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I am trying to understand which cassettes I can install on my bike. I've understood that the length of the derailleur also determines which cassettes will fit.

In my case I have a triple (see picture below), with a 10-speed cassette, in the range 12-25. My experience with this cassette (and 30-tooth chainring) in the 18% stretches is not too pleasant. Therefore I am looking for options to replace it with a cassette with sprockets up to 27 or even 30 tooth.

My bike

My main question is how I can determine which cassettes will fit on my bike. Even more I would appreciate a general answer, that explains what the limitations are of the derailleur when choosing cassettes, such that I can determine it myself.

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  • Three things: 1) The number of cogs the derailer can handle. 2) The maximum rear cog size (determined in part by the length of the derailer arm). 3) The "tooth capacity" of the derailer (also partly dependent on length of derailer arm). Tooth capacity is determined by the wrapping a chain through the derailer and pulling it just barely taut (with the other end of the chain anchored somehow), then pulling the chain until it won't go any further, counting the number of links that pass through in the process.
    – Daniel R Hicks
    Dec 6, 2018 at 18:04
  • I am not sure if I understand 3, but this seems something that can be fixed by getting a longer chain, right? Then, for 2, how does the derailer arm length relate to the maximum cog size?
    – Bernhard
    Dec 6, 2018 at 19:10
  • @Bernhard longer cage means more chain slack that can be taken up. Larger difference between largest and smallest sprockets or chain rings creates more slack when in small/small combination
    – Argenti Apparatus
    Dec 7, 2018 at 1:55

2 Answers 2

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You can't work out from first principles in advance what kind of cassette your derailleur will work with--you need to know the model, and then look that up to find out what Shimano's recommendations are.

Daniel R Hicks describes how you can determine whether a cassette that you've already installed will work.

"Tooth capacity" is also sometimes referred to as "chain wrap" or just "wrap." This is how much slack the derailleur can absorb; this is a function of the length of the derailleur cage (the distance between the jockey pulley and the tension pulley); the longer the cage, the more wrap. This is expressed as a number of teeth, figured as follows: 

(Big ring - little ring) + (big sprocket - little sprocket)

So if your bike has a 50/34 in front and a 12/25 in back, your derailleur needs to be able to wrap at least 29 teeth; if you switch to a cassette with a 30t low gear, you'd need 34 teeth of wrap.

I'm not sure about the exact derailleur you've got, but apparently Shimano has made some version of a 10-speed 105 derailleur (possibly newer and different from the 105 you have) where the short-cage model can handle 34t and the long-cage 40t. So you're probably OK.

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  • Agree you need to check the specs, but the last 10 speed 105 was 5700, with largest sprocket of 28T
    – Swifty
    Dec 6, 2018 at 21:14
  • 'Total capacity' is the term used by Shimano in their docs
    – Argenti Apparatus
    Dec 7, 2018 at 1:50
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Looks like a 105 RD 5600, you can check that by looking for stamped numbers on the onboard face of the derailleur. Checking the specs at http://si.shimano.com/pdfs/si/SI-5VN0B-001-ENG.pdf gives this table:

enter image description here

So the max recommended cassette is a 27 tooth for that derailleur, whichever cage length you have. Shimano recommendations are often said to be conservative and you might only find a 28 nowadays that’s bigger than your 25. You could experiment with a 28 if that’s all you can find and discover if it will work on your particular bike. Make sure the chain is long enough if you increase tooth count.

My guess is that you have a GS cage looking at the photo, so total capacity should be accommodating

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