[pianotech] Thoughts on Bearing Angles

[Gene Nelson]

Hi Jude,
As a beginner to this I have often thought about this length of wire between the bridge pins.
When I was trying to figure out how to set bearing I had studied several procedure and formula.
In my mind, if I decide on 1mm bearing, I would pull the string and cut the bridge till the string was 1mm above the bearing point of the plate and touching the leading edge of the bridge at the same time. Assuming the top of the bridge is flat (paralell to the original string plane), the sine of the front and rear string segments would be the same but the angles would differ and the total would be their sum - depending on the tail length it may or may not add up to 1mm. 
Figuring the individual angles omits the bridge segment but setting bearing does not.
Would this take into account the bridge segment?
Gene
  ----- Original Message ----- 
  From: Jude Reveley/Absolute Piano 
  To: Piano Tech List 
  Sent: Friday, November 07, 2008 8:10 AM
  Subject: [pianotech] Thoughts on Bearing Angles


  List,

  There is the well known diagram for downbearing that is often referred to in classes and discussions and is often cited as the geometric basis for calculating downbearing force. See page 37 in the Nov. 1995 issue of the journal as an example.  

   Using the following model:

  Overall Length=100mm; Speaking Length=60mm; Backscale=40mm; Offset from staight plain=10mm

  According to my calculations this would yield a bearing angle of 23.5 degrees with 9.5 at the front and 14 at the back. What I don't like is that in this traditional model, the length between the front and rear bridge pins is not accounted for but would certainly increase our rear and thus composite bearing angle. This seems somewhat significant since we base our calculation of the force on the bridge using this angle.

  Am I missing something? Thoughts? Comments?

  Cheers, 

  Jude Reveley, RPT
  Absolute Piano Restoration, LLC
  Lowell, Massachusetts
  (978) 323-4545
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