Park Tool TM-1 calibration
- Calibration of a used Park Tool TM-1 spoke tension meter -

Recently I got a moderately used Park Tool's TM-1 spoke tension meter with a request to recalibrate it back to match factory spoke tension charts. Since I couldn't assess the wear degree of the tool, I was curious what the deviation in readings would be from actual tension and if the tool is still reliable across different spoke models. In the article you will find how a worn Park Tool TM-1 affects the tension readings and if it is possible to get it recalibrated back to the factory values.

The tool: Park Tool TM-1

As mentioned, a package I received, included a moderately used Park Tool TM-1 spoke tension meter, inarguably one of the most used tools of this trade. Known for its practicality, robustness and lightweight construction, when calibrated on point, it fairly accurately and reliably measures the spoke tension of a wheel.

Park Tool TM-1 spoke tension meter
The Park Tool TM-1 spoke tension meter that needed to be calibrated once again.

Factory service & repair

Park Tool is aware of their tool coming out of calibration as they say: "Over time, especially with the repetitive use experienced in a shop, the spring on the TM-1 Tension Meter can come out of adjustment or wear out completely, causing the TM-1 to come out of calibration and no longer read spoke tensions correctly."

However the spring wearing out as they state is considered as wear and tear and as such not covered by their warranty. But nevertheless they still offer aftersales service and repair of their tool. Visit their link for their factory TM-1 service & repair.

Since they offer a limited number of third-party service centres around the world, for some Park Tool TM-1 users, a self service becomes the only sensible option for their tool maintenance.

Tension meter inspection

Since I wasn’t aware of the actual wear degree of the tool, I first inspected all the vital parts of the spoke tension meter.

As shown on the image below, the client made a remark that the triangle tip of the scale is leaning towards the negative value of -1, when the spoke tension meter spring is released. Looking at my own Park Tool TM-1 tension meter, I should assume this must probably be a standard flaw of this tool, but nevertheless, I was able to move the triangle tip a little towards the zero value (0) on the scale. However, this does not affect readings accuracy of the tool, it is more of an aesthetic defect.

Park Tool TM-1 scale
A standard flaw of the scale on a Park Tool TM-1 tension meter. The tip of the triangle shows a negative value on a scale.

Next, looking at the front of the tool, where the spoke is inserted in for checking the spoke tension, all the pivots were ok, meaning they weren’t showing any sign of an excess wear and were still almost perfectly round.

Park Tool TM-1 pivots
The front of the tool was almost intact and wasn’t showing any signs of an excessive wear.

Tension meter calibration

Packed with convenient cross-spoke tension chart, Park Tool TM-1 is a great time saver. Just find your spoke dimension in this colourful table and you are good to go as all the reference numerical spoke tensions are listed corresponsive to the readings on this spoke tension meter scale.

But is the tension meter still accurate and reliable to the point after some moderate use? And if not, how does this wear degree affect deviations in readings from the actual tension, put on spokes.

At this point, my own spoke tension meter calibration device comes in action. Adjusting a spoke tension on a spoke to match almost the exact value you are looking on the Park Tool TM-1 scale is a pretty straight-forward process. To re-calibrate this Park Tool TM-1 spoke tension meter, I opted for a widely used, classic J-bend Sapim Leader 2.00mm round spoke.

Park Tool TM-1 calibration device
Adjusting tension on a spoke is easy with a spoke tension meter calibration device.

Note: For more information about tension meter calibration process, proceed to the following article: Tension Meter Calibration: How to calibrate a spoke tension meter and make your own spoke tension chart

The next step was to start taking readings for spoke tension values that were on factory tension chart table diagram, enclosed with the tool. Only by comparing factory readings with the actual readings would tell me if the tool shows spoke tension correctly or not.

Every reading was taken multiple times, with a controllable, consistent release move of tool’s spring. Also, each time, the tool was placed with the center pin aligned to a center point of a spoke length as on the image below.

