Cassette ratio change question

[donfisher]

Thanks in advance for an answer to this one:

My commuter came with a 10sp 12-30 cassette, I need to replace it and find that 12-30s are all out of stock or from a groupset noticeably more expensive than I need.

Will a 12-28 replacement fit straight in and work or will any part/setting on the rear mech need to be adjusted? Presumably the cage on the rear mech being long enough for a 30 will mean that a 28 isn’t an issue? Not bothered at all about a change in ratios, the 30t has been unused on the last three I’ve gone through.

Cheers

[yellowjack]

Derailleur alignment ought to be unaffected. So long as it's a compatible 10 speed cassette the spacers should be identical and chain line and limit screws shouldn't need tweaking.

The only possible issue might be chain length. One-for-one swaps (same ratios) mean you stick the new chain on with exactly the same number of links as the old one. If your derailleur spring is stiff, you may find you can run it on the same length chain without problems. You may have to chop out a link to keep it tight though. There'll be an online calculator to work it out, or a diagram of how to decide on the appropriate length of chain. Too short a chain will cause premature wear and possible damage in certain (extreme) gear combos.

[donfisher]

yellowjack said:

The only possible issue might be chain length.

Good point, thanks. The cage has always seemed to point quite far forward in the higher part of the cassette so that might not be an issue. Even then, no problem to sort if it's not right once swapped.

It would be straight swap, Tiagra 10sp to Tiagra 10sp, just with different sprocket sizes.

[gazza285]

I wouldn't worry about chain length, your smallest sprocket hasn't changed size, so the chain is not going to be any slacker than before.

[jesusbuiltmycar]

You may need to adjust the B-Limit screw on the back of the Deraillieur.

See: https://www.youtube.com/watch?v=4sm-tTbfamM