There IS a weight, hung on a horizontal rod...that approximates the flexion in an actual cast. And there IS a relationship between that weight and the force causing the flexion on the rod in that cast with that line weight. That relationship can range from nearly identical to proportional in either a linear or more complex fashion.
Assume a useful, proportional relationship. In implementing the model, one could have cast leads weights representing the F[210], F[240], F[280] for people to use. Then they could simply set a rod horizontally, hang each weight on the tip and measure the CHORD to the hanging tip There is a right triangle in there and the chord is the hypotenuse, so given the chord the lever length is easily calculated.
That way, one can predict, within the limits of accuracy of the model, that throwing 300 grains of line-fly-whatever...that rod is going to shorten to a 66". And throwing 240 grains it will shorten to only 68" (or whatever). And throwing an 11 wt line when the first 30 feet is out...the rod will be flexing to, say, 64".
The L[weight] IS a measure of stiffness...exactly as the ERN is...but in this case at actual casting flexions. AND, because we have defined a standardized set of Forces (represented as weights) we can reverse it to predict rod flexion in actual casting for different line loads.
So, a rod COULD be marketed as an L[280-10 wt] = 66 plus a slope-of-curve figure (taper) which predicts more or less further shortening at further loading .......or as L[240-9 wt] = 67, L[280] = 66, L[330-11 wt]=63.
After all, when we buy cars we see and compare charts of HP, Torque, turning radius, MPG,0-60 time, stopping distance, etc. We KNOW what we are getting at least in terms of performance. Don't hold your breath that the rod companies are ever going to want the actual casting parameters laid out in such a concrete fashion. But it IS do-able.
Shortened lever length, a measure of stiffness, is THE single most important casting parameter. Taper, in it's relationship to that lever length as more or less lever length due to less or additional Force...is the next most important parameter. After that, rod weight, moment of inertia (swing weight) are minor issues. I will clarify that with further discussion.
VI The Haul
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