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Bent fork with 531c tubing - Reset to proper dimensions?

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Bent fork with 531c tubing - Reset to proper dimensions?

Old 04-30-15, 01:01 PM
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Bent fork with 531c tubing - Reset to proper dimensions?

There are a number of threads on alignment of forks and frames. This one is a rare Trek 1984ish 760/770 fork that was like finding a needle in the haystack. I am tempted to bend it into alignment but would like your opinion. My justification is that these forks were bent to their initial positions and this part of the fork (current unacceptable bend) is not in that location. Cold working may not be an issue nor should there be any real risk that could not be observed earlier than catastrophic failure.

The steerer is not bent, which is often the case in frontal impacts, as indicated when putting a straight edge along multiple location around the circumference.

Of course, how the correction is made is critical to final quality of results. Lets put that aside unless there is a reason to ensure minimal negative results in durability and reliability. OK now that I think of the suggested approaches maybe we should include discussion. What came to mind was the application of heat. I have a concern using heat as opposed to cold setting: too much could change the mechanical properties of the material.

Open discussion time. Please provide justification of your opinion. Don't really want statements like "just junk, throw it away and buy a track fork with the right dimensions" without good justification based on somewhat scientific based reasoning. That implies knowledge of properties of materials and how they change with mechanically manipulation. Experience is worth a pound of theory so it is very welcome with examples. Can you tell your talking to an engineer? Now I really don't expect very many response!

there were a couple of cracks in the paint but remember, this is imron paint. checking under the paint revealed a uniform surface. The paint was going to be stripped anyway so the surface will be sanded and examined for micro fractures. Doubt I will find any.

first the bend:
[IMG]WP_20150430_008 by superissimo_83, on Flickr[/IMG]

Now the "crack" exposed area:

[IMG]WP_20150430_002 by superissimo_83, on Flickr[/IMG]
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Old 04-30-15, 01:28 PM
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it might buckle. It's bent at the thin end that isn't meant to be bent. Not sure how you are going to actually bend it back. Usually when fork blades are bent, they are supported really well.

I am also an engineer, but this is not something that is going to reduce itself to some sort of thought experiment. You might be fine. It's the lack of certainty that would keep me from riding this fork. Personally, if I really liked that fork I would get new blades and use the old crown.
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Old 04-30-15, 01:42 PM
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@unterhousen - that was my backup plan, new blades, old crown. Should I get new DO's too? They are Campy from the era. I am trying to minimize my uncertainty of what I will end up with. Greatly appreciate your response.

I can always bend them back and see what I end up with and go from there.
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Old 04-30-15, 03:20 PM
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well, there are an infinite number of those dropouts available. So I would make those blades into key rings with a bottle opener.

As you say, bending them back isn't going to hurt anything. If it goes smoothly, then you can consider riding the fork
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Old 04-30-15, 09:40 PM
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I'm sure the fork can be bent back to OEM spec (whatever that was) but I am not sure that there won't be any concerns about integrity after. This kind of question is not a simple black/white one, like what cable is compatible with which lever. The photos lend very little specific information, no linkage with the paint crack (more like a chip off area) with the location of the bend. And poor views of the actual bend.

So without much more info, best attained with the fork in hand, I can't really say much about possibilities or estimates of this or that. If you want more hard advice provide more data.

I will say that recycling the crown and steerer has it's issues too. Do you know what filler is in place? Is the steerer still straight? Andy.
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Old 05-01-15, 11:01 AM
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I didn't think the photo would be very effective and I am not sure what photo would be. Perhaps a profile?

The crack in the paint was where the chipped area is. I used a knife blade to remove the paint away from the center of the crack to expose the metal without touching it.

All paint will be removed. I can then look at the surface more carefully and get an idea of what brazing material was used. My suspicion is brass like the 1984 Trek 610 I have stripped.

The steerer is still straight. The second thing I checked with a straight edge.

About the only dimension I know about the fork is that it had a 38 mm offset. Crown is Tange C-14 investment cast. Tubes are 531P (Trek Cat info)
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Old 05-01-15, 02:38 PM
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Originally Posted by SJX426 View Post
All paint will be removed. I can then look at the surface more carefully and get an idea of what brazing material was used. My suspicion is brass like the 1984 Trek 610 I have stripped.
The steer tube and blades would have used standard, low-fuming bronze. The dropouts used nickel-bronze.
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Old 05-10-15, 06:58 AM
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Update on the fork.

Today I decided to take a chance on bending it back.

First was to put the steerer in the vice:

[IMG]P1020996 by Patrick Boulden, on Flickr[/IMG]

Then get a BA 2x4 in put in in position after putting in a sacrificial hub (bad races)

[IMG]P1030002 by Patrick Boulden, on Flickr[/IMG]

[IMG]P1030005 by Patrick Boulden, on Flickr[/IMG]

Get in position and push on this small lever

[IMG]P1030007 by Patrick Boulden, on Flickr[/IMG]

After about for adjustments, I think I have it nearly straight

[IMG]P1020998 by Patrick Boulden, on Flickr[/IMG]
[IMG]P1020999 by Patrick Boulden, on Flickr[/IMG]

Pictures are hard to get aligned right but based on the straight edge, it looks really close. Next is to check for offset.
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Old 05-10-15, 08:53 AM
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I hope it is right, and that it rules.
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Old 05-24-15, 08:54 AM
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SJX, I'd like to see your pix and see if it worked decently. Out on iBob I ran into dddd when we were both in our civvies, and we talked about tube stresses when bending forks (get two Bobbish engineers who bike together - what else was there to talk about??). One thing we agreed on is that due to the relatively large front-back chord of a fork blade, it is a lot more stressful to bend one at the thick end than the thin. Also more difficult.

I have a 1984 610 fork that I had bent backwards to reduce trail (long story!) At this point I'd like to have it bent forward to the original offset and probably a cm more forward to reduce trail. I don't know what will happen to tire clearance, but I am curious.

Having stripped yours and taken a gander at the brazing et cetera, what did you think?

What size frame did yours come from?
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