Originally Posted by
BabyCowHK
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.
I agree with you, several Shimano rear derailleur accept bigger biggest cog that what Shimano specify. I know the the RD-R8000-GS medium cage that according to Shimano is limited to 34t can work with 36t.
But I have an Ultegra RD-R8000-GS medium cage on my Birdy and its not too close to the ground, its even slightly higher than the R&M factory mounted RD-T610-SGS, so I see no reason to take the risk of using a short cage instead.
For the chainrings, the difference between brands is as Glye says for more sophisticated chainrings like narrow-wide and even more for double chainrings that have grooves and pins to help and speed up the chin going from the small to the big chainring. Each brand has its own patented tricks.