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#21
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Broken steel road bike fork
On Sep 29, 10:40*pm, jim beam wrote:
On 09/29/2009 03:15 PM, Jobst Brandt wrote: Jay Drew wrote: What is feathering? A fading out of one cross section into another where they meet: *http://www.gtgtandems.com/tech/brazing.html wrong. *feathering is simply filing the edges of the lug. The sudden step in cross section causes a stress concentration that leads to cracking. which is why it's not a straight cross section, it's got curved features that mitigate - effectively radius reduction, a standard fatigue mitigation technique. Not surprisingly, jim beam has things exactly backwards. When blending a transition from a larger sized piece (like the lug) to a smaller sized piece (like the fork blade) the idea is to increase, not decrease the radius. The ultimate in radius _reduction_ would be a sharp corner, which generates a high stress concentration. Large radii are the opposite and generate lower stress concentrations. You can play around with different radius values at http://www.fatiguecalculator.com/cgi-bin/findkt.pl to investigate this. For example, for D=25 mm, d = 23 mm and r=1 mm, the stress concentration factor is 1.84 - that is, peak stress in the area of the shoulder will be 1.84 times the stress further away. If you reduce the radius (as jim seems to recommend) to 0.1 mm, the stress concentration factor jumps to 3.52 - Frank Krygowski |
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#22
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Broken steel road bike fork
On 09/29/2009 09:04 PM, Frank Krygowski wrote:
On Sep 29, 10:40�pm, jim wrote: On 09/29/2009 03:15 PM, Jobst Brandt wrote: Jay Drew wrote: What is feathering? A fading out of one cross section into another where they meet: �http://www.gtgtandems.com/tech/brazing.html wrong. �feathering is simply filing the edges of the lug. The sudden step in cross section causes a stress concentration that leads to cracking. which is why it's not a straight cross section, it's got curved features that mitigate - effectively radius reduction, a standard fatigue mitigation technique. Not surprisingly, jim beam has things exactly backwards. When blending a transition from a larger sized piece (like the lug) to a smaller sized piece (like the fork blade) the idea is to increase, not decrease the radius. The ultimate in radius _reduction_ would be a sharp corner, which generates a high stress concentration. Large radii are the opposite and generate lower stress concentrations. you're a fraud and moral retard krygowski. you knew exactly what i meant before you tried putting false words in my mouth - the concept of decreasing radius sharpness is something so simple and obvious, even a failed phd candidate like you should be able to grasp it. You can play around with different radius values at http://www.fatiguecalculator.com/cgi-bin/findkt.pl to investigate this. For example, for D=25 mm, d = 23 mm and r=1 mm, the stress concentration factor is 1.84 - that is, peak stress in the area of the shoulder will be 1.84 times the stress further away. If you reduce the radius (as jim seems to recommend) to 0.1 mm, the stress concentration factor jumps to 3.52 you're a fraud and moral retard krygowski. resign and stop ****ing up people's careers. |
#23
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Broken steel road bike fork
On Sep 29, 6:52*am, raamman wrote:
On Sep 29, 12:37*am, wrote: The shimmy was the wheel moving back and forth due to the fork leg being fatigued. *There hasn't been any accident damage since I put this fork on the bike. *The crack was almost all the way through, starting from the BACK of the fork leg. well, there you go, "steel is real" has been the mantra- but the phrase " suseptable to rust" could be added. The fork looks very clean, so I wonder how it happened to escape your notice earlier. I am glad you escaped any mishap. It doesn't have anything to do with rust or cleanliness. There is a bit of surface oxidation inside the fork, but it's totally superficial. There is a stress concentration where the fork crown meets the blade. The joint was possibly overheated during manufacture leading to a weakness at the braze. The crack initiated at the rear of the fork, at the place where the crown meets the blade, and propagated forward. You can kind of see this from the pictures http://www.bitrealm.com/misc/fork/p1000784.jpg Along the broken surface, at the upper right it's fairly dark, which indicates it's been cracked for a while and exposed to atmosphere, dirt etc. This is where the crack follows the line of the crown. Along the front of the fork blade, the line of breakage is more smooth than scalloped, as if it were just torn away, and the surface looks cleaner. This is what was still attached until the final break happened. If the OP could take an in-focus close up picture of the fracture surfaces (ideally in side lighting, with a tripod) it would probably show the differences in texture of the initial and final fractured surfaces clearly. It looks like the fork was cracked almost halfway around for a while before the final rupture. Any material can fail if there is a flaw in the design or manufacture. Steel is no panacea, but neither is anything else. iIt's probably because it was steel that the fork was "rideable" for a while even though cracked, but whether that is good or bad depends on your ideas about fault detection. (That is, a piece that cracks instantly is bad, but a piece that lulls you into still riding it around even though it's about to finally give way and drop you is also bad.) Ben |
