Building up a trail bike and was wondering what is the biggest cassette you can get away with while running a medium cage rear mech, probably an x7.

Any thoughts.

Cheers

Mechs aren’t rated for size but tooth difference they need to wrap the chain for. So it will depend on on the range not just the size and how many rings up front and the range of the rings up front.

will have single 32 ring at front and 9 speed at back so what do you reckon the range could i get like 11 – 32, 11 – 34?

You’d get away with 11-36 if you so desire!

cheers frankie, thats what i wanted to hear!

(32+11)-(32+34)=23 You will be fine easy, even a short cage rear mech should be able to cope with that tooth difference, although it would be worth doing a cautious test with the 34 tooth if you fancied trying that.

explain that wee equation to me, seems useful to know.

It’s chain capacity.

Mechs are spec’d for max sprocket, road ones at least.