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Compromised Fork?

Old 05-12-13, 11:06 AM
  #1  
RapidRobert
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Compromised Fork?

I have a custom steel frame made in 1982 by a famous framebuilder currently still active. First damage I ever did to it was a couple months ago, when I hit a curb and bent the fork back very symmetrically straight back (and tacoed the wheel). No frame damage or bending. Didn't wipe out (my lucky day!). The pic below shows the fork on an optical breadboard I used to measure the bends.

I sent the fork to the man who built it 30 years ago to straighten it. In the middle of the job, he called to tell me he saw a crack in the Imron paint up in an area at the front of one blade by the crown, in an area not affected by the bend. He said he wouldn't return the fork to me, citing his potential liability. I convinced him to look closer, and he media blasted the paint away from the area. He saw nothing, but insisted the fork was compromised to the point of un-useability. I suggested that if he returned it to me, I could have it examined professionally by NDT methods to look closer for an invisible micro-crack that might be there. He finished straightening the fork and shipped it to me.

I saw no crack. I have 30+ years experience inspecting mechanical parts and laser optics, and know how to look for a crack. I read up on non-destructive testing, and bought a kit to do a liquid dye penetrant test. Again, no crack indicated. I emailed the result of my test to the framebuilder who called and said he was pissed off that I didn't have a professional do the test as promised and wished he never took the straightening job.

Is this fork compromised? I plan to reinstall it without repainting so I can watch the area for a while. A replacement fork from the same guy would be $1000 (done when he might have time next year), plus whatever it would take to have his guy repaint ($500?) and more months. The frame is iconic, and slapping some aftermarket or alternative fork on it is not an option.

Thanks for any and all comments!
Attached Images
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Bent Fork.jpg (95.4 KB, 102 views)
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Bent Fork Top.jpg (95.5 KB, 88 views)
File Type: jpg
BG Fork Test.jpg (97.8 KB, 85 views)
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BG Fork Test Detail.jpg (97.4 KB, 99 views)
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Old 05-12-13, 11:18 AM
  #2  
unterhausen
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I wouldn't straighten a fork, and I will not ride a bike that I am not 100% certain about. Having said that, the paint will crack long before the steel. I don't think this is a particularly good application for dye penetrant. I think microscopy is the way to go.
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Old 05-12-13, 11:24 AM
  #3  
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Why isn't it a good application for dye penetrant? You suggest I look at it with a microscope. Why would a microscope show a crack that wouldn't be indicated with dye penetrant?
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Old 05-12-13, 11:34 AM
  #4  
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$1500.00 for a replacement fork ?

Fork blades are bent during the process of building them and if there is no structural damage / issues at the fork crown, then that fork can be re-shaped / bent to it's proper alignment.

Steel is a forgiving material.
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Old 05-12-13, 12:02 PM
  #5  
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You should have had the builder make you a new fork, instead you did an end run on him and are now asking the internet for advice. Who's the professional?
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Old 05-12-13, 12:46 PM
  #6  
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One other area was indicated to have a potential crack. It was down the blade about half way and also on the front. I experimented with lighting for the pics below to best show scratches in the paint that go across rather than along the blade.

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BG Fork Crack B.jpg (68.8 KB, 33 views)
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Old 05-12-13, 02:01 PM
  #7  
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I'm with 65er...$1500 for a replacement fork?

also, I concur with UH in post #2.
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Old 05-12-13, 02:07 PM
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Originally Posted by Live Wire View Post
I'm with 65er...$1500 for a replacement fork?

also, I concur with UH in post #2.
I have access to high powered inspection microscopes. What magnification is recommended?
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Old 05-12-13, 02:13 PM
  #9  
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Originally Posted by Sixty Fiver View Post
$1500.00 for a replacement fork ?
At that price, you'd think the fork was made out of kryptonite. Must be one heck of a builder to command that sort of price; and he's not going to be any too happy building the OP a replacement fork.
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Old 05-13-13, 07:17 AM
  #10  
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Seems to me that the OP could still have a pro do the crack presence testing. In fact he might have some friends in the business, given his claimed experience in the field. So that aspect is not "lost" IMO. I do understand the builder's situation and am surprised that he agreed to straighten the fork, given the builder's first comments. Once the fork is out of his hands all control is lost.

