# Calibrating a spoke tensiometer



## eddie m (Jul 6, 2002)

I have an uncalibrated spoke tensiometer. What is the most accurate way to calibrate it?
FSAs are calibrated by putting a spoke in a rigid frame and measuring the the stretch of the spoke to calculate the tension. The seems just a little beyond my fabricating capability. 
What if I put a spoke in a rigid frame and tensioned it, then plucked it and compared its frequency to a known source? I can't accurately judge the frequency of a spoke in a wheel, but I think that is because harmonic frequencies complicate the problem. Would a spoke in a rigid frame have those same harmonics? Is it reasonable to expect that I could get and accurate enough frequency and tension measurement that way?

em


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## wankski (Jul 24, 2005)

i dont think so... how would u know what frequency, if u could get a measure of it, corresponds to what deflection??

just beg borrow or steal another (good) tension meter and do it that way...

which one are u using? 

many ppl have two tm-1s... one is never used and kept for calibrating.... i guess they are cheap enuff that u can do that..


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## bobski (Aug 12, 2005)

I'm just curious...do you tensionmeter users rely on the absolute reading? I never bothered to get mine re-calibrated ever since I purchased it years ago, so its accuracy is questionable. However, I've never paid too much attention to the absolute reading. I use it mainly to help me achieve even tension, so I pay more attention to the relative reading between spokes.


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## wankski (Jul 24, 2005)

well, personally both... on low spoke wheels its pretty important to know if you are at 90 or 110... personally without a tool i cant tell

but from there on its about relative tension of course... these things i use aren't that accurate i know, so i don't care about 95 vs 90 or whatever, on a decently heavy rim and 32 spokes relative tension within a certain tolerance is good enough... that changes tho a bit on light rims and low spoke count... ime.

i think the tool has helped the WW builds i do be dead reliable.. jumping over speed humps, pot holes etc hasn't been able to pop them outta true under a 80kg rider... i'm sure the tension and evenness of the spokes is responsible for this.


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## 4slomo (Feb 11, 2008)

It shouldn't be too hard to calibrate a tensiometer, all it is doing is measuring deflection in a loaded spoke. 

Securely suspend a hub, stick a spoke in it, attach a known load to the other end of the spoke, apply the tensiometer to the loaded spoke, adjust the tension screw until the reading agrees with the known load.


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## Zen Cyclery (Mar 10, 2009)

4slomo said:


> It shouldn't be too hard to calibrate a tensiometer, all it is doing is measuring deflection in a loaded spoke.
> 
> Securely suspend a hub, stick a spoke in it, attach a known load to the other end of the spoke, apply the tensiometer to the loaded spoke, adjust the tension screw until the reading agrees with the known load.


That is how I did mine.


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## eddie m (Jul 6, 2002)

wankski said:


> i dont think so... how would u know what frequency, if u could get a measure of it, corresponds to what deflection??
> 
> just beg borrow or steal another (good) tension meter and do it that way...
> 
> ...


If I know the frequency, I can calculate the tension, and compare the known tension to the instrument reading. AFAIK putting the tensiomter on a spoke of known tension is how all of them are calibrated.
I'm trying to calibrate an instrument I made myself. I don't think a Park is accurate enough to calibrate mine.

em


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## eddie m (Jul 6, 2002)

bobski said:


> I'm just curious...do you tensionmeter users rely on the absolute reading? I never bothered to get mine re-calibrated ever since I purchased it years ago, so its accuracy is questionable. However, I've never paid too much attention to the absolute reading. I use it mainly to help me achieve even tension, so I pay more attention to the relative reading between spokes.


I use mine for both. If I only used it to measure relative tension, I wouldn't need to calibrate it.

em


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## eddie m (Jul 6, 2002)

4slomo said:


> It shouldn't be too hard to calibrate a tensiometer, all it is doing is measuring deflection in a loaded spoke.
> 
> Securely suspend a hub, stick a spoke in it, attach a known load to the other end of the spoke, apply the tensiometer to the loaded spoke, adjust the tension screw until the reading agrees with the known load.


