ah ok, cheers for that....
next stupid question...
could i bung on a 34 or 36tooth in place of the 32*tooth?
*31 now
Not a stupid question but the answer is probably no, because if the front mech outer cage position is right (i.e. 2mm clearance with large ring) you are likely to find the inner cage clashing with e.g. a 36T middle ring. But to be sure you can either find out what the official spec for outer/middle ring minimum tooth difference is for your front mech, or alternatively measure how much clearance exists between the inner cage and the middle ring currently when you are on the large ring - for every 2mm you can have an extra tooth (for the middle ring).
In terms of ring/spider compatibility the chance is good for the same bcd, but unfortunately IME never guaranteed even when they are from the same manufacturer (e.g. XTR rings on XT cranks) let alone different manufacturers. On the other hand I have never had problem running higher speed (e.g. 9 speed) chains on lower (5/6/7/8) speed chainsets.
