Originally Posted by
cxwrench
Heat dissipation is most definitely part of it. That's why Shimano, for example, does all the fancy stuff like finned pads, Ice Tech rotors, and ceramic pistons. That's also why carbon brakes exist for cars and there is super high boiling point brake fluid, fancy brake ducting, and Mercedes spent millions designing their wheels. Larger rotors have more leverage, so more 'power'. All the other stuff makes sure the 'power' is there for more than 1 or 2 braking events.
On the pad. I know heat dissipation is a thing. I have an "ICE" shimano rotor on the front. What I'm saying is that that is not the reason bigger rotors are better. Boiling brakes is when the pads transfer too much heat to the oil. Hence finned pads.
A bigger rotor is better because it has more leverage which means that for a given force it will stop you quicker. Or for a given brake length, you will need to apply less force than on a smaller rotor.
It is friction that stops you.
I mentioned the small front wheel on my bike.As such, having a smaller front wheel will mean that the rotor goes through more revolutions for a given brake distance, but it also means that the lever compared to the circumference of the wheel is much larger than rotor size alone will give you. That said, all that can be somewhat ignored, because no one use brakes like that. It's about stopping the mass traveling at X speeds. Friction does that, and yes, friction creates heat. A bigger rotor means that that friction exerts more force to stop you for a given input (i.e. a given amount of friction) because the lever - that is: The force lever from the axix to where the force/friction is applied - is longer.
If it was all about heat dissipation, you could simply make a very holed surface (to maximise surface area), but make it a thick disk, and only make it, say, 3 inches in diameter.
I don't like rim brakes over disc brakes, but the only reason they work (so-so), is because the lever arm where the friction is applied is so far away from the axis. If you attempted to use the same level of force on, say, a 120mm disc, it would stop your bike very, very slowly. That is because the force arm is so short.
Heat dissipation is obviously a thing, because where you have friction, you will have heat. That's a byproduct of stopping power though.
People have exploded their tyres going downhill with rim brakes.
One of the ways to ride more efficiently, faster, and taking care of your rims or hydraulic disc brakes is to brake late and brake hard. Hovering with your hand on the brakes to take top of the speed is a surefire way to overheat things, and your average speed will be lower to boot.