I don't want to do that. First, there is he problem of accurately determining the weight, then I'd need to build a frame to hold it. Last, my tensiometer uses a weight to bend the spoke, so I would need to build a frame that holds the spoke horizontal. To me, it seems easier to just measure the stretch of the spoke and to calculate the tension, but even that may be too difficult.

em


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## asad137 (Jul 29, 2009)

eddie m said:


> If I know the frequency, I can calculate the tension, and compare the known tension to the instrument reading


True, it's simple physics -- for simple geometry, such as a fixed-diameter rod. A butted spoke might cause errors in the calculation.

Asad


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## Lab Worker (May 31, 2004)

I couldn't think of a nice, simple, accurate and repeatable way of calibrating a tension meter. Hanging a weight from a spoke would work however the weight needs to be quite heavy (ie: around 90kg / 200lbs) to be in the working range of the tension meter and I didn't really want a 200lb weight potentially dropping on my toes every time I calibrated!

I ended up building a jig to measure spoke stretch under load and it doubles as a way of calibrating my tension meter - this won't be much good to you though unfortunately :-(

If it's just a one-off calibration you want I'd find someone who *does* have a calibrated tension meter and compare readings.


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## eddie m (Jul 6, 2002)

Lab Worker said:


> I couldn't think of a nice, simple, accurate and repeatable way of calibrating a tension meter. Hanging a weight from a spoke would work however the weight needs to be quite heavy (ie: around 90kg / 200lbs) to be in the working range of the tension meter and I didn't really want a 200lb weight potentially dropping on my toes every time I calibrated!
> 
> I ended up building a jig to measure spoke stretch under load and it doubles as a way of calibrating my tension meter - this won't be much good to you though unfortunately :-(
> 
> If it's just a one-off calibration you want I'd find someone who *does* have a calibrated tension meter and compare readings.


I was thinking of building something like that, but I was going to determine the actual tension by measuring the stretch of the spoke. That would be hard for me to build, and I'd need to buy a test indicator, so I'm trying to avoid that. The only instrument I would compare mine to is an FSA, and I've never seen one of them around here.

em


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## rruff (Feb 28, 2006)

Lab Worker said:


> I couldn't think of a nice, simple, accurate and repeatable way of calibrating a tension meter. Hanging a weight from a spoke would work however the weight needs to be quite heavy (ie: around 90kg / 200lbs) to be in the working range of the tension meter and I didn't really want a 200lb weight potentially dropping on my toes every time I calibrated!


I haven't built one, but if I got bored I'd use a weight with leverage. Rig up deal that gives you ~3-4 to 1 ratio and the weight becomes manageable.


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## Lab Worker (May 31, 2004)

rruff said:


> I haven't built one, but if I got bored I'd use a weight with leverage. Rig up deal that gives you ~3-4 to 1 ratio and the weight becomes manageable.


I looked at that but after fabrication time was taken in there wasn't much of an advantage over the design I settled on and built which can be put away in a drawer when not in use and substitutes a compact loadcell for a 50lbs weight.


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## rruff (Feb 28, 2006)

It looks very cool. What would it cost to set up a rig like that?


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## Lab Worker (May 31, 2004)

rruff said:


> It looks very cool. What would it cost to set up a rig like that?


I did all the design and machining myself which kept the cost of the jig down. The expensive part was the loadcell which is more accurate and repeatable than a hanging-scale and can be left on for long periods of time without creeping. I'd originally planned on using much cheaper strain gauges but I couldn't get the repeatability I needed so I switched to the loadcell.

I built the jig from 35mm cold rolled steel and I machined some slots on either side to mount dial gauges and a second loadcell to apply force perpendicular to the spoke. The slide used to apply tension runs on a thrust bearing so it's easy to apply tension up to about 1000N by hand but I machined a 19mm hex so that a socket or wrench can be used.

