I'm looking to replace the cassette on my bike with a smaller version, as I find I'm only ever using the top ring of my chainring (except once recently during rain-ageddon).
What do I need to know when buying and fitting a replacement? Are there known compatibility issues?
I currently have a Shimano 7-speed MF-TZ21 (apparently "14/28" means something in this context).
In case you haven't guessed: I don't know a lot about cycle part replacement.
I had the same problem (48x38x28 chainset). It turned out that MF-TZ21 is actually not a 'cassette', but a 'freewheel'. Your options for that are very limited: In the UK, Raleigh is distributing a 7 speed 13-24T model for less than 10 quid. SunRace is still producing 7 speed freewheels, but the closed-spaced 12-?? model wasn't distributed in the UK: Check eBay Australia maybe.
I ended upgrading to hand-built wheels with a standard Shimano 8/9/10 speed 'freehub'. Using simple spacer rings, these accept 7 speed cassettes compatible with the MF-TZ21, i.e. I got the 12-21T Shimano CS-HG50 and love the resulting gearing. You might need to fine tune your rear derailleur after installation.
The 14/28 is the number of teeth on the smallest and largest cog of the cassette. From your description you want to make at least the second number smaller, possibly the first number.
As long as your replacement says that it's Shimano compatible (and 7 speed), you should be fine. Count the teeth on the cog that has the most teeth that you actually use...make sure the new one has at least this many teeth. I would go a few extra teeth but my legs get fatigued now and again and I just go into pleasure-commute mode. My off the cuff recommendation would be to look for a 13-26 (Amazon has these for less than $25 bucks). I'd say the 13-23 would be too much of a jump.
More information on the topic from the oracle Sheldon Brown: http://sheldonbrown.com/k7-7.shtml
Depending on how many teeth there are on the most largest cog, you may need to take a link or two out of your chain. Replace the cassette first and see (or better yet, take this opportunity to replace your chain as well).
Happy Riding.
