> The top octave, which has no agraffes on the bridge, has a LOT of >deflection in the normal sense. With the tool leveled on the speaking >length, the >bridge measures higher in front, and the strings certainly angle their way >down >to the duplex bar in front of the hitch pins. Hi Ed, some questions if I may. Higher in front? Like the front bearing angle is greater than the rear? What is the angle from the speaking length to the segment on the bridge top? And what is the angle from the speaking length to the back scale? How much bearing does this little sucker have in the treble? >It looks a little excessive but >the top octave on this little piano really screams. Maybe it is worth >mentioning that the C8 string length is 2.20 inches! (Do I hear the scalers >starting to scream, too?) Just short of 56mm. I've seen worse, but that's pretty abusive. > Assume all measurements with the Lowell gauge begin with leveling it on >the speaking length, and all deviations are in contrast to this >baseline. At >C6 the string definitely rises from the back of the agraffe to the >back(distal) side of the bridge, so the agraffe would pop up if the >threads broke. Or rather rotate back? >However, from the back edge of the bridge to the hitch pin, there is a >downward >angle to the string, so I read this as indicating that the total bearing >pressure >at this string is downward. I agree. What's the net bearing angle between the speaking length and the back scale segment here? >The same holds true for the next octave or so. > At C5, I notice that the downward angle in the backstring has approached >zero, yet there is still a certain amount of rise from the back of the >agraffe >to the distal edge of the bridge. This may be an area where the bearing is >actually trying to lift the soundboard. Or it may result in a zero pressure >either way. Hard to tell. It wouldn't be lifting the bridge directly unless there is negative net bearing, but would be applying torque on the bridge - trying to rotate the top toward the rear. The curve of the bridge through that section will greatly resist this rotation, but it will eventually deform the bridge. The lack of net bearing in this area is hardly a surprise. > My feeling is that the agraffes were an acoustical endeavor and that the >board originally had coventional downbearing throughout. I think you're correct. >The one section near C5 >where the the backstring and speaking length were at the same angle, >interrupted by the small rise from agraffe to bridge, is where I normally >see boards >flatten out with age. Indeed. > Now, about those cut-out bridges. In the area of zero deflection > (C5), I >notice that there is a small break in the glue joint between the notch in the >rib and that long continuous maple plate. This break is wider at the front, >making it appear that the bridge has attempted to roll back,(distally). For >this to happen, I believe that the downbearing had to have been lost and the >bridge was slightly rolled backwards in this section. This is also the most >tonally deficient area of the piano, which, without a doubt, is either simply >coincidental or irrefutable proof that bridge roll kills sustain. Take your >pick. >Regards, > >Ed Foote RPT I vote that bridge roll does not kill sustain, but rather that bridge roll is the result of structural failure, and it is the structural failure that causes bridge roll, loss of crown, loss of bearing, and loss of sustain - assuming there was crown, bearing, and sustain in the first place. Ron N
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