Unison speaking lengths - was Ditch pin bushings

[Overs Pianos]

Joseph and list,

Joseph wrote re the piano shown in the image at 
http://www.overspianos.com.au/testa.html

>Ron O.,
>Just curious, It appears from the picture that each unison has a longer
>speaking length for the right string than the left string. Is that just an
>optical confusion, as it were?

Well observed Joseph, I had to look at the image again to see what 
you were referring to.

Yes it does look odd in the image, but there's a reason behind it. In 
fact the bridge pin groups are increasingly inclined anticlockwise, 
from perpendicular at C88, then gradually increasing down to the last 
note before the break to the agraffe section. The idea of this (which 
Samuel Wolfenden wrote about in his work Treatise on the Art of 
Pianoforte Construction) is to arrange the bridge pin groups parallel 
to the capo bar. This is intended to make the unisons equal length, 
which it nearly achieves but not quite (this is a very recent 
discovery).

Recently, when revising the bridge pin layout for our new piano no. 
003 I discovered that this practice, while greatly improving the 
situation compared to using squared bridge pin groups, tends slightly 
to overdo it. Since the unison spacing at the bridge is greater than 
that at the capo bar, when the angle of the bridge pin group is 
inclined at an angle parallel to the capo, the right hand unison will 
be about 0.25 mm longer than the left. If we were to arrange the 
bridge pin line perpendicular to the speaking length (as is typical), 
the left hand unison would be longer than the right by about 1.25 mm 
in a 225 scale just before the capo/agraffe break (the longer the 
piano scale the worse the situation will get). As a working solution 
to this problem, I wrote a formula (in the spreadsheet I have 
designed to calculate everything to do with the scale layout) to 
derive the correct bridge pin angle (which will be nearly parallel to 
the angle of the capo but slightly less) for each unison group to 
achieve equal length unisons. We have been arranging the bridge pin 
group parallel to the capo for many years. Our piano 003 will be the 
first to have truly parallel unison lengths for the capo sections. I 
am looking forward to hearing this piano (two weeks to go). The 
'cleanliness' of the treble may further be improved. We will see.

Ron O
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Website:  http://www.overspianos.com.au
Email:        mailto:ron@overspianos.com.au
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