Upgrading 9-spd 12-25 Cassette

The top-3 cogs are built together as one piece, which is why you can't find just the 27t alone. But hypothetically, if you were able to replace the 25t with a 27t cog, that would be a very big jump from the next one down on your existing cassette (a 23t) up to the 27t. You could replace the 21-23-25t arrangement with a new 21-24-27t if you could find one, or better yet just replace the entire cassette and keep the 12-25t to use when your fitness improves or when you expect to ride flatter routes.

That sounds like a good plan. And you caught the true underlying problem, my level of fitness. Thanks for the help!

if you can find a 28t cog (i have used a 7sp cog for this with excellent results) AND a 9sp spacer you can put those behind your 12-25, BUT you need to remove one cog from the other end, preferably the 14t and your new setup is:

12 13 15 17 19 21 23 25 28

my 9sp system works fine with this configuration for our once a year hillclimb.

all's'miles

curby

Since cassettes are wear-out items, recommend you keep your 12-25 until you're ready to replace it, then get the 12-27. I ran a 12-25 for years with my triple, but last time at the LBS they only had a 12-27 in stock......the 10sp is of course more popular now. Anyway, there's no downside to the 12-27 option, because you don't lose any midrange gears, just the big three change. Difference between a 25 and 27 isn't huge, but on a 15% grade when you want (or need to) stay in the saddle, lower is better.

Hi, I've just bought a 12-27 cassette for my trek 1500 ultegra bike. very hilly where I live, most of my cycling pals have compact gears, so gettin up the hills is easier than my current 12-25 setup. Hope to fit and try before end of the week, i will let you know if and whether the differance is worth the £40 odd I've just spent.
Also not sure yet if I need a different chain to go with the new cassette, don't think I do. 😎
Wayne.

curby said:

if you can find a 28t cog (i have used a 7sp cog for this with excellent results) AND a 9sp spacer you can put those behind your 12-25, BUT you need to remove one cog from the other end, preferably the 14t and your new setup is:

12 13 15 17 19 21 23 25 28

my 9sp system works fine with this configuration for our once a year hillclimb.

all's'miles

curby

I did the above with three 12-25 cassettes I had. I purchased 28t sprokets,
ditched the 14t sprokets, and installed the 28t.

So now all of my bikes have the same size cassette as above, 12-28.

maander`

let us know how it works out...

curby said:

maander`

let us know how it works out...

I don't have a problem with this combination. It's a welcome change to
the 11-32 cassette I had on one of my bikes. On rolling roads the big
jumps from the 21 to the 24, or the 24 to 28 can upset your rhythm.

When the cassettes need changing I'll keep the 28 cog, as I'm it won't be
as worn out as the rest of the cassette (even though I do ride in the Cotswolds).

Harris Cyclery sells individual cogs.
Shimano Hyperglide Sprocket - harriscyclery.net