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I recently rode my bike on some steep hills. After descending on a 4 km road with some sections at 21% slope, I realized my rims were pretty hot not only where the brake pads operate, but even up to the sprokes connection.

Though I realize that keeping the bike moving will allow for a more effective cooling, should one let the rims cool down to preserve braking efficiency in such situations?

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    You need to modulate your brakes, and not use them too much. Overheating a hydraulic brake can boil the brake fluid leaving you with a loss of braking. Overheating a bicycle rim can cause it to fail. – Batman May 8 '17 at 15:53
  • Use other techniques to slow down too - sit up and catch air with your chest to trim some speed. – Criggie May 8 '17 at 19:53
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    @Criggie: Sitting up may have a bad side-effect by removing load from the front wheel and inducing uncontrollable shimmy! (Speaking from experience.) – Carel May 9 '17 at 9:58
  • @carel yes that's possible, more so on a bigger frame bike. The trick is to move forward, which means sliding your backside forward on the seat and therefore restore the weight through the hands. Shimmy can be controlled /reduced by pressing a knee into the top tube. – Criggie May 9 '17 at 10:20
  • @Criggie That sounds very bad for the case when you suddenly have to make a full stop, after all for braking fast you want your weight towards the back. – Nobody May 9 '17 at 12:23
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Your braking technique is probably bad with respect to overheating. Continuing to ride could help, but only if you do periodic breaking, i.e. you don't break at all until you reach max safe speed, then you brake hard to slow maybe by 10km/h, repeat. How much speed to lose depends on slope (how fast you gain speed again) and load (how much energy is dumped into your brakes to slow the same amount). If you brake more in one go, then your brakes have more time to cool in between but your average speed is also slower. In the absolute worst case (heavily overloaded/bad brakes/too steep) you stop almost all the way and your max safe speed is also pretty low. Make sure the periods of braking are much shorter than the periods of free-rolling.

If your rim brakes become too hot you risk melting the tube (or otherwise damaging the rim, but I suspect first to go would be the tube) and at least disc brakes also just stop braking at all once they are properly overheated (I can tell you from experience, this is no fun).

If the above technique doesn't suffice, then it's too steep and/or there is too much load on your bike for this gradient and you must periodically stop and let the brakes cool or just walk your bike down the hill.

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  • Braking technique is important but check your pads for fouling by grease or if they're still soft enough. Replace in case of doubt. Clean the braking surface on the rim with a clean cloth with just a drop of white gas or alcohol to remove greasy deposits. And most important, brake short and hard but don't let the brakes drag. – Carel May 8 '17 at 16:27
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    In other words - stop feathering the brake on descents. Hard on - hard off works better than partially on. Also can alternate between front and rear (as long as its not equipped with "SureStop" ! ) – Criggie May 8 '17 at 19:55
  • @Criggie Not sure I would ever alternate between front and back. Using both at the same time gets you roughly twice the braking power over a single one, so for a given target speed difference you need only half the time. Using those assumptions, it doesn't seem to make a difference if you are alternating or not. But in reality I usually have this problem on bad roads where it's much safer to use both brakes at the same time. Or at least to me it feels safer, I guess that would be another question. – Nobody May 9 '17 at 12:20
  • @Criggie Everyone says this, but I haven't found a good explanation of why it's true. Assuming the same average speed, the average thermal power must be the same (from conservation of energy), so why is brake dragging worse? The only things I can think of are that a) at higher speeds, you get more help from air resistance (less work for the brakes), b) the airflow over the brakes is greater at higher speeds (better cooling), and c) heat is dissipated more rapidly when the brakes are hotter (during "hard" braking). But I'm not sure how much difference these things make in practice... – Will Vousden May 10 '17 at 12:08
  • I found a report that examines the merits of the two braking strategies (dragging vs. pulsing) in the context of trucks. It concludes that brake pulsing leads to a very small reduction in average brake temperature at the end of a long descent (about 5%), but it doesn't explain the mechanism for this. Their finding about maximum brake temperature doesn't apply to bikes, I think, since the modulation and proportioning are completely different (manual instead of pneumatic). – Will Vousden May 10 '17 at 13:06

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