[CAUT] pre-stretching new string?

[Richard Brekne]

Hi Mark.

Just a thought here... You say the string had 1 mm of positive 
bearing... and now the surface is 3 mm lower.. i.e. 2 mm negative.  That 
essentially means the plane of the string has gone from being pushed 
upwards to being pulled downwards... and the net deflection is increased 
by 1 mm.  Pitch has to go up some small amount by this change depending 
on the length of the string.  On the other hand, the strings offset 
through the bridge pins is lessened significantly shortening this 
length.... which will lower the tension quite a bit more then the net 
increase in vertical deflection (negative tho that may be) would cause.

I dont know how you are measuring changes in bridge dimensions... but I 
find it difficult to believe that the overall thickness of the whole 
bridge / soundboard changed so much that the height of the bridge 
recessed by 3 mm. That would mean something like a 5-6 mm shrinkage in 
the entire thickness yes ?  If you take the panel at a liberal nominal 
10 mm and the bridge similiarly at 40 then you are talking about a 10 % 
shrinkage.  This tells me something else is going on.... i.e. the front 
and rear termination heights are not static either.

And just for the record... consider the consequences of the vertical 
force on the bridge caused by such a change. Gets interesting real quick.

Cheers
RicB

    I just went down and measured note #40 on a 1963 Baldwin L we have
    yet to
    re-string, which had 1mm positive bearing when we prepped it last
    summer:

    The bridge surface is presently THREE MILLIMETERS LOWER ( 1/8th" )

    i.e.: TWO MILLIMETERS NEGATIVE, relative to the front and rear
    terminations.
    (RH 34%)

    (an improvement from measurements I last posted 05/03/07; RH 9%)

    .2mm cap rise I can see, given the approximate 6mm thickness of this
    cap at
    note #40, but 3 whole millimeters drop?

    Three millimeters, now that's a meaty number.... where'd all that
    wood go?

    (inquiring minds, even wandering ones, need to know ;>)

    best regards
    Mark Cramer
    Brandon University

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