i have a medium cage sram x-9 fitted but will be buying 3 new chain rings for my bike, i currently just use 2 and a bash but want more gears therefor getting rid of the bash, will a medium cage be ok to use for a big chain ring or is that what the large cage is for or does it all depend on the chain length? i tried to fit my chain on before and it dragged my derailleur right across and looked stretched, how do i go about this thx

you'll need a longer chain.

the key thing is the maximum difference in tooth numbers front to rear that you'll run as that's what the long/medium/short cage is there to cope with.

so medium will be fine, just need a new chain then, as mine hasent been replaced for a while, infact its been that long i cant remember taking bits off it, that must be the problem, thx alot anyway 🙂

well, probably;

Mech capacity required (in teeth) = (Biggest front cog no. of teeth – smallest front cog no. of teeth) + (biggest rear cog no. of teeth – smallest rear cog no. of teeth )

Then use a mech >= the required capacity.

For example:
standard mtb triple front with usual cassette

Capacity = (44 – 22) + (32 – 11) = 43T

and I think an SRAM medium will cope with 44T so you shoudl get away with it but you may find big/big and little/little cause issues for you.

oh, and if the chains knackered and you're fitting a new chainring you might want to think about the cassette and other chainrings being too old too – you'll have problems with a new chain and knackered rings.

yea ill be getting new rings and casette and jockeys also

if your capacity is limited, its not the end of the world.

it SHOULD be long enough that the BIG ring, and the BIG sprocket can be selected without over stretch, and the smallest ring and smalles sprocket can be selected without it doubling back on itself and dragging the chain acros the chain as the mech folds up, however if not, just run a middling chain, and dont use Big ring to big sprocket (which you shoudlnt be doing anyway really)

the problem arises where if you forget, and DO select to big a selection, it will bust the chain/rip the mech off/both.