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#11
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Broken steel road bike fork
On 29 Sep, 16:29, Tim McNamara wrote:
Chrome plating *can* result in embrittlement but does not necessarily do so. *Millions of bikes and other load bearing appplications are chromed without such failures happening. More likely the cause of embrittlement is what causes it with non- chromed forks. Overbrazed. Current common practice is to use oxy-acetylene torch and spelter wire because it is quicker overall. I suspect the higher temperatures obtainable with oxy-acet ensure a deeper penetration of the spelter, alloying with the steel, causing the embrittlement. In skilled experienced hands, oxy-acet does produce excellent results when there are no distractions. I would certainly favour a builder which had the certification of qualification from a technical college in the use of oxy-acetylene for brazing if I was unable to locate one with historic recommendation. That (college cetificate) does not necessarily qualify him to braze cycle forks. And be careful that builders although say they do the work, leave spotty Nigel to do the preperation. Hearth brazed forks heated with natural gas may also suffer with brass embrittlement due to excessive duration in the flame. Good preparation is the key. No cutting with abrasive grit, only by file, plenty of flux in the joint and the use of granular spelter inside the end of the fork reduce the heat exposure of the steel so keeping the alloying depth to a minimum at the edge of the crown. I think it be the filling from the end of the crown rather the end of the fork to be the killer. Nigel, may of course used carbarundum instead of the file and scraper to prepare the forks and crown and if he's applied a paste flux, how much water? The reason for the failure seem pretty self-evident in the photo. *Note how the crack followed the shore line of the socket so perfectly, and note the relative thicknesses of the metal. *Note also the lack on a reinforcement tang on the inside of the fork leg. Overbrazed. for the future, consider quality cfrp - unless abused, it's pretty much "fatigue proof". More clueless advice from "jim beam." overbrazed. |
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#12
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Broken steel road bike fork
On 29 Sep, 21:12, Jobst Brandt wrote:
Frank Krygowski wrote: 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 steel. *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. Lucky you caught it when you did. *Last year, our old custom tandem let go of both blades simultaneously. *The warnings I got were nowhere near as explicit as yours. *Fortunately, we were moving only about 10 mph at the time. In my case, the bike was built by Jim Bradford, formerly of the Atlanta, Georgia area. *It was the last bike he built before getting married and leaving on a honeymoon. *I suspect he slapped in whatever fork blades he had lying around in order to get the job done, because the wall thickness was roughly one-third of the proper gage for Reynolds 531 tandem forks. I wish I could offer advice on how to choose a competent and ethical frame builder, but I can't. BTW, I think you shouldn't have too much trouble finding a replacement. *I was able to buy a replacement tandem fork from Tandems Ltd in Alabama. *For a single bike I intend to build, just last year I was able to buy a chromed Japanese fork from Citybikes in Portland, Oregon. *http://www.citybikes.coop/ This is a classic fork failure in which the lug interface between fork blade and fork crown had not been feathered after brazing. *This failure could be seen coming while "Cinelli" fork crowns that are internally lugged, do not reveal their trend to failure until after fracture. *However after failure they were obvious failure points. Many joints between lugs and frame tubes may be shiny chrome but they have discontinuous transitions that cause fracture as in this one. Jobst Brandt you're as bad as beam. The error is in the brazing. Plenty of aero crowns and forks survive despite their 'design flaw'. I used a 'full aero' crown on a Raleigh Competition for five years with 531 light gauge forks with no ill effects. I repainted the frame and forks in the last year and so know, because I gave it a careful inspection, that there were no faults whatsoever in the framebuilding. There did not appear to be any dressing of the lugs following brazing, the cutouts were clear and the depth was consistent all around with a tiny fillet. One 'fault', seat stay end cap had a little gap in the braze, the gap was irrelevant and sealed by the paint. The dent I put in the seat stay (crash) could not be covered by paint. |
#13
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Broken steel road bike fork
On Sep 29, 1:12*pm, Jobst Brandt wrote:
Frank Krygowski wrote: 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 steel. *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. Lucky you caught it when you did. *Last year, our old custom tandem let go of both blades simultaneously. *The warnings I got were nowhere near as explicit as yours. *Fortunately, we were moving only about 10 mph at the time. In my case, the bike was built by Jim Bradford, formerly of the Atlanta, Georgia area. *It was the last bike he built before getting married and leaving on a honeymoon. *I suspect he slapped in whatever fork blades he had lying around in order to get the job done, because the wall thickness was roughly one-third of the proper gage for Reynolds 531 tandem forks. I wish I could offer advice on how to choose a competent and ethical frame builder, but I can't. BTW, I think you shouldn't have too much trouble finding a replacement. *I was able to buy a replacement tandem fork from Tandems Ltd in Alabama. *For a single bike I intend to build, just last year I was able to buy a chromed Japanese fork from Citybikes in Portland, Oregon. *http://www.citybikes.coop/ This is a classic fork failure in which the lug interface between fork blade and fork crown had not been feathered after brazing. *This failure could be seen coming while "Cinelli" fork crowns that are internally lugged, do not reveal their trend to failure until after fracture. *However after failure they were obvious failure points. Many joints between lugs and frame tubes may be shiny chrome but they have discontinuous transitions that cause fracture as in this one. Jobst Brandt- Hide quoted text - - Show quoted text - What is feathering? |
#14
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Broken steel road bike fork
On 29 Sep, 22:34, jay wrote:
