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Discussion Starter · #1 ·
Does anybody know a simple, reliable method? The only things that occur to me would be to measure a photo or try holding a straight edge parallel to the headtube and measure against the ground and neither idea sounds very good. While I`m here, any tips to make measuring fork offset easy?
 

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Use an angle finder. Seems low tech, but it works fine. You can get them at any hardware store. You need at least two wheels and the fork installed to measure using an angle finder, and the value changes with tire size and pressure. As far as fork offset goes, I don't know of any easy method.
 

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NeoRetroGrouch
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rodar y rodar said:
Does anybody know a simple, reliable method? The only things that occur to me would be to measure a photo or try holding a straight edge parallel to the headtube and measure against the ground and neither idea sounds very good. While I`m here, any tips to make measuring fork offset easy?
- Find a digital level that is short enough to fit on the head tube (or fashion some kind of spacer with known parallel edges). Set the bike on the floor and mark the position of the front and back wheels. Measure the angle. Turn the bike around and place on the same two spots. Measure the angle. Average the two measurements.

- Install a centered QR in the fork ends. Place the steerer tub on a block on a flat counter so that the QR is parallel to the counter. The block must have parallel edges and be thick enough to hold the legs and crown off of the counter. Measure from the counter to the center of the QR. Turn the fork over. Measure from the counter to the center of the QR. Take 1/2 of the difference between the two measurements.

TF
 

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TurboTurtle said:
- Install a centered QR in the fork ends. Place the steerer tub on a block on a flat counter so that the QR is parallel to the counter. The block must have parallel edges and be thick enough to hold the legs and crown off of the counter. Measure from the counter to the center of the QR. Turn the fork over. Measure from the counter to the center of the QR. Take 1/2 of the difference between the two measurements.

TF
This is assuming the "counter" is flat. I'd say unless you have a good granite counter top, this method is unreliable. Averaging the values will not help.
 

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NeoRetroGrouch
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AteMrYeats said:
This is assuming the "counter" is flat. I'd say unless you have a good granite counter top, this method is unreliable. Averaging the values will not help.
Where did I say to average the values? All you need is for the system to be 'flat' enough to ensure that the measurement is close to perpendicular to the steerer tube and to put it in the same spot for both measurements. - TF
 

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TurboTurtle said:
Where did I say to average the values? All you need is for the system to be 'flat' enough to ensure that the measurement is close to perpendicular to the steerer tube and to put it in the same spot for both measurements. - TF
Close to perpendicular is not perpendicular. A measurement off by two or three mm is not very accurate.

You're right about the "average" comment.
 

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NeoRetroGrouch
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AteMrYeats said:
Close to perpendicular is not perpendicular. A measurement off by two or three mm is not very accurate.

You're right about the "average" comment.
And what does the angle have to be to change the answer by 1mm? - TF
 

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If I assume a 72 degree HTA and the fork is in a straight line through the steerer tube to the dropouts, 1 mm horizontal difference in the distance from a perpendicular line from the top of the crown of the fork to the center of the dropout would be 10 minutes (1/6 degree).
 

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don't bother....

As someone who worked in the machining and precision measurement field for over 20 years, I can tell you that you're wasting your time with any of the suggested methods. A good machine shop with a granite table could set the frame up and get you an accurate answer, but no home method will give you a reliable answer to a fraction of a degree.

Most modern frames will have HTAs in a very narrow range of 71-74 degrees and very few will be at either of those extremes.

The magnetic protracter isn't a bad idea, but there's still a good chance of a 1 degree error and also a chance it won't work at all if the head tube doesn't have enough length of straight surface to place the protractor against.
 

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Discussion Starter · #11 ·
Thank you. The idea for putting a squewer in the dropouts and measure/flip/measure sounds fine. For the angle, I dunno- I`l keep thinking on that one. What I want to do is try to get an approximation of the front end geometry on one particular frame I have that I like the handling of, mostly out of curiosity. Yes, 71 to 74 degrees pretty much covers all head tubes, but that`s still a fairly big range. I don`t need to know to the minute, but if there were a simple way to measure to 30 min or one degree... Maybe there isn`t a simple way. In that case, it isn`t the end of the world.
 
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