Published by Tristan on 14 Dec 2008 at 03:15 pm
Spoke Tension Meter Calibration
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- 89kgf applied to the spoke
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- 89kgf applied to the spoke but with the tension meter applied the reading increases
With every wheel I build I include an information sheet which shows the lateral and vertical trueness of the wheel as well as the spoke tension on each side of the wheel. I’ve been doing this for just shy of 3 years now and although I’ve taken some flak over the geekyness of it the concept seems to be catching on with a NZ bike shop doing a similar sheet and even Park Tool writing a spider chart for their TM-1 tension meter. As they say copying is the best form of flattery :-)
3 years ago most bike shops didn’t have a spoke tension meter, let alone know how to use one. Luckily things have come a long way and now I even get requests by home mechanics to buy TM-1′s.
But what good is using a precision device like the TM-1 if you don’t know how accurate it is?
Park Tool offer a re-calibration service for the TM-1 but I wasn’t about to send my tool back to the USA and be without it for a month so I designed and built a jig to be able to do my own calibration (as well as run stress / strain tests on spokes – but that’s another story.)
The jig is a very solid device made from cold-rolled 32mm steel square bar. At one end the spoke is held by it’s j-bend, at the other end a load cell is attached to a linear slide running on two parallel guide pins and a thrust bearing. By turning the knob the spoke is put under tension and can be measured by the load cell. The load cell is a very accurate and repeatable strain gauge which has almost no temperature or humidity dependence. My loadcell is a Seltron STC-100 and is it’s regulated and read by a Rinstrum R310 display – both of these are very expensive bits of kit.
Here is shown a typical calibration process. The loadcell is zeroed before a spoke is attached to the device then tension is applied. The spoke is read with the TM-1 and the units are compared with the tool’s conversion table. I typically test two different spoke tensions on three types of spoke: DT Swiss Revolution, DT Swiss Competition and Sapim CX-Ray but the jig will accept any j-bend spoke and has future capability for spokes such as Mavic Zicral.
In doing calibrations for the time the tension meter with typically over-read or under-read by up to 10kgf. My personal tension meter is calibrated on a monthly basis and rarely needs any on-going adjustment so it seems there is just a settling process for the tools.
This procedure is not limited to the TM-1 and will work with any brand – please contact me if you have a tension meter you wish to calibrate.
Tristan on 01 Jan 2009 at 2:22 pm #
After this post I received a couple of questions from a gentleman about the comparison of the Park TM-1 and DT Swiss tension meters and the lack of stiffness in the Park TS-2 truing stand.
The DT tension meters have similar definition to the Park. The DT Tensio’s have less deflection at the spoke and use a dial or digital indicator to amplify this small amount of movement to something readable. The Park uses a long indicator arm to do a similar job. The upside of the DT is it’s smaller size. A downside of the Park is the long lever arm adds to the unit’s spung weight which can affect the accuracy of the reading depending on how the arm is released – the DT also suffers this problem but to a lesser degree.
There are two parts of using a tension meter to measure: Repeatability and accuracy. Repeatability is making sure that the tool returns the same reading when used 10 times in a row; accuracy is knowing this reading is correct. All of the tension meters I’ve used are highly repeatable when used correctly however no matter what tool you use it will need to be recalibrate with use and age. Precision engineers have their torque wrenches re-calibrated yearly and any weight scale used for trade needs to be calibrated and certified. You can have the most expensive tension meter in the world but if it’s not calibrated then it is not accurate – this is why the DT Tensio has a calibration set-screw and why the Park’s spring-stop is adjustable.
The Park TS-2 truing stand (which I use) is no where near as stiff as the no-longer produced TS-3 but when using dial indicators for the final truing the stand does not twist because there is no load on it thus the readings are accurate. Even if the stand was completely rigid a small 5kg lateral load applied to the rim by your fingers will result in about 1mm of movement – 100 times the resolution of the dial indicator. No mater what truing stand being used when measuring with dial gauges it’s important to rotate the wheel by turning it at the hub rather than the rim as this reduces the chance of inaccurate reading.
Links:
DT Tensio analog:
http://www.dtswiss.com/Products/Proline/DT-tensio-analog.aspx
DT Tensio digital:
http://www.dtswiss.com/Products/Proline/DT-tensio-digital.aspx
Park TS-3
http://img.epinions.com/images/opti/21/ed/Park_Truing_Stand_With_Dish_Tool_TS_3_Shop_Tools.jpg
Park TS-2
http://www.parktool.com/products/detail.asp?cat=16&item=TS-2
Cédric Lemaître on 27 Nov 2009 at 5:32 am #
Hi Tristan,
Do you have somes links where buy the component to calibrate TM?
Cédric
Tristan on 27 Nov 2009 at 8:27 am #
Hi Cedric
I designed and built this machine myself – as far as I know there is nothing ‘off the shelf’ which will do a similar job.
Park Tool suggest the following:
“Calibration
The TM-1 Tension Meter is calibrated at the Park Tool manufacturing facility. Generally we do not recommend readjustment of tool. If the tool is worn or damaged and appears to be inaccurate, return to Park Tool for recalibration. Park Tool will recalibrate and return the tool for a reasonable charge.
It is recommended for professional mechanics and service departments use a non-riding wheel to help in tool calibration. Remove the axle from a wheel so that it is not rideable. Mark a spoke as a reference spoke and measure this one spoke. Write the date of the measurement directly on the rim. This same spoke can be used to double check the original calibration. If the tool is reading different than this “reference spoke”, the spring tension can be changed. If the tool is reading low, take the tool off the spoke and turn it upside down. Remove the spring from the fixed moving stud, and then thread the adjusting screw inward toward its stud. This will decrease tension on the TM-1 spring, resulting in a higher deflection reading. If the TM-1 is reading high, increase tension at the adjusting screw. “
Cédric Lemaître on 28 Nov 2009 at 8:47 pm #
Thanks Tristan,
I understand the process but i would know where buy this :
- loadcell is a Seltron STC-100
- Rinstrum R310 display
Cédric
Tristan on 28 Nov 2009 at 9:01 pm #
Cedric
You’ll need to find a commercial / industrial company who sells equipment for scales and loadbridges. I used Gilbarco Veeder-Root, they’re a global company but not sure if you have them in France?
http://www.gilbarco.com/page/home
Or buy online from someone like http://www.transducertechniques.com/
I didn’t know much about loadcells so I used the experience of Gilbarco to help me chose the correct loadcell and display, and to correctly calibrate them…of course this cost more than buying online though.
Cheers, Tristan