While, as you say, a bicycle rim is essentially a giant disc, it is a disc with some big limitations. For one, whether it be a clincher or tubular rim, it is a thin-walled and very light disc. Then, attached to this giant disc is a very temperature-sensitive device, a clincher tire, or a glued tubular tire. These highly limit the full functionality of the rim as a giant disc rotor.
Also, I have used carbon disc brakes very extensively (in aircraft) and I can say that even highly engineered and highly refined carbon disc brakes have some very significant shortcomings. I have also used disc brakes very extensively on a series of different road
bikes. While I would agree that disc brakes are on a bike may not be super aero or super light, they do stop and modulate very, very, very well in a remarkably wide range of harsh and challenging environmental conditions. And those are just mechanical discs. In my experience, disc brakes work far better than any other bicycle brakes in ALL conditions, whether they be on AL or carbon rims. Once you try them, it is very hard to go back.
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