Anyone got an idea of what temperature disc brake calipers reach. I am guessing if you can boil dot4 fluid in the alps then 446degrees or higher?

I know touching the disc is a no-no after a long descnt but does the caliper get hot as well?

wouldn't have thought its that high* – if there's any water in the fluid (ie all the time in the real world since DOT absorbs moisture) – the fluid boils much lower than that

*usual stw uninformed teeth sucking opinion masquerading as an answer…

Callipers get hot as well. They must do to boil fluid. The disc can and does reach hundreds of degrees C

Comparison Table here reckons wet (with water contamination) dot 4 will boil at 311 degrees.

Mmmm, interesting.

Anyone ever got dot 5 to boil?

nobody has got brake fluid to boil on a mountain bike.

all 'oh my brakes have boiled' situations are a result of water ingress into the fluid, which flashes off giving crazy pump up/levers to bar scenarios.

My mate who did his phd in train brakes says that brake fade is due to organics in the brake pad volatilising and creating a film of gas between the pad and the disc, so you get the wooden unresponsive feel. Boiled brake fluid? I'm a 14 stone scaredy cat, use my brakes all the time in the alps, never had this..

have a look at this…

Ashima disc rotor test

The rotor starts to glow!

I have blued the discs on the tandem and the calliper was well over 100C. The lever feel altered but the brake did not fail. Close to boiling for sure.

That video is scarey, I know it's unlikely but imagine if you looked down and saw your rotor glowing like that!

Riding in sheffield I managed to turn the anodising on my hayes pads a funny colour, thought I saw the disk glowing once, but its hard to tell under orange streetlamps.

Hot enough to cut through a camelback hose (and ruin the rotor) anyway.

I've boiled (all be it the water or the fluid) formula the ones to the point that they no longer worked, I have also very nearly done this with hope tech moto with the vented rotors.

There is a reason why some alpine guides use cable discs.

ha ha ha. i guided in the alps and there's no effing way you'd catch me using cable discs. i'd suggest this was a cost and maintenance thing rather than the fear of boiling fluid.

i have completely cooked a number of sets of discs, and it was all following long periods of use without bleeding. if you bleed regularly with fresh and NEW fluid most brands will be fine assuming a sensible rotor size for cooling.

i guide in the alps and there's no effing way you'd catch me using cable discs

+1!

I don't know how hot the rotor gets but my calipers get well above 100°C judging by the way rain boils instantly off them :o)

Turned plenty disks blue but never totally failed a brake or suffered lever pump really. Actually, having just thought about that turning blue will be related to a change in heat/temper of the material. Could probably compare the colour the disk turns against a chart to estimate the temperature. If I recall correctly bluing probably happens at well over 300°C. Alternativly, go riding with a remote heat spot checker and point it at the disks at the bottom of a big hill.