Spoke tension and tensiometers

If you'd like to learn more about wheels and wheelbuilding please take a look at this wheelbuilding book.
Wheelbuilding tensiometers

It's always a hot topic on any cycle maintenance forum with people asking should I buy a tensiometer? and the closely related question are my spokes too tight or not tight enough?

I've had many years experience building wheels without a tensiometer and I also have lots of experience of using tensiometers. In this article I take a close look at spoke tension and the tensiometers from DT-Swiss, Park Tools and FSA.

No tensiometer, no worry

Sometimes too much information is a bad thing. When I started building wheels the subject of tensiometers never entered my thought process, in fact I didn't even know of their existence. Along comes the Internet and it brings us tensiometers! Today there are dozens of discussions covering all aspects of the wheelbuilding process and this information overload leaves many people slightly concerned about the quality of the wheel they've just built and in particular is it tight enough. Building wheels in the 80's was so much easier!

Without any knowledge of tensiometers and tension values I built my first wheels, put them on the bike and went riding. Not just ordinary riding, back then I rode a lot of cyclo cross, including five finishes in the Three Peaks Cyclo Cross (a long tough race with mountains and lots of rocky trails) The wheels worked perfectly and the tension was miraculously correct. Why, because there's a broad range of acceptable tension and it's easy to drop into this zone and end up with a good wheel. Going to higher tensions does not lead to stiffer or better wheels. Loosely tensioned wheels can be problematic. As you build the wheel a loose wheel feels loose as the spokes squish together when squeezed and the spoke wrench turns easily, then after a little more tightening and stress relieving the spokes firm up and become taught, then you try for a little more tension and you reach the stage when it's noticeably harder to turn the spoke wrench and the spokes start twisting a little (all this assumes you've correctly lubricated the spokes and rim). Then you decide to stop and call it done.

Back in the 80's the only wheelbuilding book worth reading was by Jobst Brandt (The Bicycle Wheel) and it's still a very good book today. Jobst described a method of obtaining the correct spoke tension which was was the maximum the rim can withstand without buckling by incrementally tensioning and stress relieving the spokes until the stressing operation caused the rim to lose its shape and buckles, at which point you back off half a turn on the nipples and call it done. It's not a technique that can be used today because the rims are too strong and you will never be able to make the spokes tight enough to cause the buckling effect, but back then with shallow section single cavity road rims it was possible to use this technique to achieve maximum tension on lightweight road rims. I was fortunate to know one of the UK's legendary wheelbuilders, Joe Thompson who lived nearby who often called in the workshop and we drank tea and chatted about cycling and wheels. Joe built his wheels tight and even now I can still hear him saying nice and tight, and of course Joe never used a tensiometer. So along with Jobst Brandt and Joe I became accustomed to tight wheels, but as you'll see later when I eventually took a tensiometer to them the tension value was 125kg which is the norm for a good wheel.

Note : If you assess tension by giving the spokes a squeeze then be careful with bladed spokes because you are flexing them across the broad stiffer section and they will feel tighter than they actually are.

That's how it started for me and continued the same way into my professional building, no tensiometer and no idea of the numeric tension value in the spokes. The wheels I built for myself met my requirements and if they were good enough for me they would be good enough for paying customers. I rode my wheels hard and couldn't break them (I tried) and customers never had any problems with them (paying customers will always get in touch if anything goes wrong) and my confidence level was pretty high. Any lingering doubt was finally put to rest as I used the same techniques for building the wheels for several high profile cycling teams (mountain bike downhill race).

What all new wheelbuilders require is confidence, especially first time builders. These people should concentrate on the important aspects of the build which is uniform tension, fully stressed and with no residual spoke twist, and there's no ambiguity when it comes to achieving this. The spoke tension lies into the good zone and they have a perfect wheel that meets their requirements and their confidence level is high. This first wheel is a bench-mark for all the other wheels you build and you'll no doubt be comparing this to other wheels you see. Wheelbuilding is all about accumulating knowledge.

I get to inspect a few wheels from first time builders and they are very good. Some could do with a little tension increase but it's not a big deal. I see plenty of wheels when doing race service and often ask myself the question how does this wheel still hold up? they are way out of true with several very loose spokes (finger tight nipples) and the only reason the rider decided to do something about it was because they saw me fixing wheels for free. Without my intervention something bad will eventually happen but right now this wheel was still being ridden - and you are losing sleep worrying about whether the spokes in your wheel need another 1/4 turn or not!

