Strings riding up (was Tuning stability)

[Ron Nossaman]

>I wonder what
>basis there is for our notion that bridge tops should ideally be flat.

Fred,
 From a practical engineering standpoint, the flat top is a balance or 
compromise between termination quality and durability. Presuming positive 
front bearing (angle between bridge top and speaking length), it gives a 
more defined and positive termination point at the edge than a top that is 
curved to be tangent to the string plane, while being less easily crushable 
(and cheaper to build) than a top that is grooved or "V'd" down the center.

>In fact, maybe a mild curvature would be better
>theoretically, both for transfer of energy and for durability. Possible, 
>I'm not
>saying it's so.

I don't think so. It's that mild curve in existing bridge caps, with loose 
pins, that makes the false beats we love so much to hear in the treble 
third of the scale.


>    How is load transferred through bridge to board?

The string moves the bridge, and the bridge moves the board, which moves 
the strings, which move the board, etc. - until the string energy is 
absorbed in the internal friction of the system and dissipates as heat. A 
piano is, after all, a low efficiency space heater.


>Harpsichords (not
>counting 20th century anachronistic designs like Neupert and Sabathil) have
>the string bearing on a mild point of the bridge, 1 - 2 mm behind the 
>bridge pin
>(with a definite gap between). The flat bridge top is definitely a piano 
>thing, and
>probably mostly by accident of practical considerations (planes make flat
>surfaces).

Not by accident, I think, but by practical necessity. We're dealing with 
much higher tensions and hugely greater energy input from hammers than 
harpsichord builders have to deal with.


>One may be reducing the loading of the bridge a wee bit by removing the
>curved part in the middle of the bridge.

I don't really think so. We are, however, redistributing the string load 
that will be there with the new strings, to the outside edges of the cap 
rather than in the center. That's bearing load. The cyclic crushing of the 
cap at the pins will continue whether the bridge is resurfaced or not.


>    Following this general avenue of thought, I wonder what measurement made
>with the downbearing gauge would reflect the reality of deflection and 
>loading
>best, given the reality of string grooves in a bridge top (and a likely 
>curved
>profile). One question that can be addressed is whether and how much the
>string deflects just behind the notch/pin, which will give a notion of 
>whether front
>termination is good.

Try this. Pull a treble string. Straighten a short length of #13 wire 
(smaller than what you took off), and lay it in the string groove on the 
bridge top. Using a knife blade, steel scale, or anything else thin and 
stiff enough to control, push down lightly on that straightened string in 
the center of the bridge, and at various points between the center and the 
pin until you get to the point opposite the pin, or the notch edge - 
whichever comes first. Observe the range of angles the string shows, 
relative to the bridge top, as it follows the tangent of the curve in the 
groove. At the position where the pin is, and just behind it, you'll see an 
angle that far exceeds any down bearing angle that ever existed in that 
piano. There is no way that edge will be a good front termination, because 
it won't be touching the string except when the bridge is in the expansion 
cycle


>But perhaps a comparison of speaking length level, back
>length level, and measurements with one leg on the middle of the bridge and
>one on speaking length, and another with middle of bridge and back length,
>would give a better picture of how much the string is being deflected/how 
>much
>it is loading the bridge.
>    Anyway, a bit of food for thought.
>Regards,
>Fred Sturm
>University of New Mexico
>_______________________________________________
>caut list info: https://www.moypiano.com/resources/#archives

Ron N