Spoke Tension

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Spoke Tension

Postby Dial » Tue Oct 14, 2008 10:10 pm

If I weigh 100kl shouldn’t the tension on my spokes be 100kl, if not why not?
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by BNA » Tue Oct 14, 2008 10:13 pm

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Postby Mulger bill » Tue Oct 14, 2008 10:13 pm

I'm no engineer, but isn't the load shared between more than one spoke?

FWIW, your bike actually hangs from the rims, it doesn't sit on them.

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Postby Dial » Tue Oct 14, 2008 10:25 pm

Your right, it does hang and the weight is shared so it wouldn’t be 100kl/spoke, it would be an equation depending on the number of spokes
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Postby Kalgrm » Tue Oct 14, 2008 10:35 pm

I doubt you weigh as much as 100 kilolitres. 100 tonnes is pretty heavy, dude!

I'd believe 100kg though .... ;)

I don't believe spoke tension is calculated by the rider's weight.

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Postby Dial » Tue Oct 14, 2008 10:41 pm

Geez I hate the metric system, it's just to easy to calculate

Thanks Kilogram
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Postby RobS » Tue Oct 14, 2008 11:35 pm

Spoke tension has more to do with the rim. The recommended spoke tension isn't always given through! While the load hangs from the top spokes the tension is high enough that the lower spokes never become slack.

I found this some time back. The static spoke tension is superimposed on top of the the values shown in the figures:
http://www.astounding.org.uk/ian/wheel/
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Postby AlanAtom » Wed Oct 15, 2008 8:44 am

Gee, interesting read!! From the analysis, it implies that the heavier you are, the more spoke tension should be put on otherwise, the load at the bottom spokes will overcome the spoke pre-load and buckle. Or use stiffer rims to reduce load propagation to the spokes.
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Postby RobS » Wed Oct 15, 2008 1:11 pm

> From the analysis, it implies that the heavier you are, the more spoke tension

For a scaling of load point of view you would think so, however,...

> the load at the bottom spokes will overcome the spoke pre-load and buckle.

The analysis is for 1000N (~100kg) load on *one* 36 spoke wheel and the tension only drops by 350N or so in comparison to the 1000N or so spoke tension. If you took say a 20 spoke wheel the load per spoke load increases by a factor of 1.8, but, if you also take into account two wheels at a worst case R:65%/F:35% distribution that factor drops to 1.2. More revealing is the largest increase in spoke tension is only 40N. I suppose that's what I've learned from the analysis - there is good distribution of load in the load carrying spokes. So even if some spokes did go loose the stress in the others does not increase much. You might get this when you hit a bump for example. It would be interesting to see how things change with you tilt the bike over on a corner.

The spoke tension appears to be more a function of limiting stress an fatigue at the nipple holes of the rim. That's one reason why the rim is involved. A good dump of recommended spoke tensions can be found at:

http://www.cyclingnews.com/tech/fix/?id=tm_1

I've read people talking about hub stresses but I don't know how valid they are.

From the spoke's point of view I think they should handle 2000 to 3000N depending on details. So for the most part the spoke tension idles at a good safety factor below the maximum.
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Postby wintal » Wed Oct 15, 2008 1:58 pm

RobS wrote:Spoke tension has more to do with the rim. The recommended spoke tension isn't always given through! While the load hangs from the top spokes the tension is high enough that the lower spokes never become slack.

I found this some time back. The static spoke tension is superimposed on top of the the values shown in the figures:
http://www.astounding.org.uk/ian/wheel/


The link you posted disagrees with your comment that load hangs from the top spokes. His analysis looks fairly convincing to me, and it argues that the load stands on the bottom spokes.

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Postby Kalgrm » Wed Oct 15, 2008 2:16 pm

There are some wheels on the market with kevlar spokes (I'll go searching in a minute). Essentially, they use string to construct the wheel. If the spokes at the bottom of the wheel applied compressive forces to hold the hub up, these wheels would simply fail.

The analysis linked to above needs to be redone with radial forces modelled, not the force in the x direction only.

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Postby wintal » Wed Oct 15, 2008 2:26 pm

Kalgrm wrote:There are some wheels on the market with kevlar spokes (I'll go searching in a minute). Essentially, they use string to construct the wheel. If the spokes at the bottom of the wheel applied compressive forces to hold the hub up, these wheels would simply fail.

The analysis linked to above needs to be redone with radial forces modelled, not the force in the x direction only.

Cheers,
Graeme


Hmm, I don't think I buy that argument... all the spokes remain in tension at all times in his modelling, so it wouldn't preclude kevlar spokes working. What he shows is that after applying the compressive forces, all spokes are still under a tension.

I agree that there may be radial forces at play though, so the analysis might be flawed. Might try and dig up some more references.

