Quote:
Originally Posted by ysu
I'm genuinely interested how can the quick hard braking be better cooling wise! I've heard it a number of times, too, btw.
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Disc brakes operate on the principle that the big heavy iron rotors can absorb a lot of heat, and then reject it fairly quickly and consistently.
Think of it like a medium sized lake, 40+ ft deep, a body of water will collect heat and hold it, but if you dive down even a few feet, the temperature drops significantly.
With short braking hard braking zones it's like a short burst of intense sunlight on the lake, the top layers will get hot, but ultimately all that heat will escape quickly as the sun disappears.
Compare that to exposing the same lake to a long exposure, maybe slightly less intense. That heat will soak in, it'll travel deeper into the water warming up the whole body, when the sunlight disappears it will take longer for that heat to escape from the surface of the water as it has to travel back up to the surface.
With long slow braking zones you're heating up the core of the discs, the depths of the lake, more than a shorter harder braking zone. Yes for one stop, maybe you've lowered temperatures, but over the course of a lap, over the course of a session you've brought up your disc temperature to a higher average than someone who brakes 'harder'. There's a time element to the equation of "Q=mc delta T" that's difficult to describe and works in conjunction with the heat transfer through the thickness of the disc.
This is why brake discs sucked in the '60s and '70s, they didn't size them appropriately to handle holding all the heat, the lake was not deep enough and the core temperatures would just build and build until failure. They had to absolutely baby cars back then to even get them through a moderately long race. Mark Donohue wrote in his book how much better and more reliable the brakes got when they made them thicker and heavier racing in Trans-Am, great read.
It's also why brake discs have a wear limit related to thickness, once they've lost enough material it's like the lake evaporated and the threshold for overheating has lowered to what the OE deems a dangerous level. The purpose of getting a BBK is to make the lake bigger, it's not going to decrease your stopping distances, it's there to increase how much heat you can dump into that lake before trouble hits.