But I have to say that I might have straightened the fork if it were mine and depending on the results, reinstalled it. I have done much the same before. Andy.
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Old 05-13-13, 09:39 AM
  #11  
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OP; Not intending to jab at you and apologies in advance; but I would have to wonder of the outcome if the original builder had simply been asked for his assessment of what it would take to make it right for proper, safe riding. If you trusted him to build the frame, you should now trust him to repair it, etc.

I would guess that one most likely response would have been that he would have simply made a new fork for you; matching the original sizing and matching as close in appearance and style to the original as possible (for example the crown or exact drop outs may no longer be available).

On the other side of the coin, that bent fork looks pretty common in terms of crown, blades and drops. Pretty sure you could buy a replacement in the $100-125 range. There are a lot of nice new or used C&V forks for sale. Then a simple repaint it to match the frame.

Re the $1,000 quote. My gut feel is that this is the "go away and never call me again" quote.

Again apologies in advance.

/K
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Old 05-13-13, 07:15 PM
  #12  
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The quote for a replacement fork was made early in the conversation about options, right after being told he thought there might be a crack by the crown. We discussed alternative dropouts and crowns. The paint cost would be for the whole frame. It wasn't a "go away" quote. That price for a fork from him is reasonable to me, and commensurate with the inflation adjusted price I paid for the frame in 1982.

I have to add that despite this issue with the builder, I totally understand his position and am sympathetic. This frame and fork will NEVER be on Ebay. Worst case is it will hang on my wall as loved art right next to the 1977 Witcomb USA frame (built by P. Weigle) I bent at RAGBRAI the following year. I'm very happy he returned it to me, and I'm taking extreme steps mitigate the risks. I'll probably be examining the area on this fork using holographic interferometry, a simple technique once my studio is back up and running. Here's a link describing the technique: http://www.keytometals.com/page.aspx...ite=kts&NM=203
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Old 05-14-13, 08:46 PM
  #13  
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Robert,

I'm not sure that exact crown is still available but if my guess is right, any builder/ person can buy "said builders" current fork pieces/kits directly from said builder's website and would likely cost you between 300-400 to have another builder build a copy for you and have it painted. The biggest thing is getting the rake right as said builder's offsets are based on each frame rather than a standard dimension as he is very particular about the front and center dimension. Also, I don't think said builder would mind if you used his name ATMO. It's not like the fork failed and you're railing him on the web. Save the bent fork for when you retire the bike as Art.

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Old 05-14-13, 09:07 PM
  #14  
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Originally Posted by RapidRobert View Post
I'll probably be examining the area on this fork using holographic interferometry
Wow.. I thought I was a bike nerd... just buy a new fork and dip it in a can of red paint.
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Old 05-19-13, 01:01 AM
  #15  
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Originally Posted by FixedFiend626 View Post
Wow.. I thought I was a bike nerd... just buy a new fork and dip it in a can of red paint.
I'm not a "bike nerd", I'm a holo-laser geek! As obscure as it gets. I was over-partyin' on a bike party ride, lost attention and smacked a curb. I could've been chewing pavement, paying the dentist and who knows what other surgeons but no ... this fine fork absorbed the shock and certainly saved my idiotic lidless ass. After 30 years of proud service. I'll gladly spend twice your budget for a whole new bike to repair it. Just sayin'.
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Old 05-19-13, 02:02 PM
  #16  
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Tech Gigs Obviously pay better than mere mechanics and bartenders..

maybe you can sort out your desires for the '42' bike .. Frame and Fork that you now desire..


