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If your hydraulic brakes get hot, can this heat ever travel to your brake levers. Can your levers feel hot or is this my imagination?

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    Your hands can get hot, from the muscles working, the friction between hand and lever, and the lack of airflow. – Daniel R Hicks Mar 14 at 11:59
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    The fluid will expand a bit and your hands will feel the levers becoming stiffer which feels like they're pushing back out on their own a bit. Could that be causing a perception of heat ? – Criggie Mar 14 at 20:47
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No, the thermal conductivity of brake hose/fluid is way too low.

I've run the numbers on this -- very roughly to make the calculations simpler. I've assumed the hose isn't cooled by airflow for some reason and a solid hose or equivalently that the fluid has the same thermal conductivity. Even the lever is assumed not to lose any heat to the air. All these assumptions maximise the amount of heat delivered to the lever.

Lets assume the hose is PTFE, which decomposes above about 200°C and has a thermal conductivity of 0.5 W/m.K (the highest value I saw). I'll take a hose outside diameter of 6 mm, length 1 m. To feel warm the lever would have to get to something like 40°C, giving a temperature drop of 160°C at the point where the caliper end of the hose got so it it would be giving off toxic fumes, i.e. the absolute maximum.

Working this through, you'd be conducting 2 mW of heat to the lever. If the lever is 100 g of aluminium with a specific heat capacity of 900 J/kgK, this 2 mW it would take 12.5 hours to warm by 1°C.

Some steels have 100× the thermal conductivity of PTFE. Brake cables could conduct 200 mW for the same caliper temperature, and only take 7.5 minutes to warm by 1°C - but still an hour or two of non-stop braking to get from ambient to warm.

  • Wait, PTFE decomposes at 200°C? I've been using my 3D printer with PTFE in-nozzle lining heated up to 235°C or so, and many people do this with no ill effects. I haven't noticed any physical problems with the tubing when I pull it out. Should I be worried? – Dmitry Kudriavtsev Mar 15 at 4:45
  • @Dmitry if you bought the nozzle like that, and you're using it as specified, don't worry. I've seen various figures and this is the lowest, but was on the page I got the thermal conductivity from. Non stick cookware is also used to similar temperatures. I've toned down the wording of that bit – Chris H Mar 15 at 6:44
  • @Chris H I always loved "word problems" in the math and physics classes i took during my school days. Felt it gave a little substance to the lean purity of an equation or a set of given numbers to crunch, That they often seemed to slow-up those fellow students truly gifted in math only fed my favor of them. Anyway, i liked how you approached this question. Giving you an SE version of an A: upvote. – Jeff Mar 15 at 6:52
  • @Jeff, thank you. Doubly so as I gave a lecture on the topic this week and used an example problem (heating a room, rather than bike-related) and I'm glad to hear it suits some people – Chris H Mar 15 at 7:37
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I've never heard of anyone having that issue before. The heat would have to travel all the way up the hose, even then still having to warm up all of the body of the brake lever before heating the part that you would pull. If you're pulling your brakes very hard for a long time, then it's more likely to be related to the strain of pulling them or cutting circulation.

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    Since the brake fluid is constrained in a narrow tube, the transfer up it would have to be entirely by conduction: there'd be no appreciable convection. That conduction would be slowed by the cooling effect of air passing over the brake line. So I agree that it's very unlikely that brake heat will reach the levers. – David Richerby Mar 14 at 11:51

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