?extra concert maintance&string?

[Ron Nossaman]

 We blame the 'cutting' action of the wire during tuning, 
>especially when the bars are soft. This deterioration also leads to the 
>development of string noise. When the 1986 D was rebuilt again last year, 
>after being reshaped the  capo' and duplex bars were hardened. The damage 
>sustained between the 1993 restring and last year on the (radius reduced 
>but unhardened) bars was incredible.

* More on the tuning thing further down. On the capo, I would think that
reducing the radius without hardening the material would inevitably
accelerate wear, from both play and tuning. By the way, how do you harden
v-bars, and how are they tested for hardness, before and after?


>The friction between the front and rear rows of bridge pins (ie. between 
>the string and the bridge wood) will always result in less load on the 
>rear pins when the string pulls through to the speaking length from the 
>back length, and consequently less damage.

* Therefor, the counterbearing bars closest to the tuning pins would wear
deeper grooves from tuning abrasion than in the capo. Is that the case? The
same logical cause and effect relationships have to work the same way at
every friction point along the string don't they? I don't doubt that tuning
causes wear at bearing points, I was just giving the amount of play
considerably more weight in the process than tuning. I also seem to recall
getting a lot of argument on this list against the notion that strings
render through bridges in the first place... ever. That said, what kind of
"load" are you talking about here, tuning load, or playing load? Obviously,
the playing load is much heavier on the speaking side, which was my point.
As for static load during tuning, it gets a little more complicated. When
you raise pitch during tuning, you put more temporary load on the front
pins. When dropping pitch, the load will be temporarily higher on the back
pins because it will take some time for the section tensions to equalize. It
seems to me that any wear on bridge pins as a result of tuning would tend to
average out between the front and back row, which would leave play as the
primary cause of the depth of wear in the front row, and the difference in
wear between the front and back.    


>Further, a higher string offset angle (as it crosses the bridge) and a 
>higher bridge pin inclination angle, will also increase the load on the 
>pin. A string offset of 10 degrees combined with a string tension of 175 
>pounds will result in a lateral force of over thirty pounds to be exerted 
>on the bridge pin.
>
>Tension on bridge pin (in pounds) = [Sin(string offset angle) + 
>Tan(bridge pin inclination)] x (string tension in pounds)


* The math says a little over 30 pounds, but with a Young's modulous of
around 28 million psi, it should be more because of the stiffness of the
wire. I never bothered to try to work it out, but it might be interesting to
see what we really have there. In any case, the side bearing on the pin
would be greatest when the pin is perpendicular to the direction of the
pressure. The greater the pin inclination, the more the side bearing is
vectored down to the bridge. which is the purpose of the pin inclination in
the first place, and the lower the load on the pin will be. 



>I believe technicians tapping the strings down on the bridge is a major 
>cause of damage to string termination points. When we rebuilt the ABC's 
>1986 D in 1993, the strings had been 'tapped' down previously (by another 
>technician). The string speaking lengths met the bridge wood nearly 3mm 
>in front of the pin (I will publish a slide of this on our web site 
>eventually).

* Here here! Couldn't agree more. I've been preaching this for many years,
but it seems to be contrary to what most techs "know". As a passing
thought... is anyone out there tapping pins on a Wapin bridge?



>Many manufacturers appear to be careless about maintaining a uniform 
>bridge pin angle and string offset angle. We use the same specification 
>for all rebridged instruments, ie. 10 degrees of string offset, and 
>twenty degrees of bridge pin inclination from the vertical. We believe 
>the implementation of these figures will prevent the strings climbing the 
>bridge pins.

* I don't believe it's possible for strings to climb bridge pins if there is
zero, or positive bearing on the bridge, if the bridge pins are inclined
over about 10 degrees (I like about 15), and the bridge top is at level, or
a positive bearing angle to the speaking length. What I see is bridge tops
grooved by dimensional changes in the bridge with humidity swings, tapping
of strings by techs, and possibly to some degree by play. That groove isn't
flat, but has a section like the long axis of an oval. It's much deeper at
the notches than in the center of the bridge top. When the string bearing
angle is low enough that the string contact point on the bridge is farther
back on the bridge than the pin and notch, the string pivots vertically at
that point and scrubs up and down on the side of the pin, wearing the track
deeper into the pin. Since the point of side bearing on the pin doesn't
correspond with the down bearing point on the bridge, the pins can flagpole
from side to side during play and produce false beats. That's why touching
the side of the speaking length pin of a string with false beats with a
screwdriver or something will usually clear up the beat as long as you hold
it. Tapping or pushing the string down on the bridge may temporarily keep
the string down on the bridge and appear to clear up the false beat, but it
won't stay there. Increasing the bridge pin angle would hold the string down
on the bridge better, because it vectors more of that side bearing down to
the bridge top, but the trade off would have to be accelerated deformation
of the bridge top at the notch, wouldn't it? I note that the grooves left by
strings in bridge tops are, unless butchered by tapping techs, nearly
equally deep front and back. I think the wood movement of the bridge with
humidity swings is pushing the string up and down the bridge pin and
crushing the edges of the notches in the process, as well as accounting for
the fact that the track left on the bridge pin is taller than would be
accounted for by the diameter of the string involved and the depth of wear.
It occurrs to me too, that the Wapin system I mentioned earlier wouldn't
exhibit this humidity cycle crush on the front notches nearly as much as
conventional pinning. That's not an endorsement, just an observation.  



>When drilling the bridge pin holes, we attach a bubble gauge to the drill 
>to maintain the correct angle.

* That's what I do in the piano. When making bridges with soundboard
replacement, I do them on the drill press.
>
>Ron E. Overs, Sydney
>

 Ron