Originally Posted by
hueyhoolihan
if i'm not mistaken, the length of a cage on a derailleur does not dictate the maximum tooth count it can handle. it dictates the amount of slack the tensioner (cage) can take up, which is a function of the tooth count difference between the highest and lowest gears.
max cog is more a function of geometry involving the jockey wheel, the derailleur hanger, the derailler's parallelogram and the rear axle.
more useful might be success stories i've heard of a 105 with a 32t cog. but it will really have to be tried on the bike it is intended to be used on to be certain.
Maximum capacity is calculated by adding the difference between the largest and smallest chain wheels to the difference in the largest and smallest rear cogs while the maximum cog speaks to the derailleurs physical limits.
On a bike with a 34/50 and 12-30 the capacity is calculated as 16 (50-34) plus 18 (30-12) which is a capacity of 34.
The 105 SS has a max cog of 30 and max capacity rating of 34.
The 105 SG is rated at 40 with a maximum cog of 32
Sometimes you can exceed the cog limit on a derailleur and this can be affected by the hangar length which also determines the maximum cog size a derailleur can handle.