<!doctype html public "-//W3C//DTD W3 HTML//EN"> <html><head><style type="text/css"></style><title>Re: The Richard Gertz myth</title></head><body> <div>Hello Don,</div> <div><br></div> <blockquote type="cite" cite>Very interesting demonstration - thanks for posting it!<br> <br> Can you tell us the approximate sample rib / soundboard crown radius?</blockquote> <div><br></div> <div>Crown radius of rib no. 6 is 15.9 metres or 52.2 feet. The crown radius of this sound board changes from bass to treble. So each rib has its own jig for machining the gradually reducing radius from bass to treble. The smallest radius, on the highest rib, is 13 meters or 42.7 feet.  On our 225 we use a smaller 11.5 metre radius in the treble, but our 225 rib profiling jigs are about 20 mm too short for forming the ribs for a D, so I used the profiling set I built for the 280 piano, which are a little large in radius I think. I'm reconsidering the 280 sound board layout yet again, even before I've built the first instrument.</div> <div><br></div> <blockquote type="cite" cite> Or if you could provide the unloaded measurement of the difference from the bottom of the soundboard strip (at center) and the cord line to give an idea of how much crown was in the sample.</blockquote> <div><br></div> <div>Crown height at the middle, relative to ends was 5.5 mm. This rib is loaded in a D with 17 Kg at 0.75 degrees downbearing. Using this test load caused the rib<font color="#0000FF"> (with both ends firmly supported)</font> to sink 1.75 mm under load at the middle. To derive the unstrung down bearing settings, we calculate the down bearing in mm required to achieve the downbearing angle<font color="#0000FF"> (for the load we have chosen)</font>, then add the amount the soundboard drops under that load. This figure should result in an at-pitch down bearing which is within 0.5 mm if we have done our homework correctly.</div> <div><br></div> <blockquote type="cite" cite>What style rib was it - crowned on top?</blockquote> <div><br></div> <div>Yes. It was an I-rib, which has the crown machined onto the top of the center web, so the top flange of the I-rib is glued onto the center web to form the crown radius. The bottom flange is flat in the unloaded condition. So an I-rib gets deeper towards the centre<font color="#0000FF"> (33.5 mm overall depth in this case)</font>, where it needs to be stronger since the centre of a rib is where the maximum stress in extreme fibre is encountered.</div> <div><br></div> <blockquote type="cite" cite>  Was there any "compression" style crowning in the combination?</blockquote> <div><br></div> <div>No, it was fully rib crowned. But it wouldn't have mattered wether it was rib, compression crowned or a combination of both. All styles will pull in under the down bearing load in the same way. I've got compression crowned models which do the same thing.</div> <div><br></div> <div>The myth of rib expansion, I believe, has come about through the use of those neat little piano-exhibition demonstration models which represent a small crowned sound board model with no ribs. Without ribs a sound board panel will push out. But when the ribs are added, substantially more overall offset of the sound board assembly is positioned below the line between the end support points. Therefore, a ribbed board will always pull in under load<font color="#0000FF"> (unless it had huge crown and very shallow ribs)</font>. Nevertheless, rim rigidity still remains an important factor regarding tonal sustaining qualities, but it works in the reverse way that most of us have assumed<font color="#0000FF"> (to answer your post 'Thump', when you mentioned the outstanding sustaining qualities of M&H, and its attribution to the Gerz system - M&H also have substantial rims which are a great help, and the rods certainly will help to improve rigidity to some degree, as Phil Ford mentioned, but not in quite the same way as advertised)</font>. Furthermore, I can now safely conclude that Ron N. and Del were correct when they claimed, on several occasions, that the rim has little influence over the maintenance of sound board crown.</div> <div><br></div> <div>Sorry for not answering your post Phil. You raised some excellent points which I hope to reply to soon.</div> <div><br></div> <div>Its interesting and surprising stuff.</div> <div><br></div> <div>Ron O.</div> <x-sigsep><pre>-- </pre></x-sigsep> <div><font face="Times New Roman" size="+1" color="#0000FF"><b>OVERS PIANOS - SYDNEY</b></font></div> <div><font color="#000000"><i>   </i></font><font color="#0000FF"><i>Grand Piano Manufacturers</i></font><font color="#000000"><br> _______________________</font></div> <div><font size="-2" color="#000000"><br> </font><font color="#000000">Web http://overspianos.com.au</font></div> <div><font color="#000000">mailto:info@overspianos.com.au<br> _______________________</font></div> </body> </html>