theapodan

I wrecked and bent my (steel, canti brake) fork to the side badly a couple of months ago. Clamped it up in a jig and unbent it, aligned forkends, set overlocknut distance. Measured side-to-side error was within +-1. 0mm. Buttoned her back up and I could not notice in general riding.


Since then, I feel like I now notice a slight pull to the left, and my fork appears to have relaxed towards the initial bend, although I haven't pulled it to measure, and I threw away the paper I'd made all my measurements on before.


If you bend and then unbend a steel fork, is there any memory in the metal such that it might relax towards its initial bend over the course of a couple months or am I just insane?

trashbiker

I can't comment on memory or anything, but typically the advice is not to unbend bent forks. I can't give any metelaurgy knowledge to back than up, but the three mechanics I've worked with/for have all said that. On my own bikes I like to implement jank solutions, but steering and brakes are something I never play it loose with. The fork has both. Depending on wheel size and steerer type a replacement might be quite affordable so I'd recommend looking into that.

soyabean

I do this sometimes with curb finds and you need to do a bit more than just eyeball it.

Sometimes you need to mess with the non-accident side of the fork as well, just to get them both to equalize.

I remember the last time I really had to do this was use two poles that slipped over each fork end to act as cheater bars.

In any case, this fork is shot and you're simply doing the best to salvage the bike.

unterhausen

I have heard people say steel can move after it is cold set. All I can say is that I've never seen it. At one time, most steel bikes were cold set at the factory, some were cold set a lot. When I worked as a mechanic, we used to unbend lots of forks, and I don't recall any issues

When you bend steel, the metal undergoes elastic deformation and plastic deformation. Most of the elastic deformation is released right away, that's why you have to overbend. It seems plausible that there is still some elastic deformation present when you are done, but it's difficult for me to understand why that would make it move back to the fully damaged state. If that was so, wouldn't forks get better when initially bent in a crash?

Crankycrank

Check the fork joints, steerer, fork crown as these may have been damaged and slowly coming apart. Also check your frame for alignment as whatever bent the fork may have bent the frame too.

Wildwood 

Once steel is set (correctly and without damage) it stays. If it is not done correctly or if the damage (in your case) is too severe then there may be future issues. Fork legs do not 'rebend' themselves, unless something bad is happening. Steel has no memory if cold set.

this image appeared recently in C&V and while it may not represent your specific situation, it conveys a worthy message. Rider was not hurt.

grumpus

I've seen a fork that acted somewhat like this, turns out there was a brazing fault that was hidden inside the external crown. Because the bronze filler had not fully flowed into the joint the fork leg was hanging on by a small area that had adhered, but mainly the paint. So check the joints at crown, steerer and ends for movement, just in case.

Schweinhund

Mechanical relaxation, steel can do this after bending.
Rebending is sometimes necessary.

3alarmer

...I have realigned (by bending) dozens of steel bicycle forks. None of them have shown the slightest tendency to return to where they were.
AS stated by another poster, you need a properly designed fork jig to do this. I don't have any idea what sort of jig you were using.

If you notice a pull to one side, there's a very good chance your realignment procedure was not 100% successful in the first place.

But steel bicycle forks are pretty correctable, if not too far gone. The reason for the advice given by the several mechanics quoted in post # 2 is liability and insurance. I won't realign them for other people, for this reason. Not worth the exposure. There are other possibilities, like maybe your fork was too far gone for correction by bending, possible slight bend in the steerer where it meets the crown, etc.

maddog34

The material will more easily REBEND in the direction of the previous bending.
The grain structure of the steel has been pulled apart some on the outside of the bend, and some was compressed on the inside of the bend... Repeated bending of the steel will eventually open a crack...

Riding and Braking forces are causing the rebending, and that rebend will occur in general alignment with the previously disturbed grain structure.
If the bike was never ridden, tthe fork would remain as it was after the "straightening."

"i straightened that bracket last night, but it's bent again after just a few minutes!!!"
Seem it many times...
The degree to which, and rapidity it happens with, depends on material strength, alloying elements, hardness, and use... but it WILL happen easier than the first bend..
usually when someone says "I can't straighten that".. they really mean: "I Won't Bother straightening that" because they know it would just bend again, or break.

If there are Compression Wrinkles in the tubing, most refuse to straighten a fork... the Wrinkle bends are tiny, very tight radius, bends. Microscopic Cracks are already present in most tubes that show wrinkles. Paint separation is an outward sign of Stretching, or severe bending... Repeated load cycles will open the tiny cracks into larger ones......... nose diving into pavement is not on my bucket list.

theapodan

This makes sense, although some (including me, initially) have had good experience unbending forks.

I'm going to pull the fork tomorrow and measure. It's possible I'm just psyching myself out with the perception that something is diifferent, but maybe it's no different than after I unbemt it and I've just noticed the behavior for the first time because I'm trying to learn hands free.

I would just swap it out as some suggest, I've even got a parts bike with a compatible fork, but I like that the fork has a plush ride, matches the bike, and I bought an all-steel bike precisely so that I could bend stuff back and get back on the road if I had an accident.

FBinNY 

Steel does not have memory in the sense you're thinking.

Once cold set to a shape, that form becomes the zero point for load and flex considerations.

That is unless there are cracks, which would imply weakness in some directions.

So, either

It's your imagination, or

Something like some hard impacts bent it anew, or

There is some kind if crack you missed.

Check everything, including carefully measuring it's current state. Then, keep a close eye on it. If it continues to move, scrap it before it scraps you.

3alarmer

.
...since the compression forces you mention in your wreck were from one side, you might also want to take a look at the alignment of the head tube. It's possible to twist a frame in an accident like that, and it's not always obvious to the eye. But if you think it was riding fine after your fork realignment, and now it's gradually starting to pull to one side, that's unlikely. As others have said, there are brazed joints to consider, as well as some small wrinkle or crack you might have overlooked.