On Sep 29, 1:12*pm, Jobst Brandt wrote: Frank Krygowski wrote: 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 steel. *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. Lucky you caught it when you did. *Last year, our old custom tandem let go of both blades simultaneously. *The warnings I got were nowhere near as explicit as yours. *Fortunately, we were moving only about 10 mph at the time. In my case, the bike was built by Jim Bradford, formerly of the Atlanta, Georgia area. *It was the last bike he built before getting married and leaving on a honeymoon. *I suspect he slapped in whatever fork blades he had lying around in order to get the job done, because the wall thickness was roughly one-third of the proper gage for Reynolds 531 tandem forks. I wish I could offer advice on how to choose a competent and ethical frame builder, but I can't. BTW, I think you shouldn't have too much trouble finding a replacement. *I was able to buy a replacement tandem fork from Tandems Ltd in Alabama. *For a single bike I intend to build, just last year I was able to buy a chromed Japanese fork from Citybikes in Portland, Oregon. *http://www.citybikes.coop/ This is a classic fork failure in which the lug interface between fork blade and fork crown had not been feathered after brazing. *This failure could be seen coming while "Cinelli" fork crowns that are internally lugged, do not reveal their trend to failure until after fracture. *However after failure they were obvious failure points. Many joints between lugs and frame tubes may be shiny chrome but they have discontinuous transitions that cause fracture as in this one. Jobst Brandt- Hide quoted text - - Show quoted text - What is feathering? JB flapping. |
#15
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Broken steel road bike fork
On 29 Sep, 23:15, 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 like I said, JB flapping. That's about fillet joints not lugged, as in the OP's photographs. The sudden step in cross section causes a stress concentration that leads to cracking. No, you're flapping. and maybe you're cracking. *Most better metal workers are aware of this. Sadly the sloping fork crown of 1960's Cinellis were not feathered and cracked for those of us who rode much. Should have chosen a better metal worker, rather than one who would fit such a terrible fork crown , and then overbraze it so you could prove you rode much. *After two of these failures I got a Masi Fork crown instead. * Feathering can be done as Hetchins did with curlicues and fade-outs, or as Masi did with tapering pointed lugs. The ovality of the fork blade spreads the strain so there is no extraneous stress level. |
#16
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Broken steel road bike fork
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. Most better metal workers are aware of this. Sadly the sloping fork crown of 1960's Cinellis were not feathered and cracked for those of us who rode much. that's not feathering - that's radius reduction. After two of these failures I got a Masi Fork crown instead. good for you. of course, if you'd understood what you were looking at the first time, you'd only have suffered one failure... Feathering can be done as Hetchins did with curlicues and fade-outs, or as Masi did with tapering pointed lugs. nope, that's radius reduction, not feathering. hetchins and masi [and many others] do feather, but they do it with a file, and it's not a function of lug shape. |
#17
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Broken steel road bike fork
On 09/29/2009 01:12 PM, Jobst Brandt wrote:
Frank Krygowski wrote: 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 steel. 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. Lucky you caught it when you did. Last year, our old custom tandem let go of both blades simultaneously. The warnings I got were nowhere near as explicit as yours. Fortunately, we were moving only about 10 mph at the time. In my case, the bike was built by Jim Bradford, formerly of the Atlanta, Georgia area. It was the last bike he built before getting married and leaving on a honeymoon. I suspect he slapped in whatever fork blades he had lying around in order to get the job done, because the wall thickness was roughly one-third of the proper gage for Reynolds 531 tandem forks. I wish I could offer advice on how to choose a competent and ethical frame builder, but I can't. BTW, I think you shouldn't have too much trouble finding a replacement. I was able to buy a replacement tandem fork from Tandems Ltd in Alabama. For a single bike I intend to build, just last year I was able to buy a chromed Japanese fork from Citybikes in Portland, Oregon. http://www.citybikes.coop/ This is a classic fork failure in which the lug interface between fork blade and fork crown had not been feathered after brazing. no it isn't jobst. 1. you don't know **** about fatigue. 2. you've failed to observe the fatigue interface. 3. you don't understand the nature of the loading forces. 4. you're completely ignorant of the effects of chrome plating. This failure could be seen coming while "Cinelli" fork crowns that are internally lugged, do not reveal their trend to failure until after fracture. However after failure they were obvious failure points. er, this is not a cinelli fork. and the failure is not the same. Many joints between lugs and frame tubes may be shiny chrome but they have discontinuous transitions that cause fracture as in this one. no, they cause fatigue. fatigue causes fracture. what did you learn at stanford jobst? analysis and logic were obviously absent from your curriculum. |
#19
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Broken steel road bike fork
On 09/29/2009 02:16 PM, Still Just Me! wrote:
On Tue, 29 Sep 2009 05:42:51 -0700, jim wrote: for the future, consider quality cfrp - unless abused, it's pretty much "fatigue proof". The fact that it was steel and demonstrated fatigue instead of just failing catastrophically (like crappy fiber does when it fails) is the only thing that saved him. er, so a failure is better than a non-failure??? because you're confused between fatigue and over-load or abuse??? jeepers dude, you're winning that stupidity contest again! As always, your advice is useless. no, i cast pearls before ignorant incapable swine. |
#20
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Broken steel road bike fork
On 09/29/2009 06:52 AM, raamman wrote:
On Sep 29, 12:37�am, 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/p1...k/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. 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. no ****. I am glad you escaped any mishap. |
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