Originally Posted by
FBinNY
Yes, you're sketch is wrong, but you did clarify that A & C are along the string, so you have it.
BTW - the triangles aren't congruent, they're similar.
Note- this is my method for measuring with the fork off the bike, but I wouldn't take the fork off to do it this way. On the bike, any variation of the flip the fork and average is faster and comparably accurate (if done right).
So, how about a bet.
We'll need a fork of known rake and a volunteer who doesn't know the rake. Let him measure the rake by the various methods proposed. I'll bet a night's beer supply that he comes closest to the right answer by my method.
Any takers?
I'm your huckleberry.
I started with a 1" carbon threadless road fork from a Bianchi. I put an axle size rod through the dropouts, made a loop in some kite string and pulled the string taught over the top of the steerer and taped it in place on the back side so it would stay. I picked a point above the crown race and just below a sticker on the steerer as my measuring point.
The distance to the measuring point is 6.198", and the inside of the string was at .308" from the steerer on a line perpendicular to the steerer. I took a second measure mid string thickness, which came to .325". The total steerer to axle length was right on 31.5".
To make things easier, I dumped the numbers into this tool, and used the resulting angles to construct the larger triangle with a 31.5" hypotenuse. The resulting lower legs were 1.563" and 1.65" respectively. The steerer was 1", so I added .5" to both for rakes of 2.063" and 2.15" inches. Multiplied by 25.4 to get I got 52.4mm and 54.61mm.
http://www.calculator.net/triangle-c...ts=d&x=74&y=23
Next, I got a stainless small pot from the good folks at Farberware and a rubber tipped clamp. I put the pot on my glass desk top, put the steerer horizontally across the pot with the lip of the crown race against the lip of the pot, and leveled the fork rake down as best I could, then clamped the steerer mid span to the pot and glass. I then took a piece of stiff junk mail and marked the center of the dropout on the vertically held junk mail on both dropouts. I got lucky and they were exactly the same.
Then I did the same thing, but with the rake pointing up. I took the same junk mail and made marks the same way as before. This time right and left were different because I did not get them perfectly level. No matter, I averaged the two rake-up marks and made another mark between them.
Then I took out the calipers and measured between the average upper mark and lower mark. That came to 3.375". Divide that by two is 1.6875", and multiply that by 25.4 gave me 42.8625mm.
During all this screwing around I looked at the sticker for the first time since I pulled it out of bike. It says "Offset____43.0".
As CNY and I made clear earlier, trying to make and use exact measurements from acute triangles has problems. The triangles I measured in the first test were only about 3° on top. Measuring from a round tube to a string took patience and three hands, and the string itself was very hard to judge its real location. A finer string would make it less of question where to measure, but both my bottom and mid string measures produced results that were much larger than the real rake. Working backwards, my measure to the string should have been only .234", not .308". That .075 difference changed the top angle by nearly .7°.
The pot, clamp and cardstock method was faster to set up, was very repeatable, easy to measure and required virtually no math - had I a metric caliper I would have only had to divide the result by two. And I was accurate to the manufacturer's label by .1375mm.
There is no question which one I'd use next time.
Of course, I could have made this all up just to sound smart.