[pianotech] Measuring Crown Radius

[Ron Nossaman]

On 7/15/2011 1:57 PM, John Delacour wrote:
> At 07:27 -0700 15/07/2011, David Love wrote:
>
>> The discussion was whether a rib bent and clamped to the rim while
>> bent supports load before it goes negative.
>
> There are two things I don't understand here. First, what has
> "supporting load" got to do with the function of a soundboard?

In the days before carbon fiber, a working traditional crowned (by 
whatever method) board is typically expected to have both positive crown 
and positive bearing. If a board is maintaining crown under positive 
bearing, it's supporting load.


>and
> second, why would anybody put a piano together in such a way that it was
> possible for the soundboard to "go negative"?

Beats me, but it's been done forever, and still is.


> Probably the ideal situation is a soundboard which gives the proper
> pressure (admittance) at the bridge when it is almost flat. What degree
> of crown that soundboard will have when the strings are off will depend
> on its compressibility, and that is not an easily measurable quantity.

Which is why I said I find unloaded crown measurements in old boards to 
be of no use to me.


  >I
> can't see any virtue in having any considerable degree of crown on the
> board when the tension is on.

Personally, I want the loaded crown to be at least as high as the 
bearing offset - so it can't go negative. Other than that I don't find 
any clear evidence that it makes a difference.
Ron N