Down bearing

[Keith Roberts]

Daniel,
No, I don't think we misunderstood you. You said the board pulled away from
the strings as you destrung the piano. The bridge has to be thicker than the
distance between the string and the sound board to have downbearing. In your
case, because of the slant(downward slope towards the hitch pin) of the
bridge, the rear bridge pin pulls the bridge upwards causing the front pin
to rise higher than the string plane. Yes it appears normal to the gauge.
Pay attention to Dale Erwins post on stringing the board from underneath
before and after.
No, the plate does not flex enough to change the crown of the soundboard or
downbearing. Nose bolts are supposed to stop the plate from flexing to much
and cracking. Sometimes they are needed but are never used to distort the
plate to affect bearing.
Boards are crowned by arcing (cutting not bending) the ribs to a 40 to 60 ft
radius or drying the board more than normal for gluing and the swelling of
the board after makes the crown. Or both.
Check the list archives for discussions on soundboards.
Keith R
----- Original Message -----
From: "Daniel Lindholm" <mailinglists@home.se>
To: <pianotech@ptg.org>
Sent: Sunday, March 31, 2002 2:40 PM
Subject: Re: RE: Re: Down bearing


> Hi!
>
> It seems like (because of my not-so-good english) that my
> total post have been misunderstood.
>
> The problem isnt to measure the downbearing. There is
> really not any problem at all. I'm just wondering if the
> stringtension can make the soundboard compress through the
> lenght of the piano increasing its crown when strunged.
>
> Just to give you an answer about the downbearing. We
> noticed about 4mm difference when the strings were up to
> tension and when the strings were taken away. We usually
> notice this kind of behavior when we remove the strings.
> Its not a question about IF the downbearing decreases when
> the strings are removed. Its rather a question about how it
> is possible.
>
> So, could it be that the string tension actually do
> compress the cast iron, pushing on the soundboard and thus
> increasing the crown when strung?
>
> When I get back home I'll take some photos where I can draw
> some arrows etc, just to make sure that there are no
> missunderstandings because of my way of explaining (in
> other words my bad english ;)
>
> Best regards,
> Daniel Lindholm / Sweden
>
> > David,
> >
> > When apparent downbearing is small it is very easy to be
> fooled by these
> > commercial 3 feet gauges sold. They will show you a gap
> behind the bridge,
> > where it goes in a slant, and you can take that for down
> bearing showing.
> > After that, if you use the string method You will see
> that the sting goes in
> > a straight line from one end to the other.
> > Is it what gives your input unclear ?
> > If you move the gauge at every possible place before and
> after the bridge,
> > you will have a better figure and see bridge roll, the
> slant on the top of
> > the bridge and how it is related to "measured" down
> bearing.
> >
> > It is way more precise to have some home made gauges of
> different lengths,
> > that can help you to ascertain the existing bridge and
> strings segment
> > geometry.
> > BTW if your gauge have only 2 feet  I can't see how it
> works
> > The measuring gauge with a bubble, while not so accurate
> as we can think,
> > can add some information too.
> >
> > I feel it is necessary :
> > - to learn to use the sound as a DB appreciation (but
> this seems to me a
> > long process to learn for a beginner (?))
> > - To check crown BEFORE & AFTER unstringing.
> > - To use the same method of measure before and after if
> you want to compare
> > things and try to deduct something.
> >
> > I remember reading some very instructive lectures on
> these subjects in old
> > (! 8 years) PUT journal issues, and these are now
> available on the CD ROM
> > reprints. at these times it helped me to understand why
> what different
> > gauges where showing was different of the real picture.
> >
> > Illustrations are needed there.
> >
> > Hope and so on .
> >
> > There is an old adage that says, the less you know about
> DB and crown, more
> > happy you live !
> >
> > Much respected by many "restorers" there.
> >
> > Another says " In the factory, it worked to put 1mm
> there, 2 there and 3
> > there, so it may work the same 40 years after that "
> >
> >
> > Regards
> >
> > Isaac OLEG
> >
> > Francium
> >
> >
> >
> >
> > > -----Message d'origine-----
> > > De : owner-pianotech@ptg.org [mailto:owner-
> pianotech@ptg.org]De la part
> > > de Daniel Lindholm
> > > Envoyé : samedi 30 mars 2002 23:11
> > > À : pianotech@ptg.org
> > > Objet : Re: Re: Downbearing
> > >
> > >
> > > Hi David
> > >
> > > When the strings where on we used one of those metal-
> > > thingys with two leveled 'legs'. One leg on the sounding
> > > part of the string and one on the part of the string
> that
> > > rested on the bridge. If you can wip it, it tells you
> that
> > > you got positive downbearing. If it stands firmly on the
> > > string, it tells you that you either have zero or
> negative
> > > downbearing.
> > >
> > > As for asking our instructor, we did. He couldnt come up
> > > with any better theory than we did with the cast iron
> and
> > > soundboard actually getting compressed between the cast
> > > iron pins and the tuning pins, increasing the positive
> or
> > > negative crown of the soundboard, altering the
> downbearing.
> > >
> > > I suspect that even though it has alot of negative
> > > downbearing right now, when the strings are off, it
> would
> > > get back to what it was (positive) before removing the
> > > strings, if we restrung it and tuned it.
> > >
> > > > Hello Daniel -
> > > >
> > > > At this point I would ask you how you went about
> taking
> > > downbearing
> > > > measurements.  The rest of your question seems to
> reflect
> > > some basic
> > > > confusion which might be best addressed by your
> > > instructors.
> > >
> > > Thanks,
> > > Daniel Lindholm / Sweden
> > >
> > >
> >
>
>
>