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Need a cassette here for an alexrim. How do I choose one? Need lockring too, will also need to know which one and do I need to consider what my derailleur is with the new cogs (for compatibility)[9 spline][1]?

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    How many cogs were there on your old cassette? What is the exact model number of your derailleur? Or last resort, how many clicks are there on your right-hand shifter?
    – Criggie
    Commented Jun 19, 2023 at 2:16
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    Worth noting that at least for newer Shimano cassettes, all specs are written on it. You typically have the model number on the lock ring (assuming it is the original one), for example CS-5800 and besides the obvious number of cogs, you also have printed the amount of tooth on every single cog, the one on the largest cog is usually good enough to indicate the correct cassette variant (28, 30, 32, ....)
    – DoNuT
    Commented Jun 19, 2023 at 6:53
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    @DoNuT good point. I suspect OP doesn't have any cassette. Its also possilble the rest of the bike is incomplete too - we don't know what else is there.
    – Criggie
    Commented Jun 19, 2023 at 9:58
  • @Criggie Yeah, I was too optimistic in the OP asking for a replacement since components are already present, but that might not be the case. I'd say without any information about the rest of the groupset (or whatever is present), it'll be hard to give any advise. My educated guess is that a Shimano 12-speed-cassette is needed and I'd probably pick a 11-34 but that won't help to solve the case^^
    – DoNuT
    Commented Jun 19, 2023 at 19:10

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The depth of the cassette is related to the number of gears, but 8/9/10 all use the same width of cassette, simply making the chain thinner and space between cogs smaller.

A new cassette will come with a lockring, so that's covered.

You need to match the number of cogs with your shifter, and have a chain the "correct" thickness. There may only be half a millimetre difference but that's enough to make a transmission work or not.

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    In the Shimano world, the 8 and 9 speed cassette are the same width, but a Shimano 10 speed (road) cassette is actually 1.0 mm narrower (go figure?). A Shimano 10 speed road cassette will also come with a 1.0 mm spacer which goes on to the freehub first then the cassette, followed by the lockring (spec is 40 N-m or 29 lb-ft of torque on the lockring).
    – Ted Hohl
    Commented Jun 19, 2023 at 4:40
  • @TedHohl do you have a source for that?
    – ojs
    Commented Jun 19, 2023 at 5:30
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    @ojs but since you asked, here is one of my favorites on this subject. You’ll have to scroll a bit, because there is a lot of history in this one. slowtwitch.com/Tech/Cassette_How-To_-_Part_2_3257.html
    – Ted Hohl
    Commented Jun 19, 2023 at 7:17
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    @TedHohl thanks for the link. That was new information for me, and a bit surprising because it's well known that 8/9/10 freehub width is the same, and that 11-speed MTB cassette on 11-speed road hub requires a spacer.
    – ojs
    Commented Jun 19, 2023 at 11:12
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    @ojs You're welcome. It was a surprise to me the first time I realized it myself. I rode 9 speed well into the 10 speed era, and jumped directly to 11 speed, so was late to learn this nuance. In fact, the Shimano road 10 speed cassette on an 11 speed freehub requires two spacers, both a 1.0 mm one and a 1.85 mm one (2.85 mm total). The 1.85 is the width difference between the 11 speed and 8/9/10 speed freehub, and the 1.0 is the width difference between the narrower 10 speed cassette and the 8/9 speed cassette(s).
    – Ted Hohl
    Commented Jun 19, 2023 at 13:25

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