dynaryder

Ok,I'm looking to re-gear two of my bikes;my Sirrus so I can handle snow(with cross tires) come winter,and my Cross Comp because the cross double(39/48) + DC hills is starting to suck. The Sirrus has a Tiagra rear derailleur,the Comp a 105,both long cages,and both have 12-25 cassettes(Sirrus 8sp,Comp 9sp). The plan is to put a 12-32 on the Sirrus and 11-34 on the Comp. But when I checked the specs on Shimano's site,I became confused. It said both derailleurs had a max sprocket of 27T,but a total capacity of 37T. I'm guessing the 'max sprocket 27T' would mean they couldn't handle anything bigger than a 27,but what's the 'total capacity' mean? Also,I swear I saw a bike on a site somewhere(can't remember the brand or model) that was running a 11-34 cassette with a 105 rear. So will these derailleurs handle these MTB cassettes,or do I need to swap them for MTB derailleurs? Thanks in advance.

The Great Stonk

capacity is the range of gears the mech can handle, 12-32 cassette with a 39/48 double requires a mech with a capacity of 29T (the number of teeth in the smallest gear combo taken away from the larges combo, in this case, (48+32)-(12+39)

the number basically tells you how much slack the mech can deal with, the longer the cage, the more gears. as for max tooth, you may be able to get away with it, but dont be outraged or angry if it cant handle it. worst case would be having to buy a cheap mtb mech, say a deore which will handle things no problem.

HardyWeinberg

Total capacity = biggest chainring - smallest chainring + biggest sprocket used w/ biggest chainring - smallest sprocket used w/ smallest chainring

(note there's slack in the last 2 terms for just choosing not to use certain chainring/cog combinations).

I would assume a long-cage derailer would work w/ 32 or 34t rear cogs, but don't know for a fact. I am using a 28t rear cog w/ short/mid-cage 105 rear derailer, no problems there. I do hear there's more to it than just cage length, though.

well biked

The total capacity is referring to the chain wrap capacity of the derailleur, and the max sprocket capacity is the largest rear cog the derailleur will work with without having the upper pulley run into the teeth on the large cog. The max tooth capacity is pretty straightforward (although the 27t capacity is conservative on Shimano's part, you could probably run a 30 I believe). The chain wrap capacity is figured like this: large chainring minus small chainring plus largest cog minus smallest cog. This is a long cage vs. short cage issue: long cage derailleurs can handle more chain slack than short cage. Bottom line: you'll need a "mountain" rear derailleur to run those 11 x 32 and 11 x 34 cassettes-