SBinNYC

There's a practical limitation on braking power on a bike that has nothing to do with the brakes but rather with weight distribution. Brakes stop the bicycle's motion not the rider's. The rider's motion is stopped by holding onto the bike. This presents a problem. The braking force acting on the rider is trying to lift the bike's rear wheel. This is countered by the rider's weight. Braking deceleration must be less than (l/h)g to avoid lifting up the rear wheel on level pavement, where g is gravitational acceleration (21 mph/sec), l and h are the horizontal and vertical distances of the center of mass of the bike and rider with respect to the point of contact of the front wheel with the ground. For most practical geometries, this limits the maximum braking deceleration to 0.6 g or approximately 12 mph/sec. This corresponds to approximately 0.8g deceleration that may be applied by the wheels to dry pavement before they start skidding.

One can design a braking system that could produce any deceleration rate. However, weight distribution limits their effectiveness. All quality bicycle brakes meet this upper limit with minor variations. You're going to have to adjust your riding habits, if you want to have the security that you can stop on a dime while taking that downhill at 30+ mph.