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I have a 10-speed Shimano Tiagra groupset. I want to change the cassette to 9-speed. If I make no other changes, will the derailleur shift correctly through the 9 gears?

To clarify - No I don't want to change from 9 speed to 10 speed, I only want to change my cassette to a 9 speed. I'm running a 10 speed groupsets and only want to change the cassette. I know I will have one more kick on the shifter but will the other 9 gears change smoothly .

Criggie
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Neil
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  • Is there a question here? – Adam Rice Oct 20 '23 at 16:15
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    Does the previous question and answers answer your question? https://bicycles.stackexchange.com/questions/13294/moving-tfrom-shimano-9-to-shimano-10-speed – Ted Hohl Oct 20 '23 at 16:23
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    Can you please expand on the intent here ? Are you trying to use a 9 speed cassette you already own, or can get cheap? Is the aim to get a cassette with a larger-toothcount low cog than tiagra offers? – Criggie Oct 21 '23 at 00:05
  • Thank you for sending me all your messages they were very helpful . –  Oct 21 '23 at 18:49

2 Answers2

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A shifter pulls or releases a fixed amount of cable each time you hit either of the buttons. That's the cable pull. For each mm of cable pull, the rear derailleur moves a certain amount laterally - the actuation ratio describes how much (e.g. if it's 1.5mm, then each mm of cable pull causes 1.5mm of lateral movement at the RD).

The 9-speed cogs have a larger distance between the cogs than the 10-speed cogs. Thus, if you simply put a 9-speed cassette in the rear, the RD has no way to know that it should move a different amount. Thus, the chain will not shift correctly. There is no way this will work.

The fancy electronic groupsets could, in theory, be programmed for different numbers of rear cogs. In practice, the major manufacturers don't allow this functionality for business reasons. Alternatively, if you have friction shifters, this won't be a problem - however, there are no integrated friction shifter/brake lever setups.

In any case, what is the reason to think about putting a 9s cassette on? If the thought is that 9s chains are more durable, I believe this isn't true; the durability of chains from 9 through 11 speeds has increased with each iteration, probably due to better material quality, better low friction or anti-wear coatings, and probably other factors I don't know.

Weiwen Ng
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No, the shifting will be utter garbage.

8, 9, and 10 speed cassettes are all the same width, and cram in an extra gear or two by making the space between them thinner.

At best, mixing 9 and 10 speed, the biggest and smallest cogs will line up perfectly and the middle cogs will be exactly wrong, leaving the chain to flap between 4/5/6 with no firm seating onto any of them. If the chain does happen to seat, it will pull-off easier, and will also be going through steeper sideways-angles and will wear faster.

If your cassette is worn, replace it with another tiagra 10 speed, along with the chain.

Criggie
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