Vorpal wrote:On a long descent, I alternate between brakes, front - rear - front - etc.
If I am not familiar with the descent, I am cautious with my speed.
I do that too. If you have a load on, and the hill is above a certain gradient, it is the only (safe) way of moderating your speed.
On some alpine passes the estimated terminal (ha!) velocity of a loaded touring bike with no brakes is something like 70mph. Unladen it is bit less but 50-60mph is easily possible. In my impetuous youth I have (unladen) gone past cars at about that kind of speed.
One thing I will say for old (rubber) brake blocks is that they give an early warning of the rim getting too hot. What usually happens is that when the rim starts to get too hot, the brake block starts to soften/melt at the surface. This causes several things to happen; the wear rate of the brake block goes up, the brake modulates differently (because the friction is no longer the same) and most importantly the noise of the brake blocks is changed.
The last of these is most important; it tells you when you need to change to the other brake when alternating brakes as suggested above. The point at which this happens varies with the brake block type. On steeper hills you may find that you have to alternate brakes with increasing frequency. When it gets to less than 2s bursts on each brake you are pushing it.
If the hill is very steep then you can get both brakes making that noise. At that point you have two choices; neither are very good ones, but you must decide quickly what you are going to do because it will only get worse if you carry on dragging the brakes. If you carry on using the brakes much longer you may get a blowout (which I think is most usually initiated by the most stretched part of the inner tube, which is pressed against the hottest part of the rim, tending to split when it gets hot). [FWIW if you fit tubes that are not undersized , using lots of talc, they are less stretched in the rim well and less likely to fail in this way.]
The two remaining choices you have are
a) to let the bike run so that both rims cool off, (even 5s may be enough if you are doing a reasonable speed and the air can cool the rims effectively) or
b) to use both brakes to stop the bike, and hope that you don't get a blowout before you stop, or that it will happen when you are going slowly enough for it to be manageable.
I've stopped before now on a steep descent and taken the opportunity to admire the view, (holding the brake on to stop the bike from rolling away) only to find that the brake blocks have melted and welded themselves to the rim; the bike wasn't about to roll away.... IIRC this happened with kool stop brake blocks. I didn't like the way they felt when they got too hot, either, (it seemed like they started to melt too suddenly) so I stopped using them for touring in mountainous areas after that.
Note that as beneficial cooling airflow goes, so does aero drag, more or less. This means that aero rims are (weight for weight, surface area for surface area) liable to be less well cooled than boring old square section ones.