Why go to a bigger diameter disc?
TOP Bigger is better! Right?!?!? Not necessarily. A bigger diameter disc does provide more power and better cooling (because any particular point on the disc spends more time in the cool air dissipating heat buildup) you do end up with less modulation (brake control). Traditionally, downhillers, free riders, and tandem riders can all benefit from larger diameter discs. However, riders who spend more time in technical situations need to be aware that the larger diameter disc (like a lower bottom bracket) has a better chance of being struck and damaged by a boulder, tree stump or similar object. Take a look at motorcycles and the diameter of their brakes.
