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Quantifying the Amount of Spine in Fly Rods

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Merlin
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Re: Quantifying the Amount of Spine in Fly Rods

#11

Post by Merlin » Thu Apr 01, 2021 4:21 pm

Ok that's it.

Well, if I put aside exagerrated commercial arguments, there is a misinterpretation of a small change in deflection during a test (1.8%) if you turn the section upside down. This is not the sign of a difference in stiffness but it comes from the fact that the (horizontal) line passing just in the middle of the section does not correspond to one of the main inertial axis. So it takes a different position depending on the orientation of the section. Below you can see the position of that main inerial axis (horizontal lines). The mistake is to measure the deflection on a geomatrical axis going through the center of the section and not on an inertial one.
Reuleaux.JPG
Now imagine the main inertial axis is the same for a given load and the section on the right hand side seems to deflect more than the other (both axis are in the same position for a given load), hence the wrong conclusion. So no, the rod is not faster in one direction (forward vs backward).

The technical argumentation is just BS unfortunately. The only true thing is that this type of cross section offers the highest stiffness for a given weight, but that remains marginal compared to a round section and the drawback is more shear stresses (well known attribute of triangle section for cane rods).

The important thing is not an ideal section, but the way you use mandrels and prepregs to make a fine rod.

Merlin
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Re: Quantifying the Amount of Spine in Fly Rods

#12

Post by gordonjudd » Thu Apr 01, 2021 4:40 pm

James,
I found the D-flex site and see this is a re-birth of the Reuleux cross-section rod that was started years ago, not the one that was using an oval cross-section as inferred in my previous post. Ultrawave rod aside, there seems to be no shortage of ideas revolving around using different cross-sections in fly rods.
Image

In theory the Releaux shape should have the same bending stiffness in all planes since it has triangular symmetry. The same is the case for a 5-sided bamboo rod since it is composed of 5 equilateral triangular cross-sections . As I remember it was supposed to have different spring constants in the forward or backward bending planes because graphite has different Young's modulus characteristics in tension vs compression. Thus there is a second order effect going on that is not included in Tutleman's FLO approach.

That said, I would think it would suffer from the same scrim overlap variations that is present with conventional rods and thus would have some measurable spine effects. I don't know how the varying Young modulus characteristics would impact Tutleman's theory, but I would think you could still use his approach to finding two FLO planes.

The only way to make sure would be to test it. The supposed difference in the stiffness could also be measured by finding an FLO plane and then doing spring constant tests in the two direction to see if there is a measurable difference in the two spring constants.

Gordy

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Re: Quantifying the Amount of Spine in Fly Rods

#13

Post by Paul Arden » Thu Apr 01, 2021 5:59 pm

It must be about 35 years ago that I bought a Bruce and Walker Hexagraph. I can’t remember what the advantages are now that it was supposed to give. I try to keep an open mind but we’ve seen Trident, Arctic Silver, the chest reel, the carrot rod, Boron... polyurethane, skinny flylines, self-loading rods, invisible tippet material :D

It’s hard to keep up with all these technical advances!

Cheers, Paul
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Merlin
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Re: Quantifying the Amount of Spine in Fly Rods

#14

Post by Merlin » Fri Apr 02, 2021 12:06 pm

I think they forgot that there is a flexural modulus. The compression and tensile modulus are specific of pure tension or pure compression of the whole section. For flexion there is a specific one (Young), and all is measured on probes. The flexural modulus is not a combination of the other two.

The section profile has always been a topic of discussion. In the cane area first, and despite the fact that regular sections are neutral bending on the paper (either three, four, five and six strips), they are not in reality because cane exhibits a significant variation of modulus all around the culm (e.g. 25% variation), which pushed designers to use strips sequencing to reduce it. It is possible to minimize the assymetry for a cane rod, but you need information on strips modulus to get the best sequencing. At the end of the day you still have some stiffness assymetry in real life.

Same issue, although on different technical grounds, for synthetics: the number of wraps dictates the amount of assymetry, not including resin losses and pressure issues.

In conclusion, all rods are "D rods", D meaning different stiffness along the main inertial axis. There is no rod bending differently forward and backward, if you find that it is because you cannot consider the actual inertia axis, but only the geometrical ones. I'm surprised to see such technical misconception used to fool customers with supposed to be advantages coming from a section shape. You may have the best one, but if you miss the taper it is useless.

What's next? Oh yes: the constant loaded frequency rod. That should be a revolution. The problem is to make it.

Merlin
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Re: Quantifying the Amount of Spine in Fly Rods

#15

Post by gordonjudd » Fri Apr 02, 2021 2:26 pm

What's next? Oh yes: the constant loaded frequency rod.
Merlin,
How about the no counterflex rod?

At first when I saw this claim:
Image
The Dual-Action of a D-Flex rod dampens the oscillation more rapidly, generating a superior energy transfer into the line for greater accuracy, distance and control. The result is a supremely crisp, snappy recovery that's unmatched by any circular rod, and an incredible payoff of smooth power and precision.
I thought why would you want a rod with internal damping that was close to critical damping and thus would slow the recovery time from max rod flex (MRF).

