[pianotech] Measuring Crown Radius

[David Love]

I think you need both (crown with load and without load) along with the
bearing measurements before and after as well.  An analysis of the rib scale
is also helpful even with pure compression crowned boards and certainly with
hybrids.  The ribs still do a fair amount of work even in compression
crowned boards once glued into the rim.    

Measuring the crown under load doesn't tell you much.  You could have
negative crown on a board that's way overloaded that might respond ok once
the load is adjusted to the proper level--damage aside.  The crown with
bearing measurement tells you more, but still not enough.  Measuring the
crown with tension and without tension and the bearing change with and
without load tells you quite a bit about how the board is responding and
whether it can withstand any significant load or not.  Insuring that your
limited and randomly chosen measurements actually reflect the reality of the
load distributions is another matter.  More is more in this case.  

David Love
www.davidlovepianos.com



In the many instances of trying to diagnose soundboard problems in 
pianos, nearly everyone, certainly every manufacturer's rep, has told me 
you can't tell the condition of a soundboard under string load. I find 
this to be entirely backward. An unloaded board will tell me virtually 
nothing about it's load capacity, while a board with positive bearing 
load retaining positive crown is an indication of a structurally viable 
board. Anything an unloaded board seems to be telling you is untrustworthy.
Ron N