[CAUT] pre-stretching new string?

[Richard Brekne]

Hey there Mark...

Let me get a couple things straight... :)...

1:  Am  I to understand that there has not been strings on this 
instrument during the entire period. ?
2:  Your measurement of positive bearing.... just curious here... how 
far from the initial point of contact of the string to the bridge to the 
point where you measured a 1 mm gap.

The negative bearing bit is no problem... I mean a 2mm drop down to the 
bridge from a straight string is what it is. As for the reverse crown .. 
I think you perhaps may need to hold on a bit.  You may have had 3 mm of 
crown to begin with...... and only slight bearing. (not unusual in an 
old piano me thinks)  The plate could initially have been installed such 
that if the panel ever did flatten dead out (or even come close to dead 
flat)  the strings would have negative bearing. Thats been done before 
no doubt. Negative bearing doesnt necessarily equate to negative crown.

I'd like to know what a straight edge placed on the top of the panel at 
your 9 % RH reveals.... positive, negative... or nearly flat crown.  An 
unloaded board moving 3 mm towards flat given the humidity change you 
site should surprise no one me thinks.  Yes ?

Perhaps I've missed something in the reading ?

Cheers
RicB


    Hi Ric,

    bearing was measured without strings on, via a bearing string stretched
    between terminations. When it initially measured positive, the bearing
    string indicated a gap of 1mm at the read duplex peice, with the thread
    barely contacting the bridge.

    (primitive but useful, nonetheless, I will get a Lowell guage now that
    Piantek has them available)

    Now, there is a gap of 2mm between the bridge surface and the
    string, with
    the string touching the rear duplex peice.

    I believe Ron is correct about the reverse crown, however, what
    perplexes me
    is is how the board could collapse to such a degree with no strings in
    place.

    Remember, both positive and negative measurements were taken w/o
    strings in
    place. So, there is no string length, no angle, no deflection, no forces
    downwards or sideways and no pitch to factor.

    The only factor that changed was RH, dropping to an alarming 9% in
    January
    (we called in the engineers)!

    As for bridge shrinkage, well I can only grin. Ron mentioned
    measuring .2mm
    of cap expansion, and I'm sure that's accurate (less than 5% of it's
    thickness) , the root of the bridge being vertically laminated would
    reacte
    differently.

    So there's certainly no accounting for a 3mm vertical drop there.

    All I can feature, is that a mildly crowned 44 year old board (from the
    desert-on-one-side/tropical-rain-forrest on the other school of
    soundboard
    crowning) dipped below-the-line as it's MC dropped.

    What would make things interesting, is if this same board would go
    positive
    as it's MC rises.Not likely, but a 3mm would be pretty hard to
    explain any
    other way.

    thanks,
    Mark C.