Originally Posted by
rahill
Swept area increases linearly with radius, assuming the width of the disc remains constant. Mass, neglecting the spindly disc arms, would also increase linearly, assuming constant thickness. The size of the pad obviously remains the same, so there is significantly more area for cooling on a larger disc.
The disc can conceivably run much hotter than a rim before anything bad happens. I'm pretty happy with two discs, having tried them on several steep and fairly long descents with no fade whatsoever (fingers crossed after saying that!).
"swept *area*" is the key: area is radius squared * 3.14.
And of course mass is a cubed formula so it too would increase dramatically.
What *does* increase linearly is the brake surface velocity. Larger disc (or rim) swipes faster by the brake pads than a smaller disc. The time between swipes does not change, just the speed of the swipe. Thus the time to cool is the same. But stipulated that larger disc will have more radiation surface to lose heat (as well as more mass to heat slower and then cool slower).
Agreed that disk can likely run hotter before failure. Dual disc brakes was one of my selection parameters when we purchased our tandem.