Originally Posted by
Duragrouch
I have wondered about that. One one hand, a 20" rim has a lot less mass with which to dissipate heat versus a 26" or 700c. But on the other hand, the ground braking thrust produces less wheel torque, so shouldn't need to squeeze the brake as hard. Makes sense, as the total "work", i.e., the braking energy, is force x displacement, so it should even out, but again, 20" is less mass to take the same energy.
The energy to dissipate depend of the total mass (bike+cyclist+luggage) and deceleration, its independent of the wheel size.
For a small rim size (like for a small disc size), the rim become warmer than with a bigger one. But for the rim; heat goes to the tire and inner tire that also become warmer which can be damaged in case of extreme braking like in case of long downhill.
Warmer weather will increase the risks since the heat dissipation depend of the difference between rim temêrature and ambient temperature.