Why would spoke tensions be uneven, despite equal thread engagement?

Question

I'm building a wheel. I've used a nipple driver to equally engage all nipples on the spoke threads and used this as a starting point for tensioning the spokes.

Despite this, after applying a couple of turns on all nipples, some of the spokes have some tension in them, while others are completely loose, with the nipples still rattling in the rim. This is true even for spokes on the same side and in the same orientation.

What are the likely causes of this and how can I diagnose the problem?

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Some notes on my specific situation:

• The rim is 50 mm deep, carbon, 32 hole (Duke World Runner)
• The hub is a SON28 15 mm 6-bolt dynamo hub
• The ideal spoke lengths, according to Roger Musson's calculator, are 260.6 mm and 261.6 mm for left and right respectively
• I rounded up and chose 262 mm CX-Ray spokes for both sides
• The nipples are Sapim Polyax 14 mm
• The spokes are laced in a standard 3-cross pattern

Here is a photo of the wheel, showing where some of the loose spokes are:

Answer 1

I can think of a few causes related to the parts and material:

1. Manufacturing tolerance of the spokes. They are not exactly equal length.
2. Manufacturing tolernace of the nipples. I assume they are drilled then tapped during manufacturing and likely this results in some variance.
3. Rim radial runout. Rim when not under tension may be not perfectly round (and this is OK)
4. Manufacturing tolerances of the hub
5. Spokes-hub interface not properly bedded in yet. This will not be bedded in until you tension and de-stress the wheel

And a few other reated to the wheel-builder:

6. Your error in counting driver revolutions when engaging the nipples. This can't be ruled out especially because an equal number of turns of two small fasteners does not always have the same result as you may not notice when exactly the threads engage and do a few turns with zero effect...
7. Suboptimal spoke length for an asymetric wheel build. This last situation will be very obvious if all of your loose spokes are attached to one hub flange and all the tighter ones are attached to the opposite
8. Improperly laced wheel. This can be easily ruled out with a manual inspection

Out of all of the above I would probably guess 7. This is the most common reason some spokes are loose and other are tight in the beginning of the build. This is also a reason which will have the biggest variance (sometimes more than 1mm). On asymetric builds this is hard to avoid because optimal spoke length produced by calculators is "theoretical" and often has high precision downto .1mm yet spokes are sold in length which are increments of 2mm...

For example:

If I know I need 291.3mm spokes on drive side and 292.5 on non drive side, I am likely to just buy a pack of 292s...

Answer 2

Manufacturing tolerances is my best guess. The rim is not a perfect theoretical circle, its wall's thickness is not perfectly even all the way around, spoke holes are not drilled perfectly identical, and no two spokes or nipples are perfectly identical if you zoom in far enough.

Spoke tension is sensitive to small imperfections like these. This may not be a perfect example of, but there's something called "the butterfly effect", when small variations of input in a system cause a huge, and impredictible change in the output.