Rod Loading - Spring v Lever

[crunch]

This is a model which "explains" to me the difference between higher IP rod 1. and lower IP rod 2. when identical casting stroke is used until RSP in the picture. There is more potential energy in rod 2. but more kinetic energy in line which is in rod 1.



I am not sure anymore but I still think rod 1. will straighten faster from that point unless higher air drag against line prevents it.

Also same model explains lighter line and less MOI in rod 1.

Esa

[crunch]

Esa (crunch), the wrong way round. The correct way is that spring is responsible for "first" 11m/s and then the lever puts the 21m/s on top of that.

Sakke, actually I hope I am wrong because I also hate how much credit "loading rod" has in various discussions elsewhere.

But if lever "first" accelerates line 0 to 21 m/s it makes 12J. Only 12J because too much is lost when loading rod.

When total 32m/s stays the same at 28J and if "spring" accelerates line 21 to 32m/s isn't it 16J then?

I know in practice it isn't like that but perhaps the "truth" is somewhere in between?

Esa

[sms]

The 11m/s is the velocity the line reached from a standstill with the spring only.
In the cast with the same bend, the velocity of the line was 32m/s. So the difference between 11m/s and 32m/s comes from the (flexible) lever.

[James9118]

Hi Grunde,

These are the energy figures my spreadsheet kick out:

The 'mass', 'spring' and 'drive' curves relate to the flexible lever.

The 'KE rigid' is obviously the broomstick.

Both casts share the same drive conditions.

[sms]

Grunde,

Assume same force profile for the hand in later and rotational direction for both bendy rod and broomstick and same tip travel distance from A to B. Now assume that the broomstick hand does y-direction compensation to make SLP. Both rods are weightless. Skip air resistance too. Lossless spring in the bendy rod. Same line velocity or not?

I think it should be the same since the work done is the same.

[Magnus]

Good thread reminds me how ignorant I am.

[Aitor]

I would agree that the bendy rod allows us to use a wider arc while keeping the tip path as straight as possible (until RSP that is!) WITHOUT moving the rod hand along a curve (lifting towards RS positions).
Still in summary of all this I see the tighest loops coming of the stiff rods usually - because of smaller cf.

Makes sense?

Only that last part makes sense.

When we compare a single characteristic between objects (in this case the effect of flexibility) the usual approach is to keep the rest of conditions equal.

[Bernd]

Aitor,
that does not change the fact you won't be able to use a longer tip travel with the bendy rod than I can use with the stiff rod if both are of the same length.
And we both would have the same kind of tight loop.
Well, honestly I think I would get the tighter one (because of less cf).
Do you agree?
Greets
B

p.s.: When you want to keep the rest equal then why do you change the size of arc?
If you can change arc, fine, but I can change rod hand path either (AND keep arc constant) :p ...

[Aitor]

This is a model which "explains" to me the difference between higher IP rod 1. and lower IP rod 2. when identical casting stroke is used until RSP in the picture. There is more potential energy in rod 2. but more kinetic energy in line which is in rod 1.



I am not sure anymore but I still think rod 1. will straighten faster from that point unless higher air drag against line prevents it.

Also same model explains lighter line and less MOI in rod 1.

Esa

Esa,

Rod1 will straighten faster... applying more force to the line over a shorter time.
Rod2 will straighten slower... applying less force to the line but over a longer time.
IMHO same impulse to the line in both cases.

[Aitor]

Grunde,

Assume same force profile for the hand in later and rotational direction for both bendy rod and broomstick and same tip travel distance from A to B. Now assume that the broomstick hand does y-direction compensation to make SLP. Both rods are weightless. Skip air resistance too. Lossless spring in the bendy rod. Same line velocity or not?

I think it should be the same since the work done is the same.

That's exactly the question.

[John Waters]

Hi Grunde,

If I incorporate a step in my delivery cast, not a swoop, but a rotation and vertical lift of my elbow, is the distance used in your definition of force profile (like that term - very descriptive) the total distance the hand travels? In this case, the sum of the two linear distances comprising the step path of the hand.

Love the focus on the hand and the path it follows as that determines how any rod (fast or slow taper) performs.

John

[Merlin]

I’m coming back from holidays and see this topic is still alive. I’m pretty sure we already discussed that point many times so let make a summary of what we have to bear in mind.

Warnings:
• Do not consider that the energy which is temporary stored in a rod is completely given back to the line. Some is lost in loop shaping; and some is lost in counter flex.
• Do not rely on deflections to get an idea about the amount of energy stored in a bending rod. A small deflection in a stiff rod can store more energy than a large one in a soft rod.
• You cannot compare the energy situation of the broomstick with the one of the flexible lever: in other words you cannot say “this energy comes from the lever” and “that energy comes from the spring”, it is all blended together.
• There is no such thing as: the amount of stored energy is always close to 15% to 20%. It all depends on the tackle and the casting; its range is something like 5% to 35%.

