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I was thinking about buying Shimano Deore groupset to upgrade my ride, checking both older M610s and newer M6000s. The hitch is I want 2x10 set commuter flavor. That means narrow ratio cassette, like 11-25, 12-28 or 11-32. It seemed pretty logical to look for medium cage (GS) rear derailleur version since:

  1. SGS (long cage) was designed with 3x10 in mind (as far as I heard).
  2. I'll be getting transmission capacity like 26, 33 tops which feels too low for SGS with 43 cap.

However, if you look at GS version at Shimano's web site, you'll see:

  1. Low sprocket_Max. 42T
  2. Low sprocket_Min. 42T

As far as I understand it, I should not be using any cassette lesser then 11-42t, which is quite weird to say the least. So, have anyone got experience with using narrow ratio cassettes with RD-M6000-GS, or maybe give some build advice?

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  • I suppose unless you go to super-narrow cassette ratio like 11-23 the tolerances will be totally fine. Here's mine somewhat relevant anecdotal experience: I've been running RD-M9000 GS over 10 speed 11-36 cassette without any issues for a couple of years. This RD is rated for 40t min / 42t max, but we all know how conservative Shimano is in this regard.
    – Klaster_1 Нет войне
    Commented Jun 7, 2018 at 8:09
  • I see. Thank you. It's weird they officially state that GS only for 42t. Makes no sense since their M610 GS RD officiall support 36-32 sprocket size.
    – user2838376
    Commented Jun 7, 2018 at 11:18
  • Went into "Dealers manual" (that's how they call normal manual) for clarification that 42t min|max isn't mistypo. Unfortunately it is not: "11-42T is the only cassette sprocket compatible with RD-M6000-GS. Cassette sprockets with other tooth combinations cannot be used. If a product with an incompatible tooth combination is used, the chain could become derailed from the outside of the smallest sprocket and the user could fall off."
    – user2838376
    Commented Jun 7, 2018 at 14:43

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The min/max sprocket size spec of 42 does actually make some sense if you think about how the parallelogram of the derailleur works.

On derailleurs designed for large sprocket ranges the cage is moved a long distance vertically as it moves horizontally. At the inner position, the cage is too low and too far away from the axle to work with a small lowest gear sprocket.

That said, you could probably get away with running 2-4 teeth smaller than the specified minimum. If you want to go to a large sprocket of 30 or less teeth, you should be looking at a different derailleur.

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