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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?
  • 11
    Brake as little time and as hard as (safely) possible. Continuously dragging the brakes downhill is a bad idea on any vehicle. – Carel Oct 16 '18 at 12:45
  • 4
    @Carel Why? Because of overheating? I really want to see a study/paper that explains why this is, but I can't find any. – Will Vousden Oct 17 '18 at 12:45
  • Overheating on long descents is certainly a problem. Overheating with rim brakes can cause inner or tyre failure, and overheating hydraulic disc brakes can boil the fluid and cause brake fade. Pumping the brakes to allow heat dissipation avoids both. – Useless Oct 17 '18 at 13:30
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    Hmm, in aviation, carbon-fibre disc brakes are recommended to be used "short but hard" while steel discs are recommended to be used "soft but long". – Agent_L Oct 18 '18 at 9:24
  • 1
    @WillVousden : That's what any cycle race club will teach you in their beginner's course to avoid bad habits. On a 500m long descent your rims will get warm but won't certainly overheat. But try it on a long alpine style descent of ten or more km you'll run into trouble. – Carel Oct 18 '18 at 15:28
43

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:

  • lightly but constantly brake maintaining more or less constant speed; or
  • gain some speed, brake hard, repeat

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.

  • 2
    Very good comment on the overheating issue. Overheated brakes lose the braking capability significantly. – Crowley Oct 16 '18 at 16:33
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    @Mike I've heard a lot of people claim that intermittent braking limits overheating, but why? Do you have any explanation or references? To a first approximation, the total energy lost during the descent must be fixed, so why should one braking strategy be better than another? – Will Vousden Oct 17 '18 at 11:17
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    @WillVousden If you're descending a 10% grade at 40 mph, without pedaling you're dissipating energy at a rate of about 1,500W - into the air, via drag. Play with some numbers here: bikecalculator.com/veloUS.html Every Joule of energy you don't dissipate into the air via drag has to go somewhere else. So if you slow yourself down with your brakes, the energy that doesn't get dissipated into the air goes into your brakes and rims or disks instead. Your gravitational potential energy at the top of a hill is constant - it's going to go into your brakes as heat or into the air as drag. – Andrew Henle Oct 17 '18 at 15:19
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    @gschenk I don't see that. The working surface of the pad is small, so air cooling is inefficient there. Heat-conductance cooling to the rim should be much faster actually, and the rim itself is in either case cooled well by lots of surface to the air. – leftaroundabout Oct 17 '18 at 18:14
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    @AndrewHenle Thanks for providing some numbers. It occurs to me that braking hard at the end of the descent may minimise the total work done by the brakes (since you're making optimal use of drag), but presumably maximises the the temperature that they reach, since you're dumping a huge amount of energy into them in a very short time (over which they don't have much chance to dissipate it). Conversely, you can maximise the work done by the brakes and minimise the temperature they reach by going down the hill very, very slowly using the brakes. – Will Vousden Oct 17 '18 at 20:26
53

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.

  • 74
    You can get the least amount of wear on the brake pads by crashing into something. – ratchet freak Oct 16 '18 at 16:18
  • 12
    You can get even less wear by never riding the bike and instead spending time researching more bikes to buy. – Adonalsium Oct 16 '18 at 20:26
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    I find the question interesting from both a cycling and driving perspective, and from a driving perspective, brakes are not cheap to replace, especially if your wear patterns requiring turning the rotors which in turn brings rotor replacement closer down the line. But perhaps this is worthy of a separate question on a different stack as the answer could be quite different for cars. – Michael Oct 17 '18 at 21:20
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    @UKMonkey You try doing an emergency stop on your bike at 70kph and let me know what your breaks (and face) are like at the end - that's the sort of power that will melt rims; not continuous breaking. I've done descents at 60 mph - that's 90 kph. You're wrong and don't know what you're talking about. – Andrew Henle Oct 18 '18 at 10:54
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    @AndrewHenle This braking strategy relies entirely on being able to descend really fast, because that's what generates the required aerodynamic braking force. What if it's unsafe to descend at more than, say, 35 km/h, because the road isn't straight, or has a bad road surface? – Will Vousden Oct 18 '18 at 12:31
17

There are really two optimal solutions:

  • Go as fast as you dare, perhaps slightly braking, maximize your frontal area (thereby maximizing air resistance) then brake at the end quickly. The idea here is that you are obtaining part of the braking force from air resistance due to the maximized frontal area and maximized speed.
  • Go very, very slowly, braking all the time. The idea here is that the slower you descend, the less the braking power needed is. Power is force times velocity; force stays the same for a given hill, but velocity is something you can affect. Because braking power is lower, the heat dissipation required per unit time is lower, and thus, brake temperature is lower.

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.

6

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.

  • 1
    Nice analogy with atmospheric reentry! Also, interesting point about pads dissipating heat better when not in contact with the disc. Do you know how much difference this actually makes? – Will Vousden Oct 18 '18 at 10:34
1

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.

  • The question was about brake wear. It would probably be a good idea to at least mention it in the answer. You also got the effect of speed mostly wrong, check the answer by Andy P. – ojs Oct 17 '18 at 21:21
  • @ojs I agree that this answer doesn't talk about brake wear and that it should. However, it seems to agree with Andy P's answer, to me -- Walto also recommends not braking continuously. – David Richerby Oct 18 '18 at 9:07
  • The conclusion is correct, but the theory behind it is wrong. – ojs Oct 18 '18 at 19:58
  • @ojs Could you explain what's wrong? I'm not seeing it. – David Richerby Oct 18 '18 at 23:29
  • @DavidRicherby the answer suggests that the reason for going faster between braking is more efficient cooling, not that there is less energy heating brakes in the first place. – ojs Oct 19 '18 at 20:03
-1

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.

  • Emergency room visits are free in most developed countries, technique is important (hence the question) and most rim-braked bikes can't be upgraded to discs. – David Richerby Oct 21 '18 at 10:49
  • @DavidRicherby Emergency room visits are not free any where. Even if you didn't pay yourself the staff get paid for treating you. If insurance pays then insurance rates eventually go up, and if the state/country pays then the everyone pays for it in the form of taxes. In all cases more emergency room visits mean you pay more taxes,insurance,co-pay, or etc which all come out of your pocket. – cybernard Oct 21 '18 at 17:40
  • @cybernard Of course. But the answer suggests that an ER visit will cost me as much as a nice bike and that simply isn't true. Directly, an ER visit costs me nothing; indirectly, my contribution towards all the ER visits for bike accidents in my country this year will be much less than the cost of a new bike. – David Richerby Oct 21 '18 at 17:49
-3

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.

  • 1
    You've completely misunderstood Andy P's answer. You can't ignore air resistance: above about 20km/h, air resistance is the greatest force opposing a cyclist. When you go down a hill and stop at the bottom, you have a fixed amount of potential energy that you need to dissipate. This goes almost entirely into your brakes and into overcoming air resistance. Because air resistance increases super-linearly with speed, going faster means that more of your fixed energy budget goes to air resistance, so less goes into the brakes. – David Richerby Oct 21 '18 at 14:34

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