Strechy spokes or what?

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Strechy spokes or what?
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Posted: Feb 23, 2020 at 3:28 Quote
In december built up a new rear wheel with a Stans Neo hub, 2mm Sapim Leader spokes, DT Swiss Pro Lock brass nipples and a DT Swiss FR560 rim. The spoke tension was very equal and the wheel was true to about 0,3mm thanks to a truing stand with dial indicators and a spoke tension gauge. I stressed the wheel multiple times during truing to set the nipples into place, so it wouldnt shift when i start using it. The spokes on the steeper drive side were tensioned to just under the permitted maximum of 1200N.
Since then ive gotten about 400km on the new wheel and just the other day I noticed how ridiciolusly low the spoke tension is. Its so low that I dont even understand how the rim can still be in one piece since there have been some significant hucks in the last few weeks. I dont have a tension gauge available right now, but i would guess it to be around 300 to 400N.

Any ideas why that would happen?

Posted: Feb 28, 2020 at 16:09 Quote
Your nipples bed in and you need to retention your wheel it happens with all new builds

Posted: Mar 3, 2020 at 4:54 Quote
cyberoptixs wrote:
Your nipples bed in and you need to retention your wheel it happens with all new builds

Yes, but not that bad. When you buy an entire new wheel that doesnt happen.

Posted: Mar 3, 2020 at 5:18 Quote
Definitely, absolutely NOT stretchy spokes. That doesn't happen.

Possible causes:

1. Something wasn't fully seated. Sounds like you addressed this, so it's probably #2.
2. The nipples are rotating and unthreading.

Did you lubricate the spoke threads? Oil or grease helps during the build, but increases the likelihood of detensioning. I've built some wheels with and some without. Some, but not all, wheels built with greased threads needed retensioning once or twice, but are easier to adjust many years later.

Solutions:

1. Higher tension.
2. Threadlocker (ex. Loctite).
3. Nipples with a more aggressive friction feature, such as Pillar Taper Grip.

Posted: Mar 3, 2020 at 8:33 Quote
I didnt use oil and the nipples have thread locker in them. The spokes were also tensioned to their max.
I suspect the nipples rotating, even though this should be mittigated by the thread locker. The wheel has taken a few good impacts, but I rule plastic deformation of the spokes out, since i am only about 78kg geared up.
Another guy told me thinner spokes are the way to go since they have more elastic deformation under the same tension, but i have my doubts. 2mm spokes are not insanely thick and rather common, so they should work.

Posted: Mar 3, 2020 at 9:51 Quote
The only possibilities are that components seated into place, there was plastic deformation of the metals, or the nipples backed off. It's not the former two, so it's the latter. Being frustrated because it shouldn't have happened isn't going to stop it from happening again.

"Maximum tension" is not really the maximum. When you ride, your spokes experience a lot more tension than the static load. Rim manufacturers make some assumptions about the peak loads and choose a static load accordingly. They've probably chosen a reasonable number for the static maximum; I just want to illustrate that the static maximum is far from the failure load.

Your friend is correct that thin spokes are stretched a greater distance at the same tension and are less likely to fully detention for a given load. Spokes behave like springs: thin spokes are soft springs and thick spokes, like yours, are stiff springs. Thick spokes should still be able to hold tension, though, so maybe start by removing the spokes from the nipples and reassembling with threadlocker.

Posted: Mar 3, 2020 at 10:19 Quote
I know about the implications of spoke diameter and how it changes the behavior, but and heard the no full deload before, but that doesnt make sense. When you have two opposing spokes tensioned with lets say 1000N each. If you now load the hub with 1000N in the direction of one of the, spokes, one will be fully deloaded, no matter the diameter, since elongation is proportional to the cross section.

Anyway, I just retensioned the wheel half an hour ago, lets see how well it holds up this time.

Posted: Mar 3, 2020 at 10:25 Quote
That's not correct because forces do not act on one spoke in isolation.

Imagine if the the spokes were extremely elastic, were extremely short prior to installation, and could stretch more than the radius of the wheel. The rim could bump all the way up to the hub without any spokes becoming fully detentioned.

Obviously spokes are not this elastic, but it illustrates the effect of stretch in the spokes and it shows that a load greater than the static tension does not necessarily cause any spokes to fully lose tension.

Posted: Mar 3, 2020 at 10:32 Quote
Yes, I know what you mean, but with thinner spokes you will get bigger deflection of the hub in the rim, therefore more movement. The movement is directly proportional to the spoke cross section.
Ergo: The hub wont move as much with thick er spokes.

Posted: Mar 3, 2020 at 10:54 Quote
Yes, but the properties aren't linear. As the wheel deforms, the load quickly begins to distribute to neighboring spokes.

Another thought experiment:

When the wheel is in its static, undisplaced condition, some spokes are pulling and some are pushing. When a small load is applied, some spokes begin to pull a little more in the direction opposite the applied force and some spokes are still pulling in the direction of the force - i.e. not all spokes are "fighting against the bump force" and the wheel is still "fighting against itself".

Now imagine infinitely elastic spokes. Imagine an impact that displaces the hub beyond the rim (impossible, obviously, but bear with me). As the hub continues to move away from the rim, eventually all spokes are pulling essentially straight opposite to the force - i.e. perfect alignment and perfect recruitment of all spokes. Obviously this won't happen to such an extent, but it shows how the more the wheel deforms, the more spokes are engaged in resisting the force and the better the alignment of these spokes. The behaviour is more complex than the model you have in your mind.

For a given spoke tension, a wheel built with stiff spokes is more likely to fully lose tension in one or more spokes.

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