As a new builder you should be monitoring your wheels and inspecting them by spinning the wheel in the bike and checking the lateral trueness. If your wheel works then great. If you didn't get your spokes tight enough then one or two spokes will lose tension and the wheel will lose its lateral trueness, solution: increase the tension. You'll soon know what is good and what is bad.

Tight spokes

Can very tight spokes damage the rim? People are paranoid about rims cracking and eyelets ripping out yet in my experience this doesn't happen. Occasionally I'll see a forum post about someone with a cracked rim but since I don't know much about the original build it's difficult to draw any conclusions. I'll wait until the rims in my wheelbuilds start cracking then I'll give it some more thought. In the book I've shown a cracked MTB rim from a wheel that I'd built and I asked the manufacturer who claimed said it was due to inappropriate use, it was a lightweight mountain bike rim and the customer used a narrow tyre with very high pressure and rode LOTS of miles, if it were used as intended with a larger tyre at a lower pressure it would have been okay. Current rims are well designed and cracked rims are rare, although some rims that are not well designed with insufficient spoke bed thickness can crack, but these are soon identified and the manufacturer gets a revised design out. Mountain bike riders should pay close attention to recommended maximum tyre pressures because over inflating a large volume tyre will cause the rim to crack, not necessarily at the spoke eyelets but the surfaces of the rim channel (often hidden under the rim tape).

I have also inspected wheels built with very lightweight MTB rims with the spoke tension WAY beyond the rim makers specification. They weren't all this high, but generally they far exceeded the rim specification. You would have difficulty obtaining this tension by hand but it's quite easy for a wheelbuilding machine which built the one shown in this example. These particular wheels were very popular and I did think they would eventually fail but there was not a hint of any problems and if there were problems then the MTB forums would be alive with comments. On this one I backed off the tension but there will be other examples like this being used at the higher tension.

This does not mean you have to get your spokes super tight, once you reach around 125Kg then going beyond this achieves nothing (not better, not stiffer, not stronger), but be careful about getting spokes too tight on a lightweight road rim or an old generation rim used in a restoration project because it may buckle under compression during the building.

Tensiometers

Commercial wheelbuilding adds extra pressure because you need to be even more sure that the wheels you build are good and you'll know this if you ever build a wheel for a friend. Errors in wheels you build for yourself are not an issue because it's your fault and your problem. In the commercial world poorly built wheels will tarnish your reputation and lead to costly rectification work. All the customer wheels I built without a tensiometer over many years were fine and I had no concerns about reliability - until I started to build on Stans ZTR rims.

The wheelbuilding specifications for ZTR rims (pdf) strongly advise spoke tensions that are a LOT lower than those for other rims. This was the first time I gave any thought to numerical values for spoke tension and that I should build to the makers specification to ensure reliability of customer wheels. I could just build up a set as my normal builds and go ride but it would take a long time to show up any issues related to over tensioning. So a tensiometer was required; i) to see what tension my normal builds were and ii) to ensure I built the ZTR rims to specification, and I suppose iii) to answer the questions I was getting from readers of the book (very early versions of the book (1996/1997 editions 1 and 2) had no mention of tensiometers because the book just documents how I build wheels and back then I never used a tensiometer).

The DT-Swiss tensiometer

The DT-Swiss tensiometer is expensive but for me it's a business expense so not a concern. As soon as it arrived I took it to a pair of mountain bike wheels I'd just built. Both rear drive and front disc tensions were all around 125kg and since the wheels were tension balanced by tone there was never going to be any tension variation between the spokes. So the wheels were perfect.

Since I'd purchased it I tended to use it, I saw it as a quality assurance check to ensure all the customer wheels were built to the same specification - or so I thought.

I had an issue with a wheel built with DT Competition spokes. All my experience told me the tension was right yet the DT tensiometer said it was way too low. It took a while to figure it out and the reason was the spokes were made undersize at 1.75mm instead of 1.8mm (this is probably within the spoke manufacturing tolerances). There was no way of telling the true tension because the readings from the DT tensiometer are cross referenced to tables and the table assumes your DT Competition is made to 1.8mm. If you tightened your undersize spokes according to the chart the result would be over tensioning them. I started to measure the diameter of the spokes (all spokes in the same box were made the same) and used this to make compensations, i.e. if they were undersize then I'd take this into account when looking up readings in the tables - all very hit and miss.

