>>If the force parallel to the pin is higher than the friction generated by >>the normal force then the string will want to move down the pin. If the >>friction force is higher than the force parallel to the pin then the >>string will want to stay where it is (even if it's above the bridge cap). > >And when you throw in vibration from play, and the movements from thermal >effects, the forces will overcome friction and the string will stay with >the bridge cap. There is vibration from play. I don't know how much variation in the forces at the pin it yields. I would think it would be fairly small even for a hard blow. It might be enough to overcome the frictional forces. I suppose it depends on how close the so-called Parallel force is to the Frictional force. If it's close, then the periodic increase from the vibration should be enough to overcome the frictional force and cause the string to move. If the frictional force is significantly higher, because of geometry, materials, or surface condition, then the force increase from the vibration wouldn't cause the string to move. I don't know what you mean by movement from thermal effects. > We know that over time, strings will render across the bridge, which > takes overcoming friction at the pins. That ought to work vertically at > least as well as, probably better than, horizontally. They will, but not always. I think once again it's a matter of whether or not the forces are high enough to overcome the frictional forces. If the mismatch in tension between speaking length and back length is high enough to overcome the friction at the pins then the string will move. If that mismatch was just below the amount needed to overcome the friction, then the periodic increase in force from the string vibration would cause the string to move. If the mismatch is far below the amount needed to overcome friction then string vibration would not cause it to move. >>On a practical note I have to agree with David I that my experience in >>prepping new pianos is that on tapping the strings down (and I'm talking >>about a light tap) I often see a noticeable downward movement of the >>string and will often hear a noticeable improvement in the tone. I don't >>have an explanation for why strings on newly strung pianos would end up >>some distance off the bridge cap, but my perception is that they do. > >There are two possibilities that I can think of, given positive front and >overall bearing. Either the string isn't touching the cap at all, in which >case a feeler gage should slide between the string and bridge cap in the >middle of the section between the pin rows, Which would mean what, in your opinion? That the forces aren't in fact sufficient to keep the string seated against the bridge cap, or that there hasn't been sufficient time for the vibration and thermal movement to cause the string to move down against the cap? >or the notch edge is already crushed to where it is just barely touching >the string at the pin. If side bearing and down bearing are sufficient to keep the string against the cap, why wouldn't they also keep the string firmly down against the notch edge? > It is my opinion that the second condition, and a less than solid pin, > accounts for this fuzzy tone. The string is contacting the notch edge too > much to let the pin flagpole enough to beat, but not enough to prevent > pin flagpoling altogether, which is why seating the string "clears" it > up. If you take a string off of a new piano, you'll almost certainly see > a groove in the bridge cap. It will be wider (and deeper) at the notch > edge than in the middle. It seems that if there were a well defined groove into which the string wasn't firmly sitting (which raises the question of how it became a well defined groove), then tapping it down would push it straight down into the groove which would also seem to be moving it down away from the bridge pin, which I wouldn't guess would clarify the tone. To keep it firmly against the pin and the notch it would have to move down parallel to the bridge pin. This would seem to require widening of the string groove in the hard maple. I wouldn't think that a light tap could accomplish this. Phil Ford >>I don't do too much prepping of new pianos these days, but I'll be happy >>to try to stick a feeler gage or piece of paper under some strings the >>next time I'm around an unprepped new piano. I'll report back. >>Phil Ford > >Thanks, I'm interested. >Ron N
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