The way I've always understood it is that your BS is measured striaght from the WMS to the very rearmost of the bead. like your straightedges for measuring show (excluding the limp ruler
). Offset is a measurement of how far off the centerline you are, but to convert it, you're supposed to take the nominal rim width (8" for example), add in 1" (for both beads), and then that gives you the total wheel width to divide in half and find the centerline with.I've never known wheel construction to play a part, I've always thought you add in a half inch per bead to get the total width, since that is the "standard" (that's been the standard in all the conversion calculators at least).
I think people get misled by leaving off the bead width. For example, on a wheel known as being 15x8, they assume 4" is the centerline, and so 4" BS equal 0 offset...not correct. Have to add in the inch of bead width, centerline becomes 4.5, now a 4" BS wheel is somewhere around -12.5mm offset (offset by half an inch, negatively).
But yes, I do get your point that if the bead is designed differently, then the BS of two different wheels could be different. I would think if it was different enough due to the bead, that the offset would have been different as well, though. Since offset factors in both beads, or is supposed to.
