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Rod deflection and acceleration

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Lou Bruno
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Rod deflection and acceleration

#1

Post by Lou Bruno » Wed Aug 28, 2019 6:58 pm

I'm doing some research on fly rods when I came across this statement, "the larger the rod deflection is when unloading begins, the larger the acceleration is due to the spring function of the rod."

So, if soft rods deflect more, do they generate greater acceleration?

Lou

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Re: Rod deflection and acceleration

#2

Post by Paul Arden » Wed Aug 28, 2019 7:56 pm

No :D
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crunch
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Re: Rod deflection and acceleration

#3

Post by crunch » Wed Aug 28, 2019 7:59 pm

Short single hand rods don't but long double hand rods might. When rods evolve lighter they are easier to accelerate line using lever effect and when some bend is good to make line loop narrow a light rod needs to be softer because lighter rod moment of inertia cause less bend.

Esa

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Re: Rod deflection and acceleration

#4

Post by Paul Arden » Wed Aug 28, 2019 8:09 pm

A stiff rod bent to the same deflection as a soft rod will have stored more elastic potential energy.

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Lasse Karlsson
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Re: Rod deflection and acceleration

#5

Post by Lasse Karlsson » Wed Aug 28, 2019 9:04 pm

Paul Arden wrote:
Wed Aug 28, 2019 8:09 pm
A stiff rod bent to the same deflection as a soft rod will have stored more elastic potential energy.

Cheers, Paul
And that helps us flycasting in what way?

Cheers
Lasse
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Lasse Karlsson
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Re: Rod deflection and acceleration

#6

Post by Lasse Karlsson » Wed Aug 28, 2019 9:07 pm



The stiff and the soft spring cast together




Don't haul if you want spring...

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Paul Arden
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Re: Rod deflection and acceleration

#7

Post by Paul Arden » Wed Aug 28, 2019 9:31 pm

Lasse Karlsson wrote:
Wed Aug 28, 2019 9:04 pm
Paul Arden wrote:
Wed Aug 28, 2019 8:09 pm
A stiff rod bent to the same deflection as a soft rod will have stored more elastic potential energy.

Cheers, Paul
And that helps us flycasting in what way?

Cheers
Lasse
More stored energy will (all else being equal) result in a greater acceleration of the rod as it unloads. Personally I think it's a bit of a red herring. 20 odd years ago when I wrote the flycasting manual I said the same thing :p

However it's not all black and white. The rod is both a spring and a lever. It's mostly a lever of course, but it's still got some spring properties!
Fly casting works because we use the weight of the flyline to bend the rod, we then force the rod to unbend which casts the line for us. We bend the rod by moving it against the flyline's mass. We unbend it by stopping. The most important part to learn in fly casting is an effective stop.

OK now I've told you this, let me also tell you that it isn't true. What this is, however, is a nice way to visualise flycasting. It gives you 'feel'. If this was actually all there was to flycasting then we would be able to stick the rod in the ground so that it was standing upright, and by pulling the line against the top of the rod until the rod bent to it's maximum, then releasing it, we would be able to make a long cast. Truth is that the tip of the line won't even reach the rod. Try it.

So leverage is also important. Indeed it is an interesting combination, for it is possible to cast with rods which don't bend (broomsticks, short pieces of timber and one or two brands of flyrods), but if you have tried this you will rapidly come to the conclusion that a rod which actually bends is a far better tool.
http://www.sexyloops.com/flycasting/tbasic1.shtml

That first bit was reasonably OK. The following text may not be so true now we know about "stopless" :kungfu: Oh and the tailing loop is a terrible picture :laugh: I will bring this up-to-date over the next 12 months but I will keep the integrity of the original in place so we can see how we have learned over the last 21 years.

