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While removing Shimano 105 cassette, I noticed a spacer sitting on the hub between the largest sprocket and the place where spokes attach to the hub (not sure how is it called).

I checked the 105 exploded diagram and there is no spacer behind the largest sprocket so I was wondering, what is it for? Is it just a spacer the wheel manufacturer put there?

Edit: Sorry for not including this earlier: the cassette is indeed 11-speed (11-34) and it's a road bike.

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  • I have updated my answer, I'm pretty sure this is fine because the manufacturer was mixing Shimano components to get 34t which usually isn't available in a full 105 build, it is compatible according to their chart. If you ever swap the cassette and go for a <34t road cassette, the spacer probably needs to go out. I have 11-speed 105 on a 11-speed hub and there is no spacer on my bike.
    – DoNuT
    Commented Aug 23, 2023 at 9:06

2 Answers 2

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These are used when the freehub body and cassette size are not matching, there are various combinations depending on the scenario

  • 10-Speed cassette on 11-Speed freehub body requires both 1.85mm (FW8991) and 1.0mm (FW4671) spacer
  • 9-Speed cassette on 11-Speed freehub body only requires 1.85mm (FW8991) spacer
  • 34t cassettes on 11-speed road groupsets generally require a 1.85mm spacer because these are built using MTB specifications and may be used on both road and MTB setups (the former requiring an additional spacer)

Updated based on the OPs update

As the OP has stated in an update, he is running 105 with a 34t cassette (which can often be seen in compact builds because it offers 1:1 gearing in your lowest sprocket for climbing). If CS-HG700-11/CS-HG800-11 are used on a road hub, the additional spacer is required, also stated on Shimano's product page

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    Note that 11-34 11-speed road cassettes are in fact sold with the spacer (example: bike-components.de/en/Shimano/…). The strange part for me is that there is to my knowledge no 11-speed MTB derailleur that can handle such cassette, so it's where considering max compatibility just increases complexity.
    – Rеnаud
    Commented Aug 23, 2023 at 7:44
  • @Renaud I think we just need to know what the OP is running to understand the reasoning behind. Maybe just a pre-5600 (9-speed) bike with a modern replacement wheel, who knows...
    – DoNuT
    Commented Aug 23, 2023 at 7:55
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    which is exactly what my answer says, without the Shimano-terminology ;)
    – Rеnаud
    Commented Aug 23, 2023 at 9:13
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    This answer is written as though 11-speed 11-34 is somehow not "road" or somehow out of place on 11-speed 105, which is not true. It's there because it's always there when any 11-34 cassette is used on an 11-speed road fhb.
    – Nathan Knutson
    Commented Aug 28, 2023 at 4:03
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    @DoNuT I don't claim to understand all the design factors, but it's something along the lines of if you were to play the same trick with too small of a largest cog, it could interfere with the hub flange.
    – Nathan Knutson
    Commented Aug 28, 2023 at 7:33
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If have an 11-speed groupset, and the largest sprocket is equal or larger than 34T, the cassette is in fact built using MTB specifications (MTB Hyperglide freehub bodies have kept the "historical" HG freehub body, that is used on road bikes up to 10 speeds: larger sprockets can have overhang above the hub).

So if you want to use such cassette on the longer 11-speed road freehub body, you need to add a 1.85mm spacer.

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