What tolerances should I use for spoke tension when building a wheel

Question

I am replacing the rim on my rear wheel with a new one. I've brought up the tension on the spokes close to the final tension by truing / increasing tension / truing / increasing tension.

I've got myself a park tool TM-1 tension meter. I've been measuring the spoke tension, and they have a deflection reading varying from 19 to 21 or so, giving tension between 85kgf to 105kgf, using double butted 2.0/1.8 spokes according to the park tool deflection to kgf conversion chart.

How close of a tolerance between spokes tension readings should I allow for the final tension? How should I even out the tension to the ideal tolerance?

Answer 1

This blog post (google cache, original gone) on the Park site suggests a range of 20%, this thread has people saying a range of 1 on the TM-1 is fine (edited in some quotes):

I sort of try to finish within a 0.1 mm error radially/laterally and under 0.5 notch (of the tension meter) of standard deviation for all spokes.

I get DS tension under 1 notch on the park tensionmeter.

Average: 14,25 (notches of the park tension meter) Standard deviation : 0,58 notch

That's what I generally use when building a wheel. I'll look up the chart and get something like 15.5 for a 1200N tension, so I'll aim to have all spokes between 15 and 16, and most of them close to 15.5. With a TM-1 I know my repeatability on measurements is only about +-.25 anyway, so if I measure the same spoke several times I'm likely to see 15.5, 15.75, 15.5, 16... and say "buggrit, that's a bit over 15.5". But for any single measurement I could in theory chase the 15.5 target and go 1/4 turn looser or tighter, squeeze a few spokes then re-measure. But that wouldn't necessarily tell me I'd helped the situation, and might not even be accurate to within the tolerance I think I'm aiming for.

The actual process is pretty repetitive:

1. get the wheel fairly round, focusing more on up-down than left-right circularity (hops are hard to get out later)
2. run around and tightening spokes half a turn at a time to get the tension slightly under my target (say 14 using my 15-16 target)
3. get the wheel true to 0.5mm or so
4. Then I'll run the tensiometer over the whole wheel to see how even the tension is. If there's a big problem I'll back everything off a turn and try again. Otherwise I'll back off any over-tight spokes a little, tighten the loose ones and stress relieve the wheel, hoping it comes right. Then I'll re-true it. And repeat this step. Again and again, until it's all good.

Once you get the hang of it a lot of those steps will happen once at most. Normally I'll hit each spoke maybe twice with the tensiometer, and possibly a couple of spokes more than that. A good build would be tightening by feel, getting it true with a couple of tweaks, measuring and it's done. But that's very rare unless I'm on my third identical wheel in a day. If I'm fixing an existing wheel I might end up working 4-5 spokes 5 times or more trying to get it right.

One test I suggest you do is just to write down the tension of each spoke on your wheel, then put that bit of paper in a safe place. Then do it again, so you're measuring without looking at the earlier measurement. Then compare the two sets of measurements. That will give you an idea of the variability of your measurements of spoke tension. There's no point trying to get more accurate than that with a wheel, because you're chasing a random number.

Answer 2

Your results seems reasonable to me. While the TM-1 is a useful tool, getting repeatable measurements out of it depends a lot on the skill of the user. I use mine more as a double check to make sure that I've got enough tension in the wheel and that the tension is roughly balanced. If you start out with a straight rim and build a wheel that is true, that's actually a much better test of even tension than the meter.

Answer 3

I hadn't realised the TM-1 was that vague! I use a Spoke Tension Gauge app on an iPhone which gives very consistent & repeatable results once the spokes are sufficiently tensioned to make a clear note when plucked. I can usually get even tension within ±20N with DT Competition spokes and ±40N with Revolution; the limiting factor is that the smallest turn of the wrench makes a bigger difference than that.

The only time it's given me trouble has been the left-side of a Mavic Reflex rear wheel: the low upper-limit on maximum tension (900N) meant the left-side spokes were barely measurable when the right-side spokes were maxed out.

In my experience if I calculate what the spoke tensions should be on both sides and set all spokes to that the wheel is true/straight/correctly-dished. I only use my other gauges now to verify my calculations…