If you have the wrong effective rim diameter (ERD), there’s a good chance you’ll compute the wrong spoke length. A lot of times you can use values from the manufacturer but it’s best to double check the numbers with Google. After a while you’ll find some brands, like Stan’s NoTubes or WTB, are very reliable in their ERD numbers (contact me to order Stan’s rims).
You’ll get the most accurate ERD numbers if you measure your rims so I always do. My tools are DIY, which is inexpensive. Here’s how I make them: take two black 310mm Sapim Leader spokes and cut off the elbow leaving a 300mm rod. Screw a silver nipple to each rod using a bit of Loctite so they never move. For my process I make sure the spoke penetrates the nipple until it’s flush with the bottom of the screwdriver flats — spokes stretch a little under tension so they’ll end up in a good place. That’s it. If you’re precision-minded, you can ensure nipple geometry isn’t a factor by making a new set of measuring rods any time you build with a new variety of nipple.
Usage is straightforward. Insert your measuring spokes into opposing spoke holes, counting them to make sure you’re not off by one. Pull the spokes tightly across a ruler. Use one with 0.5mm resolution (such as this one) or eyeball to the same. For most builds the spokes will overlap on the ruler, in which case you deduct the overlap distance from 600mm. If the spokes don’t overlap, add the gap distance to 600mm. Perform at least two measurements 90° apart and average the results to get ERD.
If you’re building with nipple washers, remember to increase spoke length to compensate. As an alternative, you can simply install nipple washers on your measuring spokes and build nipple washers right into your ERD. With this direct approach, no compensation is required.
Most spoke calculators will give you lengths to the tenth of a millimetre, which you’ll need to round to the nearest available length. Since measuring as above targets the bottom of the acceptable range, don’t round down aggressively for low tension builds or on the low tension side of a wheel. When shopping here, you get to round to the nearest millimetre compared to traditional vendors that stock spokes in two millimetre increments. In a followup blog I’ll expand on the topic of rounding, which matters more when available lengths aren’t as good. Stay tuned.