Line configuration affect on rod

[George C]

When we simplify the rod as a spring, the softer rod stores more spring energy for the same load.

Hi Torsten
Can you explain why this is?

Thanks
George

[Paul Arden]

When we simplify the rod as a spring, the softer rod stores more spring energy for the same load.

Sorry in that last sentence I should have written “bend” not “load”. You are correct George.

I believe that stored elastic energy would be the same between softer and stiffer rods under the same load. I don’t know if or how the dynamic loading changes this.

Cheers, Paul

Ah yes. Because PE= 1/2 k.x^2 ?

[Merlin]

Hi all

Comparisons are not so easy. Considering a given cast for a given mass of line and two graphite rods (905 type), one rod being faster than the other, then the casting model says that there is:
• A larger tip speed for the faster rod at launch time
• More elastic energy stored by the faster rod with a slightly smaller deflection
• The swing, spring and whip relative shares are very similar

That is not straightforward.

Merlin

[Vinny]

I never thought so much thought went into this. My adjustments are based on the way my loops look. I really am a “simple man”

[George C]

When we simplify the rod as a spring, the softer rod stores more spring energy for the same load.

Sorry in that last sentence I should have written “bend” not “load”. You are correct George.

I believe that stored elastic energy would be the same between softer and stiffer rods under the same load. I don’t know if or how the dynamic loading changes this.

Cheers, Paul

Ah yes. Because PE= 1/2 k.x^2 ?

Now I'm even more confused.
Isn't K the spring's constant and isn't this value different between a stiff rod and a soft rod?


My thinking is that at rest a bent rod has no kinetic energy (relative to its surroundings) and only potential energy. For a given load on the rod at rest the stiffer rod will bend less but contain the same potential energy as a softer rod under the same load.
In a dynamic situation the rod contains kinetic energy and potential energy. Since the lever arm of the bent stiff rod is longer than that of the softer rod I can see where at any given moment one rod may have more kinetic and less potential energy than the other but I can't see where this is a simple relationship that suggests one rod will produce more line speed than the other.

What am I missing, here?
Thanks
George

[Paul Arden]

That’s what I initially thought too, George. But then Torsten said something different. So I looked up PE and found it’s half the spring constant multiplied by the square of deflection. Something I probably knew 40 years ago but have long since forgotten. And then Daniel gave an opposite example.

I’m looking forward to being re-educated.

Cheers, Paul

[Merlin]

Hi George

Let’s try answering your questions

When a given mass is moved by a given force by two levers of the same length but different stiffness isn’t the load the same???

I understand the “load” as the force generated which is the product of mass by acceleration applied to that mass by levers, but nothing can tell if that acceleration is the same for both levers since they bend. This is why we have to use some model to figure out what can happen.

Does a softer rod under load hold more elastic energy than a stiffer rod under the same load?

If the load / force is the same, and assuming k1>k2 then F=k1*x1 = k2*x2, with x1<x2; and since elastic energy are W1 = ½ k1*x1^2 and W2 = ½ k2*x2^2, then W1 =1/2 F*x1 and W2=1/2 F*x2. Since the deflections of levers are different then the elastic energies are different and W1<W2. So the answer is yes without any other consideration than a static situation.

Or do the rods hold the same elastic energy just in a different shape???

The answer is no in that case, the shape is another story.

Isn't K the spring's constant and isn't this value different between a stiff rod and a soft rod?

Yes, this is the usual convention but a fly rod is a hard spring and that can be expressed by such an equation for an applied force:
F = k*x + k’*x^3; k’ being the non linear part of stiffness.

My thinking is that at rest a bent rod has no kinetic energy (relative to its surroundings) and only potential energy. For a given load on the rod at rest the stiffer rod will bend less but contain the same potential energy as a softer rod under the same load.

We have just seen before that the stiffer rod contains less elastic energy because it bends less (x1<x2), so the first part of the sentence is correct. When speaking of potential energy you speak of elastic energy in fact (what else?), so your statement about potential energy is wrong.

In a dynamic situation the rod contains kinetic energy and potential energy. Since the lever arm of the bent stiff rod is longer than that of the softer rod I can see where at any given moment one rod may have more kinetic and less potential energy than the other but I can't see where this is a simple relationship that suggests one rod will produce more line speed than the other.

For sure, that part cannot be deducted from simple considerations but IMHO the larger speed obtained with the stiffer rod is mainly due to the fact that you need to impart more energy to the rod and line system to follow a given casting input (e.g. given rotation speed history) with that rod. For comparable rods (same make), the stiffer rod has a larger MOI and needs more energy to follow a given input, and that one of the reasons why one gets a larger tip speed and a larger elastic energy for such rod. However a key parameter is the “loaded” speed of the rod and that involves rod action somewhere. Faster rods are more on the tip action side and can remain faster under load by comparison to softer / butt action rods. This influences tip speed.

Merlin

[VGB]

I never thought so much thought went into this. My adjustments are based on the way my loops look. I really am a “simple man”

Simple is good for learning how to do. In real time you have limit your cognitive load to that essential for conducting the task

[Paul Arden]

the stiffer rod has a larger MOI and needs more energy to follow a given input, and that one of the reasons why one gets a larger tip speed and a larger elastic energy for such rod.

Hi Merlin, the stiffer rod feels “lighter” when casting and can be rotated quicker. I assume this is because torque at the butt goes more into rotation speed at the tip and less into bending? I remember casting an actual broomstick for the first time and being surprised by how “light” it felt (despite being very much heavier).

Cheers, Paul

[Paul Arden]

I never thought so much thought went into this. My adjustments are based on the way my loops look. I really am a “simple man”

Even the simple things can be made to be very complex.