<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> <html> Hi List I wanted to try and better explain why I am having difficulty in accepting that the ribs in a CC board do not support crown against down bearing. Please follow the following reasoning and diagrams and you will see where I am stumped. First is a panel that is simply bent across the grain and held into place with a cable appropriately attached on the ends of the panel. Obviously, when down bearing is applied, the outward pressure on the cable is increased. This entire situation is going to be the same if one first dries out the soundboard, then securing that same cable tightly, let the panel take on enough MC to form the same crown. <img SRC="cid:part1.3F9BA64F.78B112F6@grieg.uib.no" height=305 width=482>   Replacing the cable with a piece of wood doesn't really change a whole lot conceptually.  The rib will  be under the same amount of tension as the cable was, though the amount of physical strain may differ.  Pressing down on the panel is definitely going to be resisted by both the panel and the rib. If the assembly is only resting on supports at the edge, down bearing might even result in some downwards bending of the rib.. but I will have to try this to see. Still, the panels edge will <<bend>> in a downwards direction, and there might be some reaction in similar direction by the ribs. In any case, the assembly as a whole will resist any down bearing for the same basic reasons as above. <img SRC="cid:part2.3F9BA64F.78B112F6@grieg.uib.no" height=174 width=472>     Now if the rib is secured to the panel such that it bends a bit in the same direction as the panel, (i.e. glue say about 10 inches in on each side)  then the only thing that has really changed is that the rib bends upwards a bit... as in the below drawing. Yet the same basic condition exists. Downwards pressure in the panel increases the outwards pressure, yet this is resisted just in the same way as in the previous two examples, so in response both panel and rib will feel an increased stress and will strain against that stress until one of the pieces fail, which of course will be the panel. But until that point is reached, the entire system will resist down bearing vigorously. No doubt the rib will straighten at some point, and the relative tension/compression on the top and bottom sides will equalize, but it will have to experience an increase in overall tension for this to happen, which it will resist. The panel will also straight out at somepoint... but nobody believes for one second that it wont be under stress when its flat.. So why should the rib be any different ? Opposite side of the same coin. Both will physically strain against the stress, the panel a good deal more then the ribs, but the stress levels are the same on both. <img SRC="cid:part3.3F9BA64F.78B112F6@grieg.uib.no" height=319 width=506> The only thing that remains is to simply glue the entire rib to the soundboard, but I cant see that that changes anything. If the above three steps hold, then it seems in error to say that the only thing that supports crown in a CC board is compression in the panel. The entire system, panel and ribs alike will strain against down bearing. Ok... so this seems to make perfect sense to me, and though ribs are not acting as beams in their support, they can neither be said to be purely fighting crown. Quite the opposite. The fact that the rib is bent in the process, and of course resists this bending can be said about the soundboard as well. But I dont see that simple bending of either describes their condition adequately.     Cheers RicB -- Richard Brekne RPT, N.P.T.F. UiB, Bergen, Norway <a href="mailto:rbrekne@broadpark.no">mailto:rbrekne@broadpark.no</a> <a href="http://home.broadpark.no/~rbrekne/ricmain.html">http://home.broadpark.no/~rbrekne/ricmain.html</a> <a href="http://www.hf.uib.no/grieg/personer/cv_RB.html">http://www.hf.uib.no/grieg/personer/cv_RB.html</a>  </html>