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Stress-relief demonstration suggestions?
Hmmm . . . does squeezing stainless steel spokes render them
immortal? The idea is that when spokes are bent to form the elbow, potentially fatal stresses are formed at the bend, stresses that can be relieved by giving the tensioned spoke a good squeeze. Unfortunately, theory and data are controversial. A test or demonstration would be nice, but testing spokes is difficult, since even unsqueezed spokes seem to last for millions of wheel revolutions. (Ten thousand miles is about 53 million inches, which in turn is about two-thirds of a million spins of a 700c wheel--and 500 hours at 20 mph. Setting up a test for a hundred plain and a hundred squeezed spokes would be daunting.) Can anyone suggest a practical test that I can use to convince a pack of skeptical high-school physics students, one way or the other? I think that I have the raw material, some widely available pieces of stainless steel wire that come with three smooth u-bends from the factory. It's so cheap that I should be ashamed to steal them in boxes of a hundred, but the students need to learn the basics of scrounging. Any ideas about how to demonstrate over-tensioning stress-relief using paper-clips? Carl Fogel |
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On Fri, 31 Dec 2004 15:57:52 +1100, Bruce Graham
wrote: In article , says... Any ideas about how to demonstrate over-tensioning stress-relief using paper-clips? Your first long jump is to assume that paper clips are made of the same stuff DT et al. use. (not that I would know). Dear Bruce, Once the thought struck me, my first check was to google--paper clips are made of stainless steel. Some months ago, I mentioned that Wheelsmith says that they use 304 stainless steel, while DT claims to use 18-8 stainless steel--pretty much the same stuff, expressed differently: http://groups.google.co.uk/groups?q=...4ax.com&rnum=2 or http://tinyurl.com/4yr6b In any case, I don't think that anyone has claimed that bicycle spokes are made of an unusual kind of stainless steel with unusual stress or fatigue characteristics. Certainly no one has argued that spoke squeezing works only on one brand of spoke. If paper clips are reasonably similar to spokes in material and manufacturing, then their advantage (apart from being free) is that they're available in a wide range of much thinner wires, which presumably would fatigue more easily and quickly than spokes. The idea is a test that would eliminate the question of whether squeezing is seating things (as opposed to relieving stress) and that would be quick enough to run repeatedly. The only spoke tests that I know of were performed about 20 years ago, were cut short due to time problems, and involved no attempts at stress relief. (The fellow who has the spoke test data offered to send it to me, but then never got around to it and hasn't replied to my second email--of course, prettier girls have stood me up with less excuse, so I'm not heartbroken.) Carl Fogel |
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On Fri, 31 Dec 2004 16:33:32 +1100, Bruce Graham
wrote: In article , says... Hmmm . . . does squeezing stainless steel spokes render them immortal? The idea is that when spokes are bent to form the elbow, potentially fatal stresses are formed at the bend, stresses that can be relieved by giving the tensioned spoke a good squeeze. Unfortunately, theory and data are controversial. A test or demonstration would be nice, but testing spokes is difficult, since even unsqueezed spokes seem to last for millions of wheel revolutions. (Ten thousand miles is about 53 million inches, which in turn is about two-thirds of a million spins of a 700c wheel--and 500 hours at 20 mph. Setting up a test for a hundred plain and a hundred squeezed spokes would be daunting.) Can anyone suggest a practical test that I can use to convince a pack of skeptical high-school physics students, one way or the other? I think that I have the raw material, some widely available pieces of stainless steel wire that come with three smooth u-bends from the factory. It's so cheap that I should be ashamed to steal them in boxes of a hundred, but the students need to learn the basics of scrounging. Any ideas about how to demonstrate over-tensioning stress-relief using paper-clips? Carl Fogel Thinking some more about this... Most of my fatigued spokes happened in a batch, 11 failures ocurred on two bikes on a 2500 Km tour in France (3 on my wife's bike and 8 on mine - all drive side rear and all failed around the spoke bend near the head). The spokes were not DT, Sapim or Wheelsmith and had been stress relieved by me. It seemed to me that, compared to a DT spoke, the failed spokes had a sharper transition to the head, maybe causing an extra stress concentration. I