Home built carbon fiber wheel loosing tension

Tech Tech Help
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I have a tension ratio, which I understand to be a function of mainly the hub and the rim, of 100%:52% (DS:NDS). Meaning that if I got the NDS up to the 80kg range, that means I’d have my DS at around 160kg which is way beyond the manufacturer’s recommendations of maximum 135kg so I don’t think that’s feasible in my case.

Would that help though? From what I have gathered so far a thicker spoke does not make for more tension. 100kg (random number) of tension is 100kg of tension regardless of how thick or thin a spoke is. If I understood @Nick_Coward correctly thicker spokes would indeed make for a laterally stiffer wheel but not change anything vertically. Again, assuming that I understood it correctly, it is the insufficient non drive side tension that is making the wheel loose tension.

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One more thing that I can think about in the pursuit of increasing (NDS) spoke tension is to try and achieve the manufacturer’s maximum spoke tension with the tire mounted and pumped up to the typical pressure that I run it at. I have measured some spoke tension loss between the measuring without the tire and with the tire mounted and pumped up to pressure.

Would this be a valid approach or is this not a good idea?

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100kg of tension is 100kg of tension. However for a wheel in equilibrium the forces must balance out. The NDS spokes pull at a flatter angle than the DS ones and therefore for the horizontal forces to balance the tension will be lower on the NDS.

There are geometric ways to improve this situation (offset rim drillings, high/low hub flanges etc) but you can also change the spoke quantity or thickness. The Modulus of elasticity of steel spokes is pretty uniform, if there is less cross-sectional area of steel spoking on one side the thinner or fewer spokes will stretch more (experience more strain) and be under more tension at equilibrium.

Laterally stiffer wheels have better (wider/flatter)bracing angles and more or thicker spokes. Tension, within limits, does not have much influence. I suppose vertical compliance is more complex but my experience suggests that thinner, more butted spokes are nicer to ride.

I can only suggest that you try it! If you tension your 12 drive side Champions (less than a Euro each?) to 120 I would expect your 12 NDS CX-rays to be at least 70 but I can’t predict how much - there is an argument here on the cross sectional area:

You could of course try CX-sprints but they are pricey and may not make enough of a difference.

I’m at the limits of my physics so happy to be corrected but found the below helpful if you like that sort of thing.

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I would say this ratio is pretty normal and the wheels will hold up. We work with DT Swiss hubs alot and they make for a reliable wheel. In fact most of the Zipp road wheels that come for service have tension ratio of 100%:0% DS/NDS and yet the customers don’t feel any misbehavior nor unexpected wheel failures. We have plenty of wheels under customers riding them for years built with CX rays and DT hubs with DS/NDS tension ratio of aprox. 100/57. The ratio is highly dictated by the hub flanges design and you don’t hear DT Swiss wheels/wheels with DT hubs are unreliable. Yes, it could be better but still it works good enough.

As far as I know the LB rims have assymetric drilling if not ordered otherwise.

Not a good idea IMO as the spoke tension would rise above the limit when you unmount the tire which could result in a rim damage.

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It seems that I might have solved the mystery in the process of trying to bring the wheel back to tension.

I took off the inner tube and the tire and started taking a baseline tension reading with the parktools meter. Much to my surprise, the values that I got were between 8 and 9 for the NDS and 14 and 15 for the DS. This is pretty much exactly what I built them up with. According to the conversion table (just as a rough reference point) that should correspond to around 70kg for the NDS and around 130 kg for the drive side. So regardless of the absolute accuracy of the park tools meter, the measurement stayed the same.

I then mounted the GP5000 tire that I’m using and pumped it up to the maximum 8 bar (I ride it at 4.5) to seat it properly. When I then measured the tension again, lo and behold I was getting around 3-4 on the NDS and 10.5-12 on the DS. So a significant drop in spoke tension.

In principle this should not be surprising since the air pushes in all directions, including towards the hub, but the amount of tension loss did surprise me.

Just to check, I lowered the pressure to the 4.5 bar that I run while I ride and i measured again. Now I got 4-5 on the NDS and 12-12.5 on the DS.

So in short, I think I was chasing a ghost.