Could I use the DT tensiometer for checking Sapim spokes? The Sapim Race spoke is dimensionally the same as a DT Competition and the Sapim Laser same as a DT Revolution and the steel has to be very similar. I needed to assess the tension in the wheels I built with Sapim spokes. Better ask Sapim at their Belgium factory and their reply was:

No, you can't use the DT charts as these charts are based on DT spokes which have a totally different behaviour than the Sapim spokes especially for the Laser spokes. Best is that you send us the meter and that we calibrate it for you on Race and Laser spokes.

Sending the tensiometer to Sapim in Belgium was not an option. I couldn't figure out why they needed to physically check it because all the DT tensiometers must be identical since DT does not issue customised cross reference charts for each of their tensiometers. I didn't pursue this one.

A few months later I was chatting with a guy who managed a wheelbuilding operation (machine built wheels), they used Sapim spokes and had their DT tensiometer calibrated by Sapim who issue a revised cross reference tension chart. He sent me a copy of his chart. Whether it's good for my (or your) DT Tensiometer is anyone's guess and I'm not bothered because it's just another chapter in tensiometer guesswork. For information here is the chart supplied by Sapim, the marks around the 120-140Kg were written on by the guys at the wheelbuilding factory, however there seems to be an error in the chart since the figures for the G13 Strong spoke are the same for the super light Laser spoke.

The Park Tools tensiometer

This popular tensiometer uses the same measuring principle as the DT tensiometer where a reading is taken and looked up in a reference table to obtain the actual tension. I purchased one just to see what this low cost tensiometer is capable of.

The notable point is that Park supply one table that covers all spoke manufacturers. Compare this with DT who supply a chart for their own spokes and how Sapim say that chart is incompatible with their spokes yet the Park happily covers both manufacturers. I think the low precision of the Park tensiometer would not be able to differentiate between the different spoke manufacturers.

The FSA Jobst Brandt tensiometer

This tensiometer was originally designed by Jobst Brandt and the FSA device is virtually the same as the one shown in Jobst's wheelbuilding book "The Bicycle Wheel".

The only seller appears to be Ric Hjertberg, that's Ric founder of the Wheelsmith spoke company and previously working for the FSA company, and now operating on his own as Ric the Wheel Fanatyk. Ric describes the FSA tensiometer on his site. The analogue tensiometer shown here has now been replaced with a digital version using the same design.

It works on a similar principle to the DT and Park Tools tensiometers by measuring the spoke deflection from an applied load but with two significant differences. Jobst Brandt discusses it here on an Internet discussion group...

The problem with all tensiometers that I have seen offered is that they measure across the spoke so part of the reading is spoke thickness and none of these instruments allows zeroing the gauge on the spoke. Besides, Hozan in particular and others I've seen use too large a test load, one that will affect tension of the spoke being measured.

For this reason I designed a precision gauge that measures from the support side and uses a dial micrometer so that small deflections can be used. It also zeroes on the spoke and, because it uses such a small deflection, bending stiffness over a 100mm span is small enough to make the difference between 1.6 and 2.0mm diameter spokes insignificant. The instrument is shown in "the Bicycle Wheel" and was in production at Avocet but shops were unwilling to pay $300 for this precision calibrated gauge. 100 of them were subsequently sold by DT in Europe.

I have offered the drawings to several bicycle tool folks but they don't think there is a market for it. My impression was that they didn't believe there was any benefit from it in the first place, from the questions they asked. I use mine with every wheel I build to sample tension level. I pluck spokes to make the wheel uniform because I can do that more easily. ...my tensiometer was never calibrated and I just use the dial readings to characterize tension. 0.013 right side rear, 0.025 front wheel. That doesn't help you much but it works great for me. The Avocet units had metric dials by Starrett. Mine has an inch dial by Mitutoyo.

See the full discussion.

I've highlighted two important comments:

There's more from Jobst on tensiometers here.

The FSA tensiometer is one of the best available and the most accurate in determining tension, but the makers only claim +/-10% accuracy. This page is taken from the FSA tensiometer user manual:

Instructions - About Tension Measurement

We list the absolute accuracy at +/-10% because it's unrealistic to pretend higher accuracy is possible. Here are some of the reasons why:

I'll also quote this from the FSA manual:

Uniform tension of same-side spokes is a goal of well built wheels, not perfect trueness. Thanks to the rim's inherent stiffness and imperfections, a perfectly true wheel can contain spoke tension imbalances which makes the wheel less stable in the future. Thus, it is acceptable to trade-off perfect trueness to achieve consistent spoke tension to produce a more stable and lasting wheel.

