 Answers 2

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Assuming compatible parts, going back and forth for days without being able to find an adjustment that works implies a problem with either hanger alignment or cable friction. Shimano 12 is a little sensitive to good adjustment, but not to that extent.

One of the easy mistakes to make wrenching at home in the 11 and 12 speed era is assuming you have some license to not worry about hanger alignment. You must worry about it or else things won't work. You mentioned rub between the cage and the second largest cog. Usually problems like that are only even possible with a tweaked hanger.

Cutting your RD housing to match a previous derailleur and assuming it will be fine is faulty logic, because the location and angle of the housing entry may be different. It's also incorrect that the housing being too long is the likely culprit if there were a cable friction problem. It's more common for issues to result from the rear loop being too short, which causes friction as the cable passes over an overly sharp bend near the RD's housing stop. Good setup usually means having it be the minimum length needed to still allow the housing to be running straight into the RD stop for about 12-15mm, which also usually makes all the other curves nice and smooth. Allowing it to be a little long is a very marginal disadvantage compared to how the extra friction in that area can impact shifting performance when it's a little short. You should be worried about friction where it enters the RD and little else.

The chain length and B-gap need to be right for the new RD. But as long as you've set the B-gap to the obvious line as indicated on the cage, it's probably fine.

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    I just wanted to thank you for your consistently very informative answers that reflect a wide range of practical experience as well as knowledge of specs etc.
    – Armand
    Aug 14, 2021 at 22:14
  • @Armand Thank you! Aug 16, 2021 at 4:29
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Thank you all for your answers. Turns out the issue was the small part that comes with the derailleur which is located on the screw that attaches the derailleur to the hanger. It was on the wrong side of the hanger. That's all.

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