Greetings, I, like Dale, prefer to measure ratio by hammer rise per key depression. If the key moves x amount, and the hammer moves y amount, then there is a ratio that I can use. I found that my results were more consistent when I added 10 grams to the SW before measuring . The additional weight removes some of the variability through additional compression of felt and leather. I also stick a needle out the side of the hammer to most accurately measure how much it really moves. The point of a needle, on a scale in mm, gives information precise enough to distinguish the difference in action ratio at the beginning and end of the keystroke. They are usually different, and correlate , I think, to the capstan's relationship to the "magic line", and perhaps to the relationship between the whippen radius, (flange pin to jack contact) and the line between whippen center and hammer center, (the "other" magic line"). Regards, Ed Foote RPT http://www.piano-tuners.org/edfoote/well_tempered_piano.html
This PTG archive page provided courtesy of Moy Piano Service, LLC