Strings riding up (was Tuning stability)

[David Skolnik]

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Fred -
I wrote this earlier, before other responses materialized, but it's my 
daughter's birthday, so I don't have time to edit for redundancy, accuracy 
or intelligibility.   I'll let you, or someone else do that.

At 09:25 PM 4/5/2004 -0600, Fred Sturm wrote:

>    I am thinking in practical terms: if one chooses (as I do) to 
> resurface the bridge
>top, re-notch and repin, should one make a great effort to get the top 
>back to
>flat? Perhaps not. I will theorize that a curved profile will work just 
>fine, as long
>as, using the string test, a string run from capo or agraffe touches front 
>of notch
>before the rest of the bridge top.

Fred -
Back in the days when this thread was just Tuning Stability, on 3/30 to be 
exact, I asked you and Mark Cramer what you felt was achieved by planing 
out the string grooves.  While it might afford you an opportunity to 
justify the time spent keeping your plane blades sharp, I couldn't see that 
it would have any tonal effect.  I visualized a string groove that, even if 
more pronounced at the edges, nevertheless showed some compression across 
the entire bridge width.  Certainly, any instrument for which you were 
about to undertake such sort of restorative measures would be expected to 
have been around for long enough to have developed such indentations.  As 
such, the groove would provide you with enough of a guide in planing.

If, in your statement above, you are envisioning the intentional 
introduction of a curvature into the bridge surface profile, I suspect that 
there are very practical reasons why it would have no greater success than 
a flat profile, given that, for a variety of reasons, even the latter ends 
up inconsistent.  Even theoretically, I think you DO want a flat surface, 
at least at the front, since without that, you have cannot create an 
angular relationship between string and bridge, but rather a tangential 
one, which would have its own potential termination problems.

>In fact, maybe a mild curvature would be better
>theoretically, both for transfer of energy and for durability.
and
>One may be reducing the loading of the bridge a wee bit by removing the
>curved part in the middle of the bridge.

As Tom Lowell explained to me, as far as bridge loading is concerned, the 
angles created by the straight line, possibly only imaginary, between the 
front a rear bridge pins, in relation to the front and rear string 
segments, are what determine the loading.  Anything can be happening on 
that bridge surface ("to the moon and back" is what I think he said), but 
the loading will depend on those angles.  I got it after a while.  So 
planing the curve flat would not, in theory, affect the loading, however, 
it would, in theory, redistribute that load from the entire bridge surface 
to the bridge edges, thus increasing the compressive stress at these 
points, and potentially accelerating the fibre crushing process, whether 
from downbearing or the seasonal bridge height movement Ron Nossman has 
described.

I don't see how rounding the bridge surface would affect the transfer of 
energy, but I can see how distributing the downbearing load evenly, over 
the entire bridge width could reduce the stress on the notch edge, but, to 
a degree, that's what happens in time anyway.

>How is load transferred through bridge to board?

Not sure what you mean.

>    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. 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.

First, picture a hypothetical string and curved bridge surface, where the 
bridge deflection looks like a speed bump in a road, and where the front 
and rear string segments are on exactly the same plane. This theoretical 
configuration is only enabled by the presence of bridge pins and seated 
strings. (Without either, the string would simply follow a straight line to 
the top of the bridge profile.) If you were to zero on front and then 
measure rear with the gauge, it would read as zero downbearing.  If you 
measured the bridge string-segment in the traditional manner  (gauge feet 
spread to just shy of front and rear pins), it would seem to confirm the 
previous zero reading (again, assuming that front and rear points of curve 
fell exactly on plane line).  Using either sliding method,  two feet close 
together or one foot on sounding length, the other on bridge string segment 
will provide a picture of what deflections are present,


>    Anyway, a bit of food for thought.
>Regards,
>Fred Sturm
>University of New Mexico


David Skolnik 

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