TonyGonzales84 -> RE: vertical string pull on the soundboard (torque) (Aug. 22 2020 2:45:41)
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Some methodology notes. 1) This is hopefully not too much in the vein of, "measure with a micrometer, mark it with a chalk, cut it with an axe!" 2) I was trying to get a feel for orders-of-magnitude, and compare my results with Richard's values. 3) I measured (using a vernier caliper...ouch, my no-longer 20-year old eyes!) my 1970 Manuel Rodriguez (used for the illustrative photos -- note the pronounced doming - muy ahuevada, even with these marginal photos), and my 2012 Charles Sutton. 4) The dimension, a, is reasonably accurate, not really requiring calibrated eyeballing (i.e., it is repeatable). The dimension, b, does require eyeballing, so there's more slop in that measurement. Some thoughts. 1) Romanillos, in his book on Torres, states that Torres was the first to standardize using a saddle, to give a significant degree of freedom in adjusting string height. This was because the double-domed top is difficult to build precision into (pg 138). Romanillos mentions nowhere the desire to vertically load the top. Interestingly, Huber, on pg 42 of his "The Development of the Modern Guitar," says the saddle "(allows)... a downward bearing of string tension..." Note, La Leona is from Torres's first epoch, and has no saddle (the picture included here). 2) Ricardo mentions, upthread, that he's played a guitar with break angle = 0, and it was sonically negligible in difference with when it had a small break angle. The classical players that play and concertize with La Leona, love that guitar's sound. 3) On the problem of extrapolating a guitar's response based on a (beam-based) simplified vertical stiffness measurement, I'm not sure what to make of this (I do understand that people need quick and ready guides). I know luthiers perform tap tests at various stages of assembly, but is this type of test frequently used? It would only take a few more minutes to set up a test where the majority of the top's edges are supported; would this "higher accuracy" measurement be worth performing, since it still does not simulate the string load's applied shear and moment? (This is possibly in the mode of thinking aloud...) 4) I can envision a rig that would load the stiffened top in shear and moment, but I can't see how the top would remain undamaged (crushed and compressed top). 5) I can also envision a test rig, using low friction installed pins (waxed, oiled, etc), perpendicular to the top, to convince oneself of the string loading. The pins on the front of the bridge would be compressed due to the moments, and those on the back of the bridge would want to be pulled out; all pins would see the shear force of the full string set, T. 6) I recall seeing somewhere, online, that someone has built a modern version(s) of Torres's papier mache guitar (or something like that). I could envision that challenge as a right-of-passage for some luthiers. 7) I'll shut up, now! [8|]
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