Having field tested Your Prior Bike.. at least It was not in an Airplane you did the Hard Landing ..
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Old 06-07-13, 01:14 AM
  #17  
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I've completed my test of this fork using holographic nondestructive testing as described in the article linked above, and the result is that I see no evidence of a crack. The set-up used is shown below. The laser on the right sends its red beam up through a shutter, bends left off the mirror in the top right corner, bends down off the mirror in the top left corner, goes through a beamsplitter on the way to the mirror in the bottom left corner, bends off that up through a lens that spreads it to illuminate the fork blade. The beam reflected off the beamsplitter goes to the right, bends off a mirror, goes through a lens assembly to spread it, through the big lens, and down to illuminate the hologram recording material held in the holder at the bottom center of the table.

After the hologram is exposed for a couple seconds, it's developed like a black & white negative, dried, put back into the plate holder, and aligned accurately to produce the interference fringes seen overlapping the area illuminated on the fork blade. The fringes appear as a regularly spaced set of straight dark lines. When the blade is stressed by clamping it with a c-clamp, the fringes curve in a pattern determined by the direction and magnitude of the force, and the shape of the metal, but they remain a regularly spaced set of lines.

The image is very difficult to photograph with a cheap point & shoot camera. Laser speckle makes the image granular. It real life, the fringes I see are sharp and clear. They respond obviously and predictably when the blade is clamped. There's no deflection of the fringes anywhere in the area thought by the builder to have a crack. The pic below represents the best I could get tonight of the fringes pattern and shape I'm seeing. The area in this pic where the fringes look washed out (along the bottom of the blade) is a result of the lighting for the pic. I tried many orientations of the clamp and the fringes maintained integrity for all of them. I'm going to ride this fork and not worry about it at all.
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Old 06-07-13, 07:51 PM
  #18  
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I realized this morning that I focused on the wrong spot last night, so I just did it again for the area shown in the pic of the original post. Still nothing indicated.
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Old 07-27-13, 06:37 PM
  #19  
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I was talking to a friend the other day who saw this thread and was confused about how the fringes were actually formed, and suggested I explain it here a little better. The first picture in post #17 shows a conventional transmission hologram recording set-up. The recording material is high resolution conventional silver-halide photographic film on a glass plate, like astrophotographers used before CCD sensors. Think of the set-up as a camera, with the object inside the box and no lens between it and the film.

After the film is exposed and processed, it's put back into the plate holder. When the beam reflected off the beamsplitter re-illuminates the hologram now, a three dimensional image of the fork is seen when looking through the hologram plate. The hologram is like "a window with a memory", replaying a 3D image of the fork overlapping the actual real fork that's not been moved since the exposure. When looking through the hologram, one sees both the real fork (still illuminated by laser light) and the 3D image of it recorded by the hologram.

If the alignment of both images is accurate enough, a pattern of bright and dark lines called "interference fringes" appear on the surface of the fork when viewing it through the hologram. The fringe spacing and orientation depend on the alignment of the hologram and the real fork. Movement of one relative to the other causes the fringes to move. Movement equal to 1/2 the wavelength of the light will move a bright (or dark) fringe the distance to the next bright (or dark) fringe. In my test the wavelength was 633nm, so fringe to fringe distance was about 316nm or 0.000012".

Because the fringes are soooo sensitive to alignment changes between the real object (fork) and the hologram image overlapping it in 3D, stressing the fork warps the fringes. In my test the fringes started out as straight vertical lines. Gently pinching the fork warped them into curved lines. If there was a flaw in the tubing like a crack, the fringes going across it would be discontinuous or offset a bit. Or bend sharply. Something other than continuously flowing lines. Nothing I tried would produce such fringes. All orientations and pressures on the clamp gave smooth lines. No crack. Couple hundred miles on it now and nothing visible.

Last edited by RapidRobert; 07-27-13 at 07:23 PM.
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