My head gets all twisted when I think about braking and long descents!
The total energy to be dissipated is pretty much fixed by the height of the hill and the weight of the bike etc. This energy will be dissipated by braking, by wind resistance, rolling resistance... that ought to be about it!
Wind resistance gets quite strong when you get going fast. It also increases if you sit more upright to increase cross section. Sometimes I'll unzip my jacket a bit so the jacket puffs out and acts a bit like a parachute. Loose clothing can really increase wind resistance!
Whatever energy is dissipated by the brakes will go into heating up the pads and rims. But the pads and rims will also dissipated that heat into the air. Using the brakes to go slowly down the hill will give the rims more time to dissipate the heat into the air, so the rims will be cooler at the bottom of the hill.
So it seems like the rims will stay cool if you brake a lot to go slowly down the hill, and they'll also stay cool if you can just keep off the brakes and let wind resistance limit your speed. Of course, if there are curves or traffic or whatever, wind resistance might not be enough to avoid energy dissipation through structural deformation, definitely the least desirable option!
Seems there must be some intermediate speed which maximizes the temperature of the rims at the bottom of the hill!
Hey, for nifty gadgets, how about little drag parachutes for bikes, a little packet on the back of the rack and maybe a ring to pull down by the seat post clamp?