Cheers,
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Postby RobS » Wed Oct 15, 2008 2:26 pm

>and it argues that the load stands on the bottom spokes.

The analysis values in the article have no spoke tension, the spokes are just rods. The article shows a -350N at the bottom, the negative value implies a compressed spoke, however it is not really compressed. What you have to do is add the spoke tension to the values. Add say 1000N to the -350N and you get +650N. So in a real wheel the spoke tension will drop from 1000N in the unloaded state to 650N when loaded, but it is still in tension. You have to get the summed value below zero before the spoke actually goes loose which is a very high load. The actual spoke tension cannot go negative, as the nipples are free to poke into the tube when pushed. Does that make sense?
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Postby wintal » Wed Oct 15, 2008 2:39 pm

RobS wrote:>and it argues that the load stands on the bottom spokes.

The analysis values in the article have no spoke tension, the spokes are just rods. The article shows a -350N at the bottom, the negative value implies a compressed spoke, however it is not really compressed. What you have to do is add the spoke tension to the values. Add say 1000N to the -350N and you get +650N. So in a real wheel the spoke tension will drop from 1000N in the unloaded state to 650N when loaded, but it is still in tension. You have to get the summed value below zero before the spoke actually goes loose which is a very high load. The actual spoke tension cannot go negative, as the nipples are free to poke into the tube when pushed. Does that make sense?


Yep, that was my understanding. I think it's a semantic argument - to me this still implies that the wheel is standing on the bottom spokes. The fact that the default state is for everything to be in tension means that a reduction of tension on the bottom spokes means it's standing on those spokes.

But, if it's less contentious, I could say that the spokes providing the support are the bottom spokes. They provide support by untensioning. The top spokes are in basically the same state as they would be if the wheel was unloaded, hence are not providing any support (though they are vital to the wheel existing at all).

I can't claim this argument, I adapted it from here: http://hea-www.harvard.edu/~fine/opinio ... wheel.html
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Postby Kalgrm » Wed Oct 15, 2008 2:51 pm

Here's a proper analysis of spoke strain. Figure 11 shows that spoke never holds up the load, but simply loses some of it's tension. Unless that tension becomes compression, it is not "holding up" the load.

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Postby Kalgrm » Wed Oct 15, 2008 2:58 pm

wintal wrote:
But, if it's less contentious, I could say that the spokes providing the support are the bottom spokes. They provide support by untensioning. The top spokes are in basically the same state as they would be if the wheel was unloaded, hence are not providing any support (though they are vital to the wheel existing at all).

I can't claim this argument, I adapted it from here: http://hea-www.harvard.edu/~fine/opinio ... wheel.html
Cheers,
Leighton...

Breaking it down to its simplest case, if you tie two pieces of cotton to a hub and tie them to a rim so that one piece of cotton holds the hub to the top and one to the bottom of the rim (ie you now have a "wheel" with only two "spokes" in tension), what happens to the hub if you cut the top thread?

I'm betting the hub will fall down because the "spoke" at the bottom does not support the hub.

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Postby wintal » Wed Oct 15, 2008 3:29 pm

Kalgrm wrote:
wintal wrote:
But, if it's less contentious, I could say that the spokes providing the support are the bottom spokes. They provide support by untensioning. The top spokes are in basically the same state as they would be if the wheel was unloaded, hence are not providing any support (though they are vital to the wheel existing at all).

I can't claim this argument, I adapted it from here: http://hea-www.harvard.edu/~fine/opinio ... wheel.html
Cheers,
Leighton...

Breaking it down to its simplest case, if you tie two pieces of cotton to a hub and tie them to a rim so that one piece of cotton holds the hub to the top and one to the bottom of the rim (ie you now have a "wheel" with only two "spokes" in tension), what happens to the hub if you cut the top thread?

I'm betting the hub will fall down because the "spoke" at the bottom does not support the hub.

Cheers,
Graeme


Hmm, after thinking about it a bit, I'm agreeing with your initial comment that it needs better force modelling. The ones at the bottom are the ones that change the most (which is what I was working on really), but I suspect they're not the most important ones.

What it really needs is an analysis of the tension in parts of the rim I think, because the force will pull down through the spokes in the top half of the rim (to differing amounts, based on the angles) and that force will go through the rim until it gets to the bottom half of the wheel and it gets to the opposing force of the ground and gets transmitted back up (via slackening on the bottom spoke tensions).

Obviously, from the analysis done, the pre-tensioning of the spokes has the effect that the tensioning loads on the spokes are very spread out, whereas the compressive load is quite localised.

And because I like lists, there's some interesting outcomes from the analysis:
    Spokes at the bottom go through the largest change in load, an undertensioned spoke will fail at the bottom
    Overtensioned spokes will fail at the top.
    There's a lot more scope for undertensioned spokes to fail than overtensioned spokes


I guess I agree with the comment that wheels hang from the top now, though I feel it's a bit misleading, since _all_ spokes are really required at all times (saying it stands at the bottom is probably at least as misleading though).

