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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?

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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.

  • The GS is more useful since it also handles a 11-25 cassette without any problem. So there's no need to swap the RD if you want to use the smaller cassette and it's just a tiny bit heavier. – Carel Apr 17 '16 at 10:06
  • @Carel: I looked up the spec on the Shimano page. It says the minimum large cog for the GS is 28. I suspect it would work for the 25 as well. The idler pulley will be farther from the cassette than intended. – Ross Millikan Apr 17 '16 at 14:01
  • @ Ross Millikan: I use the combination on my bike, it works very well with 11-32, 11-28 and 11-25. – Carel Apr 17 '16 at 15:53

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