<HTML>In a message dated 8/16/2003 = 12:00:57 AM Pacific Daylight Time, fordpiano@earthlink.net writes: <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px">Subj: Re: Soundboard crown Date: 8/16/2003 12:00:57 AM Pacific Daylight Time From: <A HREF="mailto:fordpiano@earthlink.net">fordpiano@earthlink.net</A= > Reply-to: <A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A>= To: <A HREF="mailto:pianotech@ptg.org">pianotech@ptg.org</A> Sent from the Internet <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px"><BLOCKQUOTE TYPE=CITE style= ="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px; PAD= DING-LEFT: 5px">  =     Calin      As I'm sure you know the board your describing to b= e made with flat ribs will not stay straight but crown once the dried panel = gains moisture. What you're describing is the old syle  pure compressio= n type of board making. That is unless you mean that  no panel drying i= s done prior to the rib glue up. In this hypothetical case some amount of be= aring is no doubt likely to reverse the crown of the board don't you think. = And the panel will likely crack as soon as it dips below it's realitively hi= gh E.M.C at the time of ribbing....</BLOCKQUOTE> <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px">  Regards---Dale </BLOCKQUOTE> </BLOCKQUOTE> &nb= sp;   Phil Fod writes</BLOCKQUOTE>    &nbs= p;      Dale, <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px">    What makes you think it wil= l crack?  Because the board is in tension from the downbearing and as i= t gets dryer it goes into further tension? </BLOCKQUOTE> >>Phil-- My thought was that any wood will shrink over time especially= quartered wood because of it's expansion contraction properties and actuall= y that could take some time. However We've all seen old stwy board with no c= racks ,compression ridging, crown or bearing for that matter that make my st= atement  questionable. Or perhaps whatever little compression the wood = had originally has equalized or was never great enough to cause compression = set or damage.   <FO= NT COLOR="#000040" style="BACKGROUND-COLOR: #ffffff" SIZE=3 FAMILY= ="SERIF" FACE="Times New Roman" LANG="0"> Do you think that would be e= nough tension to exceed the tension strength of <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px">the board?  I wonder.  I also wonder what c= auses cracks in boards to occur.  Do you think it's from tension exceed= ing the nominal allowable strength of the wood?  </BLOCKQUOTE><F= ONT COLOR="#000040" style="BACKGROUND-COLOR: #ffffff" SIZE=3 FAMILY= ="SERIF" FACE="Times New Roman" LANG="0">   ><FON= T COLOR="#000040" style="BACKGROUND-COLOR: #ffffff" SIZE=3 FAMILY="= SERIF" FACE="Times New Roman" LANG="0">>>>Me too        Or is it from compression damaging a se= ction of the wood during a period of <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px">high EMC which then weakens it so that the allowable = strength (in tension as well as compression) is reduced, so that the next ti= me it sees a low EMC it cracks.</BLOCKQUOTE> >>>>>Everbody says so<= FONT COLOR="#000040" style="BACKGROUND-COLOR: #ffffff" SIZE=3 FAMILY= ="SERIF" FACE="Times New Roman" LANG="0"> <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px"> I also wonder how a flat board that is pushed into reverse crown by the down= bearing will behave with changing humidity.  With decreasing humidity t= he board wants to shrink.  It seems to me that this would mean that the= board would want to move up, back toward the flat position.  But the r= ibs are resisting this.  So a moment would be set up between the ribs a= nd board that would want to push the board down.</BLOCKQUOTE> >>> If it's reversed the ribs to my mind would now be acting as&nbs= p; a spring in tension and could behave the way you describe.          The reverse would be true with in= creasing humidity.  The board wants to <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px">grow, which seems to me would mean moving downward.&n= bsp; But the moments between ribs and board would be tending to push the boa= rd up.  So perhaps this configuration is actually more stable than the = conventional configuration.</BLOCKQUOTE> ... I dunno. It could = be <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px"> With a conventional board having crown and with a downbearing load I think t= he board is always in compression.  However, with this reverse crown bo= ard I can imagine that there is some humidity level where the expansion of t= he board induces a compression load in the board which just balances the ten= sion load from the downbearing so the board is in an essentially unloaded st= ate.  As Del was conjecturing in another post, the acoustical behavior = of a board in this state may be very different from a conventional board.</B= LOCKQUOTE> >>>I agree</F= ONT> <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px"> Didn't you post some time ago about a piano (I think it was a Sohmer) that c= ame through your shop that had a reverse crown which you thought was deliber= ate?  I would think that this board would be in tension.  I have t= rouble imagining how you would build a board like this to be in compression = unless the ribs were on top.  Which side were the ribs on, by the way?&= nbsp; I think I remember you saying that this piano had a very nice sound.&n= bsp; Did it have a lot of cracks?</BLOCKQUOTE>    >>>>> Yes I did post on this. It was a  Meh= lin & sons and a Sohmer. Both in my recall had postive downbearing and i= ntentionally built reversed crown boards. The Mehlin had a really dished boa= rd but had only one hairline crack at the tenor bridges end. The ribs in bot= h cases were on the bottom. I like the sustaining qualities of these pianos.=   I'm sure the performace of these differs from the conventional boards= but it's a very atrractive sound. The lack of damage to the wood in tension= I've seen doesn't support the cracking of assertions I and others have made= in regards to tensioned panels but these are only two examples. By the way = I just bought another sohmer 5ft 11" that has the same config. only it has a= ggraffes from the bottom of the tenor bridge up to the capo break. Regular b= ridge pins in the bass and top trebles.           Regards ---Dale Erwin= <BLOCKQUOTE TYPE=CITE style="BORDER-LEFT: #0000ff 2px solid; MARGIN-LEFT= : 5px; MARGIN-RIGHT: 0px; PADDING-LEFT: 5px"> </BLOCKQUOTE> </HTML>