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shimano cassette splines

When I'm installing cassette on my freehub, I need to make sure that the spaces align correctly. That is, there are 10 splines on the freehub and 10 corresponding splines on the cassette. My first attempt was just to put it in any of 10 positions but then I found out that one groove on the freehub is slightly bigger and the corresponding spline on the cassette matches it. So in fact, there is only one acceptable position. Why is this like this? Why aren't all of the splines the same size?

Naturally, I managed to replace the cassette, so I ask this out of pure curiosity because to me this "feature" seems annoying because I need to look closely to match corresponding splines, so I thought there must be some smart reason for this behaviour.

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    It makes sure the individual sprockets are correctly aligned to each other. – Michael Apr 29 at 12:48
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Early cassettes (Suntour Accushift and Shimano Uniglide being some commonly encountered examples) weren't like this. The splines were all the same and there was no hunting for the little spline or big gap.

To summarize a bunch of history, these cassettes were current at the same time as early indexed shifting. Accushift and early Shimano SIS both functioned, but not all that well by modern standards. The cog teeth on some of them had various thinned out or axially twisted bits to act as a primitive shift aid, but they were substantively like the freewheel cogs that came immediately before them.

Arguably the key innovation in Shimano's entire history, the first Hyperglide cassettes were released in 1989. Hyperglide uses a system of mapped-out chain exit and entry points (the different profiled teeth on modern cassettes) along with pickup ramping on the sides of the cogs to facilitate smooth shifts that demand a bare minimum of finesse on the part of the rider. It's the piece of the puzzle that made indexed shifting start working really well, and has been universally copied. But by nature, in order for it to work, the cogs have to be in an as-designed position relative to one another. Hence all modern cassettes only go on one way.

  • Those sprocket were riveted or screwed together – Carel Apr 29 at 13:55
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    OK but most modern cassettes are a bunch of cogs permanently fixed together in a fixed orientation with respect to one another. So the point is that the keying allows for cassettes built up from individual cogs, even though that's not needed by most people these days. – David Richerby Apr 29 at 15:32
  • Some were riveted, but some had screws to allow customization or replacement, and both Accushift and Uniglide used the small cog to act as a lockring. – Nathan Knutson Apr 29 at 16:07
  • It's not unknown for riveted sets to be unriveted to customise them: sheldonbrown.com/k7.html#custom – armb Apr 29 at 16:25
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    @DavidRicherby My current Shimano SLX 11 speed cassette has several loose cogs - so it's not entirely true to say most modern cassettes are permanently fixed together :) – Olly Hodgson Apr 29 at 21:41
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Nathan's answer above is great - just a minor ELI5 as it may be a little technical. The full cassette functions as a unit, not just individual cogs, with additional grooves / shaping to assist gear changing (look at the sides of the cogs and you will see they are not flat). The 'fits one way only' feature ensures that these gear changers work properly.

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If you consider the cassette as a whole, it doesn't really matter how it is installed on the freehub. What matters is each individual cog is aligned with relation to other individual cogs. Why it is important (re: shifting) is described in Nathan's answer.

If the cogs in our cassettes were all riveted/welded together at the factory into one solid non-disassemblable "supercasette", then we would not need these differently shaped splines on the freehub.

But, as you know, only the inner three large cogs are typically riveted together, while the rest is freely replaceable. Thus, we need these freehub splines to keep the teeth alighed properly.

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