would it fit

[Kiwiavenger]

how much differnce (size wise) is there between a 26 cog and a 28 cog on a rear cassette.

im looking at this http://www.wiggle.co.uk/shimano-hg70-i-8-speed-cassette/ to replace the standard 13-26 on my allez, im rarely out of the big cog at the front (except on the steeper hills round here) so the 28 would possibly keep me on the big ring for longer with the 11 giving me better speed on the flats/descents.

im just worried about the jockey wheels catching on the big cog and knackering it up!

 

[Smokin Joe]

It won't be a problem, a short cage mech will easily cope with a 28t sprocket. Should you have an issue with the jockey wheels fouling the sprocket a simple adjustment of the 'B' tension screw on the mech will sort it, but in my experience you won't need to bother.

Many people advise against using the big ring/largest sprocket combo, but provided the chain is long enough to stop the mech being wrapped round the cassete I have never had a problem, it seems to make no difference to chain life. The 11 sprocket will give you more speed on the descents, but TBH I think you're kidding yourself if you think it will make you faster on the flats unless you are at pro contract standard.

 

[Scilly Suffolk]

Big - big? You need to use the front derailleur more: you might find this useful; you'll see that the high and low ranges on the front overlap, rather than being sequential.

To answer your question: assuming your derailleur has the capacity for the range, use the "B" adjustment screw to raise and lower the jockey wheels.

Unless you are a competition level cyclist, I doubt you'll get much use out of an eleven tootk sprocket.

 

[TheDoctor]

On 53*13 I'd be running out of steam at 50+ mph...
Really, you don't need a 11T sprocket.
You won't need the bigger sprocket at the rear if you use your front mech.
That's what it's there for

 

[rockyraccoon]

I've got a 28 cog on my allez rear cassette (11-28). Running without any problem!

 

[Kiwiavenger]

i'm only on a compact and at the minute my max rpm is just about 100 but i get there quickly! lol. im working to keep it at about 90 atm.

sheldon brings up a 5mph difference at 90RPM which is good enough for the time being!