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I am trying to determine the proper spoke-tension target reading for a plain J-bend stainless spoke. It has a 2.16mm diameter per the little gauge that comes with the no-name tensiometer I've just bought.

The rims are DT Swiss M442 27.5". According to the DT Swiss manual, for a front wheel with disc brake the M442's max tension is 1200 N (122 KGF) and its min tension is 950 N (97 KGF).

However, the tensiometer's conversion chart doesn't list 2.16mm spokes. It jumps from 2.0mm to 2.3mm

A reading of 24 would be in the sweet spot for a 2.0mm spoke and a reading of 28 would be in the sweet spot for a 2.3mm spoke.

Should I aim for a reading of 25?

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    2.16mm is almost exactly halfway between 2.0 and 2.3, so half way on the tension sounds like a fair place to start. Your final tension should aim more for "even all round" rather than a specific value.
    – Criggie
    Jan 13 at 22:06
  • 2.16 mm sounds like a very unusual diameter. To verify that the measurement is indeed correct, I would try to measure using a more precise instrument, like a calliper, measure several spokes and then take the average of the measurements.
    – Mick
    Jan 14 at 11:32
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    @Mick Sound advice, and I did indeed do that. However, the battery is dead on my digital calipers, so I had to use an old-school analog calipers, a well-made tool, smooth as butter. I measure odd things around the house with it just for the pleasure of it :-) The diameter of the spokes is between 2.1 and 2.2 millimeters.
    – Tim
    Jan 14 at 13:26
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Assuming you are right that their actual thickness is 2.1-2.2 and it's not a measurement error, that is an odd situation.

Assuming your chart has data for 1.5, 1.6, 1.7, 1.8, 2.0, and 2.3mm round spokes, the good simple thing to do is plot the curve of the reference numbers on the chart for a given tension in the relevant range, say 100 kgf or 1000N. Then find the appropriate spot on that curve where the average measured thickness is for your spokes. That's just a few clicks in any spreadsheet software.

This will give a lot more certainty than fudging it because the formula for deflection of round shafts has the smallest inscribed diameter of the shaft to the fourth power in it, hence the readout curve of tensiometers across different gauges is pretty darn non-linear. You could probably get to the same thing by reverse engineering the chart completely by using the formula to figure out what load the chart is figuring the tool is applying to the spokes, then making your own chart column for your spokes, but that's probably way more work than needed.

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