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I have a triban RC 120 road bike. It uses an 11-34 rear cassette (11/13/15/18/21/24/28/34). I purchased an extra 8-speed cassette for use with a direct drive smart trainer I plan on buying. The only cassette available was a 12-32 cassette (12/13/15/18/21/24/28/32). If I use this with my roadbike, are there any other adjustments (to the chain or the rear-derailleur) I need to make?

I went through some of the threads here. My understanding is that since this new cassette is a 'subset' of the old one, I should be good. Is that right?

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    Although it's not exactly an adjustment, I'd use a separate chain for each cassette.
    – Jerry Coffin
    Sep 7, 2021 at 16:29

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You should be fine to remove the bikes rear wheel and put the frame on your trainer with the new 8 speed cassette left there permanently. Both cassettes are 8 speed, so that's good.

However, the trainer and your rear wheel are unlikely to have exactly the same offsets, so you will need some minor fiddling of the rear barrel adjuster on the derailleur when changing.

Try and figure out how much turning is needed, and just dial in that much when mounting or unmounting the bike.

If you do this a lot, it might pay to shim the trainer so the cassette's position matches your rear wheel, to eliminate this fiddling.

You can find more info on the rear derailleur's barrel adjuster browsing through https://bicycles.stackexchange.com/search?q=rear+derailleur+barrel+adjuster but this one stands out as useful and relevant: What does this barrel adjuster do on the rear derailleur (SRAM Apex 1)? Applies to most derailleur brands, not just sram.

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    With today's machining precision of hubs, the positioning of cassettes should not affect interchangeability of of wheels. I swap several wheel-sets of different makes (Campa, Mavic, Zipp) and put the frame on a direct-drive trainer without any need for a re-adjustment of the RD.
    – Carel
    Sep 7, 2021 at 7:56
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    @Criggie: Thank you for your reply. Any other links/videos you can point me to to understand this barrel adjustment better?
    – Sriram
    Sep 7, 2021 at 9:04
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    @Sriram added to answer.
    – Criggie ♦
    Sep 7, 2021 at 9:57
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    @Carel Although I agree that indexing is unlikely to be wildly off, the "machining precision" found in the bicycle industry leaves much to be desired.
    – MaplePanda
    Sep 7, 2021 at 17:29
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    I think every time I've seen it not work as it's supposed to out of the box, it's because someone was running a 10-speed cassette without its included 1mm spacer. There may have been one where it was because they were using a 7-speed conversion spacer other than the Shimano-designated 4.5mm thickness. I don't believe I've actually seen it happen yet that even a cheap, basic cassette hub locates the cassette wrong all by itself.
    – Nathan Knutson
    Sep 7, 2021 at 18:45
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You understand right that the different tooth counts should not require any adjustment. The b-tension will be a touch out further than it would be if it were set for the 32 rather than the 34, but that doesn't matter on a trainer.

It is always possible for the hub to not locate the cassette exactly where the trainer expects or vice versa, which ruins the adjustment-free interchangeability until they've been made to match each other. In practice that does not happen very often, and when it does happen it's usually because someone put the wrong spacer somewhere, or potentially the axle hardware on the hub was misarranged or replaced with something non-stock.

If you do find there's a mismatch in cassette location, solve the problem with spacers to create the drop-in interchangeability, or have a shop do the same. Otherwise you have to adjust both the cable tension (barrel adjuster) and ideally the limit screws every time, which is a drag. Again, usually it just works though.

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  • Thank you for your help. I tried the cassette and everything works ok. so far so good! I did use a spacer though..
    – Sriram
    Sep 24, 2021 at 7:35

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