If you just needed a jig for calibrating a tension meter it could be much simpler - I'd probably base it off a rigid 100x50x5mm aluminium channel with a simpler form of applying tension to the spoke but you'd still be faced with the expensive loadcell.

I've found with most of the tension meters that I've calibrated that out of the box they read well but after a bit of use friction in the pivots reduces and they tend to read high. Once re-calibrated they tend to hold their reading very well. I use a Park TM-1 and I check it about once a month but after the initial calibration I rarely need to make any adjustments to it.

Some more photos of the jig here: https://picasaweb.google.com/wheelworks.co.nz/SpokeTensionMeterCalibrationJig#


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## eddie m (Jul 6, 2002)

Lab Worker
If I mailed you my tensiometer, could you calibrate it for me? If it were calibrated accurately, it would be more accurate than the TM1 in your picture. It's easy enough to build that I don't need it returned. I could assign you the southern hemisphere commercial rights. 
E-mail me if you are interested.

em


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## eddie m (Jul 6, 2002)

Here is a picture of my homebrew tensiometer. It's pretty crude. To use it you hole the spoke horizontal, hang a 2kgf weight on the spoke, place the instrument on top pf the spoke and zero the dial, remove the weight and read the dial. It takes longer to write it than to do it. As crude as it is, it avoids the design flaws of a Park or DT, measures accurately, uses small load on the spoke, and minimizes friction.


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## Lab Worker (May 31, 2004)

eddie m said:


> Here is a picture of my homebrew tensiometer. It's pretty crude. To use it you hole the spoke horizontal, hang a 2kgf weight on the spoke, place the instrument on top pf the spoke and zero the dial, remove the weight and read the dial. It takes longer to write it than to do it. As crude as it is, it avoids the design flaws of a Park or DT, measures accurately, uses small load on the spoke, and minimizes friction.


I like it - certainly is simple! The design is straight forward and a lot like the FSA tension meter except they use a spring to generate the deflection force rather than gravity. Having to hold the spoke horizontal would be the biggest problem I think - this would be a pain while you're building a wheel as you'd have to keep removing the wheel from the truing stand.


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## Lectron (May 29, 2005)

Using pitch is more that good enough, and more accurate that TM-1 it-selves.

You have a tensioned steel 'string' where the steel itselves has a density of approx. 7.83 g/cm^3. 
The length can easily be measured, but remember. If the spokes are crossed, the cross 
can work as clamp-of. For butted spokes, the effective length is the butted part only (add a little). 
By plucking the cross, the doubling i mass will be canceled out by doubled tension, and formula stays 
the same The frequency you want to hear while plucking the spoke is given by the formula:










Using a tensiometer, the actual cross sectional shape of the spoke is very critical. Using
pitch, it's only mass and length. download a free tonegenerator to your puter, enter the frequency
and calibrate your TM-1. Easy.

When it comes to repetable...no rig is even close. A meter can only measure it-selves. In
this case there ain't no meter.:wink: 

Just remember to measure the correct length.


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## eddie m (Jul 6, 2002)

LW
I'm glad you like my solution, and you identified the weak point. that was a trade off for having a device that was easy to build and affected by friction. I thought of building a horizontal truing jig so that could measure tension without removing the wheel, but then it would be more difficult to true the wheel and to check the dish, so I gave up on that.

Lectron
I'm not able to accurately judge the tone of a spoke in the wheel. My ear is not good enough, and there seems to be lots of effect by the way spoke are crossed with each other. I think even a spoke in a jig still has harmonic frequencies that confuse me. 

em


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## 4slomo (Feb 11, 2008)

I finally relented and purchased a spoke tensiometer calibration rig from blackcat wheels. Now I can develop calibration curves for my tensiometer to enable accurate spoke tension measurements as my tensiometer ages. My customers like the results.


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