[pianotech] GH-1s

[David Love]

Yes, I understand that the cutoff makes for a smaller rib cross section but
it's not without other consequences.  Why not reduce the rib to 550 mm.  You
could reduce the cross section even further.  The tonal implications can't
be ignored.  We can talk about physical structure and ignore subjective
evaluation of tonal consequences but we are building a musical instrument,
not a bridge to drive toy cars over.    

Your question for comparison is a bit in isolation and so it's hard to be
very precise because I don't approach it this way but I will try just for
comparison sake.  Just so we are on the same page I'll assume you did these
beam calculations with MOE for sitka spruce.  

I actually use 1.5 degrees, not 1 degree, but since you calculated the rib
dimensions based on a load of 25 lbs I'll use that.  Twenty-five pounds load
on a rib is very low to me.  If I extrapolate that out and use the total
load on a board as I would load it then that would mean that the board would
have some 36 ribs!  Approximately 900 lbs divided by 25 lbs gives 36 ribs.
Even at 1 degree it would be about 600 lbs and use 24 ribs (I'm using 35000
lbs total tension, I don't remember what a Steinway B actually is off the
top of my head).  First of all, I wouldn't build something like that.  Nor
do I target a specific certain percentage for deflection which is a somewhat
arbitrary number depending on the amount of crown you have as you pointed
out. My approach is a bit different.

But if I had a rib as you describe and wanted a 50% deflection then my 770
mm rib with 9M radius that needed to support 25 lb of calculated load would
measure more like 15 mm wide by 16 mm high, roughly.  That's 24 ribs at 1
degree or 36 ribs at 1.5 degrees for the whole piano.  Of course, I'm using
fixed end formulas, not simple ends formulas.   

>From appearances, I'd say we're pretty far apart. 


Yours:  25 lb load, 770mm x 20W x 26H, and a 9M radius
Mine:   25 lb load, 770mm x 15w x 16h, and a 9M radius (for approximately
50% deflection)

David Love
www.davidlovepianos.com

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The cutoff makes the rib effectively stiffer while it actually reduces the
mass of the rib. A smaller cross section, shorter, lighter rib with a cutoff
will support the weight of a bigger, heavier, longer rib without one.
Similar strength with less material. Simple.


That's big of you, but we're talking about real physical structure here
rather than your subjective evaluation of tonal consequences, which I've
already read at great length.


I'd probably size the rib at 770mm x 20W x 26H, and a 9M radius.

Without another chorus of the dangers inherent in making ribs too stiff,
what numbers would you put on the rib dimensions, showing the figures from
your deflection formula. I expect you'd use the original length, right?

Ron N