Below you will find the outcomes of some 20 years of investigation (not full time I can assure you), sometimes with the help of a competition caster, and also from on line research of findings by academics.
The spring & marble, fly rod casting model
Here is a short presentation with a diagram to explain the basic mechanism of casting. It appears simple but in reality is more complex. However, the basics remain the same. Some comments about the complexity of the subject can be found at the end of the article.
To cut a long story short, it’s as if you were pushing a marble, on a table, along a straight line (e.g. in a V block), but there is a spring (the rod) between the marble (the line) and you, which is attached to your finger. As you start pushing, the spring compresses because the marble is resisting the motion (imagine a bowling ball instead) but after some time the spring decides to unload itself and launches the marble. You can imagine that things will vary depending on the weight of the marble and the stiffness of the spring. It will take more time for a soft spring to react and unload. The characteristics that influence the launch speed of the marble are the stiffness of the spring, the weight of the marble, and your input. It is even simpler than that (for a specialist) because the actual mechanical characteristic involved here is the frequency of the loaded spring (attach the spring vertically with the marble and make the marble go up and down, then measure the frequency of this motion). Frequency depends on both the spring stiffness and the weight of the marble. It increases with rod stiffness and decreases with line weight. We are referring here to the role of the “response time of the tackle”. A reasonable variation of the response characteristic can be tolerated whilst still giving a suitable line speed and it is important to know this. Only few casters are able to precisely adapt their cast to the tackle (with an immaculate sense of timing), and generally this is not necessary. Now you may perceive why rods should be quoted by their stiffness and why lines have been rated by their mass for over half a century.
The rod is not flexing and unflexing at its fundamental frequency during a cast. This is because a cast is a transitory motion, which makes a big difference. If you can imagine a rod that is not damped after line launch by the caster, then the fly rod will effectively flex and unflex at its fundamental frequency during this late part of the casting motion, but not until after the caster has brought it to the stop and launched the fly line. However, we do not observe this in practice since we tend to damp the rod systematically.
Studying a spring & marble model, mathematically, gives you two basic rules:
- Firstly, for a given input (casting style: acceleration and deceleration patterns), load influences maximum line speed: the line speed goes up and down at some point in the frequency range. However, this optimum is rarely met in practice with our tackle, we are nearly always within the range where the faster the rod is, the higher the line speed is. It is only when we use a very short length of line that the frequency is too high and that line speed goes below the optimum, and this explains why it is difficult to get suitable line speed with short lines. We might say that the rod “does not work properly”. In fact we also usually prefer a comfortable range of frequencies which are adapted to our physical capabilities and rod response. And this is what we try to repeat with the rod, line and casting style (timing, angle).
- Secondly, for a given load, changing the input (casting style) affects maximum line speed: If a caster uses rather short timing, then he is likely to prefer faster tackle or a lower line number. If a caster has a more leisurely timing, he will naturally be inclined to prefer a slower tackle or a heavier line number.
Nothing new here but the modeling with the spring & marble is consistent with experience.
In fact the actual stiffness of the spring of a rod varies with its orientation (angle of the rod relatives to the line path) and its load: a fly rod is a “hard” spring with stiffness that increases with deflection. When you start casting forward with your rod tilted backwards, you encounter the stiffness of the butt (which can be twice the stiffness measured when placing the rod vertically), and you will introduce the weaker stiffness of the rod-tip at the end of the forward cast.
So the question is how to understand why harder springs can effect performance. Firstly, hard springs deflect less for the same effort. Secondly, hard springs deliver a higher line speed as the load increases. The direct effect of line load on frequency for a given rod is not greatly influenced by the hardening of its spring, whilst the change in maximum speed, due to the variation in stiffness, is magnified when you change from one rod to another. Hardening of the spring exacerbates the difference in behavior (line speed) between the natural trends in rod characteristics: soft/slow/not so hard spring rods and stiff/fast/pretty hard spring rods. A hard spring tends to shift the line number fit higher up the rating scale i.e. it helps to carry a longer line for distance casting. Modern fly rods can be classified as pretty hard springs.
I hope that this will help you to understand the behavior of your rods: the key is to think stiffness, load and finally frequency.