“The fundamental problem with the casting model based on rod load is that nobody knows what a proper rod load is for a given cast; I don’t know of anybody able to look at the bend in a rod during a casting stroke, and tell if it is a proper load or not. Not only different rods will show different amounts of bend for the same cast, even every angler can get different amounts of rod bend for the same cast and casting distance, depending on their individual styles.”
The Technocrat
onemorelastcast.net 
One can generally tell the difference between loaded and un loaded
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All rods are “loaded” when casting, even in the hands of a complete beginner.
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A few years back I wrote an article on rod loading which appeared in the Loop. In that article I included a table which calculated the amount of force needed to accelerate a series of fly lines from 0 to 120 miles per hour over a 16 foot tip travel. I used 1 weight through 10 weight lines. Although I did not do it in that article the tip travel distance can also be varied.
Determining the force required to accelerate a fly line to a desired speed over a given path is very straight forward. So determining the proper load is easy. The problem is to be able to know the amount of bend that force will create in the fly rod throughout the casting stroke. This problem is further complicated by the fact that the amount of rod bend for the same force varies as angle between the rod butt and the fly line constantly change during the casting stroke.
One of the things I dream of doing, but which I have as yet not had the time to do is build an apparatuses that will allow me to apply constant forces to a fly rod as it is rotated through a given arc. By repeating this process for different forces and measuring tip deflection I think it should be possible to determine the best weight of fly line for a given rod by looking at which combinations produce the straightest tip travel.
But as you pointed out, the ability of an instructor to take all of these variables into consideration when observing the bend of the fly rod during a cast is asking a little too much. The best I can do is observe how well the fly line travels towards the target and listen for the whistle of an unload fly rod.
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I don’t know a Rod-Load model Aitor, do you ? I know teaching concepts including a view on the ‘load’, deflection respectively. To me this view belongs to a complete analysis of the cast, because in fact the way the caster ‘loads’ the rod indicates the quality of the energy transfer along the fly rod (without ‘load’ / deflection there is a very weak energy transfer !). The question behind your post hits the issue, since ‘the proper load’ can’t be faced staticly. But there are already some teaching concepts who explain how the motions should be adjusted to achieve a better energy transfer along the fly rod.
Cheers, Tobias
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What I call the “rod-load model” is the predominant view that sees all good and bad things in casting as related to the amount of bend in the rod.
A Spey cast works due to the anchor loading the rod, they say. If a cast fails it is due to the rod not being properly loaded.
That is like looking at the finger while this one is pointing the Moon.
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Agreed, according to Newton anchor casting won’t load the rod and a casting fail must not point to a bad ‘load’. I just struggle with the designation ‘rod-load model’ since in my book such a model doesn’t exist (or if it does it can’t face the fly cast).
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Remove “model” and put “mantra” if you prefer.
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Bill,
“Determining the force required to accelerate a fly line to a desired speed over a given path is very straight forward. So determining the proper load is easy. The problem is to be able to know the amount of bend that force will create in the fly rod throughout the casting stroke. This problem is further complicated by the fact that the amount of rod bend for the same force varies as angle between the rod butt and the fly line constantly change during the casting stroke.”
As you say the varying angle between rod and line is a problem. But a more serious one, which puts into question the practical value of a quantitative approach, is that the impact of translation in the casting stroke is underrated. Most of the force applied to the line comes from torque, but the amount of distance that torque is applied over influences the amount of work done on the line. So, a caster fond of longish casting strokes will have less rod bend than another caster with a shortish stroke, for the same line speed and distance.
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