Spoke Tension Drop with Tire Installation

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alanmclean
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by alanmclean

Hi, can you please explain why the spoke tension drops in the first place. Apologies if this is covered above but I can't find an explanation of the basic mechanism, or at least not in a simple enough way for me! I am thinking that the the rim is compressed by the air pressure and the axle to rim distance is reduced by a tiny amount, hence looser spokes. But I can't see how to compress alloy.

I am starting to use tubeless and have to use temporarily high pressures (>130psi) to pop the beads on. Will this increase the likelihood of wheels going out of true?

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Skylark
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by Skylark

That's precisely what's happening. Why can't you compress alloy? Sure you can. Materials are more elastic than is often given credit to.

The diameter of the rim is decreasing slightly when the tyre is inflated. Bummer :(

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whosatthewheel
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by whosatthewheel

alanmclean wrote:
I am starting to use tubeless and have to use temporarily high pressures (>130psi) to pop the beads on. Will this increase the likelihood of wheels going out of true?


That will not be a problem, spokes also are elastic structures. Not sure why you need 130 PSI to pop the tyre on the rim... I find 60-70 is generally plenty... the last loud click typically happens around that pressure

alanmclean
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by alanmclean

Thanks. I have two sets of road 'tubeless" wheels and regardless of whether I use tubes or go tubeless I have to go to >120psi before the beads pop in. (I have used soap at home but this is not an option on the road.) Otherwise I am left with flat spots where the bead is not fully seated. Any tips? I am almost ready to buy the big clunky Park tool to at least seat the beads when I am at home.

NovemberDave
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by NovemberDave

Try a thinner rim tape. DT and Schwalbe tapes (same tape, different packaging) are very thin, require only one layer for road pressure, and are slippery to help the tire slide out to seat. Another option is to use a thin (~8mm) wrap of tape just in the trough/channel, and then do a second wrap with a full width tape. Either should help your situation quite a bit.

jih
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by jih

Tubless tyres also typically have very tight beads once the beads are seated - perhaps this also compresses the rim some - it is a tight circle of kevlar afterall.

I'm finding that with a rear wheel that I have, with the DS built to the maximum tension of the hub (no tyre), when the tyre is on and up to pressure, the tension is very low on the NDS.

Should the maximum tensions be reached with or without a tyre?

I wonder if maximum tensions were given for tubed tyres with old-style lower air volumes. In which case, with modern wide tubeless tyres it might be correct to exceed them when no tyre is on - afterall, the wheel won't be meeting any forces from riding while no tyre is mounted.

alcatraz
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by alcatraz

How do you true the wheel radially when a tire is on? Just visually?

bm0p700f
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by bm0p700f

If the wheel is properly built the tension drop caused by the tyre should not be an issue. It's does not seem to harm the wheel.

As for tensions when building, experience counts. I go with that. For most tubeless wheels I tension the front to up to 1100N for rim brake and 1200n for disc brake wheels and the rear gets to 1300n Ds. Not every rim is stable at those tensions so not every rim is suitable for tubeless tyre use.

jih
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by jih

bm0p700f wrote:
Fri Apr 12, 2019 8:26 am
If the wheel is properly built the tension drop caused by the tyre should not be an issue.
In this context, “properly built” means with a high enough tension so that the tension is still high after the drop from the tubeless tyre?

In which case, are you measuring tension inflated or without a tyre?

bm0p700f
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by bm0p700f

I measure tension before tyre installation not after. Properly built means even tension to a very tight tollerance and fully stressed relieved. Done lots of tubeless wheels. It all I do and I never/very rarely get wheel going out of true. The insuffient NDS tension is a red herring. the wheels I rode on today have 400N tension with a tubeless tyre installed. I have test wheels which have done 20000km with that or less tension at the end. Also fitting a tubeless tyre can cause the tension to become uneven. Personally I would not even them out with the tyre fitted as different tyres will cause the tension variance to be different. Remove the tyre and the tension variance reduces again. It depends on how big the variance is though. That might indicate a fault in the rim. The test if does the wheel go out of true whwn the tyre is fitted. If the answer is yes sort it out. If no dont worry.

