Originally Posted by
oldacura
The discussions on this forum about brakes overheating causes me to wonder: Which system is fundamentally better at dissipating heat: rim brakes or disc?
A thought experiment: A tandem team descending a steep mountain road with tight switchbacks. The switchbacks are tight enough to cause the team to repeatedly have to slow way down to make the turn. This situation is severe enough to cause at least one of the brake systems to experience some failure.
They have two identical bikes - except that one has the "best" caliper rim brakes and the other has the "best" disc brakes. They descend this road on each bike under exactly the same
conditions.
Which system is likely to fail?
Brakes basically turn kinetic energy into heat and dissipate the heat to the air. A disc is steel and can get much hotter before it fails. Heat transfer occurs faster at a higher temperature differential. A rim is aluminum, much larger and likely moving at a higher rate of speed through the air. If the rim gets too hot, the tire is likely to blow off.
Under the most taxing conditions, which works better?
In dry weather, rim brakes or disc brakes will not fail if used with proper technique and in good working order. I've used cantis, sidepull, and V-brakes on fairly demanding descents without any failures. It's just a good idea to check for rim/pad wear and excessive heat.
However, I have no doubt that hydraulic brakes work better in the most taxing conditions. Everything else being equal, a tandem with hydraulic disc brakes should descend the same route much faster with more reliability. With disc brakes, it's easy to make multiple steep descents (several miles at 6% or steeper) without worrying about brake fade or worn pads. The same team will be slower and more cautious when descending with rim brakes. No one has bothered to test this objectively because there is so much downhill or enduro mtn bike results to show that disc brakes offer more power, better modulation, and better reliability (rain or shine). It's no surprise that disc brakes are also the preferred choice for motorcycles and cars.