what is downbearing?

[Don Gilmore]

Hi Jim:

Well, it's true that the downbearing force increases as the angle increases,
but not exactly in a geometric fashion.  A geometric progression would mean
that the force increases exponentially with the angle...a process which
continues forever as the angle increases.

In our case the relation is a trigonometric one.  The biggest changes occur
in the smaller angle-range.  As I mentioned before, you are already pulling
at half the string tension by the time you get to 30 degrees.  As you
approach 90 degrees, however, the change in downbearing levels off and large
changes in angle produce small changes in force on the bridge.

For example, a change from 5 degrees to 10 degrees almost doubles the
downbearing force.  In contrast, a change from 80 to 85 degrees only
increases the force by about 1%.

Take a look at a sine wave.  When it starts at zero it is very steeply
sloped upward.  At the top of the curve (90 degrees) it levels out flat.

Don A. Gilmore
Mechanical Engineer
Kansas City

----- Original Message ----- 
From: <JIMRPT@aol.com>
To: <pianotech@ptg.org>
Sent: Friday, February 20, 2004 11:55 AM
Subject: Re: what is downbearing?


> Hmm using that 5 degrees and an old Knabe I am rebuilding currently, which
> has 223 individual tuning pins,...would give us (150 lbs X 223)= 3,345 lbs
of
> downbearing pressure....no wonder there is compression set/damage!! :-)
>  I think that in general the total pressure from down bearing amounts to
much
> less than this...say in the nature of 3 to 5 lbs and range totals  more in
> the neighborhood  of 670/1115 lbs. But I certainly am open to changing my
> viewpoint. We need to keep in mind that it takes more and more lbs of
pressure as
> the degree of displacement rises...in other words downbearing is
geometrically
> progressive...isn't it??
> Jim Bryant (FL)
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