New Cassette?

[Thomk]

I changed the cassette and have 1 small problem and 1 query. Problem - when in my lowest gear on my big ring (it's a compact) the chain rubs on the top of the front deraillier, I guess because the circumference of the new cassette is slightly bigger? I'm tempted to just avoid using that gear as it's a poor choice anyway rather than mess with the front mech.
My query is - if the chain length needs adjusting would it be to take out a link or add one? (I removed a 12/25 and installed a 12/27 cassette).

 

[HLaB]

I changed the cassette and have 1 small problem and 1 query. Problem - when in my lowest gear on my big ring (it's a compact) the chain rubs on the top of the front deraillier, I guess because the circumference of the new cassette is slightly bigger? I'm tempted to just avoid using that gear as it's a poor choice anyway rather than mess with the front mech.
My query is - if the chain length needs adjusting would it be to take out a link or add one? (I removed a 12/25 and installed a 12/27 cassette).

As you suspect you are better off avoiding that gear anyway, chain rub is a common feature but treat it as an audible warning.
You've went from a smaller too a larger cassette any adjustment necessary will need lengthening of the chain, unless it was wrong in the first place.

 

[Peteaud]

My 25 to 27 didnt need a new chain.

But

Ive now junked the Tiagra and fitted Deore rear mech and a 11-34 cassette and new chain anyway