I just learned how to take measurements a few days ago, and it's not difficult. If I can do it, any lesser moron can do it.
- Take two spokes and accurately measure the length. If those are 20" rims, then 150ish is suggested.
- Attach 12mm nipples, and screw them down to where the tip of the spoke is flush to the bottom of the screw slot in the nipple head.
- Put one spoke through right next to the valve hole, seat the nipple. Do the same at the exact opposite side hole with the other spoke.
- Pull the spokes together, grab them with calipers, and squeeze the calipers snug.
- Take the measurement and write it down. Under that write down one of the spoke lenghts, then under that the other spokes length.
- Move the spokes 3 spoke-holes to the right. Take the measurements exactly the same way. Write them down exactly the same.
- Do that two more times, exactly the same.
- Add each spoke measurement together with each spoke length. You should have 4 different values.
- Add those values together, then divide them by 4 (getting the average).
- If it's above .5, round up. If it's below, round down.
There's your diameter.
