What measurement tools/methods (other than eyeballs, please) are used to determine if a fork has been bent fore/aft vs. as built?

OK, that's a weird coincidence...just posted a bent fork thread about a minute ago!

Rake, aka offset. I know they're technically different, but it's the distance between the centerline of the steerer and the dropout center, measured perpendicular.

The key is knowing what it was beforehand. If you have published geometry, use that. If it's Japanese, it's probably correct. Something from Europe, maybe less so. Otherwise, one can go off of guesstimates based on what was typical for the time it was built.

At any rate, trail is a better reprensentation of how the bike handles. It's best measured/calculated using a digital angle gauge and fork rake measurement, pluggin it into my favorite calculator at yojimg.net.

I'd be happy to consult in person at Atelier Gugie. Payment in beer, as usual.

Check the wheelbase if that is known. The do a visual check of the head tube to the fork assuming that the fork is the type that the legs start out aligned to the head tube.

For me the for sure check is to remove the fork from the head tube and check for signs of paint cracking at the bottom of the steerer tube. I may be showing my age now because it is possible that newer paint may be more flexible. Once the fork is out, it can be placed on a flat table and supported in a few places with pieces that are exactly the same thickness.

I work at a place that has a granite table and machined parallels, a good kitchen table may work and then find three of something that is the same thickness.

...in essence, this ^^^.

But since the whole deal with a fork in the first place is to hold the wheel at a certain distance from the frame so that the tire contact patch is a certain distance from the imaginary line you visualize by looking at the head tube and projecting it all the way to the ground, trail can vary by a certain amount from the original design and you can still have a reasonably rideable bicycle.

I don't think it's a good idea to discount the value of visual inspection, because most of the bent forks I've seen were easily identified as such by visual inspection.
The other thing you can do is to project (visualize) a line, using a straight edge, along the back of both fork legs. those are generally straight until you get to the curve forward at the bottom.

On a lot of older bicycles, there can be considerable variation between the fork legs. As long as the fork ends are where they need to be for the wheel to run centered in the plane of the bike, it works out fine.

Yeah, visual, to a point. Just sight down alongside the head tube to the bottom of the fork. If the fork is bent it will be at or near the crown, and the upper portion of the fork arms will be not in a straight line with the head tube. However the OP asked about telling the difference between a bent fork and one that was made that way, slightly off. I don't see that there is any difference, certainly not one you could see or measure unless it was so bad that the fork arms had rippled metal. The fork arm/HT alignment and the wheelbase and the distance between tire and DT will be whatever they are regardless of how they got that way. If it seem like so little that "it might have been made that way" then the bike should be just fine.

Mount it on the stand of your choice, like this


Fork Angle Bends 005 by wrk101, on Flickr


Then place a straight edge vertically, through the bolt of the stem, down the stem, down the center of the head tube, past the center of the fork crown, and down the fork leg. Typically, if it is bent, the fork will appear veering behind your straight edge (towards the main frame triangle/DT).


Fork Angle Bends 004 by wrk101, on Flickr

Interestingly enough, a lot of production bikes were built with the fork blades visually not quite in line with the steerer. More of an exception to the rule, however.

I havent been able to find published specs for a baseline. Subject is 82 Specialized Sequoia, 22.5" seat tube. [MENTION=111144]Andy_K[/MENTION] if yours is same size can you measure the wheelbase for me. Anyone else have one to measure?

Somebody on Flickr has a scan of a Bicycling Magazine review of the '82 Allez and Sequoia that includes specs. I'll attach the specs for your convenience, but here's a link to the album that includes the full scan: https://www.flickr.com/photos/leapin...57625500696996

FWIW, my Sequoia is the same size as yours. My notes say the wheelbase is 1025 mm, which is in reasonable agreement with the 40.5 inches claimed in the BM specs. A previous owner attached mid-fork braze-ons to mine so there's a slight chance that my fork is not in its original form, but I don't think that was done. If you want we can take both Sequoias over to the Atelier sometime and gugie can put both forks in his rake measuring thingy to get a really accurate comparison and possibly even bend yours to match mine if necessary.

That's a big help Andy. I will measure w.b. when I get home.

My wheelbase (to mid-span of rear dropout) measures about 39.5 inches. Score one for the seeing is believing crowd.

The Atelier is always open for my BF buddies.

I'll swing by with some bent parts one of these days. I like the idea of using Andy's bike as a template. Just have to be careful not to bend Andy's fork to match mine.

It might make more sense to measure fork DO to center of BB.