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Rod Design - Fundamental Modal Frequency

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Question for Frank Paul: Could you explain how you measure the fundamental modal frequency?  (Bill Fink)

    There are several ways to measure the fundamental modal frequency of a rod. First, one can put it on a electro-mechanical shaker and excite it with a harmonic (sinusoidal) signal. When the rod is strobed with a  light and "the shape stands still" at known frequencies, these will be the harmonic modes or frequencies - for most rods we can measure in the lab up to four. In my opinion only the first two modes and frequencies are important for fly rods. This is using a forced response approach by literally shaking the rod. The fundamental mode is the first one where the curve is similar to the back and forth bending that we see when we cast.

    A second way is to hold the rod butt in a bench fixture and pull the rod tip down (may 3 or 4 inches - not too far as you want to measure the linear fundamental mode) and then release it and measure the cycles per  unit of time during the rods free response. This back and forth motion is for the fundamental modal frequency. You can do this with any rod in your shop.

    Now when you put a line on the rod and then in a casters hand, that is more complicated and requires a high speed camera or vision system to measure the effective SYSTEM fundamental frequency of casting the rod. The rod will take on the same shape that one gets on the bench tests discussed above, but the measured frequency will be significantly less as noted in a previous email. This is basically due to the added line mass and physical abilities of the human caster to grip and hold the rod, including the casters physical arm mass and stiffness.  The rod, line, and caster now become a "system" and the added physical elements (line and caster) change the apparent dynamic behavior of the rod. Keep in mind, the rod does not change, but the combination of rod, line, and caster cause a change in the casting system.  (Frank Paul)

      It may be helpful to some to look at the experimental work I've done on defection and the natural frequency of vibration of a cane rod. The data is available on pages 36 and 63 in the book "Constructing Cane Rods". (Ray Gould)


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