>I accept that a vibrating string contacting the notch before the pin will >cause tonal deficiencies that would not be present if the pin were to be >contacted before the notch, however, what I'm talking about in this >thread is not a defined notch whose contact with the string occurs closely >after the bridge pin, but rather a tangent relationship between string and >a rounded or mis-planed bridge surface, the contact points of which could >(when string is not seated) take place 1/4" or more to the rear of the >pin. In such a case, it would seem that the horizontal bridge surface can >only be supporting the position of the string on the pin,(as a later quote >of Ron's states) but making no direct contribution to the string energy >termination. If that support is far enough from the pin, it would seem to >allow some slight amount of vertical sliding movement at the pin, even >with a 10 degree angle. This movement would cause accelerated wear and >flattening which could, in turn, cause tonal distortion. That's one of the traditional explanations for false beats, that the notch edge is too deep, and (with the bridge pin) forms two different speaking lengths for the string. Problem is, that it doesn't work that way. There is too much friction at the string/pin interface to allow the string to slither up and down the pin under normal conditions. >I hadn't thought of this before, and it makes sense, but, just to show the >way my mind functions (or dysfunctions), Ron says "it isn't PRIMARILY >downbearing..." and "it's MORE cyclic..." . Because hardly any interaction nature is purely binary. "Primarily" is accurate, if not precise. >So then, let's assume that the pitch IS allowed to go sharp, generating >more downward force by the string. If the front bridge pin were straight, >not angled, it would only be the direct increase in downbearing force that >would be acting to compress the wood. The side bearing force, and thus the friction would also increase with the tension. >But the bridge does ultimately push the string upwards on the pin. Even in >the case of negative net downbearing, where the seasonal increase in >bridge height and board height should, theoretically lower the pitch, the >force required to overcome the vector friction (pin angle) would probably >be enough to cause some wood compression. Even with zero, or slight (typical) negative bearing, It still takes considerable force to push that string up the angled pin. Downbearing accounts for only a relatively small part of the compression damage under the string. >If negative bearing exists, there is no reason to expect the string to >follow the bridge. Again, it depends on the degree of negative bearing and the pin angle. That force vector keeping the string in contact with the bridge top is the reason pins are angled in the first place. It's what the angle is for, with the offset angle and tension defining the side bearing pressure on the pin. All of this stuff plays a part. >>A model bridge section, with vertically laminated root and solid quarter >>cut cap, taken from about 4%MC to about 12%MC - >> >>Root height went from 0.842" to 0.868", or +0.26" height change. >>Cap height went from 0.253" to 0.119", or +0.004" height change. >>Pin height above the cap went from 0.026" to 0.119", or -0.008 height change. >> >>If the pin height lessened by 0.008" as the cap height increased by >>0.004", the point of zero relative movement between the bridge and the >>pin is somewhere a couple of thousandths into the bridge root below the >>cap. As the root height increases, the hole the pin was installed in >>increases in depth, to the same percentage of overall height change the >>root experienced. Since the pin doesn't change dimension with humidity >>changes, the bottom of the hole it's in has to recede from the bottom of >>the pin. > >Ron, this could be interesting, but you have some errors in your >numbers. Root height changed +0.026", not 0.26". One of the cap height >numbers is wrong as is at least one of the pin height number. Boy, I shouldn't try to do this stuff when I'm sick and my brain isn't working.. It's bad enough when I'm well and my brain isn't working. Let's try that again. Root height went from 0.842" to 0.868", or +0.026" height change. Cap height went from 0.253" to 0.257", or +0.004" height change. Pin height above the cap went from 0.127" to 0.119", or -0.008 height change. >Also, the other value that would establish an accurate picture would be >the height differential of the board. That's another set of problems. Since the bulk of the bridge damage comes from the expanding bridge pushing the string up the pin against high friction and down vector force from the pin angle and offset, that was what I was interested in exploring. >>If the string isn't contacting the notch edge, it's for a reason that >>tapping neither string, nor pin will cure. > >While I basically agree with you, I'm finding it difficult to reconcile >with your response to Wim's "More Wood, less pin..." questions: I don't see any inconsistency. Explain please. >>Front bearing is the angle between the string segment on the bridge top >>and the speaking length segment. > >My contention is that, since the string segment on the bridge displays >considerable curvature, it is misleading to think of angles or to assume >that the imaginary straight line between front and rear bridge pin is >meaningful in defining the angle actually formed by the two string >segments as they converge at the front pin. I disagree. Poor front termination, with the accompanying tonal problems and false beats, becomes most problematical when the overall front bearing angle (that between the bridge top and speaking length) is very shallow. A strong positive front bearing angle DOES put the horizontal string termination on the notch edge and none of this stuff even comes up. It's only when that angle becomes shallow enough that the crushed notch edge no longer contacts the string. We've gone over the basic points a number of times reducing them to ever finer isolated details. In the piano, they all exist and interact at once, each in relation to other(s). >>As I point out. I've also pointed out that this local negative front >>bearing from tapping a string down isn't to be considered to be overall >>front bearing. It results from doing something that isn't indicated from >>the cause of the symptoms - seating strings. > >This is where I either lose you or disagree. When you say "this local >negative front bearing from tapping a string down...". I'm not sure how >to interpret this. Yes, if you don't seat the strings to the bridge, you >won't read it as negative, but you have the potential problems discussed >above, relating to excess string movement at the pin. If you do seat it, >you eliminate, temporarily the sliding movement on the pin, but introduce >an unstable condition which will eventually return the string to its >neutral, elevated position. But, at the beginning and end of this comment, >you seem to again assert that the condition is caused by string >seating. I see no reason to support such an assertion. As I've been trying to explain to you, there isn't a significant sliding motion on the pin. Is there a sliding motion of the string on the V bar? >Then I understand your use of "climbing the pin" to refer to a string >moving upward on a pin in spite of positive bearing angle. And/or in spite of a positive down vector force from pin slant and offset. To me, climbing the pin means the string has moved some place where it doesn't belong and needs to be put back in it's natural place, when in fact, the low, zero, or negative front bearing angle and crushed notch edge mean that it is already in it's natural place (if not it's desired place) and tapping it down to the bridge puts it in an unnatural place under the circumstances. >Again disregarding the issue of possible loose pins, I clarify... I wonder >whether any electronic device can measure a difference in the vibrational >envelope created by a string which terminates simultaneously at the pin >and notch, and one where the horizontal termination (or support) lies some >distance behind the pin. And again, I still don't have that information. I would hope that if anyone on the list does know, he or she would answer. >Of course, my other ongoing concern regarding negative front, or for that >matter, areas of negative net bearing, is whether it should, if accurately >determined, constitute a reasonable basis upon which to reject the >acquisition of an instrument. But as Ron says, one at a time. And as I also have said, I don't consider negative front bearing to be acceptable in a conventional piano. Ron N
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