Originally Posted by
IPassGas
A bit of perspective rim vs. disc braking…
Heat capacity: Al:920 J/kg/C, Steel: 500 J/kg/C
Mass heated: Rim: 0.6 kg, Disc: 0.08 kg
Kinetic energy of loaded tandem (220 kg) moving at 16 mph: 5400 J
Temperature increase in components to stop tandem
2 rims: 5400/920/1.2 = 5 C increase (9 F)
2 discs: 5400/500/0.16 = 68 C increase (120 F)
If the riders are smaller and without gear, disc brakes may be sufficient. However, rim heat capacity is far greater.
I agree that the physics show that the energy required to reduce the kinetic energy of the system to 0 that the mass of the disc will get a lot hotter than the rim, but shouldn't there be time and cooling component over time in the above equations? The rim and disc will cool after picking up heat from the pads while rotating around to the pads again. The same amount of energy is required to stop a given mass in 0 sec as is required in, say, 2 hours, but the final temperature of the rim and disc would be much different based on time.
There would also need to be a slope and gravitational term in the equation.
I realize this is getting into the weeds and does not change your point, however, it would be interesting to have and equation that would allow us to plug in some real world scenarios to see where disc braking falls apart. Just throwing it out there to see if someone is game
