Downbearing was Re: Lowell..

[John Delacour]

At 11:39 pm -0400 3/5/07, Frank Emerson wrote:

>I have seen technical drawings that clearly indicate that it was the
>designer's intent that all of the down bearing should be at the front of
>the bridge, with zero back bearing.  In a piano built with this type of
>bearing design, and if the bridge were supported only directly under the
>center line of the bridge, when the string were drawn to tension, the
>bridge would immediate roll forward until the front and back bearing were
>equal.

Hello Frank,

Such a bridge as you describe, essentially a closed V with the two 
pins at the top and the fulcrum at the bottom, would immediately tend 
to tip forward as tension was applied to the speaking length, however 
the downward force was distributed over the bridge from front to 
back, since an increasingly strong almost tangential load will be 
acting to overcome increasing friction at the bridge.  Not until the 
tension in the back length is equalized with that of the speaking 
length will the bridge be relieved of the very real tipping force 
actig tangentially to its fulcrum.

Consider now a bridge such as you describe where the 'back' arm of 
the V is a tiny fraction shorter so that the line of the top of the 
bridge projected touches the hitch-plate bearing (or string rest) and 
leaves no gap.  The same line projected in the other direction makes 
an angle of say 1 degree with the speaking length.

Suppose that this angle produces a downward force (the down-bearing) 
of about 3 pounds-force, as it will for regular modern tensions, then 
the component of that small force that is tending to tip this 
V-shaped bridge forward is a matter of a few ounces and inadequate to 
overcome even a small fraction of the opposing tangential force of 
friction at the top of the bridge, no matter how smooth the bridge 
and the pins.

If we now replace the V-section of the bridge with the normal 
rhomboid cross-section it can be shown that any tendency for such an 
arrangement to tip the bridge is so minute as not to be worth 
considering.

At 11:39 pm -0400 3/5/07, Frank Emerson wrote:

>My pet peeves on this subject are:  Failure to distinguish between 
>front and back bearing.  Failure to distinguish between loaded and 
>unloaded bearing (before and after the full string tension is 
>applied).  Using linear dimensions to specify down bearing, and 
>totally loosing sight of the fact that these specifications are only 
>an expedient to achieve what is really important, the angle of 
>deflection, front and back.

I would go further and say that millimetres, card's thicknesses, 
bubble gauges etc. are but a means to determine an angle and unless 
they are so used they are meaningless, and the angle is just as 
meaningless unless it is used to quantify what is actually 
down-bearing, and that is a force, measured in units of force.  The 
important question is to determine the proper distribution of weight 
along the bridges to achieve optimum admittance combined with the 
desired final arching of the soundboard when the piano is strung and 
maintained in acceptable conditions of humidity.

JD