My Tension Meter

After 52 years of building wheels and never having touched a tension meter,  everything changed in December 2014 -

Based on my RoadBikeRider.com newsletter column, issue #669 dated May 21st 2015 -


The Wheel Builder – Equal Spoke Tension is King

In my last column I talked about the benefits of equalized spoke tensions. I've been building wheels for many years and writing about the process for a few of those, and the same question pops up occasionally: "What's the most important thing in wheel building?"
After I pull the leg of the questioner with answers like "a spoke wrench" or "the hub," I have to get serious and think about what is really important. 
Once we get past the unquantifiable things like "passion" and "patience,” one thing stands out above all others -- equal spoke tension
As I stated in the last column, getting all the spokes to do an equal amount of work is paramount to the longevity of the wheel. This evens out the strain on all of the spokes and reduces metal fatigue and, therefore, breakage.

Plucking Your Way to Equal Tension

I also mentioned two ways of checking relative tension in spokes -- the method of plucking, listening to the tone of the spoke and attempting to adjust all the tones (and therefore tensions) to be as equal as possible.
The other method is using a spoke tension meter (commonly called a "tensiometer"). This tool measures individual spoke tension, which allows us to compare and equalize the readings.  Of course, tensiometers are a relatively recent invention and most of us old-timer wheel builders started long before they became available.
So people like me were a little skeptical when they came on the market. For overall tension I've always used the "perceived tension" method for decades -- the gut feeling for when "tight" is tight enough. Since I started paying particular attention to the single most important thing in wheel building -- equalized tensions -- perceived tension has worked perfectly.
Recently, along came the well-meaning BikeHubStore.com proprietor Brandon Hunziker. He took pity on me and talked WheelFanatyk.com’s Ric Hjertberg into donating one of his digital tensiometers, finely crafted in his area of the Pacific Northwest.
Ric's gift arrived and, after watching the video and reading the complete instruction book, all included in the padded clamshell case, I set about checking all the wheels in my workshop.

 My Findings

From using the Wheelfanatyk's tensiometer on all my wheels, I found an interesting trend -- all my wheels were slightly on the "soft" side, or, low in tension relative to the suggested norm. (They were about 100kgf with OpenPro about 90kgf)
Low spoke tensions can lead to nipples that unscrew and spokes that fatigue and break due to greater tension swings as the wheel rotates and each spoke experiences large load-unload cycles. High spoke tensions lessen these tension fluctuations. My wheels have never suffered from these problems so as suppose I was close enough with my perceived tensions.
The rim is the weak point of any wheel, and rim makers usually specify a maximum spoke tension. Most of them specify around 100kgf to 125kgf. The term "kgf" is short for "kilograms-force". Excess spoke tension leads to cracked nipple holes in the rim, so it's crucial that we don't over-tension our spokes.
I found this use for the tensiometer to be its best feature. Ric's digital gauge reads to 0.01mm deflection of the spoke being tested. All tension meters work by using the same basic principle: they apply a calibrated sideways force to a 4-inch (10cm) span of spoke and measure the spoke's deflection, or bending. 
Then we use a supplied cross-reference chart to give us a kgf reading. All tension meters are calibrated at the factory and most can be returned for re-calibration (some geniuses even make their own calibration testers for home use).
The other use for the tensiometer is for comparing tensions so that we can adjust and equalize them. The "pluck-ping-listen-adjust" system works perfectly well for me, and the more experience one has with this method, the easier it is to use.
For the purpose of comparison, though, I used both methods during my next three wheel builds and found exactly what I expected -- the plucking method is faster than using the tensiometer and, for me, just as accurate. 
When I'm adjusting a wobble, I pluck-test five spokes around the wobble's high spot -- three on the high side of the wobble, and two on the other side. The higher-tone spokes get more of a loosening than the lower tone ones on the high side and the higher tones get less of a tightening on the other side.  Using this method we're doing two jobs at once -- removing wobble and equalizing tensions.
The Hjertberg digital tensiometer is a very sensitive instrument with its direct digital readout and its ball-bearing pivots for the spoke posts. I had to be very sure that its plunger handle wasn't touching my hand while I was taking a reading so as not to affect the reading. 
The tool can be zeroed to account for any slight bend in the spoke being tested. It's a precision instrument both in use and appearance. For the true techno-freak, an accessory SPC cable for direct output to a PC is available.
The tool is a great addition to my wheel building table as it removes all ambiguity and guesswork from overall tension. And the fewer spokes we have in a wheel, the more important equal tension becomes (re-read that bit at the top about spokes “sharing the workload”).

My Final Take

The Wheelfanatyk Digital Tensiometer is a well-crafted precision tool, made to a high standard, apparently with little regard to final cost -- but it's not the most expensive tensiomenter on the market. 
In my hands, as a home wheel builder with many years of experience, its best use will be to verify that my wheel spokes are at the intended tension, with all guesswork removed -- and that alone will provide peace of mind.
The plucking method will still be my main method of judging equal tension, as I find it very accurate (as verified by the Wheelfanatyk tensiometer!) and quick to use. 
Newer wheel builders, and others not experienced with spoke plucking, will find it an invaluable and accurate tool to help them achieve equalized tensions as well as final tension. If this tensiometer prevents just one cracked rim or one poor wheel build with all their associated inconveniences and costs, then the $295 price of this tool will have been a fine investment.

 

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