You are at the top of a hill and want to stop at the bottom. What is best for your brakes so you don't wear them down:
- brake continuously; or
- brake when you reach the bottom?
One should not trade their safety for prolonging brake pads life. Thus gaining the speed and hoping to lose it at the very end of the ride is not the wisest thing to do.
Having said that the question is rather whether to:
I'd opt for the latter since it allows the rims or discs to cool down thus having them ready and at considerable efficiency should you suddenly need them.
Furthermore, you can analyse the evolution of the roller brakes - the latter the generation the larger the radiator for the drum - it was observed that on longer descends when the brakes were partly engaged, they did overheat and lose their braking capability. With larger radiators they did overheat, much later (if ever) though.
In terms of wearing out the brakes, they will wear less when you brake at the bottom. When you descend a hill you have a fixed amount of energy to lose, and when you descend at a higher speed, a larger proportion of that energy is lost to wind resistance.
Practically however, brake pads/blocks tend to last a long time and are cheap to replace, wear is not really something to consider too much. It's better to descend safely at a speed you feel comfortable with.
There are really two optimal solutions:
The interim solution, by continuously braking at an intermediate speed, a speed at which air resistance isn't major braking factor but a speed at which the brakes are heated above the boiling point of water is the problematic strategy. It can cause very fast brake pad wear.
Depends on the hill which of there strategies should be used. The problem with the first one is that if the hill is very steep, chances are you may not descend at a speed that would offer good air resistance to dissipate majority of the energy. So, you have to estimate if you dare to choose the first strategy.
And remember to use both front and rear brakes so that you have more heat dissipation capacity.
This question is about whether you should execute the bicycle equivalent of a suicide burn. It is faster and more energy-efficient to brake as hard as you can as late as possible than to control your speed early. For a rocket this means sparing fuel, for your brakes this means producing less heat. Do note that the name doesn't come from nowhere. It's too dangerous to depend on your maximum thrust to not slam into an hard object like the Moon at highway speeds, rockets generally do control their rate of descent early on.
The other consideration is that your brakes are much better at dissipating when not in contact with the disc, so even if you brake early, it's better to break harder but not continuously.
Some modern disc brake systems feature special cooling fins to keep the temperatures in check. Greater speeds cause improved flow of air around these fins, thus improving their ability to dissipate heat. Letting your speed grow until you apply the brakes cools them down, whereas constantly applying them a bit would result in constant heat generation without the same cooling benefits.
Basically all the heat your brakes generate must be dissipated to the air around your bike, so this applies to brakes without these fins too, although it's far more noticeable with the fins.
I recommend braking down whenever you feel you're about to go too fast for your own comfort, then letting go of the brakes for a while and repeating this until you're at the bottom.
One emergency room visit will cost as much as a very nice bicycle, even if your insurance is paying for it.
Brakes exist for a reason. Use them as you wish and stop worrying about replaceable components.
If your brakes are overheating and letting go consider an upgrade. Usually to discs these days.
As Andy P points out, that ignoring air resistance, there is a fixed amount of energy the brakes have to dissipate, regardless of strategy. The more time this is spread over, the more slowly the energy is being converted into heat. At some point (at some temperature), heat will be shed into the environment as fast as it can be produced. So the harder you brake, the hotter the brakes will get, and it is temperature that causes damage and shortens component life.
There is an argument that releasing the brakes allows them to cool because the pads are no longer in contact with the rims. Well, yes and no. Not in contact with the rims means no more heat production, so of course they will cool, but rims are large heat sinks and have more surface area to shed heat than the pads which run on them. So, continuous braking means you are constantly creating heat, but it is going into both the pads and the rims; contact also means that heat in the pads can flow into the rims, even as they are turning, so the rims can actually help cool the pads during a long, controlled descent.
Bottom line: the gentle descent can be the best for both safety and equipment longevity.