12/25 versus12/27

Cardiff kid

Currently upgrading my wheels and wondering what benefit if any would I gain from fitting a 12/27 cassette on place of my 12/25. I do quite a bit of hill work on my training routes and contemplating entering a few track crits this year. Any advice welcome.

Bobbinogs

Pro: One lower gear
Con: Slightly bigger range so the gaps are more (you will lose a very useful cog right in the middle).

You may need to stick an extra link back in your chain but should be ok. Also, you may need to check your b-stop setting.

bigpikle

only advantage is if you are struggling on the 25t on hills at any point? If 25t gets you up everything you need without issues then its of no use and it certainly wont help with track crits unless they have an uber steep hill in them.

k3vinjam3s

Is recommend it. I recently went from 11-25 to 12-27 and although I don't feel much difference in the lowest gear it seems to enable me to stay in the big ring more as I have a lower gear without going big to big.

Grill

If you do the proper climbs around Cardiff and the Valleys then go with the 12-27 (You should actually go with 11-28). The bailout is more useful than you'd think on the steeper stuff.

southdownswolf

It will give you an extra 8% lower gear for the hills.

DavidJB

I put a 27 on my 53/39 for training bike because sometimes I don't want my HR going up high on steep climbs if my training doesn't fit with that. Its really annoying losing the middle gear and I never use the bloody 27 anyway lol.

Back to a 25 when it wears out!

backo

my first bike came with 12/25 and I like you had a long think when it came to replacing the chain and cassette..opted for the 12/27 and found it to be much better giving me a better range.

beaware though that SRAM and Shimano cassettes can differed despite being advertised as the same thing...for instance the 11-28 SRAM and 11-28 Shimano are different.....I now ride the latter and find it to be perfect to cover hills in the lake district to "smaller" hills in the ribble valley with 28 as a saver.

SRAM 11-28

11-12-13-14-15-17-19-22-25-28

SHIMANO 11-28

11-12-13-14-15-17-19-21-24-28

drlodge

I think having the extra lower gear is always useful, you normally just loose one sprocket in the middle of the cassette so rather having 15-16-17 you now just have 15-17. My campag 12-27 is
12-13-14-15-16-17-19-21-23-25-27 compared with the 12-25:
12-13-14-15-16-17-18-19-21-23-25

I just lose the 18T by having the 27T, and the 27T is by far more useful on those long/steep hills.

Raffles

If I had the legs that allowed me to use 12 - 23 then id be delighted as in a 10 speed system the gaps would be be real tight between cogs. Sadly I dont have those legs and I use a 5700 10 speed 12 -25 rear cog. Id be one advising against going for 12 - 27 as it widens the gaps between cogs.

Grill

@OP- With a handle like yours I assume that my initial assumption on your location is correct. Is your chainset standard or compact, and which hills do you do around here? Also, are you planning on hitting some of the big/steep stuff?

Cardiff kid

Location is correct, pentwyn area. My bikes fitted with a compact chain set (sora). Not getting out as much as would like lately, once to twice a week weather depending,(Ticking over on turbo trainner). Hills I generally hit depending on route I'm taking are caerphilly mountain via one off it various assents and rhigos mountain. I generally ride the lanes around caerphilly, rudry,basseleg,marshfield area mid week.

springtide9

The only reason for getting the 27 is if you are struggling with the 25.

If you plan on doing a few Crits this year, you might want an 11 on the rear if you are running a compact. I found last year that the cadence was rather frantic (on occasion) with a 50/12 [ although it was never a real problem ]

If you rarely use the 25, maybe even consider an 11-23 to keep the ratios nice and close. When you ride the bigger hills, just keep a good eye on what gearing you use, which is the real way to determine what gearing you need (rather than what gearing others use!)

Grill

It would come in handy for Caerphilly Mountain (from Caerphilly), but stuff like the Rhigos and Bwlch are much more forgiving albeit longer. I like to spin so prefer access to a 28, but you can still tough most of it out on a 25. I find the 28 the most useful when I'm several hours into a ride and lacking in energy.

bernithebiker

There is no one size fits all.

I use a 21-11 for normal riding (which still includes quite a few hills), and I switch to a 28-11 for the Alps.

Keep a few cassettes and use the most appropriate one.

Pross

It's probably worth getting a 27 but I would also get a closer ratio for the crits (something like an 11-23 or 12-23) switching them is a very simple 5 minute job.

moscowflyer

I think there's too little in it to warrant changing, I'd be more inclined to try and get stronger by getting up with the 25. If you did really need the extra though then I'd cast another vote towards the 11-28.

Pickled Pig

Assuming you're after Shimano 105 then you don't lose a middle gear, it's just your two smallest gears are smaller on the 12-27. I went from 11-28 to 12-25 because I wanted a 16 tooth which I use far more than I would an 11 or 28. There is a 6g weight penalty, though, if you need a 27