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The Great ERD Wheelbuilding Nightmare
I've been getting my experiences up in wheelbuilding and found quite a bit of contradictory information across sources on the internet in regards to how to measure rims for the ERD.

Nipple Method

This is the most common (and when you search 'how to measure ERD') this is likely what you're going to see first. The diagram directs you to measure the ERD by using the nipples placed into the rim. Measuring from tip to tip, then adding the length of the nipple twice (once for each side). I've done this, and end up with a tremendously higher number than I should have. Additionally, entirely clearing the nipple head will give you no play in regards to spoke stretch, and no room for error when truing up your initial build. I strongly discourage use of this method. You will likely end up with spokes so long, that it ends your wheelbuild immediately after lacing.

Edge to Edge Method

This is the method I've used to great success, but requires a slight modifier to get the ERD ideal lacing and truing. You measure from the bottom hole underneath the valve hole over to the edge of the rim on the other side underneath the rim seam. This should give you a perfectly even amount of holes above and below the measuring line (thus resulting in the truest measurement). However, we must consider rim thickness a touch and also the convex of the rim and the vector lines which the spokes actually run across (not to mention the amount a spoke has to stretch over the flange). So we touch up this bare measurement with a subtle modifier. 1mm for rim thickness on each side and 1mm for nipple head clearance on each side—giving us a total of 4mm for our modifier to add to the raw measurement from edge to edge. Again, I recommend giving your spokes some room to grow as they stretch, and allowing you to true them up without any complications. That's why, although a nipple head is truly 2mm, we are only using half of that.

Rounding Up & Down

Sheldon Brown tells you in his headline that 'longer is better' and I would like to think there must be some truth behind this, but certainly overshooting this will end your wheelbuild immediately, and may not give you any room to true up as your spokes stretch. Where I think this statement holds its greatest truth is in the rounding factor. Suggestively saying, that it's better to round up than to round down (when all your other measurements are precise). Rounding up (even if it's by 0.7mm) gives you room to balance the truing and dishing between sides, and the bringing of spokes up to tension; rather than seeing that one side is brought to crucial tension prematurely, and then has to be loosened to balance the other side so that it can be brought up to an adequate tension.

What are your thoughts on this? Are you round up or round down guys?

How do you all measure ERD?

I've been learning that wheelbuilding is a lot more trouble than anyone else lets you know it is. And that things like higher cross wheels and large flange hubs seems to require more precision and offer less room for error.
With anything new, you go through a learning curve. So as time goes on, you understand better, and get better at doing it.
One of the things I've learned recently is that you should consider both sides of the wheel when looking at a fractionated value on the spoke calculator. Seriously tempted to do away with auto calculators and start doing the math by hand.

If you have a 0.3mm overage, that's more like a 0.6 overage.

Being under is certainly the lesser of two evils.

If you're under by like 0.6mm (1.2mm combined), you can simply use 16mm nipples (with more threads) to make ends meet on thread coverage. You'll also have room for the spokes to stretch and true up, without running out of threads or having to deal with protrusions on single wall rims.

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