> I'm asking "why does the string need to contact the front edge of the > bridge?" It doesn't. I suspect you have tuned pianos in which this was the case. I know I have. As long as the bridge pin is solid in the top of the cap, there won't be a false beat to lead someone to tap the string - or pin. >> The assumption is that the strings ride up on the bridge >> pins. Assuming positive bearing and proper bridge pin angles with a >> bridge and pins in pristine condition that is not likely. > >Most of these discussions make these assumptions, or that any evident >negative front bearing is due to overly aggressive tapping. In my >experience, these are not self evident. Negative front bearing can exist >(with positive net) from relatively early in the life of a piano, due to >errors in manufacture, bridge roll, or other conditions inducing severe >compression. Then you apparently misunderstood what I was trying to explain to you, or I didn't explain it clearly. >>At the same time, compression on the bridge top (exacerbated by tapping >>down on the strings) lowers the contact point on the bridge. > >Assuming enough downbearing ( at some point in time) to compress the wood >fibers, I would place more responsibility on the seasonally induced >increase in downbearing more than the unsubstantiated certainty of >aggressive tapping. And, of course, there is the speculation that the >bridge surface itself rides up the pin in humid conditions, in turn, >pushing the string further up the pin. This isn't speculation. It can be easily enough measured by anyone willing to take the time and trouble to do so. >The question there would be whether the string then follows the board back >down in the dry season. This can also be measured. >>Unseating on the bridge top tends to occur when the contact point on the >>bridge top is lower than the indentation in the side of the >>pin. Therefore, you are much better off tapping down the bridge pin than >>the string. > >But this assumes either that the pin is not already bottomed in the hole >or that you can safely drive the pin into the bridge body, like a nail. No pin will remain bottomed in the hole with the bridge changing overall height with humidity swings. The point of zero relative movement between the bridge and the pin is typically somewhere near the bottom of the cap - depending on the type of capping material used. That means that as the bridge top is going up the pin, the bottom of the hole is getting deeper, and moving away from the base of the pin. >> At worst, it can create a further disconnect as the contact point on >> the bridge top is lowered due to further compression of the bridge top. > >Here it is again. What do you imagine (I don't know the answer) the >differential between the force needed to seat the string and that required >to further indent an already compressed piece of rock maple? I find the >Compression by Tapping argument suspect. Look up the side grain compression limit of rock maple (1470psi). Then figure the footprint in square inches of the area under a string being tapped. Make it effectively about 0.25"long and 0.010"wide. That's 0.0025 sq"*1470, or somewhat under four pounds that can be applied to the string before the crush limit of the maple is exceeded. Long term deformation will happen at a lower figure. The friction level between a new, undamaged bridge pin and a string at a 10° side bearing angle and 160 lbs of tension is over 14.5 lbs, or roughly 3.5 times the force needed to crush the bridge top under the string. An old grooved pin will have a wider string contact area and an angled depression for the string to have to be forced up as the pin is driven, so you can expect a considerably higher friction figure. By then, the string depression in the cap will be wider too, and the psi load will hopefully not be much higher than with a new pin. This is from the presumably gentler than tapping strings process of driving the pins. What do you suppose the impact psi levels are with someone tapping strings with a brass drift and a hammer? Maybe a hammer shank, since if the shank isn't damaged, the bridge won't be either. The compression limit for the end grain of that shank is listed at 17,830 psi, or over five times the side grain limit of the cap. The point is that the edge of the bridge cap has already been crushed by the expanding bridge before anyone feels compelled to tap either strings or pins. This means that the edge of the bridge (under the string) is rounded down to lower than the center, lower than a point even a few millimeters back from the edge, and lower than a line between that point and the capo. A bridge with little (but still) positive front string bearing will have strings terminating on the bridge pin, and a spot some small distance back from the notch edge, but passing slightly above the notch edge in a straight line to the capo. This string has NOT climbed the pin, and tapping either it or the pin down will NOT make it stay down. It will straighten back out and again, not touch the notch edge, and if the pin is loose, will produce a false beat. >> Furthermore, false beats are usually a product of loose bridge pins and >> a flagpolling of the pin which creates an oscillation. > >The false beats created by loose bridge pins is different from the >distortion caused by faulty termination. Yes they are, and these discussions need to start, and stay, with one or the other until some of the questions are answered. Ron N
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