Gene Nelson wrote: > What is a good way to describe or include into rib design how much the > board is displaced when set in motion by the strings? How important is > it to rib design? Displacement is in the planning, but not directly considered as such. A low spring rate (flexible) rib will displace more than a high spring rate rib, but will deflect more under the same load. Accommodate the load, crown, and deflection, and the displacement (excursion) in use will take care of itself. It's a very tiny movement compared to the excursion of the strings. > One thing that I have discovered by playing with numbers is that the > available crown of a rib could be completely used up or even exceeded > and the rib's fspl is not exceeded depending on its dimensions. That's the idea of a RC&S assembly. None of the component parts is near it's stress limit, as is the case in panel supported boards. >So - if > a rib has xmm of crown and 50% is taken up by the downbearing or string > bearing load, in a static situation is it possible that additional rib > motion due to the amplitude of the string become a sturctural issue? > Seems to me it could. Try to measure board excursion during play. In the low bass, with the panel floated, you can get visible movement, but even then, it's not a lot. If it becomes a structural issue, it's because your ribs aren't sized anywhere nearly stiff enough to support bearing load. It's a no worry zone. Choosing the stiffness and mass appropriate to the point in the scale and the expected load is problem enough, and takes you well past any concerns with soundboard vibrational amplitude being structurally destructive. Ron N
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