MrTall from the CTC website, to avoid any confusion;
Rear mechs have two kinds of capacity and the numbers of teeth can be a bit similar. So check which of these is being talked about:
1) Sprocket capacity – is the biggest big rear sprocket they can handle. It depends upon the geometry of the mechanism. Mechs with bigger sprocket capacity swing further down as they swing inward. It's also influenced by the dimensions of the gear hanger on the frame; these varied a lot in the past and road bikes still tend to have shorter ones than mountain-bikes.
Manufacturers have to be a bit cautious in their claims, so you can often use a couple more teeth than they say – sometimes a lot more. (I've used a 32T capacity mech on a block with a 36T chainring bolted on the back, and that on a bike with road dropouts!) Sometimes the smallest rear sprocket is also stated, but this hardly matters at all. Some mechs, however, will foul unusually big sizes of top sprocket and bear in mind that anything bigger than 14T is nowadays unusual.
2) Total capacity – is the biggest difference in total numbers of teeth, front plus rear, that the mech can handle without letting the chain hang loose. Say you've got a 11,13,15,17,20,23,26,32 cassette and 24,36,46 chainset. The biggest combination is 46+32 = 76T, smallest 24+11 = 35T. So you’ll be wanting a mech with 76–35 = 41Tcapacity. But again, you can generally push it a bit.
Provided the chain is made just long enough to engage big and big (if it's too short for that you might demolish the mech by mistakenly doing that!), the worst that can happen is it hangs loose and maybe rattles a bit in small and small. But modern gearing provides so much overlap you’ll never need this combination. Your mech needs only to cater for small plus “smallest-used”. Maybe you want to shift the aforementioned gears with an old Deore mech having 38T total capacity and are wondering what is the smallest sprocket you can use with that. Your starting point is still the big+big 76T: subtract 38T capacity, also subtract the inner chainring’s 24T = 14T. Voila: all except the top two sprockets will work fine on the inner ring – and as for them: vive la catène flottant! CJ
