[pianotech] Reversing Crown

[Noah Frere]

Okay got it. I thought the rim, though strong, was flexing enough to effect
crown and bearing due to the immense pressure from the strings.

On Thu, Dec 31, 2009 at 10:54 AM, Ron Nossaman <rnossaman at cox.net> wrote:

> Noah Frere wrote:
>
>> I find this confusing because I would have assumed that as soon as the
>> soundboard starts pulling away from the strings, and negative bearing sets
>> in, then the strings would be /pushing/ the soundboard down further, rather
>> than pulling up.
>>
>
> I don't see how. Positive bearing is what supplies down pressure. Negative
> bearing would pull up. The string is under tension and trying to maintain a
> straight line between capo and aliquot (or whatever).
>
>
>
>  I imagine it this way: imagine a tunpperware container with a lid that's
>> slightly convex - you push the sides together and it forces the lid up
>> higher. (the equivalent of putting on strings).
>>
>
> No. Putting on strings, assuming positive bearing, will push the board
> down, not raise it up, and the rim has nothing to do with it.
>
>
>
>  Now take the pressure off the container and push the lid down just enough
>> to make it concave, reapply pressure, and the lid will continue its downward
>> path.
>>
>
> Again, you're manipulating the perimeter, which in a piano would be the
> rim. The rim has virtually nothing to do with crown formation or support in
> a piano.
>
>
>
>    Visually this description reminds us of the soundboard, but i mean to
>> apply it to downbearing.
>>
>
> Then where is the downbearing in your illustration? There is none.
>
>
>
>  That is, no matter which direction the crown is facing, the tupperware lid
>> represents the board/bridges as a whole.   However, being neither a
>> physicist nor a rebuilder, i must have it wrong,
>>
>
> Yes, wrong. <G>
> Ron N
>
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