Why use the spindle designation number instead of the overall length?
The entire ISIS Drive system is based on the crank stops of the spindle. Everything in the standard is derived from these crank stop datum planes, including the method of measuring a spindle. Since the spline shape itself is defined relative to these stops and the taper angle is so small on the flutes, the cranks should easily slide all the way onto the spindle until they are stopped by these crank stops. Thus these crank stops consistently locate the crankarms axially relative to the bottom bracket (and hence the frame) every time a crank is installed. The overall length of the spindle has no effect on the axial position of the crank on the spindle, and thus has no meaning in the context of this standard.
