i also remember that in the 50’s and 60’s we used to build choppers w/o front brakes. if i remember, on long bikes, (picture fonda’ chopper) there was very little weight over the front wheel, and front braking made the tire skip/bounce.
i spoke to an airline pilot friend and he says they were great re reliability and stopping for commercial aviation. he was going to try and get some maint. record data for me.
if it works on the front, i’ll probably try it on the back.
why, you say?? why not say i!

-todd

On conventional brake rotors, the lever arm of the applied force is a little more than halfway between the inside and outside edges of the brake pad. The coefficient of friction is the ratio between the frictional force dragging the rotor back and the normal force of the brake pad pushing down on the rotor. The total braking torque is the lever arm times the coefficient of friction times the force exerted by the brake pad. The pressure on the rotor is the force exerted by the brake pad divided by its area. Only the part of the rotor immediately under the pad is being heated by friction (the heating is proportional to the energy dissipated, or braking torque times the rpm of the wheel) while the rest of the rotor is flying through the air and cooling off again. Hence drilled rotors, which stir up more airflow for better cooling.

With a compact, multi-disc setup the lever arm is closer to 70% of the way to the outer radius, but it’s still not very large. That means more force has to be applied in order to get the same braking torque. If you tried to apply all that force to just one small-diameter disc it would lock up. Instead, many discs are stacked to divide the force between them. The braking torque is still the lever arm times the coefficient of friction times the force exerted by the brake pads. The pressure, however, is the force divided by the total area of ALL the discs in the brake. The heating is similarly divided among the discs, so both pressure and heating rate can be brought back down to normal levels despite the high force being applied. Unfortunately, all of the discs’ area is being used for braking and is being heated, so there’s not much area left to dissipate heat. If you used the brakes like an airplane does, for one intense burst of a few seconds, then they’d have plenty of time to cool. Repeated application is likely to cause problems.

So, to recap, a compact multi-disc brake can provide the same stopping torque as a conventional disc brake, but only if the force applied is much higher to counter the smaller lever arm. This force gets divided up among the discs, so pressure and heating are not problems, but heat dissipation is because there’s so little contact with cooling air.