Originally Posted by
SoSmellyAir
In Shimano's latest (i.e., 105 R7000 and Ultegra R8000) 11-speed drivetrains, a medium cage rear derailleur handles up to a 34T max cog, while a short cage rear derailleur handles up to a 30T max cog. However, you can still use a medium cage RD with a smaller (e.g., 11-28 or 12-25) cassette, you just have to loosen the B screw as much as possible and shorten the chain.
so If I get a new bike with a 32 or 34 max cog cassette, I can swap later to a 28 and if I don't like that, swap back. That seems pretty straight forward. Thanks!