am certain of a few things, - the french word for spoke is "rayon" and they are not shaped much like a paper clip. BTW I replaced all the drive side spokes with DT on returning home, and around 12000 km, the non-drive side no-names started to fail, so I replaced them too. Dear Bruce, You mention some of the points of interest. The spoke angle and the shape of the head flare may have changed over the years, possibly for the better. The materials used and the manufacturing process may have done the same thing Even tiny improvements can mount up to much better spokes over several decades. And then there's the widespread use of double-butted spokes, which may reduce spoke failure. In general, those who believe in spoke squeezing make no distinction between brands of spokes or whether the spokes are from 1984 or 2004--the stress relief is what matters, and it renders spokes immortal. Those who are skeptical point to various improvements in what at first seems like brick-simple technology and suggest that the squeezing affects how the spoke heads fit in the hub holes. I waver back and forth, but I'm certain of one thing--the spokes don't care what either side thinks. Carl Fogel |
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Carl Fogel writes:
Hmmm... does squeezing stainless steel spokes render them immortal? Maybe you should call it "stretching spokes" rather than squeezing, since stretch is what is being done. This can be accomplished through various manual and mechanical means. TREK has a method by which the wheel is stress relieved in two operations, one side at a time. The idea is that when spokes are bent to form the elbow, potentially fatal stresses are formed at the bend, stresses that can be relieved by giving the tensioned spoke a good squeeze. Unfortunately, theory and data are controversial. Not at all. As I have explained in detail (something you could find with your skills in web searching) bending a steel wire always involves partial spring-back, which in itself proves there are residual stresses. If it were not the case, the wire would either return to its original alignment or remain in the position into which it was bent. The partial spring-back results from not all depths of the cross section having been equally deformed, the central "fiber" not having changed length while the parts on the outside of the bend were permanently stretched and those on the inside, compressed. In between these extremes various amounts of plastic length change occurred. I think that if you review this scenario and observe that there is partial spring-back, that there must be residual stress. A test or demonstration would be nice, but testing spokes is difficult, since even unsqueezed spokes seem to last for millions of wheel revolutions. Get a coat hanger and start bending. (Ten thousand miles is about 53 million inches, which in turn is about two-thirds of a million spins of a 700c wheel--and 500 hours at 20 mph. Setting up a test for a hundred plain and a hundred squeezed spokes would be daunting.) Forget about the miles for a moment and look at the material. As I mentioned, placing a wire that purposely has been made wavy, in a tensile testing machine that can tension the wire to its yield stress, (the stress where the stress strain curve begins to flatten out) will give a perfectly straight wire when released. This wire has no stresses or it would spring to some other shape. It has no reason to take a shape other than straight because all its parts were stretched to yield to have their collective "memories" erased. This is possible with ductile spoke wire because it is made to undergo forming. Can anyone suggest a practical test that I can use to convince a pack of skeptical high-school physics students, one way or the other? If you have a spoke in a tensioned wheel that was bent into its in-situ shape by bending its elbow form its original obtuse angle to an acute angle by tensioning, then it will have its outer "fibers" of the elbow at yield stress and it will remain there because the spoke is additionally tensioned. By stretching that spoke to nearly twice its static tension by "stress relieving" over-tension, the outside of the elbow must yield and when released fall back to a lower stress. That example is of the elbow, but it is true of the threads as well. The elbow, however, is more obvious because outbound spokes all get plastically deformed in the first tensioning of the wheel regardless of their prior state. I think that I have the raw material, some widely available pieces of stainless steel wire that come with three smooth u-bends from the factory. It's so cheap that I should be ashamed to steal them in boxes of a hundred, but the students need to learn the basics of scrounging. Being practical about this, I think you will see that you can do this without a materials laboratory. Any ideas about how to demonstrate over-tensioning stress-relief using paper-clips? I like clothes hangers better because they are large enough to clearly see the results. http://www.sheldonbrown.com/brandt/s...relieving.html Jobst Brandt |