... but you already knew that (I hope).

Sample readings

I measured the same spoke on a mountain bike wheel using each of the tensiometers. The spokes in this wheel are DT Competition that measure exactly 1.8mm meaning there is no look-up error when using the DT charts. All tensiometers indicated a tension of around 130Kg.

Measuring spoke tension using musical pitch

People have written smart phone apps to listen to the sound of a plucked spoke and translate the tone into a numeric tension value by trying to adapt the techniques used to tune guitars and pianos. However it doesn't work too well and it's a technique I would never use (or trust).

Jobst Brandt explains...

The frequency of a plucked spoke depends on its cross sectional diameter, its length, its tension and the adjacent spoke and where it intersects this spoke. Interleaved spokes, therefore, have a complex tone that has multiple modes, the primary mode not containing the most energy but rather a mode that resonates in the free span of the spoke. Seen on an oscilloscope, the spectrum is dirty and not repeatable. Using tone by exciting the higher modes of the spoke is useful for balancing tension but practically useless to gauge absolute tension. By plucking spokes near the nipple, the higher modes are excited which largely avoids the problem of interleaved spokes. The musically adept human ear readily recognizes the "quality" of the tone that is otherwise difficult to define analytically.

Taken from groups.google.com/d/msg/rec.bicycles.tech/

I tried a clip on digital guitar tuner attached to my wooden truing stand but only to evaluate its usefulness in matching the tone when tension balancing the spokes where you pluck the spokes to ensure they make the same sound and therefore the same tension (actual tension value not known and not required). The acoustic resonance of the wooden stand was excellent and the tuner responded instantaneously, but the read out in musical notes and flashing bar graphs was confusing and I found the tuner of little use. Using my ears I can hear the notes and it's more than accurate enough and a lot quicker.

Tensiometer calibration

How do you know your tensiometer is giving out accurate readings?

Answer : You don't unless you have some means of checking it.

Here are some examples of people who check their tensiometers. Basically they all apply a preset tension to a spoke (the same spoke and desired tension required for the particular wheel build), then a tensiometer reading is taken. The reading is just a number, that's all, and you tension the spokes in the wheel to the same number.

Shimano

Handbuilt Shimano wheels. Their tensiometer and calibration device are shown in this Shimano factory tour.

Prolite

Here is the Prolite wheelbuilding facility. All wheels are handbuilt and they use tensiometers that look identical to the DT tensiometer. Other things to note, they use oil on the spoke threads and no adhesives, each wheelbuilder takes 6 months to train - and then they build 60 wheels per day!

Calibrate your own tensiometer

You need to apply the target tension to a single spoke then take a tensiometer reading off it.

The simple way is to hang a weight off the spoke, but since 130Kg is a lot of weight to find and manoeuvre around it's not really practical. The solution is provided by Dan Burkhart who has designed this very elegant calibration tool and I'm very impressed by it.

Here is a more sophisticated calibration device made by someone who purchased the book (forgot your name, please get in touch so I can credit you). Instead of a digital scale used by Dan Burkhart, it uses an S type load cell attached to a digital display, but essentially both calibration devices are the same, although this one is a lot more expensive, but more accurate because digital scales sometimes are heavily damped making fine adjustment difficult.

Component manufacturers tension specifications

A few rim makers publish tension limits for their rims, for example Mavic say 70-90kg limit for their rims and DT say 122kg for theirs. Does this mean Mavic rims are a poorer design or made from weaker material or does it mean they haven't a clue and just made a guess! Those Mavic figures are even lower than the figures quoted for Stans ZTR superlight rims. To see the 70-90kg figure in print, go to www.tech-mavic.com Username: mavic-com Password: dealer. Select any rim then see the General Points.

If a rim manufacturer stresses the need for a lower spoke tension (e.g. Stans ZTR) and gives a plausible reason then I just build them a little lower than my standard builds.

The only tension limits for hubs that I've seen are on Tune hubs where the Tune documentation advises a tension from 1100N to 1200N depending on the hub, for example the Tune King.

Conclusion

Written by Roger Musson

[ Top of page ]