Rear derailleur adjustment between 2 wheels/cassettes

slogfester

Sorry, this might be a dumb one.

If I run 2 rear wheels, one fitted with 12-27 cassette and the other with 11-26 (both Dura Ace 7900 on a DA 7800 groupset) what if any rear derailleur adjustment will I need/expect to make when swapping wheels?

Thanks

P_Tucker

Well you could be lucky and they'll match up; if not it'll be half a twist on your downtube adjusters at worst. Best bet is to get some washers and get 'em lined up.

desweller

You might find that the position of the hub flange is slightly different - check your bottom gear for the mech cage striking the spokes.

slogfester

thx for replies. some more details/clarrification to help:

- I meant 12-27 and 11-25 cassettes
- the wheels are both DA 7900- C24 and C35 (both clinchers), so perhaps I can expect better interchangeability?

StrangelyBrown

The hub will be the main thing that determines the alignment. Don't know how similar your hubs are, but as they are both Shimano you might be lucky and require no indexing adjustment.

If you do need to adjust, as already mentioned, it should only be a minor one to get the indexing spot on - half a turn or so on the mech adjuster barrel.

As the cassettes are similar ratios you shouldn't need to adjust the 'b' tension (vertical distance between top jockey wheel and cassette cog), but, for reference, if you were going from a 23 to a 27 you almost certainly would - which is a bit more of a faff but nothing major.