In my previous article, “SPOKE TENSION - The definitive guide to spoke tensioning” , I have emphasised the importance of spoke tension in wheelbuilding process, common misconceptions about this topic and some practical implications. Understanding how a wheel and spokes behaves under tension can save you a lot of hussle. To make things easier, a regular user can take advantage of SpokeCalc's built-in feature - spoke tension calculator - to get reference spoke tension that will suit the wheel.
But from experimenting behind my workbench, I figured, it would be a worthy topic to write about spoke tension meter calibration as this is the process of getting using a high precision spoke tension meter to a full degree. In contrast to what one would think, such devices aren’t exactly plug and play. To use them across different spoke types and dimensions, you have to take some reference spoke tension readings first to even get started. And even if a comprehensive spoke tension chart or conversion table is available, your spoke tension meter still has to be calibrated and readings rechecked once a while. So let's dive into tension meter calibration process.
SPOKE TENSION METER CALIBRATION - WHY AND WHEN
Basically tension meter calibration is a process of rechecking spoke tension readings on your tension meter's dial against actual tension on the spokes. Put simply, if based on the spoke dimension you are using your tension chart says your tension meter dial should show an exact reading for a tension value, does this value align with actual spoke tension? Is 120kgF of tension on your rear wheel drive side really showing on your tension meter dial? Or does your tension meter shows you are building the wheel with 130kgF but in reality there is less than that?
Ok you probably agree with that said, but is spoke tension calibration process even necessary for fairly new tension meters? Here are some main concerns:
- Not all spoke tension meters come with a comprehensive spoke tension chart, that tells you what readings should across different spoke types and dimensions at certain spoke tension.
- Not all new tension meters are even calibrated properly. Based on some forum chat research, even newly acquired tension meter sometimes are not quite calibrated properly.
- Tension meters get used with time. With some wear and tear your tension meter readings will be off after some time, thus results misleading. The main spring of a tension meter that affects load on the spoke wears down eventually causing misguiding readings.
- Spokes differ between manufacturers. Similar dimension spokes of a different brand won't show same readings. For example Sapim CX-Ray vs DT Swiss Aerolite. That was also mentioned in one of my previous articles.
- Even spoke color matters. Once spoke has been painted, it will affect your readings, so recheck your tension meter readings!
TIP: Every spoke tension meter should be calibrated and its readings checked with a spoke tension meter calibration device. Even before taking measurements on your wheel, test your tension meter against spoke with specified tension for a quick check as it takes just a few minutes but you will be confident you will have properly built wheel.
So, how do you calibrate a spoke tension meter?
With a high quality spoke tension calibrating device. For those DIY wheel fanatics, Blackcat wheels has a very detailed and visual article on spoke tension calibration device. There are also different DIY tension meter calibrating solutions but in its basic form you should be rechecking your readings against known load on the spoke.
As I see it from my perspective you have two choices when addressing the question of spoke tension calibration:
- Based on spoke tension readings at certain tension values for specific spoke, you write down all the readings in a new conversion table. Basically, you just overwrite the old tension chart with new. You don't mess with the spring or a dial of a tension meter. This is the least intrusive way and will also protect you against warranty claims concerns if something goes wrong in the future with your device.
- Again you start with taking a reference readings for a known spoke and compare them with spoke tension readings on existing tension chart that came with your tension meter. Then you tweak with spring to make your dial show this exact reading from your conversion table. In that way you adapt tension meter reading to suit an existing spoke tension chart. But like said before, you will lose warranty, so I recommend the first option.
EASY TO GET STARTED SPOKE TENSION METER
There are probably very few cycling workshops or enthusiasts that haven’t heard of or use notorious Park Tool’s tension meter TM-1. It is inarguably one of the most used tools in its trade. Not only for its available price, but also for its usefulness and robust construction. When calibrated on point, it fairly accurately and reliably measures the absolute tension of each of the spokes in a wheel.
Packed with convenient cross-spoke tension chart it is also great for time efficient wheelbuilding. 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.
Great to get you started if you are just beginning wheelbuilding and an allround horse for many high-volume workshops. But let’s get serious now. Speaking of tension meters as high precision tools, one must look in domain devices with digital readings or at least a dial indicator of high accuracy that scales to 0.01mm. Learn to use them in a proper way and they will show you marginal changes in spoke tension and make your work a masterpiece.
