> As others have stated, one cannot disprove an event, that can be observed > and measured, by failure to explain why or how it occurs. All that is > proven is the limits of ones understanding of all of the forces at work. Exactly, so let's discover what is at work here. Next time, measure string height at the center of the bridge before seating. You'll find that it doesn't change with seating, and the string was never above the bridge cap anywhere but at the pin. This is what I think happens. Bridge height swells with humidity increases, pushing strings up bridge pins - this is verified by measuring actual bridge models through humidity swings, not by random speculation. When the bridge shrinks back down in dry periods, the string goes back down the bridge with it, courtesy of the pin slant and offset angle. The thing is designed to be a clamp for the string, and works pretty well as one. During the expansion phase, the string friction against a bridge pin courtesy of maybe a 10° side bearing at 160 lbs, is somewhat over 14 lbs. You're an engineer, check the math. That's with a new bridge pin. As the pin is worn flat at the contact point through years of both play, and being scrubbed up and down the pin by cyclic bridge dimensional changes, the friction level goes up. This results in a very high PSI loading of the string at the edge of the notch where the pin is, and crushes the wood. Any time the bridge is in a dry or drying cycle, the string, though sitting quite firmly on the bridge surface in the center, is not touching the surface at the notch edge because that surface has been crushed below the straight line between bridge/string contact, and the capo. This allows the pin to flagpole, making the horizontal termination less solid than the vertical, producing the apparent effect of two different speaking lengths, and making a false beat. This beat goes away when you touch the side of the bridge pin with a screwdriver, and returns when you remove the screwdriver. All you're doing seating these is to temporarily force the string below it's natural path where pin friction will hold it in close enough contact with the notch edge to keep the pin from flagpoling. At best, it might stay quiet until the next dry cycle, where it will again be in poor contact with the notch edge. Attached is a photo of the edge angles produced by this process. That's a straight wire held at the notch edge, tangent to the groove in the cap. Note that the angle is far in excess of anything you'll see as a bearing angle in a piano. Note also, that a string passing across that cap at a realistic bearing angle will not touch the notch edge at the pin. There will be a corresponding pear shaped wear track in the bridge pin, narrower at the top, wider at the bottom as the yearly rate of crush diminished with the widening string contact, and depth of crush. This bridge is incidentally from an old Mason & Hamlin A. So the fact that everyone, including myself, can measure, see, and hear a string moving on a bridge pin when it's seated, doesn't mean that the string had magically climbed the pins and is floating above the cap. There's plenty of evidence to the contrary, in spite of the illusion we observe. Again, you're an engineer. Look at the photo, make some bridges and run some tests of your own, and check this out. Ron N -------------- next part -------------- A non-text attachment was scrubbed... Name: Notch Damage 1.jpg Type: image/jpeg Size: 14999 bytes Desc: not available Url : https://www.moypiano.com/ptg/pianotech.php/attachments/20060913/266e9f22/attachment.jpg
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