Changing a cassette. What about the rear derailleur?

[Ghost Donkey]

Hello, I'm looking at changing my rear cassette. I've read up on the bits needed ot operate on the wheel but the one thing I'm unsure of is if I will need ot play around with the rear derailleur. I have a 10sp Shimano 105 setup and would like to change the rear cassette to offer a wider range of gears with a lower bottom end. Would the gears need re-setting up again and the chain length need looking at or is it as simple as taking the wheel out and putting it back in again when using a cassette designed for the same groupset but with different gear ratios?

 

[PpPete]

Very likely have to change the chain along with the cassette anyway.
As for the rear derailleur - depends on the model number (RD then either GS or SS then 4 digits starting with a 5) and what gears you are planning to fit.
You can check the rated capacities by looking at the SI (Service Instruction) documents from this page:
http://techdocs.shimano.com/techdoc...DER<>folder_id=2534374302051113&bmUID=jg_V34k
Those docs also show how to re-index the gears should that prove necessary.

 

[bobones]

You would usually get away with the same length chain going from, say, 25 to 28, but you can calculate the exact chain length required using an online chain length calculator. If the chain is worn it would be wise to replace that also.

In theory the indexing should also be fine, but you might need to adjust the B tension screw if the jockey wheel is too close to the cassette. Fine tuning the indexing isn't hard and it's something you should be able to do anyway. The official max cog size on a 2011 5700 105 RD is 28 teeth, but the new 5700a RD supports 30. You can usually go a bit beyond the official Shimano specs in practice. See the Shimano Techdocs for details.

 

[Ghost Donkey]

Thankyou both. I'll be going from a 25 to 28. I bought a TT bike from a local shop in the sales. As it was a stock sale I couldn't specify the cassette as you would with a normal bike order and with the ice on the roads it hasn't been ridden yet so no chain wear at all. I'll buy a new chain for a spare so if I do need more length I have that option. I've fine tuned gears before and I'll check the website to confirm my derailleur is compatible before purchasing a new cassette.