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If i was to have a ultegra 6700 hollowtech 2 double chainset 53/39t 175mm what rear derailleur would i use a medium cage or a small?
You should use a short because those front gears are not compact range of gears thus the rear are probably narrower ranged. But it would help to make sure if you knew the rear range.
 

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There are two specs for rear derailleurs - total capacity and maximum cog.

The length of the cage/arm has to do with the amount of chain it needs to take up, total capacity. I can't remember the formula off hand, but if I recall you add the difference between the chain rings to the difference in the cassette sprockets. Then compare to the capacity of short vs. medium cage. I think short's capacity is something like high 20's, 28 or 29? Meaning even an 11-26-27 (15-16) cassette with a 52/39 (13) is somewhere around the max.

HOWEVER - the other spec for a rear derailleur is the maximum sprocket size it will accommodate and it's unrelated to cage length. It may actually be functionally a little higher than the stated spec, but regardless, it will be the same for a short and medium cage derailleur of the same lineage. Both are probably spec'd with a 26 or 27 tooth max. So that will be your limitation since either short will accommodate the capacity resulting from the larges sprocket it will handle. If you believe you can use a 28 or even 30 sprocket size with the road derailleur, then that would tip you toward the medium cage, but only if you think you can exceed the stated sprocket size spec. I

If you want a larger cassette for lower gearing, you'll have to go with a mountain bike derailleur, which will give you both sprocket size and chain capacity. There's some "rules" for using shimano MTB derailleurs with shifters. I know they work seamlessly through 9 speed, but I believe you have to use a 9 speed MTB derailleur with 10 speed road shifters. All I know is that my wife's 9 speed works perfectly with her (either 8 or 9 speed) Deore XT derailleur.

But it would be very odd for you to be purchasing that crank if you want low gearing, so disregard that.

I hope this makes some sense to you.
 

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Camilo sums it up well. That said, though I can't find the "official" capacity and max cog for it, I've used a 6700 short-cage rear derailleur with an 11-28 cassette and a 53/39 crankset many times with no problems.

Not that it matters in this instance, but my crank-arms are 175mm too.
 
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