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In article ,
says... Hmmm . . . does squeezing stainless steel spokes render them immortal? The idea is that when spokes are bent to form the elbow, potentially fatal stresses are formed at the bend, stresses that can be relieved by giving the tensioned spoke a good squeeze. -snip- Any ideas about how to demonstrate over-tensioning stress-relief using paper-clips? On Fri, 31 Dec 2004 16:33:32 +1100, Bruce Graham wrote: Thinking some more about this... Most of my fatigued spokes happened in a batch, 11 failures ocurred on two bikes on a 2500 Km tour in France (3 on my wife's bike and 8 on mine - all drive side rear and all failed around the spoke bend near the head). The spokes were not DT, Sapim or Wheelsmith and had been stress relieved by me. It seemed to me that, compared to a DT spoke, the failed spokes had a sharper transition to the head, maybe causing an extra stress concentration. I am certain of a few things, - the french word for spoke is "rayon" and they are not shaped much like a paper clip. BTW I replaced all the drive side spokes with DT on returning home, and around 12000 km, the non-drive side no-names started to fail, so I replaced them too. In article , says... You mention some of the points of interest. The spoke angle and the shape of the head flare may have changed over the years, possibly for the better. The materials used and the manufacturing process may have done the same thing -snip- And then there's the widespread use of double-butted spokes, which may reduce spoke failure. In general, those who believe in spoke squeezing make no distinction between brands of spokes or whether the spokes are from 1984 or 2004--the stress relief is what matters, and it renders spokes immortal. Those who are skeptical point to various improvements in what at first seems like brick-simple technology and suggest that the squeezing affects how the spoke heads fit in the hub holes. I waver back and forth, but I'm certain of one thing--the spokes don't care what either side thinks. The shape and material of the flange enters into this too. Simple pierced steel flanges are just hell for spoke breakage, Phil Wood's flanges being the accepted ideal shape. Perhaps stressing the spokes as we do has a compound effect rather than one single mechanism? Relieving stress from forming, shaping the head/curve to better fit the flange and also deepening the groove in the flange (resulting in a broad contact area between flange and spoke rather than asimple point contact as a steel hub). It may well be that these effects are insignificantly small _until_ the momentary high loads we put on the spokes. -- Andrew Muzi www.yellowjersey.org Open every day since 1 April, 1971 |
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On Fri, 31 Dec 2004 00:10:10 -0600, A Muzi
wrote: In article , says... Hmmm . . . does squeezing stainless steel spokes render them immortal? The idea is that when spokes are bent to form the elbow, potentially fatal stresses are formed at the bend, stresses that can be relieved by giving the tensioned spoke a good squeeze. -snip- Any ideas about how to demonstrate over-tensioning stress-relief using paper-clips? On Fri, 31 Dec 2004 16:33:32 +1100, Bruce Graham wrote: Thinking some more about this... Most of my fatigued spokes happened in a batch, 11 failures ocurred on two bikes on a 2500 Km tour in France (3 on my wife's bike and 8 on mine - all drive side rear and all failed around the spoke bend near the head). The spokes were not DT, Sapim or Wheelsmith and had been stress relieved by me. It seemed to me that, compared to a DT spoke, the failed spokes had a sharper transition to the head, maybe causing an extra stress concentration. I am certain of a few things, - the french word for spoke is "rayon" and they are not shaped much like a paper clip. BTW I replaced all the drive side spokes with DT on returning home, and around 12000 km, the non-drive side no-names started to fail, so I replaced them too. In article , says... You mention some of the points of interest. The spoke angle and the shape of the head flare may have changed over the years, possibly for the better. The materials used and the manufacturing process may have done the same thing -snip- And then there's the widespread use of double-butted spokes, which may reduce spoke failure. In general, those who believe in spoke squeezing make no distinction between brands of spokes or whether the spokes are from 1984 or 2004--the stress relief is what matters, and it renders spokes immortal. Those who