I need to get a new rear derailleur. My chain rings are 53/39 and my cogs are 28/11 so if I am not mistaken I need a total capacity of (53-39) + (28-11) = 31. I need a 11 speed compatible rear derailleur. I was thinking about getting a Shimano 105 5800. On their website they say that the capacity of the SS is 32. Should I go for it or get the GS since the capacity is 37?
There is also a spec for the maximum size of a rear cog. The 5800SS allows a 23 to 28 tooth largest cog, so you are OK. The GS allows a range of 28 to 32 teeth for the largest cog. I would suggest you decide based on which direction you are more likely to want to change-if you go to an 11/32 cassette you need the GS, while if you go to tighter gearing you need the SS. These are the specified numbers and you can often make it work over a little more range.