#25
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Broken steel road bike fork
On Sep 30, 12:19*am, jim beam wrote:
On 09/29/2009 09:04 PM, Frank Krygowski wrote: On Sep 29, 10:40 pm, jim *wrote: The sudden step in cross section causes a stress concentration that leads to cracking. which is why it's not a straight cross section, it's got curved features that mitigate - effectively radius reduction, a standard fatigue mitigation technique. Not surprisingly, jim beam has things exactly backwards. *When blending a transition from a larger sized piece (like the lug) to a smaller sized piece (like the fork blade) the idea is to increase, not decrease the radius. *The ultimate in radius _reduction_ would be a sharp corner, which generates a high stress concentration. *Large radii are the opposite and generate lower stress concentrations. you're a fraud and moral retard krygowski. *you knew exactly what i meant before you tried putting false words in my mouth - the concept of decreasing radius sharpness is something so simple and obvious... "jim," I didn't put words in your mouth. I quoted exactly what you said. You had it exactly backwards. If you had any class at all, you'd have simply said "Oops, my mistake. I meant to say..." - Frank Krygowski |
#26
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Broken steel road bike fork
On 30 Sep, 13:55, jim beam wrote:
On 09/29/2009 10:01 PM, wrote: On Sep 29, 6:52 am, *wrote: On Sep 29, 12:37 am, wrote: The shimmy was the wheel moving back and forth due to the fork leg being fatigued. There hasn't been any accident damage since I put this fork on the bike. The crack was almost all the way through, starting from the BACK of the fork leg. well, there you go, "steel is real" has been the mantra- but the phrase " suseptable to rust" could be added. The fork looks very clean, so I wonder how it happened to escape your notice earlier. I am glad you escaped any mishap. It doesn't have anything to do with rust or cleanliness. There is a bit of surface oxidation inside the fork, but it's totally superficial. There is a stress concentration where the fork crown meets the blade. *The joint was possibly overheated during manufacture leading to a weakness at the braze. *The crack initiated at the rear of the fork, at the place where the crown meets the blade, and propagated forward. You can kind of see this from the pictures http://www.bitrealm.com/misc/fork/p1000784.jpg Along the broken surface, at the upper right it's fairly dark, which indicates it's been cracked for a while and exposed to atmosphere, dirt etc. This is where the crack follows the line of the crown. Along the front of the fork blade, the line of breakage is more smooth than scalloped, as if it were just torn away, and the surface looks cleaner. *This is what was still attached until the final break happened. disagree - this is not a typical fatigue fracture. *the fact that part of the crown has come away with the blade is testament to that - highly unusual. more likely is a combination of poor brazing leaving a gap between the crown and the blade. *subsequent chroming would leave chromic acid deposits [and others] which would partially erode both the blade and the crown, and initiate fatigue. *the rest is simply fatigue propagation. I have made this kind of fracture at thre other end of the forks when attempting to correct a build error. You know, cold set the forks. New fork blades ,I was bending them but they wouldn't give, used alittle more leverage than I expected and they snapped. Checked with my trusted framebuilder, and he said "Nio, they shouldn't have done that. You just cold set the forks after brazing." The coarse granularity of what was steel was caused by an excess of spelter being absorbed due to prolonged and/or excessive heat. There was no chrome, only paint. The 'steel' failed in tension with two applications of force. The point is the metal would not yield, it could only fracture. The klutz who had 'brazed' the forks had turned the advanced steel into a similar metal to cast iron. Only the colour of fracture was different. This had the slightest tinge of yellow where one would expect blue or neutral. Cast iron is grey with variations of shade in the granulation. |
#27
