Sohmer Agraffe Bridges

[A440A@aol.com]

Inre the Sohmer, I said,
>     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.

Ron asks: 
>Higher in front? Like the front bearing angle is greater than the rear? 

   Like the top of the bridge is higher at the speaking length (proximal) 
bridge pin. 
 
>>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?<< 

 Hmm, using the Lowell  gauge, which gives only a relative deflection number, 
the bridge angle is 6 marks off the level of the speaking length, the 
backstring is 9 marks.  When the legs of the gauge are 1/2" apart, each notch 
represents .001" of difference.  So,  by my hillbilly algebra,(using toes,too),  it 
seems that the back string drops .009" per half inch, which means over the 2 
1/2 " it would drop .045" in total.  Please work the angle out for me, if you 
would. 
 
I opined:
>  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.

Ron asks: 
>Or rather rotate back?

   The bridge?  Yes, I would think so.  The vector of force on just the 
agraffe, however seems that it would be forward and upwards, since it is resisting 
the upward force formed between the V-bar and the leading edge of the bridge 
cap,(which is right behind the agraffe).  The total effect on that spot of the 
bridge, however, would be downward and back.  HOwever,  to know for sure, I 
would have to ascertain the moment arm through the bridge formed by the center 
of the the agraffe/string contact and the mean point of resistance between the 
bridge and soundboard.  (I have run out of toes, so this is all pure 
speculation). 
 
Ron asks: 
>What's the net bearing angle between the speaking length and the 
back scale segment here?<<   

  umm,  I already said I had my shoes off.  Can you get the angle from 
knowing that the backstring drops .045" in 2 1/2" of distance?  (and this duplex has 
a variety of lengths, from 2 3/4" - 1 1/2".   I measured the drop on the 2 
1/2" distance). 
 
Ron writes: 
>>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.>>

That is more than possible.  I am going to hope that a new scale, lower 
tension, and tightened glue joints will make this a passable instrument.  The 
market simply will not justify a new soundboard to rectumfry a small segment's 
slight loss of sustain.  (if it was a Stei... oh well,  it aint'). 
Regards, 



Ed Foote RPT 
http://www.uk-piano.org/edfoote/index.html
www.uk-piano.org/edfoote/well_tempered_piano.html