Stuart rib dimensions

[Phillip Ford]

Ron Overs wrote:

Try your own quick test with a sample to see what I mean. Our rib no.
11 on our 225 piano supports notes C52 to F#57, and has an active
length of 580 mm. With just 1% of down bearing (which is quite
moderate by current standards) this rib and panel strip will be
subjected to a down force of 24.4 Kg. Build yourself a test rib of
the contemporary-full-dimensioned sized of 25 X 25 mm (Steinway D mid
board ribs), glue it to a 120 mm wide test panel and load it up on
the bench with a 24 Kg test weight. You'll quickly realise why the
typical piano's killer octave is well and truly dead before it
arrives at the showroom. When you consider that most makers are using
ribs smaller than 25 X 25 in the killer area, these will be subjected
to even greater stresses. As an aside, I had occasion to take a look
under a late Stuart concert piano a few weeks. The observed 20 X 21
mm rib dimensions would seem, in the light of this discussion, to be
an unusual approach.

Ron,

I meant to comment on this but it slipped my mind, so I dug this out 
of the archives.  I also went back to the archives to retrieve a 
comment by Del that on a (the) Stuart piano that he saw he was under 
the impression (I believe he said unverified) that there was little 
if any downbearing, whether by accident or design.  Do you know if 
Stuart uses downbearing?  I know he makes much of his string 
termination and keeping the vibrations 'in a vertical plane' or some 
such.  Is no downbearing part of this system?  If so, then his ribs 
wouldn't be subjected to the same forces as a piano with downbearing. 
On a board with no downbearing the choice of dimensions seems less 
straightforward than if you are selecting the rib dimensions to make 
it perform as a beam capable of carrying a certain amount of 
download.  Perhaps it would be a rib that is stiff enough to break up 
the panel when vibrating to emphasize high frequency modes, but I'm 
not sure what stiff enough means in this case.

Phil Ford


-- 
Phillip Ford
Piano Service and Restoration
1777 Yosemite Ave - 130
San Francisco, CA  94124