[pianotech] Downbearing on RC&S designs was RE: Steingraeber

[Gene Nelson]

> Look at a CC soundboard you're setting bearing on. Unloaded, it's very 
> springy. You can push down on it and it will deflect. Pound on it as you 
> push a wedge in between the bridge top and a plate strut, and at a certain 
> spot, it quits going down. That's the point where the panel is compressed 
> to where it has nowhere to go, and that's the point at which you set 
> bearing that will load 600lbs or more of string downbearing on it to keep 
> it at maximum compression. That's where the stiffness comes from. As the 
> wood crushes over time (sometimes not much time, sometimes a long time), 
> Compression set and outright crushing of the wood relieves some of that 
> compression, and the board loses stiffness. Crown lessens or even 
> reverses, downbearing lessens or even goes negative, and the killer octave 
> joins the party.
>
Great illustration. Lets see if I got it right:
>From its birth as a dry flat panel glued to flat ribs it swells as it takes 
up moisture.
Bottom of board gets compressed, top may have tension, top of rib in tension 
and
bottom in compression. The constraint of the ribs maintains this state with 
or without
string bearing load as long as the emc remains higher than glue up.
The assembly would need to be dried to 4% or so in order to
release the compression/tension.
Gene