Soundboard recrowning and rib re-arching.

[Ric Brekne]

Thumpy writes:

    Hey, Terry, yore a  real smart guy, so I know, that you know that
    when a  board is bent, 1 side goes into compression, the other into
    tension.

Everything is relative there Thumpy. You have to remember that we are 
not talking about a plain ol bent board.  Its a board which is far more 
compressed then it would be if it was just plain ol bent.  In a bent 
board the part of the board that is neither compressed or tension is 
along the exact center line thickness wise.  But if the board is 
subjected to some degree of compression independent of plain ol bending 
compression... then this has to be added into the situation as it were.  
If you add enough of this independently caused compression then the 
whole panel will be under compression no matter how much you bend it.  
Now the top half of such a compressed panel would be less compressed as 
it were then the bottom half to be sure.  Thats where the relativity bit 
comes it... you could look at things like a number line where you move 0 
around to fit the middle point.  In that case you could say the top half 
is in tension relative to the bottom half... but you cant forget to take 
into consideration the absolute degree of compression there is (or isnt) 
if you are going to speak in absolute terms at all. 

Just how much compression is created in a compression soundboard and 
whether that is enough to put the entire panel in a state of compression 
despite the relative state of affairs when the bend is taken into 
account I dont know.  But no doubt the center line is no longer halfway 
between the bottom and top sides... its been moved up towards the top 
and most certainly more then half of the panel is under compression.


    If it is then glued to a panel, and said panel is smashed flatter
    over the years, this rib must also go intocompression, somewhat, or
    no crown would be lost.

I'm not at all sure I buy this.  I'll be the first to argue that a rib 
is tensioned somewhat as part of the process of the panel taking on 
humidity and compression... I'll even go so far as to suggest that 
perhaps the rib tensions a bit more then it would if it was just plain 
bent... i.e. its center line drops just a hair leaving more then half 
its mass under tension and less then half under compression....  but 
when a compression panel flattens out to the degree that it would stay 
flat in the abscence of any bearing... well in that case the ribs are 
simply going to relax me thinks.

Cheers
RicB