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

[Ron Nossaman]

> CC boards get their stiffness as the already 
> compressed panel is compressed to the point that it won't compress any 
> more, under bearing load. 


A little more detail as illustration. Have you seen foam core 
poster board? It's a sandwich with foam in the middle and thin 
cardboard on both sides. Neither the foam nor the cardboard 
are particularly stiff by themselves, but the cardboard is 
relatively incompressible. Sandwich cardboard in foam, and it 
won't be stiff because the foam compresses. Put the cardboard 
on the outside and it's incompressibility makes the assembly 
much stiffer than the sum of it's parts. Years back, I threw 
together a demo of this. I dug a piece of 5mm plywood (Yamaha 
crate, when they were still wood), and a straight 25mm x 19mm 
piece of scrap pine out of the wood pile. The "panel" was 
about 20cm x 79cm, and I cut the "rib" to the same length and 
glued it down the center of the panel, clamped in props to 
force the assembly into a crown of about 10mm. After the glue 
dried, I tested it's load capacity. My full weight won't force 
it flat.

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.

Incidentally, Del's procedure of epoxy soaking the top of an 
old soundboard that ought to be replaced but isn't, works by 
making the top surface of the panel less compressible as it is 
saturated by the epoxy, making the whole assembly stiffer 
under load.
Ron N