Bass Bridge Downbearing/Sidebearing

[Delwin D Fandrich]

Terry,

Downbearing, or string downforce, has virtually nothing to do with bridge
pins causing the bridge cap and/or bridge body to split.  Excessive bridge
pin side-to-side offset does, but not string bearing.

We get in the habit of saying there should be very little string bearing
across the bass bridge, but this is a relative thing.  Whereas through most
of the scale, string bearing is determined primarily by the need to load the
soundboard to a certain point, in the bass it is more a function of
backscale length and the proximity of the bass bridge to the inner rim of
the piano.

In the typical short piano -- including many, if not most, verticals -- the
backscale is very short; 50 to 60 mm, sometimes less, at the bass end of the
bridge.  If these scales are given much string bearing at all, the bass
bridge will be tied so tightly to the plate it will not be able to move at
all.  With a longer backscale, and with the bridge a bit farther in, away
from the inner rim, more string bearing can be used.

As well, cantilevered bridges can only tolerate a minimal amount of string
bearing or they will seriously distort the soundboard/rib assembly over
time.  And, I'll not even get into an analysis of the soundboard panel grain
direction and the rib layout at all, which also has an effect on this issue.

Del
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----- Original Message -----
From: "Farrell" <mfarrel2@tampabay.rr.com>
To: <pianotech@ptg.org>
Sent: November 28, 2000 4:22 PM
Subject: Bass Bridge Downbearing/Sidebearing


> A while back I posted about a bass bridge that had cracked along the
> speaking side bridge pin line - the kind where a continuous line of wood
is
> destroyed and the bridge pins have all migrated so that there is no bend
in
> the string at the pin. It was suggested that a likely cause was no
> downbearing on the bass bridge. I questions that.
>
> Assuming the side bearing was appropriate, I do not understand how
> downbearing would affect bridge pin stability/longevity. The side bearing
> force vector will be there regardless of the magnatude of the downbearing
> force vector. Now I understand that with increased downbearing there could
> initially be some reduction of force acting against the pin in the form of
> friction on the bridge top, but it seems to me that would go away pretty
> fast after whacking the string a few times.
>
> My information on target downbearing for the bass bridge is "just a tad".
> Some respected convention lecturers suggest to have the string "just kiss"
> the bass bridge top. I do not think such downbearing targets will cause
> undue or increased stress on bass bridge pins.
>
> Any thoughts on this?
>
> Terry Farrell
> Piano Tuning & Service
> Tampa, Florida
> mfarrel2@tampabay.rr.com
>