Cheers, Paul
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Merlin
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Re: Rod deflection and acceleration

#8

Post by Merlin » Thu Aug 29, 2019 6:42 am

Hi Lou,

The answer is not so obvious, because there is not one single parameter involved. It is necessary to define the conditions under which you compare situations. Do you consider a defined cast (arc, max rotation speed) and a different rod, or a given rod with two different loads, or even something else?

First, you mention acceleration and not speed, and this is a very important point. Generally speaking, the casting mechanisms blurs differences in between rods in terms of speed (see Lasse's video), although the trend is always the same: a higher speed for a faster rod as long as the caster can produce the necessary power.

If we speak of acceleration, then this is more complicate. I run some cases and focused on peak acceleration (not the average)during unloading phase. I used threee loads (2 grams, 10 grams and 18 grams for the line) for a given rod and compared peak acceleration for a variable stroke (each stroke is adapted to the load). The peak acceleration increases with the mass of line, and it is the same for rod deflection given the conditions I chose. In that case, peak acceleration goes along with max deflection, but it does not change the fact that speed is also increasing with carry because of the stroke adaptation.

It is wise not to generalize something about rods :)

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life

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Re: Rod deflection and acceleration

#9

Post by Lou Bruno » Thu Aug 29, 2019 8:39 am

Merlin

Appreciate your input, I'd be lying if I said I understood the original quote. Matter of fact, its confusing. Im not sure if there is a relationship between rod deflection and acceleration (rate in change of speed)...I can either have slow, fast, or constant acceleration...to put in simple terms.

To add confusion, at least for me, the quote mentions spring effect as it relates to the greater acceleration. Won't even go there!

So, as a fly caster I can increase the bend of my fly rod by increasing the amount of line I'm casting (greater mass?) as well as how fast I'm casting (deliberately stayed away from using "speed" and "acceleration ") or a combination of both.

Have to agree, it is a generalization.

Lou

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Lasse Karlsson
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Re: Rod deflection and acceleration

#10

Post by Lasse Karlsson » Thu Aug 29, 2019 9:21 am

Paul Arden wrote:
Wed Aug 28, 2019 9:31 pm
Lasse Karlsson wrote:
Wed Aug 28, 2019 9:04 pm
Paul Arden wrote:
Wed Aug 28, 2019 8:09 pm
A stiff rod bent to the same deflection as a soft rod will have stored more elastic potential energy.

Cheers, Paul
And that helps us flycasting in what way?

Cheers
Lasse
More stored energy will (all else being equal) result in a greater acceleration of the rod as it unloads. Personally I think it's a bit of a red herring. 20 odd years ago when I wrote the flycasting manual I said the same thing :p

However it's not all black and white. The rod is both a spring and a lever. It's mostly a lever of course, but it's still got some spring properties!
Fly casting works because we use the weight of the flyline to bend the rod, we then force the rod to unbend which casts the line for us. We bend the rod by moving it against the flyline's mass. We unbend it by stopping. The most important part to learn in fly casting is an effective stop.

OK now I've told you this, let me also tell you that it isn't true. What this is, however, is a nice way to visualise flycasting. It gives you 'feel'. If this was actually all there was to flycasting then we would be able to stick the rod in the ground so that it was standing upright, and by pulling the line against the top of the rod until the rod bent to it's maximum, then releasing it, we would be able to make a long cast. Truth is that the tip of the line won't even reach the rod. Try it.


Cheers, Paul
Personally, I believe it to be a huge, big, red herring :D
Mostly because I like to haul most of my casts, and I have seen how a rod unloads when given a chance to do so only pulling a line...

Btw, I do remember, that's why I wrote to you for the first time back in the days, to tell you you where wrong, (I stuck a rod in the ground, pulled the line back, and let go and not only got a upside down loop(concave tippath anyone?) but the end of the line passed the rod and all) and that I agreed it wasn't how we cast a rod and line..

Lever and spring, yup... Stop seaching for the better spring and start seaching for the better lever :cool:

Cheers
Lasse
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http://www.karlssonflyfishing.com

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