Originally Posted by
Cyclespanner
I've been a motorcyclist for 55 years.
When the Japanese introduced disc brakes they were chrome plated steel which, to say the least, had the same 'delay' in the wet. Things have developed since then, but the Italians introduced us to cast iron discs which stopped convincingly, rain or shine. The iron surface rusted, even sitting in the garage, but that was regarded as acceptable in return for predictable stopping power.
Polished chrome has no 'grain', like a glass surface. No wonder it provides terrible wet performance.
Is bare steel any better?....if so I'd accept rust.
Several differences sabotage your point:
Disc rotors are not integral to the structure of the motorcycle wheel; your bicycle rim is the keystone to a functioning wheel.
Disc brake pads are not soft rubber, and generally have no issue quickly abrading loose rust from a rotor surface; a rusty bicycle rim will either abrade brake pads at a faster pace, or clog with the rust particles and suffer diminished performance. I'm not willing to experiment to see which result is more likely.