what is downbearing?

[Dave Nereson]

----- Original Message ----- 
From: <Alpha88x@aol.com>
To: <pianotech@ptg.org>
Sent: Thursday, February 19, 2004 9:11 AM
Subject: what is downbearing?


> Greetings,
>
>            What exactly is downbearing? Does it affect the sound of the
piano
> or is it a mechanical/ physics measurement for the playability of the
piano?
> Why is it important to measure it? Can it be calculated by a formula(e)?
What
> does it mean?
>
>
> Thanks,
> rookie,
> Julia Gottchall,
> Reading, PA
>

Picture (or go look at and fool around with) a guitar or violin.  See the
wooden bridge that the strings cross over on their way from the nut
(beginning of speaking length up by the tuning pins) to the tailpiece?  The
amount that bridge lifts the strings up from what would otherwise be a
straight line is the downbearing.  In other words the strings are stretched
in a straight line from the nut to the bridge, then go "downhill" to the
tailpiece.  The downward deflection angle between the "waste end" (bridge to
tailpiece) and the line that would exist if the strings kept going in a
straight line after they passed the bridge is the amount of downbearing.  In
a violin or cello, it's quite drastic; in a guitar, quite a bit less, and in
a piano almost non-existent, but it's there, or should be -- at least a
little bit.
     It has nothing to do with the physical playability of the piano, that
is, the action, or "touch", but has a lot to do with volume and sustain.
Supposedly.  Some pianos have no downbearing, yet still have plenty of
volume and sustain.  Supposedly more bearing means louder volume and less
sustain; less bearing (strings closer to being in a straight line) means
more sustain and less volume.  But this isn't always the case.  Too many
other factors enter in, such as the crown (upward bowing, or curvature) of
the soundboard, the quality of the wood, the density of the grain, the
resonance of the rim or case, the various qualities of the hammers and their
voicing (especially), and to a smaller degree, action regulation and the
"touch" of the player.
    It can be measured as a downward angle of deflection in degrees, or by
the amount the string "goes downhill" from the rear bridge pin to the
aliquot (if present) or hitch pin.  Others have said downbearing is the
downward pressure of the strings on the bridge.  Well, yes, but that's not
what we measure.   It's the downward deflection in thousandths of an inch -- 
maybe even a sixteenth or an eighth in extreme cases, or the angle in
degrees -- maybe a half a degree to as much as five (?) degrees that we
measure, not the pressure in pounds of all the strings pressing on the
bridge.
    If you're looking at a used piano for someone and it has no downbearing,
it could be that the soundboard has lost its crown.  (That's better checked
by stretching a string tautly across the bottom or back of the soundboard -- 
lengthwise, between the longest ribs -- and see if there's a gap in the
middle between the string and the board.)  It's a concern -- a small one if
the tone and sustain of the piano is still good or great; and perhaps a
larger concern if the piano is almost new.
    As far as formulae and what it all means -- this is more thoroughly
covered in articles about soundboard replacement, bridge recapping, etc. by
Nick Gravagne and many others in the PTG Journal.
    --David Nereson, RPT