Spine orientation

[VGB]

The same would not be true for a wrist rotation that caused the tilt of the butt to change and vary the casting plane. That would cause the tip to travel in a curved path and potentially cause tracking problems that could affect the line path depending on how big the plane changes happened to be.

Thank you Gordy, that confirms my conjecture at Post #51

Thankss Paul, my arms are getting shorter anyway. All of the tips I've replaced have been worn to the left of the tube looking from the top. I think that you are correct about the twist as most of my casts are off to one side of the rod travel, this would apply a force that is offset from the longitudinal centre of mass of the rod. I also think the lines mass would also tend to damp any twirling of the rod.

and at post 77:

It would be interesting to look at this. I mentioned earlier that the line being offset from the longitudinal centre of mass would cause the rod to twist but it may also cause it to buckle which would effect the tracking.

Vince

[Merlin]

Hi guys

I finally succeeded in writing a deflection model able to take the spine on board. For the time being it is applicable to moderate deflections (although it can likely be extended to larger ones). It takes into account the lack of symmetry of rod sections which can induce an out of plane vertical flexion, and also the rotation consecutive to that lateral displacement of the loading force, which creates a torsion effect on the rod. The question behind is to estimate by how much can a rod be twisted.

I use a simple rod with a straight profile for the time being, and I can define any spine location. For example I can define a spiral spine around the rod and of course a straight one, or various spines in various sections. The rod section is modeled by a hollow elliptic section of small eccentricity, which means that the larger dimension is just 5% more than the smaller dimension of the section area, which is difficult to detect for the bare eye. A real rod is more asymmetric than that but this modeling is convenient to test the spine effect.

The “spine plane” corresponds to the larger dimension of the ellipse, and the “natural bending plane” (NBP) to the smaller dimension of the ellipse. If we cast in the spine plane then the rod is stiffer and faster than if we cast in the NBP. Now let’s review the various opinions about that problematic.

• Putting guides on the opposite face of spine: in a rod the spine is due to an extra stiffness at the outskirt of the rod. You can consider that placing guides feet on the opposite face contributes to balance that spine and helps keeping the rod in the spine plane. Not sure it is the right option.

• Using natural bending plane for guides: in such a case a large force at tip will keep the rod in the NBP and will avoid twisting it. Same remark.

What does the modeling say? Imagine the elliptic section with the larger dimension horizontal (the rod flexes in the NBP plane). The spine is the same for all parts of the rod for this example (no spine spiral). If you rotate that butt section, then the flexion of the rod is out of the vertical plane and a torque is created at the butt. Increasing the vertical load increases both the lateral displacement and the torsion on the rod: in fact the tip section tries to come back into its original NBP because of the torsion due the combination of load and lateral displacement. I looked at two cases: the fight, with a deviation from the supposed to be vertical plane, and the cast, introducing a lateral force to keep the tip in the vertical plane as could be the outcome of the effect of the line under motion.

If you change the rotation at the butt, you discover that the worse case corresponds to a 45 degrees angle. For a moderate load (20% vertical displacement), the tip has rotated back to 10 degrees from NBP, and the torque on caster’s hand could rotate by some 8 degrees if it was not maintained stiff (from wrist biomechanical characteristics).
The more we spiral the spine along the rod, the more the spine effect disappears since it is more evenly distributed around the rod. A regular 360 degrees spine nearly cancels lateral displacement and torsion effects. In a sense that would mean your interest is to spread around the spines instead of aligning them carefully for all sections. First you have to identify them all.

The opposite solution is to align the spines and use the NBP as the preferential one for casting and fish fighting. In that case failing to use exactly that plane will tend to twist the rod. If you prefer the spine plane for casting, the twist could be severe if you catch a mighty fish since the tip will tend to be 90 degrees from the butt. Not a good idea I think.
So what should we do? First, the designer should avoid spines by using suitable number of turns of prepregs, and if he cannot for good reasons, then he should arrange spines around the rod to equilibrate it and achieve a “neutral” shaft in terms of spine. In the case of a big game rod with its specific equipment, one has to use NBP, but for a fly rod with various casting position, we should try avoiding a twist. It means one has to identify spines anyway in order to locate them properly. For the butt section, we have to use NBP to guaranty that the rod does not whirl, or at least this can be recommended since the consequences are not that large for casting, but they are for fighting. Then one should place the other rod sections at a different angle (e.g. 120 degrees, then 240 degrees and finally 360 degrees or 0 degrees for the tip).

