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s shaped rod - How does it happen?

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Bernd
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s shaped rod - How does it happen?

Post by Bernd »

Hi everyone,
here is a picture of Paul (thanks Paul and Gordy) using pull-back on a relatively stiff single handed rod. A small s shape happens.

Image

Here is a picture of Roberto (thanks Roberto and Giovanni) showing a nice s shape in his rod. It was discussed if there was pull back involved or not.

Image

Here is a video of me (thanks Aitor) using pull back on a quiet softer single handed rod. A significant s shape shows up:

http://vimeo.com/21349829


How can I produce a s shape without pull back on a single handed rod?

Thanks
Bernd
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blackwater
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Post by blackwater »

Wouldn't just having a hard stop cause the rod to form an S shape?

All the momentum is in one direction until the rod is stopped and it tries to bend in the opposite direction above the point that prevented forward movement. With the load from the line pulling the rod in the opposite direction you get the S shape.

I think the pull back just exaggerates that effect. You could probably make the shape more prominent by overpowering the rod if you dont want to use pull back. Could be why it shows up so well in the TLT photo.
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Paul Arden
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Post by Paul Arden »

I used to think it was only pull-back that would create the S-shape. However Alejandro analysed some high speed video of my "stopless" backcast, which has a relaxed grip, no hard stop and no pull-back (normally). And on that video there was a distinct S-shape pre-RSP.

I'll see if I can find the stills...

Cheers, Paul
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Post by Paul Arden »

Alejandro writes: "Look the frames of the RSP in a 170 cast. In the back cast the butt is blocked, the vector of speed of the spool goes lightly toward the caster.
In the forward cast the action is different: the spool moves ahead, compressing the rod. The rod has, in both cases, the typical form of S that indicates the existence of a wave."
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Post by Paul Arden »

Buddha casting...
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Bernd
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Post by Bernd »

Nice pictures!
Not exactly sure what in detail you mean by a hard stop. But I have seen you accelerating the rod like hell in that style when having lots of line in the air. In the very last part of the stroke the rod will be deccelerated by a dramatic rate as I understand it. :)

One thing immediately comes to mind.
When you move your arm + rod hand backwards in such high speed your body may send out an "alert":
Hey this is dangerous movement, stop it!"

I often see a very very short but strong reflex here in which the rod hand "flips" back forward a centimeter.
You probably know what I mean?

May this short impulse be part of the game here, too?

Yet I don't understand what exactly Alejandro means.
Any help to make me understand is welcome!

Thanks
Bernd
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Bernd
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Post by Bernd »

Hi Paul,
I just looked up a video I made of you last summer.
At the end of your bc stroke - while rod is around RSP - you lift your rod hand about some degree.
Seeing that I am not astonished that a small s shape happens.
On the fc (in that video) you don't lift your line hand and no s shape happens. Pretty good video. I would see the slightest s here.

Edit:
Just analysed another forward cast. In the moment you finish your fast rotation at the very end you lift the rod hand. A very small s happens!

So it seems as if you lift the rod hand a slight bit sometimes (and sometimes not). If you do the s happens. If you don't lift it, I don't see the s to show up.
I understand this lift of your rod hand as pull back. It's force added to the butt oposite directed to the tip movement at that time.

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Bernd
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Post by Bernd »

Image

Here is a sequence of pictures showing the upward movement of your rod hand during the fast wrist+grip rotation at the very end of your forward stroke.

It seems to me that this is where the s comes from.

I think this movement affects cf, too? ;)
Greets
Bernd
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Post by gordonjudd »

Wouldn't just having a hard stop cause the rod to form an S shape?

Bernd,
Exciting different types of modes in an oscillating beam rod can come from many sources.

I think that most people here understand that a rod can be made to vibrate in different "pure" modes by using different forcing frequencies at butt of the rod. Here is an example. of exciting the second vibration mode in a rod by wiggling the butt back and forth very quickly. I expect that most people have done the same thing in "shop testing" a rod.

