Originally Posted by Pete Fagerlin
False. A larger rotor will not stop faster than a smaller rotor, unless there has been enough braking prior to the test to heat soak the smaller rotor.
Larger rotor do provide more stopping power by simple physics principle of larger leverage ratio. If larger rotor does not provide more stopping power, all the motorcycle out there will use 160mm size rotor to stop a 165mph bike (not a good idea, unless you have a death wish). You'll see higher speed and mega horsepower motorcycles out there use bigger rotors to provide more stopping power and they also use dual brake pistons calipers.
Bicycles does not acheive speeds anywhere close to motorcycle speed so a simple single piston caliper and 160mm rotor is enough for 80% of mountain bike applications out there, downhill bikes will need 203mm rotors because of their heavier weight and more rolling momentum. Just because you can lock up a mountain bike wheel traveling at 15mph and weight 30lbs does not mean you can stop a 400 pound, 140 hoursepower, 120mph motorcycle with the same setup. Just think about it!
Brake rotors dissapate heat not just by its larger size/surface area but by holes/slots that is drilled into the rotor so it will have more contact area with air so heat can be tranfer faster.
Motorcycles are the forefather of moutain bikes, many breaking/ suspension theory and principals have been work out already, just look at how motorcycle is set up and it will answer many mountain bike questions.