Making a spoke tension chart
Looking at different spoke tension meter designs, one sooner or later notices, that the readings aren’t directly translated to spoke tension. In other words, your tension meter won’t directly tell you have a 120kgF force on a particular spoke you’re measuring. That’s just how tension meters work. Spoke tension is derived from a deflection of a tensed spoke. Round 2.0mm spoke will deflect in a different manner than bladed 0.9 x 2.2mm spoke. Same readings on your dial indicator of your spoke tension meter will mean different tension values of these two spoke types. The underlying problem goes even deeper as I pointed out in my previous article. According to DT Swiss technical support, same dimension spokes from different manufacturers behave differently and so it is inadvisable to use readings charts across different spoke makers carelessly. This partly explains why some tension meter makers (for example my Birzman tool) doesn’t include a comprehensive table, such as Park Tool’s, despite being in the range of at least five times more expensive
In my case, my Birzman’s Spoke Tension Meter BM-15 came only with spoke tension chart for regular round spokes (PSR 14) which looked like that:
This chart shows how three different size round spokes deflect at a range from 600N to 2000N. A quick look shows you that a difference of 0.1mm on its high precision Japanese made Mitutoyo dial indicator approximately relates numerically to a difference of 100N (9.8kgF) of tension on your spoke.
TIP: During wheel tensioning process, use tolerance markers on your dial indicator (green tips) to mark your maximum and minimum allowed tension reading for each spoke.
Since most of my projects include bladed spokes, this chart is incomplete for me. I couldn’t even start working with it. It meant I had to make my own conversion table (spoke tension chart), following Park Tool’s examlpe above. The idea behind it was easy. For each spoke model you use, write down reference spoke tension in kgF and its corresponsive readings on your spoke tension meter. For the purpose of testing, I chose bladed Pillar PSR X-TRA 1420 spoke, which is 0.95mm thick and 2.0mm wide in its center part. Then I mounted it into my own spoke tension meter calibrating device.
To make thing easier, I only took readings for desired spoke tension on both sides of a wheel, including +/- 3% tolerance, to set reference for allowed spoke tension range.
Remember I said I prefer the first option of tension meter calibration, where you don't mess with tension meter spring but instead just make another tension chart? Yep, since my Birzman tension meter didn't show distinctive differences in readings for lower tension numbers (under 90kgF), I had to unload the spring to get normally distributed readings. Warranty, unchecked.
Spoke tension meter calibration - THE ONGOING PROCESS
As with every tool, when in everyday use, there comes wear and tear. It is inevitable that the quality of the spoke tension readings will deteriorate over time. And therefore such high precision tools must be calibrated from time to time. Blindly following what your tension meter tells you can be misleading. Readings become unreliable, commonly indicating higher spoke tension compared to the actual number, therefore misleading a wheelbuilder to finish a wheel with spoke tension that will be lower than expected.
I remember when my first Park Tool’s TS-3 tension meter had worn out noticeably after 7-10 wheelsets. Based on its original spoke tension chart, that was included with the device, it showed 10-15kgF lower force on spokes compared to what tension really was for that spoke tension reading on a scale. When I found out, which was obviously too late, my alarms went off. Some of my latest wheelsets were handled to customers with spoke tension, lower than desired. Later I discovered that the initial stretch of the spring in center pivot of a Park Tool’s spoke tension was greater than later on in its lifetime. From then on, I barely fine-tuned it again.
TIP: once in a while, calibrate your tension meter to assure tension readings corresponds with numerical values of actual spoke tension.
FINAL THOUGHTS ON SPOKE TENSION METER CALIBRATION
Calibration should be a regular maintenance of your spoke tension meter. Better to have an accurate and perfectly calibrated basic tension meter like TM-1 rather than blindly trusting an expensive tension meter with a precision dial indicator and using it in a careless way by not taking any maintenance of it.
Every now or then you should simply check if your readings match with the actual spoke tension. It can be a simple process of checking a particular tension - let’s say 120kgF – with your readings on your tension meter just before starting a wheelbuilding project.
Make a specific conversion table for the spoke models you use. No need to test your tension meter across all the spoke types and dimensions or even in a very wide range of tension. Rather than that, just focus on the spoke tension values you will be using. Define your tolerances - min / max tension on that side of the wheel – and write down what your readings on your tension meter should be.
As a high precision tool, it is absolutely necessary to operate with it with upmost care and store it safely when finished with work. Most of inconsistencies in readings can be a result of poor handling and operation with the tool itself. Talking about operation with the tension meter, again, your motion when decompressing tension meter to measure deflection of a spoke should be consistent across all the spokes.
And lastly, mimic the position of a spoke in a spoke tension calibration tool to the one on your wheel. I have noticed that I get slightly different readings if my calibration device is positioned horizontally than if I position it vertically. Tension on the spoke itself is still the same, yes, but you are distorting a reading with tension meter slightly leaning on a spoke itself.