What does the spindle designation number mean?
The spindle designation number is the nominal distance between the crank stops on the spindle. Since all the spindles in the standard are symmetric about the BB shell mid-plane (theoretically the frame centerline), one-half the spindle designation number is the distance from the BB shell mid-plane to the drive-side crank stop on the bottom bracket. By adding this number to the chainline offset provided by the crankset manufacturer, you can easily figure out the resulting chainline of that particular crank assembled onto any ISIS Drive bottom bracket of a given designation.
