What is ultimate fly line weight for your fly rod? Or what is the ultimate fly rod for your line? The goal is distance.

[George C]

Hi Merlin
A lot of this is beyond me so apologies if you have already addressed it.
For the same static load hung on a soft vs a stiff blank the soft blank obviously bends more but do both blanks contain the same amount of elastic energy?
Thanks
George

[Mangrove Cuckoo]

Merlin,

OK... F, K, x...

I can get along with those... but what is K2?

If K2 is factored with x cubed... isn't this a (or the) major player???

[RSalar]

In terms of a spring — which stores more energy? The tip action rod/spring or the full flex rod/spring. Just thinking about it I would guess the more of the rod that bends the more energy it can store. What is the real answer?

It appears to be paradoxal but if I compare two graphite rods (same length, same line, similar MOI), the “tippest” one stores more elastic energy than the other one for the same series of ”virtual” casts with my 2D casting model.

Merlin

Thanks Merlin,

I’m obviously not an engineer so the math is beyond me. When I try to figure out the difference between tip action and butt action rods, I play a mental game and take the two types of rod actions to the extreme. So, for example, an extreme tip action 9 foot long rod would be like a steel I-beam 8 feet long with a 1 foot long piece of spring steel attached to the end. An extreme butt action rod would be a 9 foot long piece of spring steel the same thickness and width as the 1 foot long piece on the I-beam. So you have a 1 foot long bendable spring vs a 9 foot long bendable spring. How can that 1 foot long spring possibly store as much energy as the 9 foot long spring if both are exactly the same thickness and width?

—Ron

[Merlin]

I seee that everyone is using a static situation whilst the issue is completely dynamic.

Let’s start by the static situation and George’s question: I have a simple spring of stiffness K and I apply a force F and get an x deflection:
F=kx and the elastic energy is 1/2kx2

Now I change my spring for one having a stiffness being twice the first one: 2k. The same force F induces a x/2 deflection now since the new spring is twice stiffer. And the elastic energy is ½ (2k) (x/2)2 = ½ kx/2

My softer spring (k) has now twice the elastic energy compared to my stiffer one. Ron’s analogy is of the same kind (static) but more complex since he compares very different springs, ignoring their mechanical characteristics (see below).

But a fly rod does not work in statics conditions. I already explained that rod response (deflection, speed) is directly related to the loaded frequency of the rod and line system. That frequency depends on a parameter called the “equivalent mass at tip”, noted mo here. Then
• The fundamental frequency of the rod is 1/2pi * squareroot(k/mo)
• The loaded frequency depends on the mass of line casted (m) and is 1/2pi * squareroot (k/(m+mo)). For a similar stiffness (said differently line number), the frequency advantage is for the tip action rod because its mo is smaller.

You cannot expect to predict anything related to the rod, in particular its stored elastic energy, from its stiffness. The term which is related to the way a rod bends is mo, larger for butt action, smaller for tip action.

In the example I mentioned before, the faster rod is best suited for the cast input (inspired from a record made by Jason). The loaded frequency we are accustomed to is in the 1.25 Hz to 1.45 Hz range, that is comfortable, but if I want to get the highest speed I need something like 1.8 Hz which is far from being comfortable to cast. Actual casting conditions favor the fastest rods, and since there is a general trend fast /tip action, the chances to break a distance record with a butt action rod are very, very small.

Conclusion: think dynamic, not static

For Gary: K2 is the non linear stiffness component, which is related to the cube of deflection of a rod, and of course it influences the amount of elastic energy stored by the rod.

Merlin

[Mangrove Cuckoo]

Merlin,

Thanks again.

I am going to naively prove the opposite by stating that I think I am beginning to understand some of this!

Out of curiosity... do you know of any commercially produced rod models that have a loaded frequency of around 1.8 Hz with the mass of the head of a MED5?

[Merlin]

No Gary

The material to make such rod does not exist.

Merlin

[Paul Arden]

I’ve been fishing… variable bend matters

[RSalar]

Conclusion: think dynamic, not static

Merlin

Hi Merlin,

Let's talk about static verses dynamic. Is there really that much difference? At an point during the cast we could take a snap shot of the rod's bend, the force being applied, etc. That snap shot of all all the forces, stresses, bending, etc could easily be replicated in a static environment. Just put the same amount of force on the rod, measure the mass and load of the line based on inertia and all the other factors, etc. at that point in time. So the dynamic action of the cast can be broken down into segments of static instances.

Just my non-mathematical way of thinking about this -- and wondering what your thoughts are regarding it.

Cheers,

Ron, aka Ben D Matters

[VGB]

Let's talk about static verses dynamic. Is there really that much difference? At an point during the cast we could take a snap shot of the rod's bend, the force being applied, etc. That snap shot of all all the forces, stresses, bending, etc could easily be replicated in a static environment.

Yes, standard structural design practices based upon the Dynamic Amplification Factor indicate that the deflection or stresses for a dynamic load can be up to double that of a static load. This is based upon single degree of freedom model but there are some studies that indicate that with multiple degree of freedom structures, that the deflection and stresses may be much more than double the value of an equivalent static load.

Regards

Vince

[Merlin]

Simulating a dynamic load with static conditions does not take into account the effect of rod material under acceleration but maximizes the one of the line on the rod. It is a crude assumption.

Try to imagine how one can simulate that in static conditions:

no line cast modes.JPG (32.02 KiB) Viewed 558 times

The concept of a multiplying factor to simulate dynamics of a fly rod ignores the inertial effect at work on the rod and the fact that rod response is multimodal (up to three modes altogether).


Merlin