There are thin metal rings between sprockets at certain locations. I understand they adjust the spacing between sprockets, but why does the spacing needs to be adjusted this way starting from? The possibilities to assemble them wrongly are endless.

I was thinking maybe the manufacturing process produces sprockets of different thickness for different diameters, even if forged from the metal sheet having the same thickness (why it should?). Or is it an effort to tune the gear shifting super precisely? Last moment fixes of the design? Or there are different variants of the cassette that share some sprockets but may or may not need these rings?

Have any manufacturer ever told why do they use the spacer rings?

  • I think this is just a pragamatic way of building cassettes. Plastic rings are lighter than metal rings or distances integrated into the cog itself, modular cogs itself can be swapped out individually if you just damaged a single cog due to an accident or bad misshift. At least for Shimano, you can usually order replacements for individual cogs or spiders on the larger cogs.
    – DoNuT
    Commented Sep 6, 2023 at 18:47

2 Answers 2


I assume you're talking about a Shimano cassette, at least if we're talking about 12s cassettes. If you examine the cassette, the first two cogs do have built in spacers. The loose cogs all come with separate spacers, which I think are usually plastic. Then the biggest few cogs will be pinned to an aluminum spider.

On Shimano cassettes, all the spacers are identical. The distance between cogs is identical. No sequencing error is possible.

On older Campagnolo cassettes made in this style, the distance between cogs is not identical, i.e. the spacers are of different thicknesses. You visually identify them. The manual will tell you which spacer goes where. When I failed to follow the manual, I couldn't tune the shifting. They did make the spacers visually distinct enough that if you read the manual, people with experience putting a cassette together would not make an error. There's a ratchet inside the shifter, which I assume has differently spaced stops for all the different positions. Campy may have said that this arrangement made the shifting better. I don't really know how that could be, but OK. It probably contributed to an air of exclusivity, but for all I know it may have made some shop mechanics think that Campy was hard to work on (which is not true).

Their 12s cassettes are mostly machined from a single block of steel with a few loose cogs. I believe current SRAM cassettes are just a single block of machined steel, possibly joined to an aluminum biggest cog.

All the sprockets of all cassettes, as far as I know, are the same thickness. If they weren't the same thickness, then how would you set the chain's dimensions?

  • 1
    Correct - its purely a construction thing. The separate spacers are often dense plastic which is still lighter than steel or aluminium. I bought a cheaper cassette that had the 11 and 12T separate, but the rest was one monolithic block.
    – Criggie
    Commented Sep 6, 2023 at 19:07
  • All well and good, but I suppose the question the OP and I have is why the sprockets aren't of the proper thickness right away, thus obviating the need for spacers? Commented Sep 10, 2023 at 14:21

Typically, a cassette will have a varying number of smaller cogs that are "loose," in that they are not mechanically attached to the spider nor to each other. These loose cogs require a spacer between them to keep the correct inter-cog distance to enable proper shifting. Some loose cogs (almost always it will be the smallest two or 3) have the spacer built onto them. For ones that don't, a separate, loose spacer is provided. These spacers are speed specific: a 9 speed cassette will have thicker spacers than a 10 speed cassette, and those spacers will be different from the spacers of an 11 speed cassette. The reason for the difference is that generally speaking the overall width of a cassette remained the same even as more cogs (speeds) were added. In other words, an 8 speed cassette is the same overall width of an 11 speed cassette. To get more gears in the same space, they've narrowed the distance between the cogs as well as the cogs themselves.

For me, cassette spacers are no trouble unless one gets misplaced. It's quite obvious when you're installing a cassette if a spacer isn't placed.

However, with the advent of 12 speed cassettes, (specifically, Shimano mountain cassettes: CS-M6100 (Deore), CS-M7100 (SLX), & CS-M8100 (XT)), we now have a couple positions that require different thicknesses of spacer to be used. A thicker spacer is used between the 16 and 18 tooth cog, while a thinner spacer is necessary between the 14 and 16 tooth cogs. The 14 tooth cog has a built in spacer, but apparently not enough so the addition of a thin spacer is required. The two spacers are obviously different and the position of each isn't hard to understand given the partial built in spacer of the 14 tooth cog. I am critical of the design using two different size spacers, especially as one cog has a built-in spacer. At any rate, I throw that caveat at you regarding cassette spacing.

See the Shimano Dealers Manual for these 12 speed mountain cassettes and the section "sprocket pattern."

  • Are you suggesting that the same sprockets are used on cassettes with different numbers of sprockets, so they need thicker spacers, say, on a 10-cassette than an 11-cassette? If not (i.e., if a given sprocket can only ever be used on a given cassette): Why aren't they manufactured so that no spacer is needed? Commented Sep 10, 2023 at 14:24
  • In order to fit the additional sprockets within a cassette cluster that's the same width, whether it's 8,9 or 10 speeds, not only is the Inter-cog distance narrower (spacers thinner), but also the individual sprockets have a reduced thickness as numbers of them increase within a cassette.
    – Jeff
    Commented Sep 12, 2023 at 12:14
  • I suppose that was an answer to my question. So you say that the individual sprockets on a, say, 9-speed cassette are narrower than those on an 8-speed cassette. If that is correct -- that is, if the sprockets are produced specifically for a certain n-speed and are not used for (n-1)-speeds -- , why are they not produced with a width which obviates the need for spacers? Commented Sep 12, 2023 at 12:39
  • Part of the design in (for example), an HG cassette, is that the individual cogs are designed to be in a certain orientation with the ones next to it. This allows the shift ramps and gates to be effective. This is also why the HG freehub body has a spline that is larger than the others. The cogs of a cassette can only go on one way to maintain the correct orientation. There's even details like differing designs of (for example), a 13 tooth cog in an n-speed cassette. One design is for a 13 tooth cog that follows an 11 tooth cog and a separate, differing design of a 13t that follows a 12t.
    – Jeff
    Commented Sep 13, 2023 at 23:56
  • In direct response to your comment, yes, a cog from a 9 speed cassette is narrower than one from an 8 speed. A 10s cog is thinner than 9s cog, etc. It's all so that increasing number of cogs can fit in the same space
    – Jeff
    Commented Sep 14, 2023 at 0:00

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