R,C&S question JD

[John Delacour]

At 23:17 +0100 26/1/08, Richard Brekne wrote:

>Hi JD... What I see here is that in an RC & S board under 
>downbearing load at anything near glue up MC, the ribs will be in 
>the opposite condition with respect to which half of the rib is 
>under compression and which half is under tension then traditional 
>boards.

Well, the "traditional" board never experiences downbearing load when 
the moisture content is anywhere near what it was when the ribs were 
glued on because it relies on moisture uptake to form the crown by 
internal compression.  At the time the ribs are glued on the 
un-ribbed side of the board is slightly stretched by being forced 
into a dished table by the curved ribs, but once the glue is set and 
the assembly is removed from the press this tension is at first 
reduced and then replaced by compression as the board takes on 
moisture, vainly tries to expand along the ribs and somewhat less 
vainly tries to expand on the unribbed side, so that by the time the 
board has acclimatized the various forces have increased the 
convexity, or upward curve, of the whole structure, the greatest 
compression being at the glue line, where no expansion is possible.

Once the board is installed and the piano strung, the crown is 
pressed down by the force at the bridge and the compression at the 
top of the board is further increased.


>   The [RC&S] panel's probably reasonably significant compression 
>will be due string load forcing it (and the ribs) down.  So the 
>panel will be in somewhat similar condition to compression reliant 
>assemblies... while the ribs will be in opposite orientation.

As I understand it the RC&S board is subjected on glue-up to a 
greater tensile force on the unribbed surface but (a) since it has 
been less dehydrated and (b) because it has been forced round a 
tighter radius against much less flexible ribs, some of this tension 
will either remain or be reduced to a point while the assembly is 
free.  Compression at the glue line will exist, of course.  Once the 
piano is strung, the downbearing will press down the soundboard less, 
because the beams (ribs) are more solid, and hence there will be less 
reduction in the curvature and less increase in compression (or 
decrease in tension) at the surface of the board.  Now those who 
practice this art will be able to give actual rough values to these 
phenomena, but this method of construction seems to have as one of 
its aims the avoidance of anything close to the degree of compression 
to which a high-class traditional board is subjected.  The 
specialists will correct me if I am wrong.

As you say, a lot of things are unclear, and these discussions often 
start off with some hope of providing enlightenment, facts and 
figures, and a statement of principles but all too often, almost 
always, deteriorate into a rather vague mish-mash, a bit of dogma and 
bye-bye.  I try to limit myself to facts and experience and as much 
science as I can muster, which is not always much!  Opinion is 
worthless -- and in my view excellent piano tone is far less a matter 
of taste and opinion than you have recently suggested.  There are a 
number of measurable qualities in the sound of a good piano.  It's 
nice to have a fairly good string scale but I could name several 
pianos that won gold medal after gold medal in the old days with 
quite outrageous stringing scales and took the prizes because the 
work they did on the belly was good.  The belly itself produces 
nothing, but what it does with what it's given is what makes the 
difference between a "satisfactory" piano and something that sends 
shivers up your spine.

JD