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It is my impression (both from experience and from online opinions) that braking on a mountain bike is to be done in intermittent hard engagement of one or both brakes. The reasoning being that braking during bad traction (cornering, wet stones/roots, sand etc.) is detrimental to traction thus handling.

I have several questions regarding said practice:

  • Is this advice true for long paved descents? Where the danger of overheating of say a loaded touring bike is significant?
  • If yes, how is that possible given that brakes convert potential energy into thermal? And the former depends only on elevation and mass. I tried writing the equations for a concentrated mass descending a ramp ... but fluid dynamics, applied to bicycles, is not something I can handle. If anything, an intuitive conclusion was that dragging the brakes slows the descent enough to radiate more heat.
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    Perhaps I’ll elaborate in the morning, but braking while cornering steals some of your centripetal force (reduces max cornering ability). Yes, potential energy goes to heat energy, but it’s not a perfect, instantaneous transfer. Burst braking maximizes the peak temp of the pads and rotor so they radiate more, while reducing total braking contact time to avoid transferring heat into the fluid.
    – MaplePanda
    Commented Oct 17, 2021 at 6:36
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    @MaplePanda absolutely right. Braking in the corner tries to straighten your line out, and is a good way to cross the centerline or exit the road entirely. Motorbikes have the same issue, and their advise is to lean more and add power. We can do the first, but adding power means risking pedal strike, which almost always leads to loss of traction/control and a messy stop.
    – Criggie ♦
    Commented Oct 17, 2021 at 9:16
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    Braking in corners and/or on loose surfaces has to be done carefully and should be avoided if possible.
    – Michael
    Commented Oct 17, 2021 at 18:00
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    @fraxinus: With cars you should mainly use engine braking while descending (especially with electric cars). Trucks have retarder brakes.
    – Michael
    Commented Oct 18, 2021 at 7:13
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    @ChrisH Sure, but firstly the radiative flux will be small anyway and secondly it is the temperature that causes brake fade so we want to keep it in some safe numbers. Increasing the heat flux (be it radiative or heat conduction to air) by allowing the temperature to go high is counter-productive.
    – Vladimir F Героям слава
    Commented Oct 18, 2021 at 14:57

2 Answers 2

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In a world without atmosphere you would be correct.

However in the real world short and hard braking has several advantages:

  • You spend more time at high speed before you brake, so you lose more energy to aerodynamic drag which increases with velocity cubed. (the power increases with velocity cubed, the force with velocity squared)
  • The surface of the brake pads can be cooled by air while not in contact with the disc or rim.
  • You can relax your hands between brake maneuvers.

Of course your first and foremost priority should be to ride at a safe and manageable speed. If you crash because you brake too late or too little it’s not better than crashing because of overheated brakes. With overheated brakes you at least have good chances that one of the two brakes still works well enough to come to a safe stop.

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    "...you lose more energy to aerodynamic drag which increases with velocity cubed...". Yes, the power increases with velocity cubed, but the time spent on a certain downhill track linearly decreases with velocity, so I'd argue the energy lost to aerodynamic drag still increases with velocity squared. (The argument of course still holds, just the effect is a bit less strong.)
    – MaxD
    Commented Oct 17, 2021 at 14:25
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    Intermittent braking results in less time at high speed, assuming there is some maximum safe speed you don't want to exceed. With constant braking, you can maintain the max speed constantly, but with intermittent braking, you only momentarily hit the max speed before slowing down again. If you're comfortable descending a hill at a constant 25mph, you don't want to hit 30mph at any point.
    – Nuclear Hoagie
    Commented Oct 18, 2021 at 13:01
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    @NuclearHoagie: True, but on smooth roads you usually let it roll and then brake as late as possible before the next turn.
    – Michael
    Commented Oct 18, 2021 at 17:10
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    "The surface of the brake pads can be cooled by air" The brake pads are cooled by air ALL the time (except in a vacuum). However, when the brakes aren't being used, the rims will be dragging air past the brakes pads. Plus there is additional surface area for air to cool the brake pads...plus, that additional surface is closer to where the heat was generated...blah blah...
    – Aron
    Commented Oct 19, 2021 at 9:59
  • I am accepting this concise and insightful answer but my rethink my decision once there are more cyclists on Mars than Earth ;-p
    – Vorac
    Commented Oct 28, 2021 at 11:13
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Brake fade under excess heating is absolutely a thing - when your brakes get too hot their ability to decrease your speed will reduce. Doesn't matter if you've got disks, rim brakes, a drum brake, a roller brake, a coaster, rod brakes or a spoon brake, they all lose performance as the temperatures rise.

