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I currently have a 1x7 setup on my mtb, originally converted from a 3x7, as you can imagine. I'd like to replace my 7 gear cluster to an 8 or 9 gear cluster. Will this be nothing more than a simple swap of clusters? Or will I run into trouble putting a larger cluster on a 7 gear hub (if there is such a thing), and will I have enough width between the stay arms at the back to accommodate the larger cluster?

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    What kind of rear hub do you have? Freewheel (ratcheting mechanism in the cassette) or freehub (ratcheting mechanism in the hub)?
    – Michael
    Jun 1 at 15:39
  • Hm, not sure. I'll snap a pic of it later, if that helps. Jun 1 at 19:12
  • Yes, take out the rear wheel and take a few photos of the cassette.
    – Michael
    Jun 1 at 19:23
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Imagine you have a splined cassette hub, common these days on newer wheels. Most newer Shimano standard hubs will take at least 7-10 even 11 sprockets these days, give or take inexpensive plastic spacers to define the precise position of a cassette on a hub. Meaning you could slap a 9-speed cassette on your freehub that formerly carried your 7-speed cassette and have your machine converted from an n x 7 to an n x 9.

None of such. There are several reasons why this is just a nice dream.

  1. the 7-s chain is wider than the 9-s chain and will rub against and wedge between the two neighbouring sprockets, although quite often 7/8/9-s chains mostly fit either,
  2. but to fit the 9 sprockets in the same space as the previously 7, the sprockets are more tightly packed and thinner, 3 meaning that the motion of the RD is smaller, the steps are shorter, implying that both your shifter lever and your RD must be from the same speed-group to work with a 7-s, 8-s or 9-s system.

Conclusion: The three components belong to a family, shifter, cassette and derailleur. The FD is a component of a lesser level, fitting 7, 8, 9 speeds. You can't mix groups in speeds. Shimano does have a data-sheet on a page that informs on (in)compatibilities. It's not the letter of the law, some things may work but others surely won't.

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    As far as I’m aware 6, 7 and 8 speed have the same sprocket spacing. The cassette got wider but the chain and maybe even the rear derailleur should be compatible (the pull ratio etc. stayed the same). OP should only need a new 8 speed shifter and an 8 speed cassette.
    – Michael
    Jun 1 at 19:06
  • You give me reason for hope. Thanks. Jun 1 at 19:11
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    @Carel Certainly the shifter and cassette should be from the same speed family, not only for the appropriate amount of detents in the shifter to match cassette cogs, but also the shifter commands the appropriate amount of movement of the rear derailleur. Since cog spacing varies between 7, 8 and 9 speeds cable pull of the shifter must match. The derailleur, however in these speed classes has the same actuation ratio so it doesn't necessarily have to be from the same speed family to offer perfect shifting in another speed's group. (This assumes total capacity and min./max sprocket size WNL.
    – Jeff
    Jun 3 at 23:30

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