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#21
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MA3 rim failure, where to now
In article ,
Simon Brooke wrote: Tim McNamara writes: But you raise an interesting point, as over the years a number of British bicyclists in this newsgroup have most vehemently and even rudely refused to tolerate the notion that the bicycle wheel stands on its lower spokes. I don't quite know why this notion seems to be so hard for some to grasp Across The Pond, especially when it's so simple and so demonstrable. Simple and demonstrable, yes. Stand the wheel on the spoke, and the nipple falls out, because the only thing that holds it in is tension. What you're saying is that _less_ tension somehow translates into load bearing. It does not and it can not, and, in particular, the design of spoke nipples would not allow them to take compression loads even if the design of the wheel allowed them to. The hub is supported by the tension in the top spokes, which hang from the rim. This, of course, tends to distort the rim out of true, and this distortion is resisted by tension in other spokes. The only spokes which are doing no work at all is the spokes which _can_ do no work - because they've gone (relatively) slack. To suggest that the wheel is standing on these spokes is a semantic perversion of the most peculiar nature. Erm, yet again- you might want to read the book. And learn something about the dynamics of pretensioned structures under load. |
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#22
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MA3 rim failure, where to now
Simon Brooke wrote:
What Simon has missed is that in a spoked wheel there is no compression in absolute terms. Just a reduction in tension provides the upward force on the hub. In other words, it's hanging from the top, not standing on the bottom. Yup, we all knew that already. We did? If it hangs from the top, then the tension in the top spoke would increase with load. But it doesn't; the tension in the lower spoke decreases. A simple test, plucking a few spokes, may help convince you. -- Ted Bennett Portland OR |
#23
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MA3 rim failure, where to now
Ian Smith writes:
On Tue, 09 Sep 2003 20:35:02 GMT, Simon Brooke wrote: Ted Bennett writes: What Simon has missed is that in a spoked wheel there is no compression in absolute terms. Just a reduction in tension provides the upward force on the hub. In other words, it's hanging from the top, not standing on the bottom. Yup, we all knew that already. So, the spokes that have the same stress state whether loaded or unloaded are doing all teh work, Yes, they are. Take a bicycle wheel. Cut all the spokes below the hub. Does the hub move? Apart from some accommodation to the now out of round rim, no. Turn it the other way up so that all the cut spokes are above the hub. Does the hub move? Yes, the lower nipples slide out of their holes and the whole structure slumps. Note that on a cart wheel exactly the opposite is true, because a cart wheel is a compression structure and the spokes act in column. and the spokes in which the stress changes dramatically are doing nothing at all. The change is when they _cease_ to do work, not when they _start_ to do work. Consider a tug of war. Two teams heave on a rope, and the hankerchief stays over the line, because each team is heaving equally hard. Now suppose the North team go off and get a beer. Their end of the rope goes slack, and the handkerchief moves. Is this because the North team are doing more work? That is your argument. -- (Simon Brooke) http://www.jasmine.org.uk/~simon/ ;; I'll have a proper rant later, when I get the time. |
#25
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MA3 rim failure, where to now
"Ian Smith" wrote in message
... So, the spokes that have the same stress state whether loaded or unloaded are doing all teh work, and the spokes in which the stress changes dramatically are doing nothing at all. As is readily understood by considering the limiting case where tension = 0 in the bottom-most spoke, quite right. -- Guy === WARNING: may contain traces of irony. Contents may settle after posting. http://www.chapmancentral.com |
#26
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MA3 rim failure, where to now
"Benjamin Weiner" wrote in message
... Simon Brooke wrote: This explanation would suggest that the deformation of the rim from a circle occurs at the top of the wheel. Which is why I refer people to the FEA, showing deformation at the bottom. Testing the spoke tension in a loaded wheel by plucking spokes also shows that it is the lower spokes whose tension changes most significantly. This is a really simple experiment which anyone interested in the issue should try. And considering the limiting case where the bottom spoke has zero tension, so could be removed without affecting the system in any way at all, readily reveals the absurdity of describing this behaviour as "standing" on the bottom spokes. That is not to dispute the physics or the mechanism by which the wheel bears load. But is is reasonable to suggest, as this argument effectively does, that one can stand on a piece of string provided it is part of a system which ensures that it still remains in tension while supporting your weight? I think not. -- Guy === WARNING: may contain traces of irony. Contents may settle after posting. http://www.chapmancentral.com |
