Road tubeless and spoke tension

I have been riding road tubeless tyres over the past 8-9 months and I have been extremely happy with them. When you ride the fastest tyre you have ridden, with all the desirable qualities such as grip and comfort AND it has sealed all the 5 punctures out of 5 picked up on all sorts of roads, without ever having to do anything, how would you not be happy?

As a matter of fact, most of the people I talk to who ride tubeless road tyres are extremely happy and would not go back to tubed clinchers. That is not to say that everybody is: some have had negative experience in fitting the tyres onto particularly tight rims, others experienced a puncture that did not seal and some have never tried them, but have very strong opinions about them and are not afraid to voice them. The latest piece of scaremongery I have heard is that if you fit a tubeless tyre, this will result in a massive decrease of spoke tension, potentially leading to spoke and wheel failure.

Is that true? Yes and no: yes because when you fit any tyre to your rim, the spoke tension decreases, tubeless or not, no because it’s not massive and won’t lead to anything catastrophic. In the attempt to shed some light on the matter, I decided to take an afternoon and designed a little experiment. The same front wheel (HED Plus rim for disc built with 32 Alpina elliptical spokes with a three cross pattern) has been fitted with a tubeless tyre (Schwalbe ONE 25) with and without an inner tube and then it has been fitted with a normal clincher (Vittoria XN PRO 32) with inner tube at a set pressure of 80 PSI. Two spokes on different sides of the wheel have been marked and the tension has been measured using two calibrated tensiometers (a DT tensio and a Park Tool one). I also have a calibration chart for this particular spoke built around the Park Tool instrument, so the DT will only be used to double check the consistency of readings. It is relevant to note that this wheel has done around 10,000 Km with no problems and only a couple of checks over nearly one year of use.

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Below you can see the conversion chart I will be using, showing the dial readings on the X axis and the corresponding spoke tension in Newtons on the Y axis. For the purpose of this experiment, the actual tension in Newtons as an absolute value is not as important as the relative tensions between the readings.

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Here are the numbers:

Rim with no tyre on:  LS 20; RS 14.5

Rim with tubeless tyre 80 PSI: LS 19; RS 12.5

Rim with tubeless tyre and inner tube 80 PSI: LS 18.5; RS 12.5

Rim with clincher tyre and inner tube 80 PSI: LS 19; RS 13.

This first experiment shows that the spoke tension drops by about 70 N on the higher tension side of the wheel, regardless of whether a tubeless tyre or a clincher is fitted. The problem is clearly the tyre, not whether it has an inner tube or not. The low tension side of the wheel shows a more significant tension drop of  170 N with the tubeless tyre (with or without an inner tube) and 130 N with a clincher.

The difference in behaviour between the two tyres is very small, 40 N on the low tension side, which is something that can easily be addressed by over-tensioning by roughly 7-8%. No difference is detected on the higher tension side of the wheel.

In a second experiment, I wanted to assess whether tyre pressure has an influence on the tension drop. Much to my surprise, in a range of pressure between 20 and 90 PSI, the readings are exactly the same, as those reported above for 80 PSI. So I can conclude that tyre pressure has no effect on spoke tension within the tested range. It is in fact the very act of fitting a tyre onto a rim with the minimum pressure required to hold it in place that affects the spoke tension.

These measurements have been performed on one rim only, other rims will respond differently in terms of relative tension drop, but what seems clear is that a tubeless tyre influences the spoke tension only very marginally more than a conventional clincher with inner tube on the low tension side of the wheel.

The sensible conclusion is that rims need to be built with sufficient tension to take into account the fact that they will have to take a tyre, tubeless or not, therefore building at the max recommended tension, or even very slightly over is a necessity, rather than a builder’s choice.

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