The next bit with the tyre fitted is the wheel stiff enough to stop NDS spokes going slack. If the answer is yes dont worry, if no build a stiffer wheel not raise the tensions.

That why all the tubeless ready wheels I build are very stiff. Spoke failure for me dont happen so I must be doing something right and I simply dont worry about the things you are. How stiff a wheel is is way more important for spoke life than what tension is used. The tension should ideally be 70% 80% of tension the rim can sustain before it goes wavy. For the Kinlin XR31T that 1275N before tyre fitting. Which is what I tension them too. I arrived at that by instinct/experience, then I found a phd that backup my instincts. Happy days.

jih
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by jih

This is good information, thank you.

The concern I had was that tune hubs have a fairly low maximum tension of 1100N, which gives a tension of about 650N on the NDS. When inflated, this decreases even further of course, to the point where I was wondering if spokes might go slack under hard efforts.

The tensions are quite even, within about 2% per side. I find in general that with carbon rims it is easier to build with even tensions since the rim should have been perfectly straight out of the mould, and not cut from a helixical reel and then closed.

I think in practice everything is actually fine, probably just paranoia.

NickJHP
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by NickJHP

I just built a pair of wheels for my gravel bike, and took the opportunity to measure the change in spoke tension between the built wheels with no tyre installed and the wheels with tyre installed and inflated:

DT Swiss XM421 700c rims, 28h front, 32h rear
Shimano HB-RS770 centerlock disc front hub
Shimano Deore XT FH-M8010 centerlock disc rear hub
Sapim Race spokes 2/1.8/2
DT Squorx aluminium nipples and rim washers (supplied as standard with the rims)

The tyres used were Schwalbe G-One 700x38 Allround tubeless inflated to 40psi front and 50psi rear, and I took the measurements using a Wheel Fanatyk tensiometer with digital readout (https://www.wheelfanatyk.com/store/digi ... ion-gauge/).

For the front wheel I tensioned the disc side spokes to 123Kgf by the tensiometer (0.3mm deflection), and found that after the tyre was mounted and inflated, the spoke tension had dropped to somewhere around 110Kgf (the deflection reading was about half way between that for 100Kgf and that for 123Kgf).

For the rear wheel I tensioned the drive side spokes somewhat higher, around 140Kgf, and found that the tension dropped to around 115-120Kgf with the tyre mounted and fitted. I also noticed, comparing the dish with and without tyre, that it increased by about 0.5mm toward the drive side with the tyre fitted.

bm0p700f
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by bm0p700f

So build the wheel off dish by one quarter turn of the nds spokes. That what I do and the tyre once fitted pulls the rim back to centre again. Works Ebay time with tubeless rims.

szazbo
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by szazbo

One thing that I have run into several times is that the tires are causing the rim to go out of true. I presume that there is a manufacturing issue with the tire. Most recently I removed a IRC Pro Light tire from an ENVE 5.6 to replace it with a 5000TL. Wheel was prefectly true with the IRC. Mounted up the 5000 TL 28 and there is significant lateral runout. Checked to see if the tire was properly seated, it was. The little line at the rim was consistent all the way around on both sides. Dismounted the tire and rotated it 180 degrees on the rim. Couldnt flip it as these were disc rims and the tire is directional. Inflated the tire again, and the same thing. Removed that tire and installed another 5000 TL and rim was true again. Took the "problem tire" and mounted it onto a HED + rim. Same result there. Significant lateral runout. This was not the 1st time that I experienced this. Previously had the same issue with some 4000's over the years. Initially blamed the wheel builder, then after playing with several tires discovered the issue. My guess is that the beads are not the same length on each side of the tire. Wierd

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