When you do that you will see that it is very hard to keep the second node near the tip at the same relative point on the rod because it is difficult to maintain a constant forcing frequency that is exactly matched to the second mode frequency. Any deviation from the pure second mode frequency will excite other modes as well and thus result in a different vibration shape.

Robert Haun in his 1996 Master's Thesis at Clemson University on the dynamic behavior of fly rods explains:

It is believed that higher modes have an effect on casting performance. At the end of each forward and back cast , the rod is suddenly stopped by high decelerations. These decelerations cause an impulse that can induce broad band frequency vibration into the rod. If the frequency of vibration matches a higher order mode of natural frequency of the rod, energy will be dissipated by the rod and not transferred to the line. Therefore a model for determining the first mode as well as higher modes from analytical or numerical methods is desired. This can be accomplished through computer modeling based on the Rod/Caster (RC) system, and using experimental testing of the rod to aid in model building and validation.


Haun worked out the mode shapes for a clamped-free rod as shown below. The tapering in the diameter of the rod produces much different amplitude shapes as compared to the mode shapes you would see for a uniform beam.

Image

When you statically bend a rod its overall shape can be considered to result from the combination of different modes. Here is an example of using the fmins function in MATLAB to find fitting constants for the three mode shapes shown above to match the shape of a deflected rod. Even with just these three modes, the resulting fit is excellent as you can see the actual curve (in blue) and the curve from the fit (green curve) are almost on top of each other.

Image

If a different pull angle had been applied to the rod then the shape would change, and thus you would have to find a different combination of the above modes to fit to a new shape.

The same thing applies to the forcing function applied via the stop. A soft stop will produce a smaller range of frequencies, and the resulting second mode will likely be quite small. As Haun noted, a hard stop will excite a broader range of frequencies and will often times exhibit a visible second mode shape at RSP1. Pulling back on the rod will excite an even higher relative second mode as shown in Paul's example.

The relative amplitude of the second mode will be quite small compared to the first. Also, since the node for the second mode is near the tip (and it will move out closer to the tip with an added tip mass) its effective length is much smaller than the length of the rod and thus will not have much of an effect on the tip speed.

Tom's bench test measurements showed that the second mode tip motion could be out of phase with the first mode at RSP1 and thus it could actually reduce the tip velocity by a small amount (about 2%).

Haun stated above that the higher modes do not tend to get coupled to the line and thus just dissipate energy, but I do not know if that is actually the case. You will hear a different point of view about that observation in the TLT thread.

Gordy
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Bernd
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Post by Bernd »

Hi Gordy,
many thanks for your help. I have to reread and rereread it.
Yet I don't fully understand it.

In regard of what I have on film with Paul's casting I can see that he in the fc aims for a very fast final rotation at the end of his stroke. He does it by wrist rotation and grip rotation as far as I can see it.
It looks to me that sometimes he kind of lifts his wrist to make the hand even rotate faster. When I try to copy this I can feel the benefit immediately.
I have perfect 250 fps slomo of him not lifting the wrist as well. That's exactly when not the slightest s shape happens within a serious of casts.
As soon as he lifts his wrist like shown in the picture serious above I can see a more or less pronounced s to happen.

I was wondering if a brutal stop of the butt section would make the very butt section counterflex and rebound while the tip section is still straightening. Is that what happens?
(sorry I don't understand your explanation yet).

Thanks
B
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Post by gordonjudd »

I have to reread and re-reread it. Yet I don't fully understand it.

Bernd,
I expect that most people are not used to thinking about waveforms in terms of their time domain and frequency domain responses. They are directly related however, and engineers use the Fourier Transform to find the range of frequencies (i.e. the bandwidth) it takes to make fast changes in time. In general the bandwidth of some time change is related to the inverse of the time it takes to make the change.

If you were to stop the rotation of the rod over a second then the bandwidth of the forcing function would be in the Hz range. Stop the rod in 20 ms and you would required a range of excitation frequencies of 50 Hz. That is what Haun was referring to with his observation that:
At the end of each forward and back cast , the rod is suddenly stopped by high decelerations. These decelerations cause an impulse that can induce broad band frequency vibration into the rod.
I was wondering if a brutal stop of the butt section would make the very butt section counterflex and rebound while the tip section is still straightening. Is that what happens?

