Changing Cassettes

May I suggest you go for a 11/28 cassette? two teeth less than a 30, but you would probably get away with using the same chain you now have for your 25, tooth cassette; and your deraluer would cope perfectly with a 28, tooth sprocket.

Depends on the mech. If it doesn't have the capacity, you may find that you jockey wheels ride on the cassette. You can adjust it a little with the adjustment screw that rests on the rear dropout, but I would go with the manufacturers spec. Shimano tech manuals are easily found online - don't know about the others.

Oh and I would be astonished if you didn't need a longer chain, so make sure you factor that in to your costs.

I'm pretty sure the largest shimano short cage rear mechs can take is a 28t, it will depend what you have and check it on the manufacturers technical section.

Your chain probably could do with a few more links but as long as you avoid the big-big crossover gears you could get away without them.

I think the most likely problem you may encounter is whether or not your rear mech can handle a 30t sprocket.

Rob

You may find that the current chain is too short. If you go up to a 30 then the derailleur may be pulled too far forward if you went for a big at the front, big at the back combo.

You may need to get a new chain and leave more links in it. After fitting the new cassette see how the derailleur hangs with regard to the reference installation instructions.