>>The pin angle combined with the string tension and offset angle forces >>the string against the bridge top. It's a clamp system. > >This is a static definition. What I was trying to ask about was >"dynamic" coupling, that is, the interaction of the parts (string, pin, >bridge) in motion. Which was no where evident in the question. >In fact, I probably DON'T want to ask about this right now, as it would >likely bring us around to that discussion about how energy gets from >string to soundboard. I'm not ready to take that on. Beyond saying that the string moves the bridge and the bridge moves the soundboard, I'm not interested in that particular "discussion". I already gave my gallon of blood. >>>Does the front edge of the bridge play any role in defining the string >>>termination? >> >>Yes. It's what the pin angle, string tension, and offset angle clamp the >>string TO, ideally. > >I meant, does the front edge of the bridge have any effect on the way in >which the various modes of string vibration evolve & decay that might be >measurably altered by its absence? Again, the question you asked in no way remotely indicated that, and I don't have those answers in any case. >>>If so, how is that function affected by no contact (string climbing pin) >> >>Strings don't climb pins unless something is severely wrong, > >You say "unless something is severely wrong". What condition would >qualify for that description? Negative bearing, insufficient pin slant, as I've already said. I left out the side offset angle, sorry, that's important too. When these conditions are bad enough to cause a string to go up a bridge pin, I consider them to be severe. >What amount of negative force would be needed to overcome the design >parameters you have quoted..."10 degrees side, 20 degrees pin slant"? With a string at 160 pounds tension, with zero overall down bearing, 10° offset, and 20° pin slant, it would take about 23 pounds to push the string up the pin. With 160 pounds tension, zero bearing, 5° offset, and 10° pin slant, it would take about 10 pounds to push the string up the pin. >What if those parameters have been ignored, intentionally or otherwise? >The initial impact wave might have enough force to move an inadequately >"clamped" string. Correct, and I've heard the result in existing pianos, but the string doesn't stay up there unless the bearing is severely negative enough to overcome the clamping effect of the pin slant and offset. This may exist in some piano somewhere, but I've never seen it happen. >>but the termination edge is crushed by cyclic wood dimensional changes >>with humidity swings pushing the string up and down the pin. It's very >>possible for a string to be resting on the bridge top and not be touching >>the notch edge, but it hasn't climbed the pin. It's crushed the cap. > >This could be the case if the original downbearing was properly set and >the pitch/tension was allowed to elevate with the increased humidity. It happens whether the down bearing was set correctly or not. It's a function of friction, string tension, pin angle, and offset. A few pounds difference in the string tension doesn't change the system much. >Here are some sub-questions: > - What amount of pressure is required to crush maple cap material? Wood Handbook lists 1470 PSI as proportional limit. It will deform over time under lesser load. > - In the normal stringing process, how much force is initially > applied to the front edge as the tension is applied? It depends on how much pounding the stringer does on the termination before he's satisfied that it's finished. Hopefully, not much. >I recall some discussion about the bridge surface itself expanding upward >(apart from the soundboard's upward excursion) and, in the process, >pushing the string up the pins. I've written about this before. >If this occurred, and then, in the dry season, the bridge contracted, if >the ratio of friction to force was high enough, the string could remain >elevated from the bridge surface, even without attributing the cause to >wood crushing or vibrational prodding. Only if something was severely wrong. Otherwise, the string goes right back down the pin with the receding bridge surface. Vibration from play relieves the friction some, as does the string rendering through the bridge with tension changes. The string doesn't stay above the bridge top. >>At that point, the loose pin flexes and flagpoles and the false beat happens. > >I wonder at your conclusion, that he pin becomes loose and flagpoles, >causing false beats. I can't see that the pin necessarily becomes loose >or that false beats are the resulting symptom. My guess would be that the >lack of bridge edge support allows a degree of movement between string and >pin that would otherwise not occur. At some point, in an instrument that >has had a fair amount of use, that movement is taking place against an >abraded pin surface, which could contribute to both the unwanted string >noise and the string's ability to remain elevated. Suit yourself. I see way too much friction between the pin and string to allow the string to slither up and down the pin during play - unless something is seriously wrong. And if the string was, indeed causing a false beat by slithering up and down the string, the beat wouldn't stop when you place a screwdriver against the side of the pin opposite the string - which it typically does. >>Of course. But it's because you're inducing a curve into the string to >>force it down to the crushed bridge edge by tapping, not because the >>string has climbed the pin. > >Sorry, no. I have, with a magnifier and strong lighting, watched the >bridge-segment string set to the bridge surface from almost all the way >back to the rear pin. It can't be all about wood fiber crushing. Of >course, as I get older, and my eyes get worse, I can reasonably suppose >that I'll be seeing less of this. That's one solution! Most likely right. I'm just blind and don't know what I'm talking about. > From the rebuilder's perspective, and not just Ron N, : Whatever that means. >Are you striving for positive downbearing (with positive front bearing) >across the scale? Yes. Aren't you? If not, why not? >If so, do you know (by measuring) how often you do or do not achieve this >goal? Always. Don't you? If not, why not? >If you are willing to accept some areas of negative downbearing, are there >locations on the board where its presence would be more likely to have a >deleterious effect? In a conventional soundboard, no, I'm not willing to accept negative down bearing. I'll concede that zero bearing in the low bass of a piano with a cantilever bridge and a very short back scale may be the least detrimental approach. Ron N
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