Originally Posted by
Jipe
Shimano RD-R8000-SS short cage is limited to 30t for the cassette, it doesn't accept a 32t or 34t biggest cog while RD-R8000-GS goes up to 34t.
As said, a Shimano 53-39 crankset is a road crankset with a narrower chainline than the Birdy original crankset and might not work with a front derailleur (same as
the Dura-Ace 7800 of 50PlusCycling which doesn't work with a front derailleur). So, again, I advise to at least first try to mount a BCD130 53-39 on the original crankset.
I have to agree with you on the crankset and testing setup. Just get two cheap chainrings and do a manual shift as I previously mentioned.
However, R8000-SS-RD is fully capable of using 34T cassette. The body length of the R8000 SS and GS models are the same. Its the cage length that's making SS
officially unsuitable for cassettes larger than 30T. I know that a lot of people don't understand why we need to calculate the capacity on an RD. I normally won't advise people to do large cassette with a SS road RD.