My bike came with a 12-25 9sp cassette. It isn't worn or anything, but I've been wondering if I would do better (climbing and such) with something like an 11-32 or 11-34. What are the differences between these three cassettes? And if I did switch, would I also need a new chain (or anything else)? (The crankset is 28x38x48, if that helps.)
Thanks!
Yes, you would find it about 20% easier to climb hills if you got a cassette with a 34 tooth cog.
You could also replace your innermost chainring with one that has 24 teeth. That would give you a similar benefit. If you want to play with combinations you can do so at http://sheldonbrown.com/gears/.
If your chain is new you should not need to change it. Old chains stretch, and need to be replaced when you change your cassette.
You may need a new chain, or at least you may need to add links to your chain. I like Sheldon's method of deciding how long the chain should be:
http://sheldonbrown.com/derailer-adjustment.html#chain
Note that if your largest rear sprocket is larger, the chain will need to be longer to fit over the largest rear sprocket and your largest chainring. You also may need a new rear derailleur depending on what you already have. Certain rear derailleurs are meant for cassette sprockets of different sizes. If you have a 28x38x48 crankset, I'm guessing that your bike has a long cage rear derailleur, which can accommodate larger sprockets on the cassette.
I suppose deciding to swap cassettes or not might depend on the weight of the bike, how much you typically carry with you, and how steep the hills are that you climb often. You could always ride with what you've got for a few weeks and see how it feels. If you are not riding off road much, the 28-25 combination might be fine. You could also look at a 12-27 or 11-28 cassette (not sure if the latter comes in 9-speed), which might not require as many modifications as a cassette that goes into the 30s.
As is well known on this board, I like LOW gears.
If you find yourself wanting a lower gear, you should have one.
That being said, when I looked at 11-30, 11-32 and 11-34 8-speed cassettes(different from your 9-speed), they all had 26 as their second gear. I'd love a 34 lowest gear, but the jump to 2nd was far too much.
Even with 32 and 26 as second. I found myself too often "in the middle." So the second gear is important.
On that bike I just got a smaller chainring. You might consider that. 22-tooth chainrings aren't rare.
Either of the cassettes would have a more comfortable lowest gear than 25 teeth. Like having TWO lower gears.
Some say that's two low, but I'd bet that you would still be using your lowest gear going up Bates street. Or Negley.
If it were me, with a 28 tooth small chainring, I would definately want the 34 cassette (assuming the jump to 2nd gear would be tolerable.)
Many other folks here like to mash more than me, but others have found my "use low gears" advice helpful.
If you find yourself wanting a lower gear, you should have one.
The limit is balance - and at the speed I have trouble balancing, walking in't a big change.
@Mick, agree about the gears. I've recently started messing with my inner chainring to reduce my smallest gear ratio and I keep getting happier each time it gets smaller. I'm like, why didn't I do this a long time ago? Pittsburgh is what gears are made for!
If you find yourself wanting a lower gear, you should have one.
+1
Also unless you find yourself spinning out in your highest gear all the time I would look for a 12-32/34 or even a 13-32/34. The closer the range on the cassette the better it will shift as there won't be as large a jump between individual cogs. Having gears that are higher than you actually need is just as silly as having gears lower than what you can use.
Having gears that are higher than you actually need is just as silly as having gears lower than what you can use.
Wise words.
These days, I'm really enamored with the combination of a 50-34 compact crank and an 11-3x cassette...I've got a 2x9 with an 11-34 and a 2x10 with an 11-36, and love 'em for Pittsburgh-area riding.
Sometimes I love having super high gears, too. You can speed downhill using a very slow, easy cadence that is, at least to me, as restful as gliding, with more control.
I like gears 
I have an 11-34 or 12-34 9 speed mountain bike cassette sitting around. I think it is a Shimano LX or XT, and I believe it has lots of life left in it. I will never use it so if you want it, email me at jcm250b at juno dot com. I was going to add it to the "Spring Cleaning" thread, but didn't get around to it. But, be aware, your rear derailleur may not be large enough for it to work with the biggest gears.
Thanks, all!
Jacob: email sent.
Nevermind