So my preliminary conclusion is that one needs to adapt the rod to its purpose (fighting and/or casting), and that one should look for neutralizing the overall spine effect by rotating spines around the shaft for a fly rod. I would not recommend to align the spines of the different rod sections.

Merlin

[Graeme H]

Thanks so much for that analysis Merlin. That's very informative.

I have a question though, and it's related to the following statement of yours:

For the butt section, we have to use NBP to guaranty that the rod does not whirl,

Given that using the NBP for the butt section prevents that section whirling, what stops the next section whirling against it?

The way I picture it, stopping the butt section whirling is equivalent to taking that section out of the rod in this problem, so it would be like holding the rod at the base of the second thickest section, making it now the "new butt section". Wouldn't I also prefer this section not to whirl? (Of course, this then progresses all the way to the tip.)

Also, when I'm fighting a "mighty fish" (not as often as I wish ) the tip and middle sections of the rod are out of the equation because the butt section is doing the work. The tip and middle are pointing directly at the fish (aligned with the line) and the lower half of the rod is the only part that's bending and thus potentially creating torsion along the rod.

Cheers,
Graeme

[Merlin]

Hi Graeme

I should have explained that it was Dr Graig Spolek who experimented whirl and spine location to conclude that only the butt is important for whirling. He tested many positions of the various (4) rod sections and the three upper ones do not influence the whirling effect. I unfortunately cannot check that with my model. He insisted on the position of the spine at the hand level as the main criteria (in case the spine is not straight). If you consider that true (which I do), then the only thing left is to neutralize the spine effect around the rod shaft for fighting purposes, but maybe large deflection tests could tell a different story.

I cannot (yet) model the phenomenon for large deflections, this is why I limited the case at 20%. The situation is certainly changing significantly. The large deflection model (no torsion, no lateral displacement) I use in this case is also home made and is not fitted with smart convergence algorithm. I did compare it to exact solutions and it does well most of the time. There is however a problem in the situation you are discribing with the "mighty fish". The calculation cannot stabilize, and some day I shall have to modify this point with an adapted calculation methodology (I have it for Excel, but it is not simple to implement in a macro).

Merlin

[Graeme H]

Makes sense. Thanks Merlin.

Cheers,
Graeme

[gordonjudd]

• Putting guides on the opposite face of spine: in a rod the spine is due to an extra stiffness at the outskirt of the rod. You can consider that placing guides feet on the opposite face contributes to balance that spine and helps keeping the rod in the spine plane. Not sure it is the right option.

Merlin,
I don't understand what you mean here by placing the guides on the "opposite face" of a spine axis.

Dave Tutelman explains that stiffness of a shaft with significant spine effects due to overlap differences of the graphite layers has a two cycle symmetry when you measure the slope of the variable force/deflection curve for different rotation angles going around the blank as shown below. Thus the opposite face (180 degrees around the blank) would have the same spine effects as the zero degree face.



Thus whether you place the guides on the outside (the 30 degree point for the above example) of the curve or the inside (210 degree point) of the curve measured with a blank in a spine finder you would have the same characteristics (assuming the spine resulting from any blank curvature effects are small). Tutelman has shown the spine from blank curvature only has 1 cycle of variation so that can muddy the water in regards to finding his true flat line oscillation (FLO) plane with a spine finder.

Thus to get a difference in the bending stiffness you would need to place the guides plus or minus 90 degrees to the effective spine axis (your natural bending plane axis) to get a stiffness difference, not 180 degrees.

Gordy

[Merlin]

Gordy

Let's take the example of a six strips cane rod. Generally speaking, the rodmakers use to locate the stiffer strip which is the cause of the spine. What I mean is that they use to put the guides on the opposite strip at 180 degrees from this stiffer strip. The practice is still in use in the industry with graphite rods. Surprisingly, I never saw a mention of the natural bending plane in books related to rod making, while it may be a better option. At least it is the recommended one for a one piece big game fishing rod.

My last "discovery", putting various spines at different angles in a multipiece rod to neutralize the spine effect, is in fact not new. It has been also used in the fly rod industry for several years. So the good news for me is that my suggestion makes sense, the bad news is that I did not discover it...

