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Author Topic: bent fork (steel)  (Read 1429 times)
buttrumpus
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« on: February 28, 2008, 07:11:48 PM »

here's a question for those with some metal working knowledge. it hasn't been a good week for me, saturday i was rear ended, and today i was pinned into the riding into a pothole by some asshole who didn't want to share my lane. so this pot hole taco'ed my wheel, sent me flying over my bars, and bent my right fork blade about a 1/4'' - 1/2'' out from where it should be. So, how does one go about repairing this? The metal seems fine, looks like my wheel took most of the stress, and assuming my steerer tube isn't bent, I figure there has to be some way to do this. I can't figure out a good google search to find an answer, so help me out!
oh, this was on my very cared-for motobecane, i am very pissed.
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faultymonk
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« Reply #1 on: February 29, 2008, 12:38:31 AM »

share my lane. so this pot hole taco'ed my wheel, sent me flying over my bars, and bent my right fork blade about a 1/4'' - 1/2'' out from where it should be. So, how does one go about repairing this? The metal seems fine, looks like my wheel took most of the stress, and assuming my steerer tube isn't bent, I figure there has to be some way to do this. I can't figure out a good google search to find an answer, so help me out!

Steel in general is quite amenable to bending.  My guess is that it's probably fine to just (carefully) bend it back.

http://www.parktool.com/repair/readhowto.asp?id=40 describes how to cold set a frame or a fork.  Sheldon Brown also has a page where he describes how to cold set a steel frame - http://www.sheldonbrown.com/frame-spacing.html.

I can't pretend to understand the intricacies of how steel is strengthened or weakened, but:

http://en.wikipedia.org/wiki/Strain_hardening, http://en.wikipedia.org/wiki/Cold_rolling and http://en.wikipedia.org/wiki/Tensile_strength describes how metals can be hardened or weakened.  In particular, "while cold rolling increases the hardness and strength of a metal, it also results in a large decrease in ductility. Thus metals strengthened by cold rolling are more sensitive to the presence of cracks and are prone to brittle fracture."  Fracture = bad, at least when it comes to forks.  So it seems that tensile strength is likely more important than hardness when it comes to forks.

However, "metal that has been hardened by cold rolling can be softened by annealing. Annealing will relieve stresses, allow grain growth, and restore the original properties of the alloy. Ductility is also restored by annealing." - http://en.wikipedia.org/wiki/Annealing_(metallurgy)

So it sounds like to me that while it's probably fine to just bend the fork back to its original shape, if you're really concerned about the "strength" after bending it (i.e. tensile strength - lest it fractures/shears while riding, which is not entirely unlikely), you can restore the fork by getting a shop to anneal it by "substantially heating the material (generally until glowing) for a while and allowing it to cool slowly."  Naturally, this probably means a new paint job too.  Afro

« Last Edit: February 29, 2008, 12:58:50 AM by faultymonk » Logged

buttrumpus
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« Reply #2 on: February 29, 2008, 10:03:03 AM »

jesus, that was the best response ever, thanks for all the info. this should be an interesting series of events.
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psmith
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« Reply #3 on: February 29, 2008, 12:50:43 PM »

I have some experience in this department, so PM me and I'll send you my number or call me if you already have it. Sucks about the bike, but at least your body is intact!
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