Park Tool TM-1 calibration
The tension meter was positioned in the same place with a controllable spring release of a pressure on a tested spoke each time.

Results of the first round, reference readings of a Sapim Leader 2.0mm round spoke, are shown in the table below. As expected readings were all consistently higher than they should be, meaning the tension meter was showing a higher tension on spokes than it actually was. Also, another noticable thing - readings deviations were greater at lower spoke tension values where deviation was as much as 20% from the actual tension.

Reference readings: Sapim Leader 2.00mm

Factory Actual
Tension (kg) Readings Readings Tension (kg) Deviation
53 17.00 19.00 63 19%
58 18.00 20.00 70 21%
63 19.00 20.50 74 17%
70 20.00 21.00 77 10%
77 21.00 22.00 86 12%
86 22.00 23.00 96 12%
96 23.00 23.50 102 6%
107 24.00 24.50 114 7%
120 25.00 25.50 128 7%
135 26.00 26.50 144 7%

As I mentioned, the results I got were somewhat expected. To some degree at least. Why? With tension meter use, the spring on the back side softens up and that consequently reduces the pin pressure on the spoke inserted into the tension meter. As a result, with a lesser pressure of a tension meter, a spoke then deflects to a lesser degree than it should and the scale of the tool shows a greater tension value.

All the magic of calibrating the Park Tool TM-1 tension meter happens on the back side of the tool by adjusting the preload on the spring with the screw. By unwinding this screw you increase the preload on a worn, softer spring so that a tension meter behaves as a factory one would. Couple of adjustments were made and then I took final tension readings with the same spoke.

Park Tool TM-1 calibration spring
Adjusting a preload on a spring will help you calibrate the tension meter if it isn't completely worn out.

Now, final spoke readings were taken and results were I must say satisfying. The tension meter started behaving almost as the factory one would. Check the table below to see results. However, some deviations were still present at low tension values.

Final readings: Sapim Leader 2.00mm

Factory Actual
Tension (kg) Readings Readings Tension (kg) Deviation
53 17.00 17.50 56 6%
58 18.00 18.50 61 5%
63 19.00 19.50 67 6%
70 20.00 20.00 70 0%
77 21.00 21.00 77 0%
86 22.00 22.00 86 0%
96 23.00 23.00 96 0%
107 24.00 24.00 107 0%
120 25.00 25.00 120 0%
135 26.00 26.00 135 0%

Calibration check and validation

Performing a tension meter calibration only using one reference spoke can be sometimes misleading. The thing is that with a different spring preload than it originally was, the spring may behave differently and readings across multiple spoke models or spoke tension values can become inconsistent according to the original tension charts, supplied with the tool. Also, it was my client request to check higher tension values with some smaller diameter round spokes that he also uses frequently. Fortunately I had one Sapim Race spoke, a single butted 1.8mm round steel spoke and one Sapim Laser spoke, with its 1.5mm diameter.

Tables below show actual readings match with factory calibration of the tool. However, another small spring preload adjustment had to be done. But all in all it looked that this Park Tool TM-1 could continue doing its job!

Check No. 1: Sapim Race 1.80mm

Factory Actual
Tension (kg) Readings Readings Tension (kg) Deviation
105 21.00 21.00 105 0%
117 22.00 23.00 117 0%
131 22.00 23.00 131 0%

Check No. 2: Sapim Laser 1.50mm

Factory Actual
Tension (kg) Readings Readings Tension (kg) Deviation
107 17.00 17.00 107 0%
119 18.00 18.00 119 0%
133 19.00 19.00 133 0%

Final thoughts

With some use, a wear and tear is pretty ordinary for every spoke tension meter. Still, fortunately, in this case, the Park Tool TM-1 spoke tension meter can be calibrated over time to match factory spoke tension charts. This article can serve as a reminder for every wheelbuilder that their tension meters need a regular maintenance and calibration to guarantee accurate and consistent readings. Only then their wheel-work art may be invaluable.

Happy wheel building!

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