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Broken steel road bike fork
In article
, wrote: So, I've been noticing that the front brakes of my road bike were acting "grabby" in that the front would shimmy pretty badly just as I'm coming to a stop. The rims felt a little sticky, maybe some Gatorade got on them and it hasn't really rained in a while. I cleaned the wheels last night and rode into work today. It was still bad, but didn't seem as bad. This has been going on for a week, I didn't think anything of it. I decided to not ride after work and just come home, coming down the driveway, the grabby-ness was really pronounced, so I figure that the brake pads must be contaminated with something. When I took the wheel off, my heart skipped a beat. Without much effort, this is the result: http://www.bitrealm.com/misc/fork/p1000783.jpg http://www.bitrealm.com/misc/fork/p1000784.jpg http://www.bitrealm.com/misc/fork/p1000785.jpg It was hanging by no more than 2mm of steeel. I shudder when I think of the roads I was about to go on, including one really bad bump at the bottom. I don't think it would have held together and when you lose the front like this, it's going to be bad. The shimmy was the wheel moving back and forth due to the fork leg being fatigued. There hasn't been any accident damage since I put this fork on the bike. The crack was almost all the way through, starting from the BACK of the fork leg. I am happy for you. Now Mr. Mom. If I said it once, I said it a dozen times, when your bike acts wonky, dismount and determine the cause. Do not ride an un-diagnosed wonky bike. -- Michael Press |
#28
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Broken steel road bike fork
On Sep 30, 5:55*am, jim beam wrote:
On 09/29/2009 10:01 PM, wrote: Any material can fail if there is a flaw in the design or manufacture. *Steel is no panacea, but neither is anything else. cfrp is highly fatigue resistant - it would not fail in this manner. iIt's probably because it was steel that the fork was "rideable" for a while even though cracked, that ben, is utter garbage. *any material undergoing fatigue is "rideable" until it fails. Ah, Beamer. It's always good to have you disagree with me. When you follow up a post of mine with a message of agreement, I feel the need to recheck my calculations. CFRP is highly fatigue resistant - if it's strong enough and doesn't have any construction flaws. On the other hand, so is steel. Saying a CFRP fork wouldn't fail in this manner is a bit meaningless because the fork failed from fatigue initiated by a construction error and worsened by a stress riser (a brazing problem most likely, possibly worsened by chroming as you argue, but most chromed forks don't fail, so that brings us back to an initial assembly error). CFRP parts obviously don't fail from brazing errors, but they certainly can fail from construction errors and stress risers. My impression is that CF bike parts tend to have a fairly large reserve strength because they'll break quickly once they start breaking, so manufacturers want to avoid the start of breakage at all. (The exception might be superlight wheels, but those tend to get banged up in use, and I'm excluding crash damage from this discussion.) Any part with a fatigue crack or flaw is rideable until it fails, the question is how long it takes from the crack becoming significant until full failure. For example, this fork was cracked halfway around and still rideable (sort of) and it sounds like that was going on for several days. An aluminum part that was cracked halfway around probably wouldn't last that long before failing completely. This is conjecture on my part since I have never cracked an aluminum fork like that, but it seems to me that with aluminum frames and parts, they make the transition from cracked to doomed very quickly. This is by no means an argument against aluminum parts - you won't catch me riding with steel cranks. but whether that is good or bad depends on your ideas about fault detection. *(That is, a piece that cracks instantly is bad, that's not fatigue, that's simply fracture. I meant a piece that transitions from fatigue crack to total failure quickly. You don't want that because by the time you figure out something feels wrong, you're already on the deck. Bad old titanium BB spindles were probably in this category. Not necessarily because titanium is evil, it's just not good in that application. but a piece that lulls you into still riding it around even though it's about to finally give way and drop you is also bad.) this is what is so dangerous about fatigue and why so many millions of dollars are spent on detection every year - pre-failure symptoms are not obvious and need skill, awareness and usually testing gear to detect. undetected failure can be tragic. *everyone should periodically inspect their bike for this kind of stuff. Well, yes, though in this case the rider did notice something funny beforehand. It's always hard to know what's just voodoo and what's a real problem, but when the bike starts acting odd it's time to start taking pieces off and wiggling things. If nothing falls apart in your hands, you can always put it back together. Ben |
