I've sometimes wondered about this and sorry if I asked this before: alloy dissipates heat faster than steel and rubber pads generate less friction than the old leather-and-worse ones, so why won't you melt modern pads long before heating alloy rims enough to pop a tyre?
Just to be clear:
I think you meant to ask about conduction rather than dissipation. Dissipate refers to how quickly it gets rid of the heat, typically by radiation and that would be dependent on the colour and texture and airflow, since heat dissipates by conduction and radiation and convection.
Also, you don't say what alloy but I assume you mean aluminium, because of the context. Steel, brass, pewter, 18K gold etc are all alloys. Some alloys have specific names like copper and zinc is called brass, iron and other metals and even carbon is called steel and lead and tin is called pewter. Aluminium alloy is simply called aluminium no matter what it is alloyed with.
The CoF of leather on metal is 0.4. I just looked it up on a table but can't figure out what metal. It does make a small difference.
The CoF of rubber on aluminium is about 0.6. This means that rubber brake blocks on aluminium rims work better than leather blocks on steel and hence, will generate more heat. Hence your question about melting.
That's because aluminium is such a good conductor of heat that it transfers the heat away from the place where the heat is manufactured. It is always created in the softest of the two materials and at the interface itself. The rubber is in close contact with the aluminium. Heat can't travel backwards through the rubber but readily travels forwards into the aluminium. Once there, it quickly transfers from the surface to the entire rim. The rim is mostly open to the air and the heat dissipates very quickly into the airstream and doesn't get a chance to heat up the tyres. If you exceed the rim's capacity to dissipate the heat, then the tyre will heat up - actually the whole rim and tyre will heat up.
If you use those same rubber pads on a Chrome-plated steel rim, you have a different scenario. The rim (especially chrome) doesn't conduct heat very well and the heat can't get away fast enough from the pad. The pad melts and you have lubrication, instead of friction at the pad interface and the brakes fail. This is what happens with carbon rims. Carbon composite is a very, very poor conductor of heat and the whole system goes belly up. That's why they use non-melt pads made from cork and wood shavings. They just smoke instead of melt. Yet, the braking is still poor.
In the example above, where the bike descends slowly, the airstream doesn't cool the rims enough and they overheat.
Sorry, CoF is Coefficient of Friction, a measure of the friction "strength".
