Wheelbuilding / Wheel Building / Wheel build help

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LewisEC

I've bought some cheap rims and hubs to have a go at wheelbuilding myself. Have measured the hubs and rims, everything looks okay other than the effective rim diameter I came up with.
I measured the diameter and the depth of the rim in about 6 spots and subtracted 2xDepth to get an ERD of 576.4mm
Velocity Deep V ERD is listed as 581mm, a difference of 5mm between two 30mm deep rims seems very big so I feel like I've made an error. Any help?

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Brommers in reply to @LewisEC

I measured the diameter and the depth of the rim

The depth of the whole rim or just the spoke bed? Sounds like you might be measuring, in effect, the internal diameter of the rim which would indeed be too small. As tester points out, the ERD should be measured to where the spokes will finish, which will be the internal diameter plus twice the thickness of the spoke bed plus (nearly) twice the height of the spoke head.

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LewisEC in reply to @Brommers

Right that's exactly what I've done. I had a look at Sheldon browns page and I don't have the bits he used to measure the rim. Is there a standard spoke bed thickness measurement I can use or are the spoke length tolerances too small to risk that?

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