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I recently had one of my rims get really hot on a downhill. I had a problem with my front brake, and I had to use my rear brake only...

I did short, hard stabs instead of lightly dragging it.

I ended up walking much of the way once I found that my rear rim was too hot to touch.

I know that it was a worst-case-scenario, but I just want to confirm that I managed it the best way I could.

EDIT: To clarify, this is road-specific, not trails.

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    You are correct to assume that you can blow a tyre doing this. I've seen it happen.
    – Móż
    Oct 18, 2015 at 21:08
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    If you do sense that the brakes are getting too hot (and beginning to "fade"), come quickly to a complete stop, then wait for the rims to cool before continuing. And, as stated below, let air resistance do as much work as possible: Sit upright, maybe open a zippered jacket to catch the wind, etc. Oct 18, 2015 at 21:25
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    Question on a question - are there other brake pad composititons that would help, like the sintered brake pads for MTBs? Are there brake pads with heatsink fins?
    – Criggie
    Oct 18, 2015 at 22:15
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    @Criggie - With rim brakes, the vast majority of heat dissipation is via the rim itself, vs the pads, so changing pads will make little difference in overall heat buildup. However, some pads are less apt to "fade" when hot vs others. Oct 19, 2015 at 0:23
  • Riding downhill with just a working rear brake is the thing not to do. The front brake is essential and does most of the braking job, about 70%.
    – Carel
    Oct 20, 2015 at 7:26

1 Answer 1

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The power output from braking is linearly dependent on speed and braking force. To reduce heat buildup, you have to reduce either one of these. The options are:

  • Descend very slowly. This is boring, but works.

  • Descend so fast that air resistance takes most of the force (the force depends roughly on second power of speed). This is the idea behind the advice to brake sharply for corners: You go faster between them, which lets the rims cool.

  • Increase your air resistance. A big flappy wind jacket slows you down by a surprising amount.

  • As you already found out, dividing the load between two rims helps.

Too hot to touch isn't necessarily too hot for the rims and tires. Also, one thing to do is to reduce tire pressure before big descents, the temperature will increase it back. Jobst Brandt's posts on descending also touch the subject: http://yarchive.com/bike/descending.html

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    Brake sharp and short with maximum force, avoid 'dragging' brakes at all costs. It applies to any kind of braking, cycling or automotive! Dragging brakes will slow you down of course but at the same time they will generate copious amounts of heat. Short braking leaves plenty of cooling down time in between. Maximum force braking needs learning, training and a careful approach.
    – Carel
    Oct 19, 2015 at 7:41
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    Short braking also means you are dissipating the same amount of energy in shorter time. Unless you take into account air resistance and find out that the energy left for brakes is less.
    – ojs
    Oct 20, 2015 at 17:24
  • @Carel dragging the rear brake is a good way to descend fairly slowly while keeping the front brake available for emergency (or planned) stops. Breaking sharply with the rear brake on a downhill is quite likely to result in a skid as there's shift forwards in balance from the descent itself. Of course, as you start to increase the descent speed you reach a point where the heat becomes too much for this to work.
    – Chris H
    Nov 28, 2015 at 14:45
  • @Carel - Automotive is a bit different. You're supposed to downshift and/or let engine braking do most of the braking on a heavy descent, lest you boil your brake fluid and lose your brakes. With a freewheeling system, you can't really do this on a bike.
    – Batman
    Nov 28, 2015 at 17:37

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