#29
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Broken steel road bike fork
On 1 Oct, 09:26, " wrote:
On Sep 30, 5:55*am, jim beam wrote: On 09/29/2009 10:01 PM, wrote: Any material can fail if there is a flaw in the design or manufacture. *Steel is no panacea, but neither is anything else. cfrp is highly fatigue resistant - it would not fail in this manner. iIt's probably because it was steel that the fork was "rideable" for a while even though cracked, that ben, is utter garbage. *any material undergoing fatigue is "rideable" until it fails. Ah, Beamer. *It's always good to have you disagree with me. *When you follow up a post of mine with a message of agreement, I feel the need to recheck my calculations. CFRP is highly fatigue resistant - if it's strong enough and doesn't have any construction flaws. *On the other hand, so is steel. *Saying a CFRP fork wouldn't fail in this manner is a bit meaningless because the fork failed from fatigue initiated by a construction error and worsened by a stress riser (a brazing problem most likely, possibly worsened by chroming as you argue, but most chromed forks don't fail, so that brings us back to an initial assembly error). CFRP parts obviously don't fail from brazing errors, but they certainly can fail from construction errors and stress risers. *My impression is that CF bike parts tend to have a fairly large reserve strength because they'll break quickly once they start breaking, so manufacturers want to avoid the start of breakage at all. *(The exception might be superlight wheels, but those tend to get banged up in use, and I'm excluding crash damage from this discussion.) Any part with a fatigue crack or flaw is rideable until it fails, the question is how long it takes from the crack becoming significant until full failure. *For example, this fork was cracked halfway around and still rideable (sort of) and it sounds like that was going on for several days. *An aluminum part that was cracked halfway around probably wouldn't last that long before failing completely. *This is conjecture on my part since I have never cracked an aluminum fork like that, but it seems to me that with aluminum frames and parts, they make the transition from cracked to doomed very quickly. *This is by no means an argument against aluminum parts - you won't catch me riding with steel cranks. but whether that is good or bad depends on your ideas about fault detection. *(That is, a piece that cracks instantly is bad, that's not fatigue, that's simply fracture. I meant a piece that transitions from fatigue crack to total failure quickly. *You don't want that because by the time you figure out something feels wrong, you're already on the deck. *Bad old titanium BB spindles were probably in this category. *Not necessarily because titanium is evil, it's just not good in that application. but a piece that lulls you into still riding it around even though it's about to finally give way and drop you is also bad.) this is what is so dangerous about fatigue and why so many millions of dollars are spent on detection every year - pre-failure symptoms are not obvious and need skill, awareness and usually testing gear to detect. undetected failure can be tragic. *everyone should periodically inspect their bike for this kind of stuff. Well, yes, though in this case the rider did notice something funny beforehand. *It's always hard to know what's just voodoo and what's a real problem, but when the bike starts acting odd it's time to start taking pieces off and wiggling things. *If nothing falls apart in your hands, you can always put it back together. Ben Aluminium, its alloys and steel all have the advantage that they will yield before fracture. Faulty brazing may remove this property. I have experienced this period of yield with aluminium handlebars which were drooping from the stem, one side tore on the top and the bar dropped quite some way before I had transferred sufficient weight to stop the discection. The bars had obviously fatigued but they had also yielded at the same time and during the final give. There is no time to take action when a CFRP part is in its final stage of failure, modification with cellulose fibre such as flax may help to mitigate the speed of failure. |
#30
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Broken steel road bike fork
On Oct 1, 5:54*am, someone wrote:
... *There is no time to take action when a CFRP part is in its final stage of failure, modification with cellulose fibre such as flax may help to mitigate the speed of failure. Hey, I see a long term strategy for Rivendell! Instead of carbon fiber frames, Grant can build frames of flax fiber! Wool, beeswax, canvas and flax. It fits! - Frank Krygowski |
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