Originally Posted by
pathdoc
Why not?
While disc brakes typically have far more heat capacity than rim brakes, they do not have as much heat capacity as the tried and true Arai drum brake that has become the default 'drag brake' for tandems.
The extreme heat generated by prolonged use of a disc on a road tandem will:
1. Boil hydraulic fluid, which is why only mechanical discs are even suggested for use on road tandems.
2. Heat calipers to the point where non-metallic parts used as part of the calper or brake cable housing can begin to break down or melt.
3. Heat rotors and brake pads to the point where brake fade makes the brake ineffective
4. Heat rotors to the point where they will warp and may no longer run true even after cooling
Therefore, while discs -- just like rim brakes -- can be 'ridden' for short periods of time on a long descent, there are limitations on just how much heat they can safely handle before brake performance deteriorates. It is not as big of an issue when a disc is used as a supplemental brake to a second, primary rim brake; however, the problems associated with overheating a disc remain valid.
Conversely, a true drum brake is used only as a supplemental brake to a rim brake and the very large braking surfaces, the mass of the housing (think heat sink) and larger brake pads made them ideally suited fo drag brake applications.