Cheers,
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Postby Kalgrm » Wed Oct 15, 2008 3:53 pm

Yep, all the spokes are needed because the wheel is not a static thing. In my "thought experiment" above (wheel with only two spokes), the wheel would not roll because as soon as the top spoke is not vertical, the hub flops around. There is no "next spoke" to take up the tension.

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Postby RobS » Wed Oct 15, 2008 6:15 pm

> The top spokes are in basically the same state as they would be if the wheel was unloaded,

I just see it as a redistribution of force. If you use the idea of a vertical taught string across the diameter of if you then pulled down on the center the top half of the string would be tighter and the lower half looser. My intuition has trouble accepting the fact the bottom is providing any support (since it is a string).

One thing about the link I posted is the bottom spokes are more affected. This might be throwing you off and makes you think the other spokes are unaffected. If the rim was perfectly rigid I suspect the force distribution would be symetrical in that the force increase of the topmost most would equal the force decrease of the bottom most. In the article the rim modelled as a non rigid object as a result the contact point deforms and pushed in. The spokes being compliant (ie. springy, set by the Young's Modulus of the spoke) will lose tension quicker. The rest of the rim is not deformed as much and tends to have a more uniform distribution of forces. Another thing which supports this argument is the spokes around the 5 and 7 O clock positions have *increased* tension this would be expected of the rim is bulging out sideways at those points thus tugging on the spoke.
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Postby Kid_Carbine » Thu Oct 16, 2008 12:54 am

In the end, what it all means, in a practical sense is that a tight wheel is a strong wheel & that spokes with a degree of elasticity will share the load better than spokes that don't.

Isn't it interesting to note that wheel builders in the 1920's & '30's were building very strong yet light wheels with very thin double butted zinc plated steel spokes, yet we have several reports in recent times right here in this forum of near brand new bikes with Hi-Tech wheels built with razzle dazzle rims & not-very-elastic, & easily-work-hardened, low fatigue resistant stainless spokes being returned to the LBS after only a very short time with broken spokes.
What's even funnier is that the skilled bike shop operators want to sell the customer a new wheel instead of rectifying the actual cause of the problem in the first place.

That's progress I guess.
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Postby Dial » Mon Oct 20, 2008 1:05 pm

More interesting than I thought. Got me thinking.

Anyone who's broken a rear wheel spoke will know its not that easy to replace while on the road. Maybe someone could market a flexible wire spoke which you could roll up in your kit bag. Unroll when required and literally thread through the hub and screw into the old spoke nipple.
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Postby Kalgrm » Mon Oct 20, 2008 1:33 pm

What, like this kevlar one? :)

The reviews from 10 years ago say it's a great idea.

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Postby Dial » Tue Oct 21, 2008 8:46 am

I was thinking steel cable. That doesn't look as good as I was envisaging but it would work better.
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Postby sogood » Tue Oct 21, 2008 8:54 am

Kid_Carbine wrote:Isn't it interesting to note that wheel builders in the 1920's & '30's were building very strong yet light wheels with very thin double butted zinc plated steel spokes.

What was the definition of "light" back then?
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Postby fixie » Wed Oct 29, 2008 2:06 pm

Well the spokes were usually 15/17 guage double butted which equates to 1.8/1.5 mm dia.

I will talk about rims tomorrow. :D
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Postby Kid_Carbine » Mon Nov 03, 2008 10:52 am

sogood wrote:
Kid_Carbine wrote:Isn't it interesting to note that wheel builders in the 1920's & '30's were building very strong yet light wheels with very thin double butted zinc plated steel spokes.

What was the definition of "light" back then?


Well strictly speaking, & to answer the question that you actually asked, the 'definition' of "light" back then would be the same as today, "Of little weight, not heavy. Of little weight in proportion to bulk or of low specific gravity. Of little density" [source Websters dictionary"]

Everything is relative but by the mid to late 30's they were a lot lighter, both the wheels wheels & in fact the whole bike, than many people give credit for.
In 1939 Malvern Star advertised a fixed wheel bicycle that you could actually buy that came in at 14lbs & 14ozs. [6,407.1 grammes] ready to ride. I don't have an individual wheel weight.

After WW2 with the benefit of the wartime development of aluminium based alloys these numbers were able to be reduced even further, particularly when the rest of the world coppied VEW's 1930's idea & made one piece alloy hubs.

I have one wood rim from the early 30's that is lighter than any of the alloy rims in my collection, [up to mid 80's] but sadly I don't have the mate to it or it would be built into a wheel & fitted into a bike at the speed of light.
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