Downbearing

[Daniel Lindholm]

Hi!

I'm attending at a instrumentmaking school in Sweden as a 
pianotech (three year course). As you probably all have 
noticed the downbearing seem to get lower/totally dissapear 
when lowering the string tension/taking away the strings 
completly.

How come? At first I thought it all had something to do 
with when you dry the soundboard to open the cracks when 
repairing it. When we took apart a 185 grand at the school 
we measured the downbearing all the way of the piano and 
noticed some positive downbearing all the way through the 
piano (+1mm at least). When we had lowered the tension of 
the middle strings and started to lower the tension of the 
left strings in the middle the metal wedges between the 
cast iron and the 'front' of the soundboard (just where the 
list wich the damperwires go through) fell off. That 
indicated that the cast iron actually is compressed alot 
under the string tension. I got surprised, even though I 
can understand that is compressed somewhat under that 
enormous tension the strings have.

Then I formed a second theory about the dissapearing 
downbearing. Could it be that the strings actually 
compresses the cast iron and the cast iron compresses the 
whole soundboard giving it a part of its crown in that way? 
I cant think of any other way, but I'd like to hear what 
you all have to say about this since I got little 
experience in this area. Do you always measure the crown 
before and after the strings are taken away to make a more 
accurate calculation of the downbearing when rebuilding 
instruments? Any tips, theories etc are welcome. I'll be 
back on thuesday to reply to any comments to this post.

Note that the downbearing was measured just before and 
after the strings were taken away on that grand mentioned 
above and the difference was very big. In some parts we had 
about 2mm positive downbearing before removing the strings. 
After we removed the strings we had 1-2mm negative 
downbearing in some places.

Thanks,
Daniel Lindholm / Sweden