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4 spokes failed on unridden bike!
I've always done all my maintenance, and commuted 3,000 miles a year for 16
years now; so I've seen quite a bit. I was cleaning up a couple old bikes we hadn't ridden in a few years, preparing to donate them. My wife's road bike had a broken spoke, My old MTB had broken two. All were rear drive-side. The first job went fine. I hand screwed the spokes into the mountain bike's wheel and put it in the truing stand. As I was about to start truing it, 4 spokes suddenly snapped! That made 6 spokes on a bike I hadn't ridden! My only guess was some combination of stress-corrosion cracking (the stainless spokes had several rusty spots) and over-tightening. I am rebuilding the wheel with all new spokes (done this before too) but would sure like to know what happened. -- Robert Haston Satellite Beach, FL |
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#2
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Robert Haston wrote:
I've always done all my maintenance, and commuted 3,000 miles a year for 16 years now; so I've seen quite a bit. I was cleaning up a couple old bikes we hadn't ridden in a few years, preparing to donate them. My wife's road bike had a broken spoke, My old MTB had broken two. All were rear drive-side. The first job went fine. I hand screwed the spokes into the mountain bike's wheel and put it in the truing stand. As I was about to start truing it, 4 spokes suddenly snapped! That made 6 spokes on a bike I hadn't ridden! My only guess was some combination of stress-corrosion cracking (the stainless spokes had several rusty spots) and over-tightening. I am rebuilding the wheel with all new spokes (done this before too) but would sure like to know what happened. without seeing your broken spokes, your suggestions are pretty reasonable on both counts. another thought is that since the wheels /had/ been ridden in the past, it's quite possible that they were already fatigued but that your truing efforts raised tension sufficiently to cause the stress at the fatigue crack tip to exceed fracture. almost nothing you can do to prevent that. sometimes also things like vibation & thermal cycling can cause sudden failure of something that's not in service. |
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jim beam:
without seeing your broken spokes, your suggestions are pretty reasonable on both counts. another thought is that since the wheels /had/ been ridden in the past, it's quite possible that they were already fatigued but that your truing efforts raised tension sufficiently to cause the stress at the fatigue crack tip to exceed fracture. almost nothing you can do to prevent that. sometimes also things like vibation & thermal cycling can cause sudden failure of something that's not in service. Please explain how temperatures high enough to cause destructive thermal cycling are reached by bicycle spokes. While you're at it, also explain how vibration amplitudes high enough to cause significant damage (apart from the loading on the spokes through rider weight) figure into it. |
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On Sun, 14 Nov 2004 02:55:45 GMT, Jose Rizal wrote:
jim beam: without seeing your broken spokes, your suggestions are pretty reasonable on both counts. another thought is that since the wheels /had/ been ridden in the past, it's quite possible that they were already fatigued but that your truing efforts raised tension sufficiently to cause the stress at the fatigue crack tip to exceed fracture. almost nothing you can do to prevent that. sometimes also things like vibation & thermal cycling can cause sudden failure of something that's not in service. Please explain how temperatures high enough to cause destructive thermal cycling are reached by bicycle spokes. While you're at it, also explain how vibration amplitudes high enough to cause significant damage (apart from the loading on the spokes through rider weight) figure into it. Dear Jose, If a pre-tensioned spoke is already about to break, would the additional contraction and brittleness of a drop to sub-zero temperatures be enough to let a spoke break on a bicycle sitting in an unheated garage? On several cold mornings, I've found a broken rear spoke and the rim jammed solid against one brake pad after the wheel had spun freely while I was cleaning the chain the night before. Typically, the riding temperature the day before was around 34-40 F, but the overnight low was 0 to -10 F. Jobst indicated (I think) that a failing spoke can even break without any apparent added stress: "Spokes can break with no additional loading once the developing crack reduces the cross section to the yield point. Then the crack grows plastically until failure without change in load or temperature. I have witnessed spoke failures more than an hour after parking the bicycle indoors where the occurrence is conspicuous." http://groups.google.com/groups?q=%2...c.ne t&rnum=1 My layman's guess is that this means that a tiny crack can slowly grow while the bike is just sitting there. Come to think of it, now I'm wondering whose spokes Jobst witnessed breaking an hour after being parked indoors. Carl Fogel |
#5
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Quoth Carl Fogel: (For some reason my spell checker wants to change his=20
name to "Fogey") If a pre-tensioned spoke is already about to break, would the additional contraction and brittleness of a drop to sub-zero temperatures be enough to let a spoke break on a bicycle sitting in an unheated garage? Dear Carl, No, no way cold could do this, since the rim would contract too. In=20 fact, an aluminum rim would contract twice as much as the steel spokes,=20 since aluminum has twice the coefficient of expansion that steel does. Sheldon "That's Not It" Brown +------------------------------------------------+ | I=92m currently appearing in: | | Gilbert & Sullivan's Iolanthe at M.I.T. | | November 12, 13, 14 and 18, 19, 20, 21 | | http://web.mit.edu/gsp/www | | http://sheldonbrown.com/music.html | +------------------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide http://captainbike.com http://sheldonbrown.com |
