How many cassette teeth before longer mech needed?

[scubadude]

Title says it all really, I had a google around and having trouble getting concrete answers, anyone know off the top of their heads at what tooth count on the cassette do you need to switch from a short to long cage rear mech?

I'm putting a new rear wheel on my roadie, considering a lower gear while I'm at it. Currently running 11-25, toying with 12-27 but nervous that it'll be borderline.

[itsnotarace]

On which rear mech? The specs will be on the manufacturers site. 12-27 will be fine on more recent Shimano short cage. Not sure about other manufacturers but I run 11-28 on Ultegra and DA no problem at all.

Last thing, you will probably need to add another link or two in your chain.

[scubadude]

The current rear mech is in a bath to find the model number, early 2000's ultegra is my guess :-)

[Bobley]

Short and long cages often take the same number of teeth on the casette. older 5600 105 take 27 teeth, latest 5700 take 30. Ultegra 6500 short takes 28, medium takes 27???

Long cage mechs are for bigger chainwheel variations (i.e. triples)

[upsidedownmark]

27/28 is generally ok, even if slightly outside the spec (which is conservative). Generally it's more about the difference you need to accommodate, rather than the absolute number of teeth.

You can also 'save' a bit of difference by *never* running the big chainring and the biggest sprocket together.. which you shouldn't do anyway.

You may however need a new (longer) chain to run the 28. Perhaps. Probably not..

[itsnotarace]

The tech document suggests 28T will be fine, with a maximum difference of 14T. So it will depend what you are running for front chainrings

You may need to wind the B screw in according to Sheldon Brown

http://techdocs.shimano.com/media/techdocs/content...

[scubadude]

Brilliant thanks, that doc also confirms I've set up the indexing wrong which explains some of the noise and reluctance to get into lowest but one cog.

[robpearson]

has anyone had any experience of the SRAM wifli kit? is it just more marketing?

[itsnotarace]

Well it's a long cage rear mech and an MTB cassette so if that's what constitutes marketing then....

[2volvos]

I replaced the 24 tooth with a 28 to my old Peugeot which has a very 'small' Simplex mech. No trouble.

[deadtom]

as others have said its not so much the absolute size of the largest sprocket that determines cage length, its more the difference in size between the smallest and largest, but even that is of lesser concern than the front chainset.

I always just go with the simple rule of short, medium and long cage are for single, double and triple chainsets respectively, assuming that your cassette is of a more or less normal size and range.

If you are just changing the cassette for 1 tooth bigger, you probably don't need to worry about anything else unless you run your chain particularly short in which case you may need to extend it slightly, and maybe wind on some B limit too.