are skeptical point to various improvements in what at first seems like brick-simple technology and suggest that the squeezing affects how the spoke heads fit in the hub holes. I waver back and forth, but I'm certain of one thing--the spokes don't care what either side thinks. The shape and material of the flange enters into this too. Simple pierced steel flanges are just hell for spoke breakage, Phil Wood's flanges being the accepted ideal shape. Perhaps stressing the spokes as we do has a compound effect rather than one single mechanism? Relieving stress from forming, shaping the head/curve to better fit the flange and also deepening the groove in the flange (resulting in a broad contact area between flange and spoke rather than asimple point contact as a steel hub). It may well be that these effects are insignificantly small _until_ the momentary high loads we put on the spokes. Dear Andrew, Nicely put--not just your point about the hub holes, but point that everything could contribute. I take it that your experience with crude flanges versus better shaped holes (possibly in softer material) suggests that stress relief alone can't (and shouldn't be expected to) overcome a hard, badly shaped flange. Have these simple pierced steel flanges disappeared? Or are they steel (sorry, couldn't resist it) found on-- Well, on heavy-set bicycles widely available for under $60? (Mine is sleeping and I don't want to wake it up by tickling it with a magnet.) Carl Fogel |
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On Thu, 30 Dec 2004 23:23:05 -0800, jim beam
wrote: wrote: Carl Fogel writes: Hmmm... does squeezing stainless steel spokes render them immortal? Maybe you should call it "stretching spokes" rather than squeezing, since stretch is what is being done. This can be accomplished through various manual and mechanical means. TREK has a method by which the wheel is stress relieved in two operations, one side at a time. The idea is that when spokes are bent to form the elbow, potentially fatal stresses are formed at the bend, stresses that can be relieved by giving the tensioned spoke a good squeeze. Unfortunately, theory and data are controversial. translation: "i have no proof & have not attempted to quantify". Not at all. As I have explained in detail (something you could find with your skills in web searching) bending a steel wire always involves partial spring-back, which in itself proves there are residual stresses. absolutely fundamentally not. If it were not the case, the wire would either return to its original alignment or remain in the position into which it was bent. jobst, please please please get it into your head that spring-back is because of the shape of the deformation graph. residual stress has nothing to do with it. bending has nothing to do with it - you get spring-back in linear tension samples too - and the reason, as explained before, is that you only deform material once you're above the hookes law part of the graph. but deforming enough for yield does not magically allow the material to yield to zero and bypass hookes law on the way. is there any way to explain this to you more simply? as long as you labor under this fundamental misconception, you're always going to keep shooting wide of the mark. The partial spring-back results from not all depths of the cross section having been equally deformed, the central "fiber" not having changed length while the parts on the outside of the bend were permanently stretched and those on the inside, compressed. In between these extremes various amounts of plastic length change occurred. you do get differing degrees of deformation, but they are not wholly responsible for spring-back. see above. I think that if you review this scenario and observe that there is partial spring-back, that there must be residual stress. no, no, no. that's an absolutely fundamental misconception. A test or demonstration would be nice, but testing spokes is difficult, since even unsqueezed spokes seem to last for millions of wheel revolutions. Get a coat hanger and start bending. you're never going to "see" residual stress just by bending anything. (Ten thousand miles is about 53 million inches, which in turn is about two-thirds of a million spins of a 700c wheel--and 500 hours at 20 mph. Setting up a test for a hundred plain and a hundred squeezed spokes would be daunting.) Forget about the miles for a moment and look at the material. As I mentioned, placing a wire that purposely has been made wavy, in a tensile testing machine that can tension the wire to its yield stress, (the stress where the stress strain curve begins to flatten out) will give a perfectly straight wire when released. This wire has no stresses or it would spring to some other shape. rubbish. that wire can have residual stress much as any other. the questions is the