Originally Posted by moxfyre
It seems perfectly plausible to me that coating the rim in an insulator would have this damaging effect...
But it seems to me that some kind of quantitative argument needs to be made here. Just *how much* does anodizing impede heat transfer? Jobst Brandt is really smart, and I'm inclined to trust him, but I am extremely skeptical that the effect of anodizing is large enough to have an impact on braking performance.
Actually, dark paint/anodizing increases the heat transfer from the dark body into the environment when the body is warmer, and also increases the absorption of heat, when the environment is warmer. Studied this at Uni as part of applied electronics, as well as thermodynamics.
Anodizing is a relatively good conductor of heat (albeit not as good as aluminum or any other metal, for that matter), and the very thin layer of anodizing has more of a beneficial efect in transmitting the heat from the heatsinks (or rims) into the environment, rahter than adverse (due to lesser thermal conductivity than metal).
However, this effect is very small, and nowadays manufacturers often don't anodize heat sinks, as it's considered more expensive than just increasing the heatsink surface area by, say, 2%.