Merlin

[gordonjudd]

What I mean is that they use to put the guides on the opposite strip at 180 degrees from this stiffer strip.

Merlin,
I think that belief was based on a misunderstanding of how the stiffness varies as you go around the blank. Even with a six sided rod any net difference in the strip stiffness-es would still produce equal bending stiffness for rotation angles that were separated by 180 degrees as shown in Dave Tultleman's measurements.

Thinking a 180 degree rotation would produce significant stiffness characteristics is similar to the unshakeable belief that many bamboo rod makers have in thinking a blank will be stiffer when it is bent in a plane going through the points of the hexagon as compared to the flats. As you know, a perfect hexagon cross-section (or for that matter any cross section that can be decomposed into N congruent triangles that form a regular polygon) will have equal stiffness characteristics in any bending axis.

If the spine was coming from a blank curvature induced by one strip being shorter or having a curve in the blank when it was being glued then it would have a single cycle of stiffness variation. In that case I would think that it would makes sense to place the guides on the inside of the curve so the rod did not want to twist when fighting a big fish.

However I think blank curvature is a small effect compared to the spine-induced stiffness variations related to the material overlap (at least that is what Spolek found in his measurements) in graphite rods, so I don't think placing the guides on the inside or outside of the bending curve found in a spine finder would make any difference. 90 degrees yes, 180 degrees no.

Gordy

[gordonjudd]

My last "discovery", putting various spines at different angles in a multi-piece rod to neutralize the spine effect, is in fact not new

Merlin,
I assume that different angle approach is an attempt to minimize the whirling effect we see when there is no load on the rod.

If that is the case, I think the no-load whirling effect is a bit of a red Herring when it comes to actual casting effects due to spine as I don't think their is much a whirling effect when there is an external force on the rod tip coming from the line.

The amount of whirling will of course be related to magnitude of the tip load, but you can see the rotation of the blank had little effect on the flat line oscillation observed back at post #170. Since most distance casters rotate their wrist while casting it is probably a good thing that those rotations about the long axis of the rod have little or no effect on the tip path while casting.

Thus I would still go by Graeme's observation that:

What does worry me about multi-piece fly rods that don't have their guides aligned along one of the spines is the tendency for the pieces to separate due to twisting. If all the pieces aren't aligned on their spines, one or all of them may loosen during the session, leading to (at the very least) the rod coming apart, and at worst, breakage at the ferules.

That would be especially true at the first ferrule, so I think it makes sense to have all of the sections aligned the same way to prevent any bend induced twisting effects when fighting a big fish.

As Dave Tutelman observed in looking at the dynamic effects of spine in a fly rod compared to a golf shaft:

I get the impression that a flat line oscillation (FLO) test for fishing rods involves only the weight of the rod. That is very different from a FLO test for a golf shaft, which is dominated by a tip weight; the weight and weight distribution of the shaft is a relatively small factor in the test. I have no idea what differences this will make in any conclusion you draw.

My conclusion is that with a sufficient load coming from the line, the tip path in the unloading rod will be straight and thus any whirling effects in a typical casting situation will be negligible. Certainly if I put a small mass on the tip and vibrate the rod in the effective spine plane the tip path has negligible whirl. I don't know if you have observed the same thing.

Gordy

[Merlin]

Hi Gordy

In the case of cane, the issue is not coming from geometry but from the fact that cane is not a uniform material, the difference in modulus in between strips can achieve 25%, which is just huge. I send you a paper on this phenomenon.

For graphite, if you have an extra 1/4 turn everywere, this extra piece of layer is the spine (to make thing easy to understand, I hope). So you can imagine that there is some sense to put the guides at 180 degrees from the middle of this extra 1/4 layer.

I compared with my model the effect of rotating the rod along its axis (a common feature during fly casting) in two different cases:

First : aligning the spines. For a moderate load (20% deflection), the tip can deviate lateraly by some 2.5% of rod length ansd twist by 10 degrees maximum.

Second: spiraling the spines by 120 degrees (4 piece rod). Only tip and butt sections have their spine aligned, the mid sections are turned by plus and minus 120 degrees. The lateral deviation is less than 0.1% and the maximum twist angle is less than 1 degree..

It is then clear that spiraling the spine makes the rod "spine neutral", as if there was no spine at all. The result is better than with spines aligned in terms of tip twisting (the maximum is near the top ferrule).

Merlin