stress orientation & whether it's significant relative to its application. all you're describing is a material that has yielded. there's no way you'll ever get a handle on magnitude or relevance with this kind of hand waving. It has no reason to take a shape other than straight because all its parts were stretched to yield to have their collective "memories" erased. This is possible with ductile spoke wire because it is made to undergo forming. fundamental lack of understanding of deformation theory. the effects of deformation are cumulative in stainless wire. the entire work history of the material is easily visible under any metallurgical microscope. you're not erasing anything. Can anyone suggest a practical test that I can use to convince a pack of skeptical high-school physics students, one way or the other? If you have a spoke in a tensioned wheel that was bent into its in-situ shape by bending its elbow form its original obtuse angle to an acute angle by tensioning, then it will have its outer "fibers" of the elbow at yield stress and it will remain there because the spoke is additionally tensioned. By stretching that spoke to nearly twice its static tension by "stress relieving" over-tension, the outside of the elbow must yield and when released fall back to a lower stress. assuming you're bending the spoke elbow. if you follow the manufacturer's instructions you wouldn't do that, and more importantly, you'd notice that the settled angle of the spoke elbow in the hub hole, with the "slotting" of the hole that results as the spoke seats itself, is at about 95 degrees. which is about same as spokes are made with. that would be a wild coincidence, wouldn't you say? we /are/ assuming incompetent manufacturers after all... That example is of the elbow, but it is true of the threads as well. The elbow, however, is more obvious because outbound spokes all get plastically deformed in the first tensioning of the wheel regardless of their prior state. you're hedging as to exactly which type of residual stress you think threads have. cast the die jobst, the thread root [the critical part], do you reckon that's tensile residual or compressive residual? I think that I have the raw material, some widely available pieces of stainless steel wire that come with three smooth u-bends from the factory. It's so cheap that I should be ashamed to steal them in boxes of a hundred, but the students need to learn the basics of scrounging. Being practical about this, I think you will see that you can do this without a materials laboratory. not unless you don't understand what you're looking at. to test for the presence of residual stress, you commonly use a chemical agent that preferentially attacks material areas with higher [crystal lattice] energy. to measure magnitude, you commonly use an atomic distance measuring strategy like x-ray or neutron diffraction. i don't have a suitable neutron source in my basement. do you? Any ideas about how to demonstrate over-tensioning stress-relief using paper-clips? I like clothes hangers better because they are large enough to clearly see the results. all you're seeing is bending. the fact that you've convinced yourself that you're seeing residual stress simply illustrates just how dangerous insufficient information or comprehension [not understanding hookes law] can be. http://www.sheldonbrown.com/brandt/s...relieving.html did you read my line-by-line critique of that article? you need to update it jobst. but don't do it until you've been to the library to revise your deformation theory please. Jobst Brandt Dear Jobst and Jim, Absolutely! (How's that for tact?) Now can either of you suggest a practical way to demonstrate to a high-school physics class the effect (whatever it is) of stretching through substantial extra tension a pre-bent piece of tensioned stainless steel wire, such as a paper-clip? That is, can we isolate and magnify the effect so that it will be clear that both stretched and unstretched pre-bent stainless steel fatigue at the same rate? Or that one lasts longer, not necessarily the one that some expect? I'm not a slave to the paper-clip scheme. Razor blades occurred to me, since they're much thinner and would therefore fatigue faster and more easily, but they're not bent, and they're not under tension. Music wire has been mentioned in earlier threads, but it's tricky in that one strand is wrapped around the other. Possibly some sort of solid guy-wires whose ends are wrapped around a curve? I'm not sure if those are stainless steel. Surgical wires? Orthodontic wiring? What we're looking for is a simple way to stress a thin stainless steel wire fast enough that it will break within a reasonable time. Then we can time things and find out what the results are of stretching. Carl Fogel |
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