Thank you so much for your comments. I have read many posts on this site on this subject (and others) and marvel at the diversity of experience, opinion and debate amongst you. Its so cool that forums like this breakdown the tyranny of distance so easily, so we can share our passion.
Grand pianos are not widely available in Australia, and restorers who replace soundboards almost non existent. Steinway Grand pianos are far and few between, and even more so NY, since Hamburg is sold in Australia. They are ubiquitous in the US, so that is another reason why I seek some insight from you all in the US, who have had so much opportunity for experience with these pianos.
I measured the crown from underneath in the three widest inter-rib sections that could fit a 39" (1m) straight edge that I have on hand. Counting ribs from the front/treble end :
rib 6-7 span = 40", crown measured over 39" = 0.143
rib 7-8 span = 44.5", crown measured over 39" = 0.112
rib 8-9 span = 40", crown measured over 39" = 0.104
I acknowledge that crown may be different elsewhere.
I also took some detailed measurements of height and depth of bridge, board (measured right next to the bridge), and frame height at aliquots (or hitch pins in low tenor) :
I have marked shimmed notes. They DO align with low downbearing points. To be further noted is inversion around the upper treble strut - where the bridge is low and the frame high. The bridge height relative to frame is low due to BOTH a dip in the underlying board, and a dip in the bridge depth. Notes in this area are a definitely a little weak, but not used that much so I have not bother attempting shim on these. The bridge sits nice and high in the last octave, which I believe is by design, and rings like a bell. Perhaps the dip in the board height is indicative of a board issue, although the bridge is dropping away from peak of the crown in middle of the board (across the line of the ribs).
Regarding the determination of downbearing, I think we have a terminology issue here. In my mind, downbearing = downward force exerted by a string on the bridge, is absolutely and universally fixed for a given string tension and angle of the string either side of bridge. This can ONLY be measured when the board is loaded, since only THEN do we know the angles (tension of a given string weight, length and tuned frequency is fixed)
If all boards were engineered and built for a given make and model of piano with equal stiffness and crown, then health of a board should indeed be easily determined by measuring angles subtended by the strings at the bridge. Given that boards are made from organic material that is not uniform, is not engineered, and subject to variable environmental stress and ageing/fatigue over its life, I can surely appreciate that degree of down bearing is not a universal measure of the health of a board. Your ears are the ultimate judge of that - if it sounds good, its fine. That said, there MUST be positive bearing on the bridge. Negative downbearing is surely a universal problem.
Seeing the board unloaded may indeed give the best view of the uniformity of the crown, but technically speaking, you still can't know what is the right height to set the soundboard - leading to the final downbearing result - unless you have :
- the manufacturers specifications for stiffness of the board, including along the length of the board if a single measurement in one place is insufficient
- manufacturers specification for optimum unloaded height as a function of stiffness
- measured stiffness of the board,
plus possibly ...
- how the downbearing load varies along the length of the bridge
- how the stiffness of the board varies over the length of the bridge (if it is not uniform)
Given that you probably don't have/know any of these, you probably know the right height to set by experiment, experience and judgement - plus your ears before you pulled it down. When you do that, I am guessing that there is still risk in this, given that there is no way of adjustment/optimisation after installation..