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I've been bringing up spoke specific gravity a lot lately, but
even though you can know the finished weight before building a wheel,
if you have the materials right in front of you, you can just weigh them all.
The real purpose of this is to extract just the rim weight from an already-built wheel.
To do that, you need to know in advance the weight per 1mm of spoke
and the weight of the hub.

In the case of pre-built wheels, they use special spokes,
so without specific gravity data you can't measure the rim weight.
If you really need to know, you can just disassemble it once.

Having these kinds of numbers on hand is useful in all sorts of situations.