Rod Loading - Spring v Lever

[Stoatstail50]

Cool, we seem to be oscillating between models now... :oh:

Just as a matter of interest, if my spring has not reached its maximum extension and the force upon it is increasing will it continue to extend or will it oscillate ?

[James9118]

Mark,

In a previous post you highlighted a comment of mine about the spring reaching its maximum extension - this is the maximum for the conditions applied, not the maximum possible!

Spring extension is proportional to force, so it will continue to extend.

[Stoatstail50]

Do any of the models on here that are claimed to approximate a cast have applied conditions that reflect that effect ?

[Merlin]

Marc

The answer is yes, it is just a question of tuning parameters.

Merlin

[VGB]

I think that explains why one is at a maximum while the other is at a minimum, but I thought the maximum of one would match with the minimum of the other along the t scale, but they seem slightly out of time.

That is what I thought unusual

Trev

Have a play with this:

http://phet.colorado.edu/sims/resonance/resonance_en.html

regards

Vince

[Stoatstail50]

Marc

The answer is yes, it is just a question of tuning parameters.

Merlin

Excellent, so there is a set of perameters that produce a graph showing the car in the car/spring/brick, accelerating at one rate and the brick accelerating at a lower rate up to the point where the car decelerates ? This represents a steady extension of the spring under an increasing load.

All of the graphical representations I have seen so far show the brick beginning an increased rate of acceleration long before the car decelerates.

If it is stiff it may unload before the end of the acceleration of the car.

Actually, whatever its stiffness, as far as I can tell, all your models currently show that, if it reaches its elastic limit/maximum extension, it will unload before the end of the acceleration of the car. So far all of the graphical representations of car/brick velocities show an increase in the rate of brick acceleration before deceleration of the car. I would expect that for all but the most extreme casts, this does not happen. The spring cannot be expanding and contracting at the same time.

All this is influenced by the non linear character of the spring

Yes it is, but also by the rate of change in how stiff that spring is. This will have an effect on the rate of acceleration of the brick once the spring begins to unload.

[James9118]

This represents a steady extension of the spring under an increasing load.

Mark,

I think this is where you're going wrong with how you're visualising this. This is down to boundary conditions.

Suppose the spring is already extended before you start accelerating. The constant acceleration (constant force) will then simply maintain this extension, no oscillation.

However, because the spring is unloaded to start with the extension overshoots the position that it would be stable at, as in the case above. Because of this the force increases (force is proportional to extension), and the acceleration of the brick exceeds that of the drive, even though the drive is still accelerating at the same rate throughout.

Cheers, James.

[Stoatstail50]

I think I understand how the model functions, my visualiastion difficulties are because I cannot reconcile the model to a cast. It simply isn't what happens whatever the model says. This, I suspect is not because my eyes are fkd or I lack the wit or imagination to understand the maths but because the boundary conditions of the model are inadequate. It doesn't represent what happens when I cast....or when you cast.

[Merlin]

I'm with you Mark, I think the actual conditions are such that they do not correspond to all the variety of situations we can imagine with a model. Experience has told rod designers which stiffness to chose for which line and despite the uncertainty around this fit, generally speaking, there is a situation which is close to the optimum. On the other side the model help to understand the intricacies that can put some shadow on strong convictions.

Merlin

[Stoatstail50]

I'm with you Mark, I think the actual conditions are such that they do not correspond to all the variety of situations we can imagine with a model. Experience has told rod designers which stiffness to chose for which line and despite the uncertainty around this fit, generally speaking, there is a situation which is close to the optimum. On the other side the model help to understand the intricacies that can put some shadow on strong convictions.

Merlin

As far as whole casts are concerned I cannot see that the boundary conditions correspond to any of the situations that occur in real life.

As I understand the model, it describes something with a constant acceleration. With this as a condition and given the way that stiffness has been input up to now, I can completely see that there would be oscillations as both yourself and James have identified and for the reasons you have clearly explained, I totally accept the validity of the results given that base line.

