well, my understanding, is that it is the centroid of the area of the pad, rather than the radial center (which may or may not be the same, depending on the geometry of the pad) the piston location is "technically" irrelevant for the theortical calculation, because if you assume a stiff backing plate, all of the force from the piston is transferred through the pad and the same force is applied from the pad into the rotor, however, because the area of the pad is greater than the piston area, the pressure on the rotor face is less than from the piston to the back of the pad. Now, in the real world, pads can't easily handle offset forces, you do things w/ the pistons to fight taper wear, but the raw theoretical calc for ER is only based on pad centroid. Of course, in fairness, I don't even remember the proper method for determining the real centroid anymore, lol.
*and for those following along at home, we are definately straining at gnats here, Todd's calculator has a more than acceptable approximation. We are just being geeks, well at least I am, lol.