No.

I don't think the butt section is going through a counterflex and rebound. It is just that fast stop excites higher vibration modes that are superimposed in the shape of the rod as it unloads.

Here is a good example of how this fitting works to match the different shapes you get in a plucked string.

It is similar to matching the static bendform you get with a clamped rod with different pull angles. That shape can be associated (or fit) with different coefficients of the different modes as shown in my earlier post. A different shape will require different ratios of the different modes.

It looks to me that sometimes he kind of lifts his wrist to make the hand even rotate faster.
and
As soon as he lifts his wrist like shown in the picture sequence above I can see a more or less pronounced s to happen.

What the bend in the rod like when that lift happens? If it increasing the rotation rate, then that implies is would be adding to the bend in the rod. I think that some CA measurements would show if that lift is actually causing the rotation rate to increase. The torque that is produced by dlosing the fingers on his hand may be a bigger effect.

Usually the second mode amplitude is much smaller than the fundamental, so you do not see it clearly until the fundamental mode is at RSP1. Thus I do not see how the timing of that lift is related to RSP1. Maybe you could upload that video to Vimeo or You-tube to show what you are talking about.

I would expect the contact with the rod butt with the forearm could produce a very abrupt stop of the rod rotation. That hard stop could certainly produce a broad frequency response that could excite the second (and higher) mode.

Gordy
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Bernd
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Post by Bernd »

gordonjudd wrote:I would expect the contact with the rod butt with the forearm could produce a very abrupt stop of the rod rotation.
Hi Gordy,
the video file is quite huge. I will check if I can upload it tomorrow.

Paul usually is trying to put the rod but on the side of his forearm. To do so he is also twisting the reel sligthy inwards, too.

Greets
Bernd
p.s. Just watched the videos of Pauls loop formation showing the distance phenomenon perfectly well again. Will try to upload some more of it, too. Really interesting I think.
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Post by Merlin »

Hi Bernd and Gordy

It may look strange, but when we cast a rod, several modes are excited but may not be visible to us. The best situation is to look at a position close to RSP since we can then detect if the rod is straight or not. Paul's cast examples do show the second NF.

I had some tests made two years ago with a pro software able to pick up several modes. For the nearly totallity of the cases studied, we could detect NF1 and NF2. In one particular input situation, as we were exciting the spring just right on its NF1 loaded frequency, the third harmonic appeared with the second. I cannot draw conclusions from that, but it is interesting to know that the rule of the thumb in terms of amplitude was the following: NF2 amplitude = 10% of NF1 amplitude and NF3 amplitude was 10% of NF2 amplitude, so very, very small.

In terms of tip speed, there was sometimes a small benefit (less than 1% by comparison to linearity and one single NF), but when we excited the spring on a loaded frequency close to NF2, then there was a loss (3%) by comparison to linearity. Again, difficult to draw a clear conclusion from that.

I do not think possile to identify the way this is created, the relative importance of the NF amplitudes, by a simple reasoning.

Merlin
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Bernd
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Post by Bernd »

Merlin, Gordy,
after reading all this and a lot of other stuff as well, trying different things on the grass, I still think the 2nd nf may show up but it does not make me cast more efficiently?

Is it correct that the 2nd nf mostly seems to show up when I add quite more energy to the cast (talking about avg length of line) than I probably would have to (for example very abrupt stop)?
Greets
Bernd
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Post by Merlin »

Hi Bernd

First question: correct for me, this corresponds to both modeling and experience.

Second question: I have the same experience than you, and sometimes, if there are two much vibrations, I do not use the wrist but only the arm rotation to avoid them. No theory behind, just practice. However, this seems to correspond to Dr Haun's statements about the stop of the rod.

The shape of the rod at any time is the result of flexural waves which travel along the rod in both ways, reflecting partly on ferrules and any significative change in dimension of the shaft. Timing can be estimated around 60 ms to go from one end of the rod to the other for a 9 feet rod. These waves can form together distinctive shapes in certain conditions, which correspond to the harmonics that we can see.

Merlin
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