Down-Bearing for Old Board

[Ron Nossaman]

Hi Terry, 
I've just put an exceptionally lumpy ugly week to bed (I hope), and have
about a nickle's worth of life left - so I figured I'd just as well spend
it here.

Generally, I'll load an old board less than a new one, especially a
compression crowned one. If you had all this crown after drying the board
down to shim, it almost has to be rib crowned, and you can probably get
away with a little heavier bearing load. Dale's description of pre loading
with wedges is a good guestimation method. I'd also suggest crawling
underneath with your Kansas Straightedge (string) and checking how much of
the available crown there is (everywhere you can reach, rather than just at
the longest rib) before and after the pre loading. When you've determined
that you have enough crown after pre load that the string bearing you will
set won't push the board concave, you're there - or at least not past
"there". Note here that ending up with what you might consider "perfect"
bearing angles over a reverse crowned board is a counterproductive
approach. Also, be aware that bearing works with angles, not projected
string height at the back scale termination point, so you have to factor in
your backscale length when you're playing with the string and measuring
plate or aliquot height. The absolutely wonderful thing about doing this
stuff is that if you don't set more bearing than the crown can accommodate,
it's pretty hard to screw it up. The bearing pressure changes quickly as
the angle changes, so as a strung piano is pulled to pitch, the board
deflects and offers more resistance as the bearing angle decreases and
produces less downbearing force. The forces will balance in the middle at a
point pretty close to where you intended when you set the thing up, even if
your guestimation was a little rougher than you had hoped. The silly thing
will meet you half way and try it's best to not let you mess it up, which
is probably the only reason most of us were able to piece any sort of
workable bearing setting procedure together in the first place. 

An alternate method is to weight the bridges, measure deflection, add more
weight, measure deflection, and determine the deflection response curves of
different sections of the board. Then, from the string scale, compute
optimal finish string bearing in each section that will leave the board in
whatever state of deflection you decide you want (balancing bearing load
against deflection curve), and calculate the bridge/plate/aliquot heights
accordingly, averaging as necessary to accommodate the different backscale
lengths. 

Whatever works best for you.


Ron N