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I have this wheelset which claims to be an 11-speed HG freehub. I've been experimenting with installing 3 cassettes and I'm finding some strange behavior:

Cassette Description Needs 1.85mm spacer?
Shimano CS-M771 10-speed 11-36 MTB cassette No
SRAM PG-1050 10-speed 11-32 road cassette No
SRAM PG-1070 10-speed 11-36 road cassette Yes

Each cassette has been installed at least 5 times with no deviations in results. I've also verified that every stock cassette spacer is included, assembled in the correct order, and does not appear damaged. Mounting the 2 top cassettes with a spacer I can't get the final cog on. The bottom cassette without a spacer has a lot of play no matter how tight the lockring is. I've also tried swapping lockrings and not found a change in results.

I think the freehub might be a 10-speed - I took 3 measurements from the base of the freehub inwards of the notches, out past the splines to the very tip of the hub, and averaged them at 34.925mm long, which lines up with the 10-speed length. This would explain the behavior of the top 2 cassettes but not the bottom.

I have a second wheelset that's 8/9/10 speed, and I can mount each cassette with no spacer and no play, as expected. I think this rules out a manufacturing defect with the bottom one.

Does anything stand out to anyone? Do some freehubs fall in between 10 and 11 speed, or do some 10-speed cassettes not need a spacer to run on 11-speed hubs? I'm looking for any kind of tips more out of curiosity rather than just getting the bike running. Thanks for your help.

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The HG8/9/10 freehub is 35mm long, the HG11 road is 36.85mm long. Referring to the specifications, if mounted on 11-speed freehub, you need the spacer all the time for 10-speed cassettes (whether road or MTB).

The mention Mounting the 2 top cassettes with a spacer I can't get the final cog on. suggests that the derailleur has been set up to be used without spacer, while it should have one. That is per se not a problem, unless you have 2 wheelsets (or a trainer) and want to have a correct alignment between the two. It's apparently not uncommon that the spacer is forgotten even on new bikes, and it's not only noticed when the owner wants to have a second pair of wheels or use the bike on a trainer (but this problem arises for 11-speed road cassettes with large sprockets — ≥34T, that are built using MTB specs and need the spacer).

A possible explaination on why you need the spacer may be linked on how the cassette is built (without having the cassettes in hand, it's only speculation): the PG-1070 smallest sprocket has probably a slightly different design, that does not allow to take the additional 1.85mm — which can happen if the grooves are shorter. The small sprocket is then locked against the freehub before tightening the other sprockets.

So in short, as I understand it:

  • the design of the PG-1070 is less fault tolerant (because closer to the original design) than the two others, that can be mounted on too long hubs while still being locked.
  • the derailleur is set up for a 10 speed-cassettes mounted on a 11-speed hub without spacer.

Of course, if you have other elements, I can adjust my answer (or remove it if I'm wrong).

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    Regarding getting the final cog on, I think OP means that it physically won’t slot onto the splines, not that they can’t shift to it.
    – MaplePanda
    Commented Sep 20, 2023 at 7:32

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