Originally Posted by
Rubato
I'm about to build my 7th wheel and still learning. I bought a nice shiny Pacenti rim and the sticker reads "ERD 611." Because I'm still trying new things, I measured it, several times, in several locations, and I get an ERD of 615.
I cut 2 spokes to 200 mm and screwed a nipple on each one until the spoke was flush to the top of the nipple. I put 'em in opposite holes on the rim and carefully measured the space between them. When I total the numbers, my measurements are always higher than the published ERD.
If I calculate spoke length with the published numbers the spokes would probably reach to the bottom of the screw slot.... Why would I waste the last remaining bit of threads for such a thing?
Am I missing something simple?
ERD is specific to the way the wheel will be built. If you want your spoke ends at the top of the nipple (or maybe a little above), then that is how ERD should be measured. Apparently it is becoming common to publish ERDs based on spokes ending somewhere in the driver slot. This is partly because there is no tension on the spokes when you are measuring. Thin gauge spokes can stretch as much as 2 mm under full tension. Hence starting a little lower. If you don't mind the spoke sticking over the nipple top a little, no problem, but you risk running out of threads and being unable to fully tension the wheel. About halfway into the driver slot is a good compromise. But it is strictly user specific. Make your own decision and measure that way.
Also every rim is different, but not every rim is actually measured. So you have to expect some discrepancies from the published value. That is why the experts say measure every rim according to how you want your wheel to come out. Then there are no oops-es.