Originally Posted by
cny-bikeman
As the head tube is at an angle I don't think it important to worry about the much smaller angle between chainring tooth and dropout, especially as it will change when you rotate the fork. For an easier, more accurate method without removing the fork try this:
Use a T-square or level (or really any long piece of metal, plastic, or even a piece of stiff cardboard, at least 2 inches wide with a 90 degree corner) and lay the long edge along the center of the fork blade (and head tube if long enough) so that the end intersects the dropout center. Measure from the long edge to the center of the dropout. Done.
This assumes that the fork blade has a long section that is parallel to the steerer tube. It might a little off parallel, or fully off parallel, and you won't be able to tell in the first case. But your measure won't come out right.
You can also try to use the head tube, but the fork crown is going to get in the way of holding something against the side of the head tube to use its center. It jst isn't an easy thing to measure directly.
Originally Posted by Sy Reene
Thanks! Sounds good if the chainring tooth seleted is inline with the fork dropouts. Do you think precise enough to distinguish a 40 from 43 or 43 from 45mm rake?
Probably 40 from 43, but a 2mm difference is going to be difficult. The bottom of the chainring is a point pretty close to a right angle from the dropouts, but it isn't perfect, so that's a small error. (Something like 2mm for every 5° off of 90°.) And then you have the measuring error problem - did you measure to dead center of the dropout, was the fork perfectly centered. And you can refine the process and calculate out the geometry errors if you have a very repeatable measuring process. This method is just the best one I have heard that doesn't fall victim to all the minor curves and incompatible shapes that make more direct measuring methods more useful.