New cassette questions

[nammynake]

The cassette on my Specialized Allez is a Shimano HG-50 12-25. I'd like to have a bigger top cog to help me on really steep climbs, and the only ones I can find which appear to be compatible are these two:

Tiagra:
http://www.chainreactioncycles.com/Models.aspx?Mod...

Ultegra:
http://www.chainreactioncycles.com/Models.aspx?Mod...

A few questions:

1. Are both of these OK for a straight swap onto my bike?
2. Will I need to adjust the derailleur ?
3. Should I fit a new chain, and would it need to be longer (how many links?)
4. Will 27 tooth be noticeably different to 25? I'm talking about on >20% gradients.
5. Any advantage with the Ultegra over the Tiagra?
6. Is there a cassette with 28 tooth which would be compatible?

Thanks guys, and sorry for all the numpty questions!

[Magic919]

Check the model of rear dérailleur and see what it can cope with.

Chain rings. Double, compact, what does it have?

[Rocksteadyeddie]

A few questions:

1. Are both of these OK for a straight swap onto my bike? Yes, should be assuming you are running 9 speed at the moment.

2. Will I need to adjust the derailleur ? Possibly but you'll likely get away with it.

3. Should I fit a new chain, and would it need to be longer (how many links?) Depends how old the existing one is. For a couple of teeth extra your existing one will probably be ok.

4. Will 27 tooth be noticeably different to 25? I'm talking about on >20% gradients. Yes, you should be able to feel the difference.

5. Any advantage with the Ultegra over the Tiagra? Weight mainly.

6. Is there a cassette with 28 tooth which would be compatible? Probably not. IIRC short cage (which I'm assuming you have) Shimano rear mechs are only approved to 27 teeth.

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[nammynake]

Magic919 said:

Check the model of rear dérailleur and see what it can cope with.

Chain rings. Double, compact, what does it have?

Rear derailleur is Sora. Chainset is compact double 34/50.