Disc brake rotor size
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.
For each surface contact on a disk brake, the stopping torque generated
with uniform pressure applied by the pad is
T = (1/3) * F * mu * [(D^3  d^3)/(D^2  d^2)]
where F is the force applied on each caliper face, mu is the coefficient
of friction, and D and d are the outer and inner diameters of the rotor
annulus.
mu will change as heat is generated on the rotor and pad, but since it's
a function of the pad material, this change can be set aside for
comparative purposes.
It's clear that the larger the rotor, the greater the stopping torque.