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On Sat, 13 Nov 2004 23:29:15 -0500, Sheldon Brown
wrote: Quoth Carl Fogel: (For some reason my spell checker wants to change his name to "Fogey") If a pre-tensioned spoke is already about to break, would the additional contraction and brittleness of a drop to sub-zero temperatures be enough to let a spoke break on a bicycle sitting in an unheated garage? Dear Carl, No, no way cold could do this, since the rim would contract too. In fact, an aluminum rim would contract twice as much as the steel spokes, since aluminum has twice the coefficient of expansion that steel does. Sheldon "That's Not It" Brown Dear Sheldon, Hmmm . . . I think that you're right, but using the wrong figures. Unlike sluggish ordinary steels, stainless steel has roughly the same coefficient of thermal expansion as lively aluminum, a ratio about about 11 to 12 (not 1 to 2). Where X is the coefficient of thermal expansion (inch/inch F 10-6) . . . Stainless steel (as used in spokes) is around 11X: http://www.engineeringtoolbox.com/14_283.html You have to scroll down to see it. Nearby is a link to this table: http://www.engineeringtoolbox.com/24_95.html .. . . which shows the thermal expansion for aluminum (as used in rims) is about 12X (and ordinary steel about 7x). I'm not sure if it matters when the temperature drops that the aluminum rim is already shrunk (in compression) by the elongated stainless steel spokes (in tension). But at an 11/12 ratio, I suspect that the small difference in what would be a tiny contraction would end up negligible. Reassuringly, "Stainless steel (type 304 or 316) does not exhibit any ductile-to-brittle transition at low temperatures (even down to –400°F), and is an excellent material for frigid temperatures." www.b-line.com/engineer/TPS/BLTPS-08.pdf So maybe it's just coincidence that several spokes apparently failed during unusually cold nights. I hope so, since it's snowing lightly right now. Carl Fogel |
#8
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Hmm.. sudden temperature change maybe? Did you take the bikes out from
somewhere, and then bring them into a different temperature environment to start truing them? Just a weird guess Mike http://mikebeauchamp.com "Robert Haston" wrote in message link.net... I've always done all my maintenance, and commuted 3,000 miles a year for 16 years now; so I've seen quite a bit. I was cleaning up a couple old bikes we hadn't ridden in a few years, preparing to donate them. My wife's road bike had a broken spoke, My old MTB had broken two. All were rear drive-side. The first job went fine. I hand screwed the spokes into the mountain bike's wheel and put it in the truing stand. As I was about to start truing it, 4 spokes suddenly snapped! That made 6 spokes on a bike I hadn't ridden! My only guess was some combination of stress-corrosion cracking (the stainless spokes had several rusty spots) and over-tightening. I am rebuilding the wheel with all new spokes (done this before too) but would sure like to know what happened. -- Robert Haston Satellite Beach, FL |
#9
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On Sat, 13 Nov 2004 23:29:15 -0500, Sheldon Brown
wrote: Quoth Carl Fogel: (For some reason my spell checker wants to change his name to "Fogey") If a pre-tensioned spoke is already about to break, would the additional contraction and brittleness of a drop to sub-zero temperatures be enough to let a spoke break on a bicycle sitting in an unheated garage? Dear Carl, No, no way cold could do this, since the rim would contract too. In fact, an aluminum rim would contract twice as much as the steel spokes, since aluminum has twice the coefficient of expansion that steel does. Sheldon "That's Not It" Brown Dear Sheldon, Drat! After agreeing with you (but pointing out that stainless steel actually shrinks at almost the same rate as aluminum), I was suddenly troubled by the vague memory that Jobst has mentioned that cast control wheels (whose rims and spokes have identical material) have wiggly spokes because otherwise they'd crack when they cooled at different rates and shrank: "Now consider that die castings have cooling stresses caused by sequential cooling and that the thicker part of the spokes near the hub and the hub itself cool last causing shrink. This puts the die cast spokes in tension. That castings do this is known and that is why S-shaped spokes are used on cast iron hand wheels of old to prevent cracks that occur if they were not S-shaped. All cast railway wheels also have such ribs on the inside." http://groups.google.com/groups?selm... utput=gplain Now I need to be reassured that the thin spokes with their huge surface area don't cool and contract significantly faster than the fat rims insulated by pneumatic tires. This was a lot simpler with wooden ox-carts. Befuddled, Carl Fogel |
#10
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Might the bikes been exposed to fumes of hypochlorite? Just a leaky bottle of ordinary bleach would be enough over time to kill 300 stainless steel if the pH goes low. In solution with water, presence of the Na ion keeps the pH high enough to prevent Cl induced SCC. ClO vapors without the Na ion allows the pH to drop, attacking austinitic SS, of which spokes are made. With the oxygen free radical, chlorine induced SCC can happen at much lower temperatures like that of a garage or store room during the summer. I have seen SS spokes as brittle as uncooked spaghetti from ClO vapor after a few months of storage near calcium hypochlorite pool chemicals. Just my materials experience as applied to cycling... -- Weisse Luft |
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