However I think there is enough data already out there that tells us that the accelerations we use at the butt are not constant during a casting stroke, the more extreme the cast, the bigger the deviation from a straight line in fact.

Bizarrely perhaps, I completely agree that there may be oscillations during the casting stroke but these are likely triggered by our physical inabilty to apply force sufficiently smoothly, there are micro changes in the rate of acceleration delivered by the caster but they are so small that damping effects in the system are more than sufficient to make their effects practically imperceptible. If they are not, then we get tails or a tailing tendency because they cause waves in the line. This is another benefit of a bendy rod.

There are enormous variations in stiffness to line rating amongst rod designers which leads me to believe there is no agreement amongst either them, or the great unwashed that purchase their products, on what the optimum relationship actually is.

A caster can keep a rod bent almost indefinitely just by drawing circles in the air, the bend may displace, but it does not unload, not in any sense that I understand it anyway, until the rod is decelerated....how is such a thing possible ? When during the sweep of a double spey does the rod unload of its own accord ? When do we as casters loose control ?.

There are thousands of videos of good casts on here, many filmed in slomo....is there one I can see where we cannot confuse bend displacement, due to change of rod/line angle, and unloading, which shows the effects the model is predicting.

[Merlin]

Mark

Let me clarify some points. The “oscillations” mentioned and illustrated by James correspond to a very specific case which you can’t observe in practice. To get these conditions you need a rod that would be 100 times stiffer that what you are using today. If you would like to see a rod unloading at the end of the acceleration phase from the caster you need a rod something like 4 times stiffer that the one you are using. These are order of magnitudes to show that you have no chance to clearly see such situation in practice. Although there are differences in rods, the study made by G Spolek in the past showed a maximum factor of 2 between the stiffness of different rods for a given line (stiffness measured at the 25% deflection point). You will likely never find a factor 4 unless you compare a #12 rod with a number 2 rod. Using a #2 line on a #12 rod will give you an idea of what a broomstick can be.

When you mention you can maintain a deflection by moving along a rod, you are in a translation situation: the rod is in a steady state and is bending by inertia (rod, line) and air drag; it will unload if you stop moving it. This is a very different situation from the casting action when you put power to cast the line: then the rod will load and unload following its dynamic characteristics (loaded frequency), in other words you lose a little bit of control. I do not have all videos at hand, so I cannot tell you which one is typical of a particular condition.

The “vibrations” one can observe when casting comes from higher harmonics of the mechanical system, and have nothing to do with what James has illustrated. Our simple models cannot capture such phenomenon.

Merlin

[Stoatstail50]

I realise that we are conducting two threads on each board at once here Merlin...sorry 'bout that.

When you mention you can maintain a deflection by moving along a rod, you are in a translation situation: the rod is in a steady state and is bending by inertia (rod, line) and air drag; it will unload if you stop moving it. This is a very different situation from the casting action when you put power to cast the line: .

Not so, we can make figure of eights, circles, above our heads, to the sides, speed up, slow down, force a deeper bend, or not, this is not in any way a steady state. Further to this we are able to control the force applied during an "unbending" phase to slow the tip motion to such an extent that the line will never pass the rod tip.

This is relevant to our discussion on the other board where we say that the line will release after Maximum Rotational Velocity because whilst I agree that this is what usually happens, we should be very careful to avoid the impression that this is what always happens. When, after MRV, this line release takes place is, in my opinion, still under the control of the caster. I can have MRV and still manage other elements of the deceleration phase in such a way that although the rod has "unloaded" the line just comes to a slow and steady stop.

...then the rod will load and unload following its dynamic characteristics (loaded frequency), in other words you lose a little bit of control.

This is the bit where we depart because I would contend that a competent caster doesn't lose control, not in the sense that I understand control anyway.

