meaning of the fly-rod frequency

[hshl]

Hi all,
I've had a very interesting exchange of ideas with an experienced fly casting instructor. He stated that 'resonance' plays a very important role in fly casting in terms of comfort. It is a pity that we could't deepen it more precisely, therefore I try to find some deeper informations here...

To start with a definition of 'resonance': from my studies I remember that resonance takes place if the "self frequency" of a mass matches the exciter frequency. In civil engineering the resonance case must be avoided since it tends to damage buildings / bridges (actually in Berlin we have problems to approve some walkways over bridges for the Berlin Marathon since the exciter frequency of the runners could match the self frequency of some bridges... ).

For the fly cast I assume resonance will not damage or fly rods . I remember a paper from Merlin called "fly-rod frequency", talking about natural frequency of fly rods which lies between 150 and 180 cycles per minute (cpm).
Am I right if this natural frequency equals the "self frequency" as described above ? How does the natural frequency influence our fly cast in terms of comfort (and other connections) - especially if the natural frequency and the exciter frequency (caster's input at the grip) are the same ?

Thanks

[Merlin]

Hi Tobias

There is no chance, and no fear, to get the rod under resonance, the cast excites the rod for a transitory half cycle, then there is a pause, then another transitory half cycle in the opposite direction, then a pause, etc. Some danger would come if the rod was wig wagged all the time, but this is not the case, and even in that case, it would be very difficult to break.

On top of that the caster dampens the rod at the end of the half cycle.

Matching the frequency of the tackle gives some benefit although it remains small, the system is rather tolearant to a lack of synch.You have to consider the "loaded frequency", that is to say the frequency including the mass of the carry at the tip, and then you are in the 1.25 Hz to 1.45 Hz range for SH rod and fishing casts.

The rod can unload at a different frequency, something in between the first clamp free mode and the first free-free one, which is influencial for DH rods in particular (Hi John ).

These are typical parameters for designers, not really for fishermen, although they can get some benefit from well designed (and well cast) rods. Such parameters are more important for competition casting.

Merlin

[hshl]

Thanks Merlin.

Matching the frequency of the tackle gives some benefit although it remains small, the system is rather tolearant to a lack of synch.

Even the common cent paper "Dynamic Characterization of Fly Rods" by Dr. Willliam Hannemann - which is interesting ideed - didn't highlight a significant practical benefit (to me). But I thought that I've overseen something.

Regards, Tobias

[Paul Arden]

Hi Merlin,

How significant is the frequency in the unloading phase? Does a higher frequency result in higher line speed?

It seems to me when casting softer rods, that pull back has a more defined feel. Is this tied in with frequency?

Thanks,
Paul

[Merlin]

Hi Paul

The faster the speed characteristics are, the better the rod potential. However the deceleration rate imposed by the caster is very important, it can even include a pullback, or a kind of « kick back » like Mel’s Whuuump.

Everything being linked in a fly rod it may be that feel can be tied to frequencies, but I would think that the bending of the rod and its inertial characteristics dominate. Generally speaking softer rods offer more feel, very fast rods do not.

Merlin

[Paul Arden]

Hi Merlin,

It was very interesting with Mel. One of his teaching methods was to take a rod without a line and bounce the rod in his hand, he was talking about the stop and how it felt - rod unloading etc. He would then pass the rod around the audience who would mimic this action. I’ve used this, no doubt, thousands of times since.

There is no question in my mind that this is pull-back. We didn’t call it pull-back then, Mel thought of it as an abrupt stop, but as a way of teaching pull-back it’s second to none.

—
I would think also it’s not just soft/stiff with regards rod feel but also the action and in particular what is going on in the butt section. I can think of many rods that have little flex in the butt. These rods have feel up to a certain distance and then the feel is the same from that point on.

Cheers, Paul

[Carol]

Reading this 2-1/2 years after posting, I went digging for better understanding of vibration in materials since I am not an engineer, to say the least, and need a foundation. There are some good videos on YouTube that have been helpful to me.

https://www.youtube.com/watch?v=VIlBI6nrnRc https://www.youtube.com/watch?v=XkmgMkDKAyU
https://www.youtube.com/watch?v=9_d8CQrCYUw This last one is an intro to mechanical vibration at MIT. It's an hour and 15 minutes.