A good question.

In my case, I eyeball the fork from both sides and square on from the front. A center line, drawn through the head tube and extended down the fork blade will help to determine bent or unbent. Hard to explain how to do this but easy enough to do. If the center line of the head tube extends down the center line of the fork, you are OK for that part of the gauging.
https://cimg7.ibsrv.net/gimg/bikefor...1d0a4f5c1c.jpg

Looking directly square on at the front, does the wheel evenly split the distance between fork blades? If you, that is another positive suggesting no fork damage. If the wheel does not evenly split the distance, the problem could be either a bent fork or an out of true wheel (just turn the wheel around and install to see if the even split thing changes - if not, the wheel is probably OK...
https://cimg0.ibsrv.net/gimg/bikefor...100d2a04dc.jpg

Cracks in the paint are an immediate indicator of frame or fork trauma. When you see these cracks, you better know what you are doing to make them go away and get everything true and good to go.

Riding the bike will also help to indicate a bent something or other. If the bike pulls to one side or the other, suspect head set issues or a bent frame or fork.

Other than that, just ask us and we will all tell you what we think. In my case, I suggest a problem only if I really see something that raises red flags. Often times, it is picture distortion. Those other times - pooey stinko!

I have come to the conclusion that a straight fork/head tube looks slightly bent backwards to the eye/in photos. I don’t know why this is the case. It often seems more pronounced in larger frames.

I don’t think most people would notice if they were riding a bike with a mildly bent fork.

The fork on the Miyata looks slightly bent to me. Perhaps it was built slightly off. I’ll bet it rides fine.

In any case, if you bend the fork on a Miyata slightly backwards you just get a Colnago. The seller should this quadruple his asking price.

You would have to figure out how to vet a buyer which may or may not be easier than proving the fork is or is not bent. ;)

Learned it from here?

-Kurt

With several non-trivial caveats, namely:

1) "Proof" would require in-person inspection(not limited to buckled tubes/lugs/paint or steerer) , disassembly/reassembly, and test riding.
2) Photos commonly have distortion, and an accurate in-plane side or frontal shot can be difficult to achieve.

That said., overlaid traces to mark out the geometry can give a starting point.

Apologies for the rough qualty of the drawing using a cheap photo editor on a tablet with my finger for a stylus.
But I think you can get the idea:

https://cimg0.ibsrv.net/gimg/bikefor...edadf03d71.jpg
The lines could be finer, as well as extended longer so that they cross (or align).

I think this thread is bent.

Nice bike. Too bad the dork disc is bent.

...I tried to bring to bring a little science and optics to this topic in this other thread. But the Bent Forkers shouted me down. :twitchy: Now I just :popcorn.

The lowrider guys here do it on purpose...
 

Because in the photo, it actually looks like the fork is bent? Like that blue and pink Miyata in post #11 . Or is it blue and purple? Hell, I can't tell ;)

Yup. At least, rhat's my assessment of the situation.

Some years ago i bought a frame from a seller who is, I think, generally well regarded. He sells a lot of good old bike parts at reasonable prices, well described, good photos. It was not a cheap frame, and shipping was not cheap either. Total cost to me was around $400 IIRC. The frame turned out to be bent. Once it was pointed out to the seller, he argued the damage was very slight. He also suggested the frame was bent in shipping, or I'd bent it myself. Seriously? It had the kind of damage that can only happen in a head-on crash.
​​​​​
Well, that was a pain in the ass to resolve, and an expensive lesson, namely that it's easy to miss a bent frame or fork, even for someone with a lot of experience. So, on these forums, when i see someone making what might be an expensive mistake, i try to point it out.

Sure, it's a meme. And sometimes a joke, about as funny as "send it to me for proper disposal," which we also see every day or two. But most often, if someone suggests a fork might be bent, it's because it looks bent. And in some cases it will turn out to be bent.

40-50+ years is a long time for someone to potentially ride into a curb, especially considering the condition most of our bikes show up in.

I should post a picture of my bike. I think the fork is bent. I raced and crashed a few times it decades ago, and have massive wheel-toe overlap.
Although, I built it to be a tight criterium racer, so overlap was expected.

“Most of the time it isn’t,” according to the OP....

I recall you starting numerous threads earlier this year asking some very newbie type questions. Many here have years and years of experience riding and working on bikes and have seen much more than I and much, much more than you apparently have. I'm not a pro mechanic, but have been restoring vintage bikes for quite a while but when someone points something out that I have not noticed, I take a closer look. Even if they are wrong, I usually learn something anyway.

Be humble my friend, and keep learning. :thumb:




......................

:thumb: Exactly this, if you're not learning, you're not paying attention or capitalizing on the opportunity to do so. ;)

Even in regular photos if we see something suspect, we call it out as someone could be in grave danger.

And while it doesn't happen very often we don't want you or anybody else to be the one that eventually sells a bike that hurts or kills someone.

There have been instances where something was pointed out, many others say "I don't see it", it gets taken apart to find a cracked steerer tube, stem, crown or some other anomaly that could have been catastrophic had it been undiscovered.

Like the majority of what we do, we have best interests at heart albeit sometimes very spiritedly if not annoyingly, sometimes. ;)

I read your optical reference. As you say, the difficulty in determining if a photo of a structure shows straight lines or curves as they really are lies in the fact that we don’t know the relationship between the focal plane of the image-gathering surface and the plane of the structure being photographed. If the two planes are not parallel there will be distortion that no amount of fiddling with rulers on the monitor or paper photographic print will correct.

This effect is immediately apparent to anyone who has tried to photograph anything like a building taller than one story or a statue of a general on horseback. When you tilt the camera back “to get it all in”, you invariably find that the building or statue looks like it is falling over backwards in your picture (maybe that is the point with the statue...). Vertical parallel lines no longer look parallel and measuring with a ruler on the print will show them to be not parallel. But we know the building is standing perfectly vertically: the photo of it is not a true image. This effect, and measures to adjust for it, are described in introductory photography texts. The optical characteristics of different lenses are added to this, especially at large apertures.

Curved, raked structures like forks are even more difficult to assess in casual photographs. Perspective of curved things like a bend in the road or the graceful ellipse of a Spitfire’s wing are challenging to draw properly from an oblique view.

Also should have noted that the newbie questions are a cornerstone of what goes on here, if they ever stop, we're all in big trouble, keep em coming. :)

I totally agree, I learn quite a bit from newbie threads, not only from other more experienced members, but when I respond to a question, I often double check my answer before hitting "enter"

If there isn't an app to check fork straightness on a photo already, I'm sure it will come out soon.

yes, kind of like when you run into an old aquaintance..."hey man, good to see you, are you gaining weight or are you just losing your hair?"

^^^ Do people really say things like that to acquaintances? If I did, I would probably conclude it was the explanation for why I don’t have many friends. It sounds gratuitously hurtful. I do have an acquaintance whom I run into every few years. He went through a period where he would say, “You’re looking great! Have you lost weight?” I knew he was being glib and insincere but at least not hurtful.

”Fork is bent,” as a supposedly good-natured jibe seems particularly malicious if said publicly and indelibly when the fork’s owner is trying honestly to sell an undamaged bike.

Complaining about someone pointing out that a fork is bent is called "killing the messenger."

If the messenger brought news to the King that he knew was false, or was recklessly careless about its veracity, and knew the King was not in a position to be sure about the true state of affairs, the King probably would kill that messenger.