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I noticed a misalignment between the two pulleys on a brand new Ultegra R8000 derailleur. It's less convincing on the photo than in real life, but still very much visible. I came across this discussion, where they start off with a similar observation, but switch to noise mitigation strategies pretty soon. I also came across this Shimano manual where the bottom figure on page 12 shows the bottom pulley at a slight angle with respect to the other one and the cogs, very similar to my photo. So while I've concluded that this is not a faulty part, I do wonder why they would design it that way. I would expect this to wreak havoc on the chain, compared to a perfect alignment, so what's the benefit?

PS: Both pulleys are firmly installed, don't wobble, and turn like they should.

enter image description here

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    On the right side of the image the cage looks bent. In the referenced image from the manual the inner and outer cage appear parallel. Yours does not appear to be so.
    – mikes
    May 7, 2020 at 21:31
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    Thanks for the comment, but I would say that the cage is parallel also in mine. It's hard to tell on the photo due to the parallax, the funky shape the cage plates, and their variable thickness. And in either case, the misalignment of the pulleys is the same as in the manual, which in itself I find odd, based on my limited mechanical intuition. Maybe I just find a way to contact Shimano, though they might be unwilling to share their design rationales with regular folks...
    – mcsoini
    May 8, 2020 at 7:27
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    FWIW, I took a look at an XT 11 speed rear mech that I just bought and it has exactly the same kind of alignment as in your photo. Shimano does lot's of magic to make index shifting work seamlessly, I suspect this is just another one of those subtle tweaks.
    – Fred the Magic Wonder Dog
    May 9, 2020 at 0:23

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Upon initial inspection, the issue described may seem alarming. However, the newer Shimano derailleurs are slightly bent and twisted to aid in shifting performance and meeting the ever increasing demands of larger cassettes.

For instance, the XTR M9100 derailleur is slightly bent and twisted and angled in such a way that the lower pulley wheel is facing towards the chainring. This helps when shifting down the Cassette and prevents damage to the pulley arm when the RD is at the 51T cog of the cassette.

In a nutshell, the slightly bent and twisted cage aids in shifting up/down larger cassettes and mitigates the damages of cross-chaining (which is inevitable on 1x drivetrains).

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  • I think this is true. On my 6800 and 8000 rear derailleurs, when looking from aft, the lower pulleys do appear to be angled outwards, I.e. towards the right, away from the wheel.
    – Weiwen Ng
    Dec 16, 2020 at 16:36

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