This is utterly wrong. The brake pads cover less than one square inch of the rim. They're not some blanket that prevents heat from escaping. Your rims are losing heat at the same rate regardless of whether the brake is on or off. You don't need to "get off the brakes" so they can start cooling. They are already cooling, all the time. All this talk about the surface of the rim vs the core of the rim is just a bunch of mumbo jumbo. Metal is an excellent conductor of heat. Aluminum is used as a heatsink in many applications. The heat is distributed throughout.
Constant vs non-constant braking is also meaningless. It depends on how hard you're applying the brakes. Friction = coefficient x normal force. If you only lightly feather the brakes, there will be hardly any friction. You can apply the brake continuously for 8 hours straight and your rims will still be cool as a cucumber.
Keeping a high speed in the straights and braking at corners has nothing to do with getting off the brakes so they can "start cooling". Your rims are always cooling regardless. All you're doing is increasing your average speed and thereby increasing the percentage of energy converted via air friction vs via brake friction. Less energy going into the brakes = less rim heat. The heat that would have gone into your rims went into your face and body skin instead.
Originally Posted by
cyccommute
It does make a large difference. As pointed out above, when the brakes are off, no heat is being put into the system. Think of it this way: Pulsing the brakes hard, heats the surface of the rim. The inner part of the rim is still mostly cool. When the brakes are released, the heat easily radiates away from the rim to the air. The core of the rim will heat some but not much.
Applying continuous pressure soaks heat into the core of the rim. There is some air radiation but the constant friction keeps the rim hotter over a longer distance. Constant braking of any vehicle results in overheated brakes.
At the bottom of the hill, you’ve done the same conversion of kinetic energy to heat but with pulsed braking, you are shedding that heat all along the way. With constant braking, you are keeping the heat in the rim.
You almost got it but kind of stumbled at the end there. Part of the equation is that air resistance. At higher speeds, heat is swept away front the rim faster and the rims cool faster. At lower speed, less air flow and, thus, less cooling. The continuous braking combined with lower cooling due to less air flow results in more heat being held by the rim. Taking an hour to do a 3 mile hill, for example, would result in cool brakes but taking an hour to do that kind of downhill is a bit excessive.
You kind of got it. But it’s not the speed that results in hot brakes. It’s their application. A medium average descent speed is usually associated with constant braking to keep your speed at a certain level. High speed descents really aren’t possible with constant brake application.
And then you don’t get it. Pulsing brakes isn’t about doing it on a set time interval. It’s about using the brakes when they are needed and then getting off them so that the rim can cool. Yes, you bomb down a hill and hit the brakes so that you don’t run off the road but you do it because it doesn’t over heat the brakes. This also works for disc brakes and is probably more important there. The rotor is much smaller. It has a lot less surface area to radiate the heat and it has a lot less thermal mass for the heat to flow into. Hub mounted rotors are much, much easier to cook.