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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:
WP_20150430_008 on Flickr

Now the "crack" exposed area:

WP_20150430_002 on Flickr
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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
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:

P1020996 on Flickr

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

P1030002 on Flickr

Get in position and push on this small lever

P1030007 on Flickr

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

P1020998 on Flickr

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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Old 02-08-23, 11:43 AM
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[MENTION=28632]Road Fan[/MENTION] - 7 years later I am answering your question! Brazing was fine after all that stressing and deformation. In fact, I cleaned up a lot of the excess brazing material and filed down the parting lines on both the frame and the fork. Miraculously,, The fork came from a same size frame which was bent!

To close this thread, here is an update.
The acquaintance who builds his own frame and did some braze-on's for the 610 made a fork jig to copy existing fork dimensions to a newly made fork. He had been purchasing forks for his custom frames. He bought a tube set from RS and wanted to make his own fork. Hence the fork jig. He was kind enough to let me borrow it to check my fork out.
Since the steerer was straight, as well as the crown. the only adjustment needed were the legs. The key to using the tool is to get the offset correct for the fork, then place the fork on the jig to see if it lines up. It didn't.
the jig. The two grub screws toward the right are where the steerer rests. Adjusting them determines the offset.
Fork Jig 1 on Flickr

Fork in the jig. If the steerer doesn't touch either of the screws, the offset is off.
Fork Jig 2 wfork on Flickr
Fork Jig 3 wfork on Flickr

The center line is checked at the same time. If the DO are not equally spaced to the center line, it won't fit in the guide next to the steerer.
Fork Jig 4 wfork on Flickr

Top view of the jig
Fork Jig 5 on Flickr

Hands free with straight body and no corrections is how good it is.

P1040796 on Flickr

This effort was a risky one. In Fact Doug Fattic was quite critical of what I did. I understood the risk and was willing to take it. I think all is good. There aren't any good frame builders around me that I know of and alignment looks good to me. Is the hub axle parallel to the crown? IDK, looks like it. This coming from a guy who use to use drafting tools in his first job as a mechanical engineer and could decern hundredths of an inch mis alignment.
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Old 02-08-23, 01:24 PM
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I had Ed Litton, then working for Eisentraut, do a similar straightening of my original Mooney fork. (Left blade saw less damage, the right more.) I wasn't willing to make that a long term rider as I had a fork fail on me recently enough to make me quite gun shy so I had Peter Mooney make me another. (I'd also seen that, given how I was riding the bike, a fork with thinner, more flexible blades would serve me better.) A year later, the bare steel fork came from Peter and I had Ed paint it and the frame at his brand new shop. (For my weight and riding - and not doing the touring I thought I might do; hence the stiffer blades on the original - the new fork was sweet! And by now, that original would have had nearly 40,000 miles after that crash. Instead it served me well for 2k, then enjoyed retirement.)

Edit: the post Ed Litton straightened fork road no-hands just fine. The guy's good. It was fun watching him.
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Old 02-08-23, 06:21 PM
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What we are willing to do and what we say is a good idea can often vary... Who for the job is for comes into play often. I'll also say that what i might have suggested 8 years ago can be different today given changing circumstances.

Glad this fork has proven to be reliable and tracks straight. Thanks for the follow up. Andy
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