It should be all good, I just compared two fundamentally different situations, no tire mounted vs. tire mounted and inflated and got different readings (as I should). I am still quite surprised though that the air in the tire lowers the spoke tension by that much.

For completeness, and to satisfy my curiosity, I ran a bunch more measurements.

I did them in the following way: I order to keep the effort reasonable, I chose four adjacent reference spokes on the non-drive side (NDS) and four on the drive side (DS). I took multiple readings of each spoke and tried to operate the spoke tension meter as consistently as possible. In total, I did 5 different measurement setups in this order:

  1. without the a tire mounted
  2. tire mounted and inflated to 8 bar
  3. tire mounted and inflated to 4.5 bar
  4. tire mounted without any pressure
  5. without a tire mounted to check repeatability

These are the results:

without tire without tire 8 bar 8 bar 4.5 bar 4.5 bar 0 bar with tire 0 bar with tire without tire without tire
NDS DS NDS DS NDS DS NDS DS NDS DS
Spoke 1 8 14.5 4 11.5 5 12 6 13 7 14
Spoke 2 8 15 3 12 4 12.5 5.5 13 6 14.5
Spoke 3 8 14.5 4 11.5 5 12 5.5 13 7 14
Spoke 4 9 15 4 12 5 12.5 6.5 13.5 8 14
avg. 8.25 14.75 3.75 11.75 4.75 12.25 5.875 13.125 7 14.125

We can see that the results make sense in that the tension is highest when there’s no tire mounted and lowest at 8 bar. Interestingly, just mounting the tire without any air already drops the spoke tension by a measurable amount. The more tire pressure is then added, the more the spoke tension drops.

As far as repeatability goes, the measurement does not seem great as the first and last measurement don’t match up super nicely. Overall, I’d still say that the general observed trends should be valid.

What I now take away from this are a few things:

  1. When it comes to taking any kind of measurement, the entire setup matters
  2. Tire pressure plays a significant role in decreasing spoke tensions. Between the average 14.75 (130kg) on the DS for the first reading without a tire and the 11.75 (~100kg) with 8 bar there’s about a 30 kg difference in spoke tension.
    This is not anything to worry about, just to note when comparing spoke tension over time.
  3. Even just having a tire mounted without any air decreases spoke tension by a measurable amount.

Thanks everyone for your super valuable input :+1: and helping me successfully chase this ghost :ghost:

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Okay, I think I get it now, you were thinking the tension drop was an issue and I/ we were thinking your wheel was falling apart :rofl:

FWIW I’d be happy with the numbers you have without a tyre to give you a stable wheel build, although I try for within 0.5 of a graduation variance in similar spokes, so you might look at that. Even tension is probably more important than absolute tension, within reason, Musson says.

Happy riding, building serviceable wheels isn’t sorcery as you are finding out!

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I’d suggest you need to be careful about accounting for the tension drop when the tyre is inflated.

If you increase the spoke tension to ensure it achieve the value you want when the tyre is inflated, what will happen is the tension will be higher when the tyre is deflated. This is OK for the NDS spokes which are at generally low tension, but you must be careful not to over-tension the DS spokes. Even when stationary, if they are tensioned too much, before inflating the tyre reduces their tension, they may destroy the rim.

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I mainly didn’t realize that the tension drop between no tire and tire with pressure would be so much. So I was also thinking that my wheels were falling apart, so we were all in the same boat here :sweat_smile:

I also didn’t find this information anywhere in the Musson book, the Sheldon Brown articles on wheel building and all the youtube videos that I watched. Maybe I somehow always managed to skip that part or maybe it should be obvious… in any case, now I know to stop worrying and love the tension drop :grinning_face_with_smiling_eyes:

Fair point, once I have an wheel truing stand (I built the wheels inside the bike frame) and the motivation, I might come back to that and equalize the spoke tensions further.

Thanks, it’s been a process for sure but happy that it’s looking all good now :slightly_smiling_face:

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For sure, at that point I hadn’t the spoke tension drops by this much (30 kg per spoke for me) between the bare wheel and when a tire is mounted and inflated. I thought the wheel was actively loosing tension by the spokes coming loose and so I was just thinking of ways to increase the spoke tension.

Now that I know that the tension drop is expected and totally fine, I can rest easy and will leave the wheels alone until there’s an actual issue.

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