More on soundboard crown

[Ron Nossaman]

>As I think you would put it, this is a consequence and not a feature of 
>the design.

Yes I would, in fact, I think I have.


>Perhaps this is just adding to the stress that eventually wants to 
>collapse the board.

It would have to accelerate the collapse of the board merely because it 
adds to the initial load on the ribs and/or cross grain panel compression.


>If it's a negative thing I wonder if making the top of the rim conform to 
>the outline of the board without bending it along the grain would yield 
>positive results.

I don't know, but I doubt it would make much difference, if any.


>Obviously just angling the top of the rim, as some builders do, isn't 
>going to accomplish this.  But if you were to crown your board and then 
>measure the shape of the outline and cut the top of the rim to match 
>(which would mean considerable deviation from a plane) I wonder if there 
>would be a beneficial result.

Haven't tried it. That "considerable deviation" part would give me 
something to consider for some time before I started chiseling.


>  I still need some convincing that none of that crown is being supported 
> by an arch effect.

Then you apparently won't get it from me. I've given you what I've got.


>But this does call to mind something I hadn't thought of.  Since the ends 
>of the ribs are feathered, the line of action is rising at the end of the 
>rib.

Yes, I was about to point out that very thing - drew a picture and everything.


>   So much for the arch effect on the ribs.

Good. RIP, but not likely. Someone else will be back with it.

>As far as the panel cross grain not holding the required compression 
>loads, how much is it supposed to hold?  I agree that it won't hold the 
>entire downbearing load.  On good spruce isn't cross grain compression 
>strength on the order of 500 psi?

580 psi is the published limit at the actual crush point.


>Perhaps this is part of what's happening in the killer octave area.  Since 
>the ribs are shorter (and the panel not as wide) and the arching is 
>somewhat greater, the arch path is stiffer relative to the bending path 
>than it is lower in the scale.  When the load is put on, a great enough 
>part of the load wants to try to go through the panel that it overloads 
>it, effectively nullifying this load path, which means that all of the 
>load now has to take the bending path.  Since the ribs are not of generous 
>enough dimension to allow this the panel and ribs collapse, presumably to 
>the point where the excess load beyond the ribs capacity finds another 
>path, probably through the bridge to other ribs.

It's simple leverage, there's no arch effect. The center of the panel to 
the center of the rib is one moment arm, and half the length of the rib is 
the other (I think, if the bridge is centered on the rib). The load 
required to bend the rib and support the downbearing, multiplied by the 
leverage ratio should give the compression load on the panel required to 
maintain the load, shouldn't it? Factor in the bearing load the killer 
octave is required to carry per rib, including the leverage effect from 
farther up in the treble that you so kindly ran the FEM series on for me a 
while back, factor in a panel that is thinned at the edges so it offers 
even less support by compression than if they'd left it thick, and see how 
many soundboards are hopelessly under-engineered structurally to support 
the loads imposed in the killer octave in the long term. Lots and lots.


>I was afraid you were going to say that <sigh>.  I guess I'll have to add 
>it to the list of things to do.  I think the list may be getting up to 
>about a 5 year backlog now.

Only five? You're not trying hard enough. Of course most of mine will never 
be affordable anyway, so it's not a very realistic list.

>>But what you see isn't necessarily what you get. As Del has pointed out 
>>many times, leave the thing under load and come back next week, or next 
>>month, or next year and see if that support obviously persists. It won't, 
>>because of the compliance of the materials under the loads imposed....
>>
>>
>>Ron N
>
>Yes, quite likely.  But I probably won't be convinced until I do it myself.

This one's easy, and that five years will pass without using up much time 
at all. It's mostly waiting.

Ron N