# Park TM-1 Calibration



## ericm979 (Jun 26, 2005)

I recently made a rig for calibrating spoke tensiometers. It uses an electronic scale in a frame of 1x1 square steel tubing. At one end is a threaded hook which I used to tension the scale and the spoke. The scale's specs say it's accurate to +- .5kg though I have no way to test it.

The results are interesting.  My TM-1 has not had the calibration screw touched but has been lubed lightly with silicon spray. I did a number of runs on the same spoke to verify that the scale and more importantly my measuring technique was consistent enough. 

I'll post a couple comparisons vs the laminated chart that came with my TM-1. First, 1.8mm Sapim Race:









Over most of the range it's not too bad. But out in the 120 kgf+ range it's reading 10 kfg low. 

Now, lets look at 1.5mm Sapim Lasers:









Yeesh. Again it's 10kgf off, but down low in the 50kfg range.. A much larger percentage of error. On many of my rear wheel builds the Race spokes on the DS are in the 120kgf range while the NDS Lasers are down in the 60kfg range (on the Park charts). So both are in the least accurate zones.

What's good is that this TM-1 reads high, so I have not been over tensioning. And it's reasonably consistent. Not so good is that it's not that accurate, and the inaccuracy is not consistent across different spoke diameters. 

I have no idea how much individual TM-1s vary, so I do not recommend anyone use these charts over the Park one.


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## dcgriz (Feb 13, 2011)

Mine was reading 19 kgf too high when Sapim Race were tensioned to an actual load of 120 kgf.
No lubing of any kind was used on the sliding scale of the instrument. The slider moves smoothly across its range of motion, without any binding.
Readings have been exceptionally repeatable.

Edit to add: IMO and consistent with your findings, the most accurate use of the instrument when measurement of absolute tension is the goal, is to calibrate it for the specific spoke and projected tension at hand.


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## nsfbr (May 23, 2014)

I'd love to see some pictures of your setup! That sounds simple enough to make and I think it would be worth it to get another data set for others.


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## Mike T. (Feb 3, 2004)

ericm979 said:


> The scale's specs say it's accurate to +- .5kg though I have no way to test it.


That's the problem isn't it? It's not easy for the average guy to calibrate a tension meter. Maybe "relativity" is their best asset.


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## Enoch562 (May 13, 2010)

Here is what mine looks like. I used a 330 lb scale which had increments of .2lbs./.1kg


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## ibericb (Oct 28, 2014)

Nicely done! 

I love the complete curve. How may points and at what approximate interval did you use for each calibration curve?

Did you use a typical J-bend spoke or is that a straight segment (can't tell when I zoom in). If a standard J-bend with head, can you explain or show how you attached at the spoke head/bend end?


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## Enoch562 (May 13, 2010)

It is a j bend spoke in plate welded to a hoop. I also made another plate so I can check I9's Aluminum spokes and straight pull steel spokes. 

THe graphs belong to the person who started this thread.


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## ibericb (Oct 28, 2014)

Thanks. I missed the change in authors.


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## nsfbr (May 23, 2014)

Nice, all you need is a dial indicator and you have a do it yourself Instron!


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## ericm979 (Jun 26, 2005)

Mine is a similar. The scale looks identical but mine goes to 440kg. The frame is made from square steel tubing. To mount the elbow end of the spoke I used a piece of 3/8 aluminium plate and milled a section to 3mm thickness (typical for hub flanges) and drilled some 3.2mm spoke holes. At the other end is a 1/2 inch hole for the snap link that connects to the scale. The nipple end goes into a nipple that fits in the lower steel tube in the same way it would on a rim. Some people who have made these have used nipples on both ends but I don't have a spoke threading machine.

I took readings at roughly 10kg intervals.


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## ericm979 (Jun 26, 2005)

nsfbr said:


> Nice, all you need is a dial indicator and you have a do it yourself Instron!


If I get a chance I'll measure the spoke stretch from 0-N kg. It's quite noticeable when loosening the tension nut after measuring.


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## ibericb (Oct 28, 2014)

nsfbr said:


> Nice, all you need is a dial indicator and you have a do it yourself Instron!


Funny you mention that - I was thinking of tapping into my old workgroup from which I retired to use an Instron for exactly this purpose. I'll forego the strain data.

The one used by Boyd replaces the scale with a load cell.


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## ibericb (Oct 28, 2014)

Thanks. 440 kg? Wow! 

I like the simplicity of using a stock J-bend spoke. I was thinking of 1/8" steel plate with hole for the spoke.


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## dcgriz (Feb 13, 2011)

I used the cover of a galvanized steel electrical junction box. It's the right thickness for the elbow end of the spoke and comes ready with punch-outs for attaching it to a U clip or other fastener.


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## coachboyd (Jan 13, 2008)

Here is the one that we made, with the load cell.









The load cell and digital display were bought on ebay for around $45. Then just some random parts at Lowes totaled maybe another $20.

The metal base that everything is attached to would actually bend under higher spoke tension, which is why everything is secured to a 2X10 board. 
Some of the square tube designs here are good, but you really have to look to see if the metal is bending at any place. If the metal is bending it makes it so you can't tension up a spoke at say 120KgF, leave it in place, and check your tension meter at random times against the spoke. 

We check tension meters on this probably 4-5 times per day, but only turn the machine on once per day to ensure the spoke is still reading at the 120KgF.

On a side note. . .I know the Park TM1 is pretty much the gold standard of "I can buy a tension meter for under $50", but we just got a few of the Ice Toolz dial indicator ones in the shop and have been pretty happy with them. Similar somewhat in design to the DT Swiss or Sapim tension meter, but less than $300. Good one for if you want a better level without breaking the bank. We still use the TM1 to get tension and do 90% as the gauge is easier to read at lower tensions, but the final adjustments are done on the dial indicator ones.


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## ericm979 (Jun 26, 2005)

Boyd- I designed my fixture so the spoke load is along the centerline of the frame, to avoid twisting it. The 1" square tubing should be stiff enough so it does not bend signficantly (especially since the load is on the short side of the rectangle) but I have not measured it.

I'm curious what protocol you use with your fixture and the TM-1. Do you use your own curves/lookup chart for each spoke, or do you use the Park one?

Did the load cell come with some sort of accuracy certification? When I bought my scale I couldn't find load cells with displays at a reasonable price like you did, otherwise I'd have gotten one. Of course the scale is just a load cell and display in one case, it's the accuracy I am concerned about. It's probably more than good enough for this purpose, I just don't know for sure.


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## coachboyd (Jan 13, 2008)

With the TM1 we mainly just calibrate it to 120KgF on a CX Ray spoke since that's what we're shooting towards. Then final adjustments get done with the dial indicator tension meter.

For those, I made our own "chart". Since we're building with the same hub every day and with mainly one type of spoke, there was really only three tension numbers I needed to calibrate (drive side, nds range, and fronts).

The S beam load cell has a calibration procedure. You grab a known weight, hang it, and calibrate the load cell to that weight. In our case it was done using a known 50 pound weight.


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## looigi (Nov 24, 2010)

It would be interesting to see how your measured curves would compare to curves generated by the Park web-based calculator using actual measured spoke width and thickness. 

Park calculator: Park Tool Co. » TM-1 Tension Meter Bladed Spoke Calculator

Park also put out a downloadable Excel based calculator too.


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