#27
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MA3 rim failure, where to now
"Chris B." wrote in message
... Having reviewed a finite element analysis of a bicycle wheel under load, what happens is that the bottom of the rim deflects, reducing the tension in the bottom spokes, So the spokes are compressed. Provided you extend the meaning of the word compressed to mean "in reduced tension, but still requiring that there is a structure which maintains them in a state of not less than zero tension, otherwise the wheel will fail." Which is, to quote another poster, a semantic perversion. But to describe it as standing on the bottom spokes is like saying you can stand on a piece of string provided it's kept under tension by some other structure. I'll bet most people don't know or understand that either. Your money is safe. Most people think weight is measured in kilogrammes, after all. Apart from the Post Office, who think it's in Kelvin gramme seconds ;-) -- Guy === WARNING: may contain traces of irony. Contents may settle after posting. http://www.chapmancentral.com |
#28
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MA3 rim failure, where to now
"Tim McNamara" wrote in message
... FEA analysis, as you no doubt know already, shows that the spokes which change tension under a load applied to the axle- as is the case for a wheel in a frame- are only those spokes between the hub and the ground. Therefore it is reasonable to say that the wheel stands on those spokes It is not reasonable, because it fails to explain the observed fact that with the spokes on which the hub stands removed, the hub remains supported, whereas with the spokes whioch "do nothign" removed the hub will not be supported. It is therefore misleading. I have nothing against a simplistic description of a complex system, as long as it makes reasonable sense, but describing a hub as "standing" on spokes which patently cannot support a net compressive load does not provide any kind of illumination. It's also easily demonstrable to the average person. Pluck a few spokes at the top and bottom and sides; then put a load on the bike and pluck the same spokes again. Unless the wheel is so horribly undertensioned so that the bottom spokes go slack, the only spokes that will change in pitch are those at the bottom of the wheel. Now remove the bottom spokes and see what happens. People just love to overcomplicate this discussion. Inventing a Janet and John explanation which has to be discarded almost immediately any detail is introduced would seem to me to be needlessly confusing, since the fuller explanation isn't that hard to understand in the first place. -- Guy === WARNING: may contain traces of irony. Contents may settle after posting. http://www.chapmancentral.com |
#29
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MA3 rim failure, where to now
"Ian Smith" wrote in message
... to describe it as standing on the bottom spokes is like saying you can stand on a piece of string provided it's kept under tension by some other structure. Whereas your proposal is that the hub hangs from something below it? No, Ian, as you are aware I am from the "none of the above" school. The wheel works because all the spokes are in tension. If the rim were infinitely rigid there would be no deflection and the bototm spokes would have the same change in tension under load as the top spokes. If the side spokes were missing the rim would squash. If the bototm spokes were absent the wheel would stay up. If spokes which "do nothign" were taken out, the hub would crash to the ground. So the wheel can only work if all the spokes are in tension all the time. Once that is understood, the larger cyclic change in the bottom spokes due to rim deflection becomes an incidental though significant curiosity. That's like saying you hang from the seat you're sitting on, which is even greater nonsense. Except it's not what I said and it's not the sense of what I said either. Actually, what holds the wheel up is the shoulders of the nipples pressing on the rim. So there we have it: the wheel is held up by the shoulders of the spoke nipples pressing on the rim, and by the neck of the spoke pressing outwards on the hub. Can we all go home now? -- Guy === WARNING: may contain traces of irony. Contents may settle after posting. http://www.chapmancentral.com |
#30
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MA3 rim failure, where to now
Benjamin Weiner wrote:
This explanation would suggest that the deformation of the rim from a circle occurs at the top of the wheel. I think you (and many other people) are missing the fact that Simon (and Guy) are disputing the terminology only. No-one is claiming anything other than that there is a tension change in the bottom spokes. -- David Damerell flcl? |
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