Originally Posted by
PoorInRichfield
The size of the cassette and/or chain rings isn't really a Di2 issue. Di2 however can be programmed to "know" what gears you have on your bike which will allow it to set the shift points for what is known as "synchronized shifting". Don't concern yourself with that right now... just know it's cool
The reason you may or may not need a new rear derailleur typically has to do with the size of the largest cog on the cassette. I *think* that most Shimano short-cage rear road derailleurs top-out at around a 32 tooth cog. If you want bigger than that, you'll need a long cage.
Why kind of bikes are you looking at... pure racing bikes? Bikes that are considered "endurance bikes", like the Trek Domane and Specialized Roubaix come with gearing closer to what you're looking for.
Believe-you-me, you'll love both once you get used to them and won't go back. Electronic shifting opens up a whole new world, especially if you buy a decent computer like a Garmin or Wahoo and install the wireless adapter so the Di2 system can talk directly to the computer.
I'm kind of torn also between a "racing" road bike versus a more "endurance" type one. I'm way too slow to do actual races, but sometimes I do fast group rides with my cycling friends and I'd like to be able to keep up as much as possible. Of course, sometimes I also do "climbing" rides with these same friends where we go up significant grades, so it would be good to have adequate gearing for that as well. Ideally I'd like something that could do both - a "racing" oriented road bike with a good range of gears so I could conquer some 18/19% grades with no problem.
The electronic shifting aspect does seem pretty cool, I must admit!