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Old July 12th 03, 03:52 PM
Jose Rizal
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Default Disc brake rotor size

Chris B.:

On Fri, 11 Jul 2003 19:35:18 GMT, Jose Rizal wrote:

Doug Huffman:

The power of a brake is its convective heat transfer rate and proportional
to Q = UA(Th - Tc) where Q is the heat transfer rate, U the convective heat
transfer coefficient, A the area and a function of rotor diameter, Th the
temperature of the rotor and Tc the cooling air.

Incorrect. This is only an indication of the heat generated by the
brake, not its stopping power.

Maybe this is the bigger issue though. Since the OP wants to use disc
brakes for "long mountain downhills" isn't heat going to be his
biggest problem? I just took a few measurements and did a few
calculations and found that even a 205mm disc rotor has only about 50%
of the swept area of even a 26" MTB rim. I wonder whether a person
would be more likely to have a tire blow off while using a rim brake
or having the rotor warp.

Heat certainly is a factor on long _steep_ downhills, and the
effectiveness of the brake will depend a lot on the pad material's
ability to maintain its properties at elevated temperatures.


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