The main feature of the flexible lever is to allow applying a force on the longest possible path (if possible straight). By storing energy the spring allows to apply a force on the line while the butt rotation is decreased, this is not the case for a broomstick, and it makes a large difference.

It is difficult to compare situations, but I think the best comparison is to use a given rotation speed profile as said by Grunde. It means that to match a given situation, the energy the caster will develop depends on the tackle; it will not be the same for different rods and or line lengths. These differences may not be that large if you limit that comparison to rods of a given length.

Although the flexible lever is tolerant in terms of load, it does have an optimum load for a given rotation speed profile. This is the “sweet spot” if we can call it like that.

Now let’s come back to the issue about deflection versus performance. Among the models which are used, there are the simple linear ones describing a harmonic oscillator. In theory, we can only mimic the behavior of linear springs with such models. To have a deeper look I’m using a non linear model for non linear rods. This is a sophisticated version of my “spring & marble” one (which is similar to the car & brick). Here I use a given rotation speed profile to compare situations.

Let’s consider a bunch of rods of variable stiffness, these rods being “very linear”. We are close to the current linear modeling. I have to use two stiffness values here, the “linear stiffness” (K1) which is the one for small loads and the “non linear stiffness” (K3) for high loads, their ratio being the non linear stiffness coefficient:

K = K1+ K3 * (deflection squared)
Non linear coefficient is K3/K1 and equals 0.2 for this first example

If I use a small load (half the weight of 30 feet of a given line), then the best performance is achieved by the softer rod (smaller K1), and the worst performance is achieved by the stiffest rod (higher K1). This is what we are talking about. Now if I go to the other side of the spectrum with a significant load (twice the weight of 30 feet of line), the order is reversed: this time, the stiffest rod leads and the softest one lags behind. Bad news? There is an in between situation where if I use the weight of 30 feet of line, it is an intermediate stiffness which is the best. It means there is a stiffness which is preferable to use 30 feet of the line considered. That gives you an idea about scaling rods and lines: you may use K1 for it.

Now I change the bunch of rods for “very non linear” ones (coefficient is 0.8). The situation for small load is just nearly unchanged, not a surprise, but for the high load, here again I can find that the softest rod (smaller K1) leads in performance! But at the same time it is the rod which exhibits the higher relative deflection whatever the load is and this is valid whatever the non linearity of the spring is.

So the first conclusion is that if we happen to experience that bending rods appear to be better at high loads, this is because of non linearity. This may not be the case for rods on the linear side (typical of butt action rods).

I gave a look at the “efficiency” of the cast, defined by the ratio of the maximum energy in the line with the total energy produced by the caster (which necessitates some assumptions on the swing weight and equivalent mass of rods). The general trend is that softer rods give a slightly better performance on this standpoint, and that this efficiency generally increases with load. The typical range is 40% to 80% in this study, with a small advantage to linear rods.

I think a glass of gin tonic may help at this point.

Merlin

[sms]

Merlin,

I think that the faster the acceleration (the less the tip moves before stop), the higher the percentage of spring effect is. I was just surprised how little it was in my test. The spring energy was much higher (did deflection-force mesurements up to 2m deflection) than what was transferred to the line. I suspect the biggest culprit is the inertia of the rod itself that is accelerated in the straightening.

The speed profile (I'd use velocity profile as speed is directionless) is something I haven't used as from theorethical point of view we should look at work and thus force. And when they are the same, look at the line velocity (KE).

Counterflex amount is pretty much relative to the lost energy of the spring. Do you agree?

[Aitor]

The main feature of the flexible lever is to allow applying a force on the longest possible path (if possible straight). By storing energy the spring allows to apply a force on the line while the butt rotation is decreased, this is not the case for a broomstick, and it makes a large difference.

Yes, but this is not the advantage that Grunde is advocating, is it?

Moreover, when butt rotation is increasing the rigid rod applies more force to the line than the bendy rod. So the rigid rod applies more force at the beginning and the bendy rod applies more force at the end. What's the difference?

[Merlin]

Aitor,

So what did you understand from Grunde's explanation? Maybe I am misinterpreting your statement.

The force applied by the broomstick is higher at the early stage of the cast, but soon the flexible lever is catchng up and is able to apply twice that force at its maximum, which is close to the moment when the deceleration starts.

Roughly speaking, you get 2/3 of the flexible lever speed with a broomstick.

Merlin