If you are decribing imperceptible oscillations throughout the cast, those that have been predicted by yourself and James and the merry old model, then I agree that they are going to be there and in one sense uncontrollable ie. you cannot avoid them. This is not, however, "unloading" in it's common usage where it usually means the process of line release or line launch or whatever we want to call the stop phase. Nor does it mean "unloading" in its totality. It means a load/unload/load/unload sequence like a tiny vibration.

I think that, in the real world, these are going to be pretty much invisible if evident at all because, with a good rod, and caster, the damping effects of the system effectively remove them from the process of making a cast. If you are saying that these are big uncontrollable effects that occur with any rod then there should be evidence in the casting video libraries... and I haven't seen any sign of them.

Throughout the cast, for any given load at the tip and any given set of dynamic characteristics, the behaviour of the rod is controled by the caster in the sense that there is no other input. No caster, nothing happens, no micro-oscillations, no cast, no circles....nothing.

That the caster moderates their input to the dynamic characteristics of the rod for a particular outcome is not in any dispute from me. We change many of the basic elements of our cast to accommodate this from rod to rod.

You can tell quite a bit by looking at a rod, or its ERN, or numbers painted on the side but you dont really know what its going to be like till you pick it up and a chuck a line about. The more we cast a particular rod the better we understand its dynamic characteristics and the more fine tuning we do to our input to get the best from it. This is not about losing control, it is about improving control and a few micro oscillations, perceptible or not, are just one of the characteristics of a rod that a caster would learn to manage...they're just there, if we notice the effects, we clock the feedback and change the input. There is a feedback loop.

None of these models seem to allow for that. Without it the models appear to describe a situation where the rod unloads and line is released, launched, whatever we want to call it, completely out of the control of the caster, not just a little bit out of control, totally out of control. I'm not sure thats exactly what you intended but that is the impression that I have had over the last couple of years.

Sometimes a rod might unload of its own accord....but this is not directly because the caster has lost control of the rod it is because they have lost control of themselves.

[James9118]

I think one of the mistakes made by myself here is showing the results of the model for situations that are a million miles away from a cast.

For example, the answer I gave for Aitor's experiment showed an oscillating result. However look at the time base, I plotted this out to 6 seconds (I don't know why - I just did). This equates to a vertical drop of nearly 90 metres! So these aren't 'micro' oscillations, they're very much macro. I apologise if this has caused confusion.

In the other thread I think Merlin has pointed out that we're mostly looking at the first half cycle when parameters relating to a fly cast are inputted.

[Hal Jordan]

When you mention you can maintain a deflection by moving along a rod, you are in a translation situation: the rod is in a steady state and is bending by inertia (rod, line) and air drag; it will unload if you stop moving it.

In the absence of air drag during a translational movement it is not velocity that bends the rod, it's acceleration. The only steady state that will keep a bend in the rod in that case is constant acceleration. If I stop accelerating, even though I may continue moving forward at a very high velocity the rod will unload. In the presence of air drag at a steady state inertia has nothing to do with it. It is simply the balance of air drag and rod bend.

The reason we can maintain a bend in the rod when drawing circles or figure eights in the air is because we are maintaining acceleration.

[Hal Jordan]

I think one of the mistakes made by myself here is showing the results of the model for situations that are a million miles away from a cast.

For example, the answer I gave for Aitor's experiment showed an oscillating result. However look at the time base, I plotted this out to 6 seconds (I don't know why - I just did). This equates to a vertical drop of nearly 90 metres! So these aren't 'micro' oscillations, they're very much macro. I apologise if this has caused confusion.

In the other thread I think Merlin has pointed out that we're mostly looking at the first half cycle when parameters relating to a fly cast are inputted.

And this is why myself and a few others keep harping about stating assumptions and boundary conditions even though we are constantly put down for hand waving by people who can't seem to understand the basics like Newton's laws after several years of attempts to educate them on the pseudo technical forum. And I am not aiming that last bit at you James because you obviously understand the basics. There are others who don't.