Rocking bridges

[Robin Hufford]

Ron,

     Thanks for you interesting post and  look forward to hearing what you what
you have to say.
Regards, Robin Hufford


Ron Overs wrote:

> Robin and other interested parties,
>
> I've been following this thread all the while. No I haven't been
> ignoring you all, I've just been caught up in a local battle for a
> pianist's right to use our piano in the 2002 Sydney Festival. We just
> got approval for this on December 22, hence my absence from list
> discussion. I'm currently on holidays until the weekend, but I've got
> a computer with me since I'm designing a new grand piano at present.
>
> For those who may be interested in the concert details (or even
> attending if you're not too far around the horizon), David Bollard
> will now be playing our piano no. 003 in the 9.00 pm concert of
> January 20. Details can be found at;
>
> http://www.sydneyfestival.org.au/events_detail.asp?id=10
>
> >  Robin wrote;
> >     . .  I have repeatedly stated, as I did in the first post, that
> >the answer to
> >this question  is to be found in the analysis of motion itself,  referring to
> >the nature of translation, rotation and stress wave, another point
> >you, Del and
> >others took no pains to comment upon.
>
> Nevertheless I can't imagine how anyone could disagree with this.
> While I didn't comment at the time since I was involved with other
> matters, I haven't got figures for bridge rocking at present.
> However, since this appears to have become such a hot issue, I will
> try to devote some attention to it when I return from holidays (by
> getting some real figures). I have no desire to get into a full blown
> debate on the issue since I am not concerned if others don't share my
> view, but I would be prepared to derive a few figures on the matter.
>
> >. . . .  Ron [N], you,   at least insofar as I am concerned, to your
> >credit,  have been
> >willing to get in the trenches and debate these issues, while your
> >co-proponents are apparently busy elsewhere, this being the holidays perhaps
> >that is understandable, perhaps not.
>
> Yes sorry Robin, but we do have to earn a living also from time to time.
>
> >There is far more than merely the
> >"tension difference" inhibiting your rocking motion.  The  forces exerted on
> >the bridge which itself is stiff expressed as  the downbearing load or to use
> >Del's term "downforce" are counterbalanced by  the resisting forces
> >provided by
> >the soundboard assemply including the ribs, the strain of crownd,
> >the rim,  and
> >so forth.
>
> True, but the apparent stiffness of the bridge will not prevent it
> from rocking as some have asserted (particularly if the bridge height
> is not too low). A bridge will 'rock' just as a relatively stiff rim
> will flex when the case is leaned upon (I have tested this with dial
> gauges previously - and written about it on the list). Similarly, an
> engine block will flex when subjected to a moderate hand force. For
> those of you who are "doubting Thomas's", confirm this with an engine
> reconditioner. Place a bore gauge in the bore of a freshly re-bored
> cylinder (in order to test this the bore must be true), the bore
> gauge will be capable of supporting its own weight in the bore. Now
> flex the engine block (by hand) across the cylinder bore and
> perpendicular to the orientation of the bore gauge. The gauge will
> then fall out of the cylinder. One could conclude from this that
> rigidity is and will always be relative. Therefore, when the
> vibrating string goes through a cycle, its tension (which varies
> slightly as the speaking length is offset from its resting position)
> will cause the bridge to flex slightly backwards and forwards (in a
> vector direction parallel to the axis of the speaking length) in
> response to the speaking length deflection also. Because the vector
> force on the sound board panel is a product of the string tension
> times the SIN of the string deflection angle, the downbearing force
> will vary similarly to that of the speaking length during the cycle.
> Therefore, the board will respond to the position of the speaking
> length string segment at each point in the cycle. This is I believe
> the most important physical factor which causes the sound board to
> respond to the vibration of the speaking length segment.
>
> Now Robin, I do not at this time have numbers to support my
> philosophy here. Previously, I didn't feel it necessary to produce a
> set just to understand how the process works. Similarly, I suspect
> that Charles Darwin had a strong idea of the theory of evolution well
> in advance of his voyage on the Beagle (I have read 'The Origin of
> Species' but it was a while back). His long held view was merely
> confirmed in an ever increasing way as he gathered more information.
> I have carried the 'rocking' theory with me for at least fifteen
> years (long before I ever knew Ron N and Del existed), and all the
> while, thinking about the many different pianos we've rebuilt (as
> each one has come along) has re-inforced my view. Even Bösendorfer
> seem to be demonstrating an understanding of the principle in their
> later pianos by undercutting their dog-leg breaks to allow a more
> uniform bridge stiffness. The 'dog-leg's wider footprint, if not
> undercut, will tend to 'close' the sound at the breaks (this is just
> one more piece of evidence which supports the bridge-rocking theory -
> perhaps not to everybody's satisfaction but ah well). I have undercut
> dog-legs on many occasions, only to find that the sound improved to
> my subjective ears. But please don't accept this idea if you don't
> want to. I am not prepared to debate it ad nauseum, since I'm
> currently designing two new grand pianos. But when I get some numbers
> I will advise the list of my findings.
>
> Regards to all for the Christmas break.
>
> Ron O
> --
> Overs Pianos
> Sydney Australia
> ________________________
>
> Web site: http://www.overspianos.com.au
> Email:     mailto:ron@overspianos.com.au
> ________________________