A Long Paved descent is no different. As a rider you can choose to pedal or to coast, you can choose to be aero or sit up and catch the breeze. Much of this comes down to line-of-sight and your confidence level.

A long sustained slow drag on the brake will raise the temperature of the pad and its brake surface, and hold high. The higher temperatures means less "headroom" before the brake overheats and function is reduced.
Heat will also "soak" further into a rim, potentially upsetting glues, patches and sealants in the tyre, which can be bad for a glued-on tubular like the pros often use.

By comparison, a hard brake of a second or two duration, followed by a similar length of no-braking will keep the overall temperatures lower. Alternating between front and rear allows you to cap your speed, and reduce runaway. While one brake is braking, the other is cooling.

The system "bicycle plus rider" is notoriously hard to model well. So many models must simplify something, potentially going off-course. As a rider, you should consider testing on your own bike. Find a descent that is 5-10%, and ride it once a weekend for a month, while recording your rides on something like Strava. Alternate your methods and see which one feels best to you. Too shallow and it won't show the braking effect. Too steep is unnerving to ride down fast.

Better brake pads can be the easiest fix to brakes that heat quickly. Kool Stop is my go-to brand for everything now.

I've experienced brake fade twice. Both were rim brakes.

First time was a MTB with cantilever brakes, on a -19.5% average gradient in dirt and shingle in 11.6 minutes descending 219 metres. The brakes faded so much the levers bottomed out on the bars and I was still accelerating. The track pitched up, so I stopped and checked the rim temp, and it was very hot even through gloves. This was averaging around 6 km/h. I don't recall what pads were on this bike.

Other time was in a road bike race, at -10.3% average gradient with 503 metres of descent in 6 minutes and averaging ~55 km/h. I could smell my brake pads and feel the drop in performance. With a hard turn at the bottom I was evaluating my bailout options, including a foot drag and a complete drop on one side. Fortunately I managed to drop my speed enough to make the turn using the whole width of the roads - hooray for closed-courses! This was on stock black Shimano brake pads, normally perfectly adequate.

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    I'd say cable discs with sintered metal pads are essentially immune to fade. There's no rubber or organic material to melt or glaze over (even "ceramic" pads are resin-bonded as I found out painfully), no fluid to overheat, and no tubes to risk bursting. Descending a steep pass stuck between cars they were as good at the bottom as the top, and I could feel the heat radiating off then from 20cm away afterwards. I wasn't even half way down when splashing through puddles let to a lot of sizzling, so the discs were well over 100 °C by that point
    – Chris H
    Commented Oct 17, 2021 at 11:06
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    "They all loose performance as the temperatures rise". While true globally, it's not always true in practice. Carbon brakes are extremely thermally stable, and their wear is not proportional to the energy dissipated. (They are better for fewer longer applications, contrary to the common wisdom). But this is mostly for aircraft and such, not for bicycles.
    – Zeus
    Commented Oct 18, 2021 at 1:12
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    @Zeus I'd remark that the carbon-carbon used in aircraft and racecar brakes is a very different beast compared to the CFRP that carbon bikes are made of. It behaves more like a ceramic than a composite. The Space Shuttle Columbia disaster was due to a failure of the C/C wing leading edges after being hit by some pieces of foam (!) from the tank's thermal insulation. C/C seems to be ok for the thick brake discs on planes and cars, but I reckon a bike brake would need to be designed very different if it were made from C/C.
    – leftaroundabout
    Commented Oct 18, 2021 at 10:43
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    @JirkaHanika While it's not really helpful to a cyclist, the correct way to put it is this: Overheating depends on the difference between the power that's removed by the brake, and the power that the brake can cool away. A bit more helpful, however, is this: The height difference gives the energy to remove, divide by the time you take to go th the height difference and you get the average power you need to remove. Subtract the power lost to aerodynamic drag, and you get the power that enters the brakes. So really, overheating depends mostly on your descent rate.
    – cmaster - reinstate monica
    Commented Oct 18, 2021 at 19:39
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    @JirkaHanika That's about right. An additional problem, though, is that the cooling power also depends on the speed, although it's a sublinear relationship, afaik. That pushes the speed with the maximum heat problems further down than one would expect. So, if you go slow, you need to go really slow to avoid overheating. The other end of aerodynamic drag, sadly, is not always available, but definitely fun when it is.
    – cmaster - reinstate monica
    Commented Oct 19, 2021 at 8:23

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