I'm getting ready to buy spokes and lace up some wheels for a new build. The rims are 650A Sun AT18s, for which I can't find any info online. They're box-section rims that appear very similar to CR18s except that they're single-walled and lack eyelets.
Since I can't locate more than one extra spoke with a nipple to measure it (the method I see recommended), I'm using a brand-new brake cable, held taut while I mark the other end with a Sharpie. The outer edge of the Sharpie mark is about 2mm up from the rim bed, so I'm just adding 2mm to that measurement to include the other nipple head's height.
And that comes up to 583mm, which seems too high based on this logic:
1) that's not too far below the rim's bead-seat diameter
2) Sun's current ERD chart doesn't include an AT18 in this size, but when I look at other AT18 rims, the ERD is 13mm less than the BSD, which I would expect to be consistent since it's the same extrusion. (CR18s are consistently 10mm less than the BSD, for example.) From this thinking, I get a predicted ERD of 577 for my rims.
Is it possible that Sun's specs exclude the height of the nipple head? At any rate, I know that the "real" ERD should trump online specs that may be out of date, but I thought I'd check here first.