Ron Nossaman wrote: > > During the wet cycle, as the swelling bridge is pushing the string up the > pin against down bearing, side bearing, and pin inclination, the wood > surface of the cap is crushed under the string. Since the most resistance > the string offers to being pushed up the pins happens AT the pins, rather > than in the center of the cap, the cap edges crush more than the center. > The string is no longer lying on a flat cap. This is important. In the dry > cycle, the soundboard crown is less, the bridge is shorter, so the > downbearing angle is less across the bridge. At some point, it's likely > that the string isn't touching the bridge surface at the edge of the notch > where the pin is, but it is touching a little further back on the bridge > because the edge is crushed down below where the pin inclination can force > the string under tension. This has been up before and is a nice piece of reasoning, but there are other things that can and do cause identical wear at the bridge. Given a string at piano tension that is deflected by a piece of wood approximately the same amount that is the average in a piano soundboard assembly, and given similiar vibratory forces and wear exerted by any tunings, the bridge cap will have the same tendency to wear more along the edges then at the center. And the reason is quite simple. A flat wooden surface of about 1 inch wide pushing up at the string will attempt to create two bends in the string, at the front and back edge of the bridge. The string on the other hand would quite prefer to be bent just once and at the exact middle of the surface pushing up on it. You take a string at low tension accross a bass string making machine and take such a flat surface and push upwards.... you will actually see that in the middle of the bridge (assuming the tension is low enough) leaves the surface of the bridge. Increasing tension will simply make this same effect less visable to the eye, and the inclusion of bridge pins in affect is as much an attempt to equalize downward pressure on the bridge by the string over the entire surface of the bridge as it is anything else. All this (and probably more that none of us have thought of yet) must be taken into account if we are to finnally come up with the definitive answer to this question. False assumptions are all too easily fashioned, even under the most ideal conditions for analysing a problem. My point is that the grooving in the bridge as we observe it is also explainable in this manner. Rons points are backed up with some scetchy math and seem reasonable enough and I have no doubt that the added affect of effectivly increasing and decreasing pressure on the bridge by the string due to the climatic forces he describes are in this picture as well. I think its important to seperate all the above from the actual question of whether or not the string actually moves up and down on the bridge pin itself if we are going to get to the heart of this matter, and thats what I attempted to do above. > The string hasn't climbed the pin. It's > horizontal termination support just no longer coincides with the notch > edge. If the pin is even a little loose at the bridge surface, it will > flagpole and produce false beats. Seating the string will knock it down on > the bridge, creating a slight negative front bearing angle between the > speaking length and the length of string that wasn't touching the bridge > prior to seating. The false beat may go away temporarily, but it isn't > fixed, and will return as the piano is played and the string tries to go > back to it's natural straight line between termination points. This is where Ron and I get into difficulty. I will aggree with his description above if it can be shown that negative front bearing is actually created. It should however be relatively easy to demonstrate this by taking the front bridge pin out for a second and looking at whether any such negative bearing is indeed present without the influnce of the bridge. One would have to take steps to insure that the string leaves the bridge in the path the pin forces it to take, but there are a number of ways of insuring this. Alternatively, it is a simple matter to calculate the angle the string has from front termination to center of bridge. This is junior high geometry. If the lowest part of the bridge grove is below that line, then Rons theory becomes very much more interesting, as it is the only thing so far put forward that could explain this (aside from over ambitious string seating proceedures). If on the other hand the lowest part of the bridge grooving is not below the hypotenus of our triangle created by the string and the position of its termination points.... then it is less likely that Rons theory are so relevant after all. Certainly this latter situation coupled with the fact that a string is up a tad on the bridge pins and away from the surface of the bridge requires a different explanation then what Ron puts forward here. Another point. We havent even really taken into consideration the fact that the pins apparently exert a constant downward force on the strings to meet the edges of the bridge. In a very real sense string seating seating would seem to be an unneccessary proceedure from the get go. The fact that it "is" lends a lot of credence to Rons reasoning... I mean what else has been put forward to explain howcome the bridge pins dont force the string down to the bridge. But then on the other hand when it comes right down to it.... Rons reasoning doesnt really account for that any better either. The pin angles remain the same...the same downward force is exerted... why does a lowered notch make any difference to begin with... regardless of what causes it ? What remains in all this is for someone to do some hard science to actaully find out by good old fashioned observeration under controlled scenarios what the frig is indeed going on when these differeing forces are applied. Simple math may point us in some direction that seems reasonable... but it may just as easily lead us astray. > > The false beat is there in the first place because the bridge pin is loose, > the bearing angle is low, and the cap is deformed so the horizontal > termination point is behind the (-20°) vertical. The notch in the pin may > eventually be a factor, but these kind of beats will often show up very > early in a piano's life before any significant wear damage accumulates on > the pins. > I aggree (by experience) that it seems that loose bridge pins are definatly involved in false beating. I dont see that it has been demonstrated that the rest is true. I suspect that bridge pin notching due to wear is more akin to the front termination wear in the manner it contributes to falseness. > > Seating strings won't cure any of these conditions, so why is it so > universally insisted upon? It's quick, easy, and gives the immediate > illusion that the tech is improving something. At least that's why I used > to do it before I decided it wasn't a long term fix. Seating strings wont cure the situation of the lowest part of the bridge grooving being below the above describe hypotenus. Indeed I aggree that this would be a potentially aggravating proceedure. Yet assuring bridge contact in the case wear the bridge grooving is NOT below this line would certainly be recommendable given what we presently "know" about the matter. In summation.. its not so much that I directly dissagree with Ron on this. Its just that I fail to see that he is drawing a complete enough picture. He seems to be saying that no condition exists where bridge pin seating is an appropriate proceedure. But what I see coming out of his reasoning is a perfectly logically single condition where it is not helpfull to seat the strings. So... how do we explain then the situations that quite obviously occur where the bridge grooving is well above this hypotenus line, yet the string has somehow "climbed" (to put a point on it) the bridge pin. Perhaps the notching of the bridge pin itself plays a role here ??? In anycase... SOMETHING is causing the situation where the string is essentially riding on the bridge pin alone for its rear termination. I dont see that we have figured out exacly what that is yet And I dont see that there is but one single (and as yet unconfirmed) condition described so far where advise to not seat the strings is valid. > > ------------------------------------------------------------------------- > > That's the gist of it. When a tech can touch a screwdriver to the side of a > bridge pin opposite the string and make the false beat disappear as long as > a light pressure is applied (never touching the string directly), what > indication is there that seating the string will fix it? None.... I thought that test was already accepted as an indication that we have loose bridge pins.... an entirely (if somewhat related) different subject matter. > > > Ron N Grin..... -- Richard Brekne RPT, N.P.T.F. Bergen, Norway mailto:rbrekne@broadpark.no http://home.broadpark.no/~rbrekne/ricmain.html
This PTG archive page provided courtesy of Moy Piano Service, LLC