But then when you look at their casting videos it does appear that there may be something to this claim. Why that would be the case I have no idea.
The D-Flex shape eradicates the unwanted 'Spine Effect' of conventional rods, and brings something completely new to the fore. The triangular geometry—unlike any circular rod—naturally forms a performance-boosting Triple Spine, which generates a self-stabilizing effect and resists ovalisation for perfect straight-line tracking.
That triple spline claim is something I would like to see put to the test as well. Would that imply there are 3 FLO planes in a D-flex rod as compared to the two that Tutleman has described and has shown to exist for conventional rods?

Gordy

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Re: Quantifying the Amount of Spine in Fly Rods

#16

Post by Merlin » Fri Apr 02, 2021 7:16 pm

Gordy

For a given mass (area), the extended Releaux section is 18% stiffer than the circular section in given conditions, the ones selected by authors of the paper from Liverpool University. This ratio is not applicable to any section dimensions (size, wall thickness) but gives you an order of magnitude. So you can design a rod of given stiffness (line size) and get a lighter rod, something like 8% maybe. Your rod will then be faster, with a reduced equivalent mass. That means a little bit less counter flex and a faster rod.

Generally speaking the faster the rod is, the less vibration there is. Speed contributes to damping (very high frequencies never show up). A smart caster can control vibrations by the way, let them pop up or kill them, and that can impress people on a video. Exaggerate that and you have the kind of commercial arguments used by the maker. The trend is favorable, but this is not going to improve your cast, it can make it more comfortable, no more (energy transmission bla bla bla is just BS). The energy left in the equivalent mass has to be spoiled anyway.

FLO: there are only two orthogonal planes and two orthogonal main inertial axes, whatever the shape of the section is. The argument used here looks like the “backbone theory” of 5 strips cane rods aficionados. Triple spine, that must be better than anything, the name suffice to justify the claim.

I can see the same future for this rod than the one of the deceased hexagraph.

Merlin
Fly rods are like women, they won't play if they're maltreated
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gordonjudd
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Re: Quantifying the Amount of Spine in Fly Rods

#17

Post by gordonjudd » Wed Apr 07, 2021 9:26 pm

The mistake is to measure the deflection on a geometrical axis going through the center of the section and not on an inertial one.
Merlin,
Is your reference to an inertial axis the same as thing as the neutral bending axis (i.e. the axis that has no tension or compression stresses)?

I assume that for a material with the same elastic modulus in compression or tension the neutral bending axis would be expected to correspond to the geometrical center of a circular cross-section.

How does the neutral bending axis shift away from the geometrical center for a circular cross-section due to the fact the material is stiffer in tension as compared to compression?

How do you determine that shift for an asymmetrical cross-section like the extended Lissajous?

Gordy

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Merlin
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Re: Quantifying the Amount of Spine in Fly Rods

#18

Post by Merlin » Thu Apr 08, 2021 7:11 am

Gordy

You mean the "extended Reuleaux", not the Lissajous. Well I do not know exactly, the "neutral axis" (which is an inertial one) shifts towards the tensile side which is something like 10% stiffer than the compressive one. This figure comes from measurements and is quite different from the assumption used in the document presenting the D flex rod (30%). So their claim is a bit optimistic.

IMHO a section presenting no horizontal symmetry, like the equilateral triangle or the Reuleaux triangle with the apex up or down, exhibits a slightly different position for the neutral axis in case of difference between tensile and compressive modulus (there would be none if the moduli were equal). Under load, the neutral axis takes a slightly different position and the geometrical axis follows, so you find a small difference in bending. The authors found 1.8% difference in apparent stiffness for a 30% difference in moduli. Reality is about 10% for moduli so the actual difference in apparent stiffness is likely very small.

The only interest I find to the Reuleaux section is the possibility to make very fast competition rods and come closer to the best casting potential. Now the question is to know if a thin walled Reuleaux section can withstand a competition cast without breaking.

Merlin
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Re: Quantifying the Amount of Spine in Fly Rods

#19

Post by gordonjudd » Sat Apr 17, 2021 4:13 pm

For a given mass (area), the extended Relueaux section is 18% stiffer than the circular section in given conditions,
Daniel,
Actually, if you check the calculations in their report and find the dimensions for a Releaux annulus (not the extended Releaux section) that match the area and wall thickness of the circular annulus they specified the reverse is true.
Reuleaux_annulus.jpg
Reuleaux_annulus.jpg (46.44 KiB) Viewed 15 times
The area moment for a circular cross-section with the same area and wall thickness would be pi/4*(r2.^4-r1.^4) or pi/4*(.005.^4-.004.^4)=2.898e-10.

The area moment ratios (I_re/I_cir) for the Reuleaux and circular annulus shapes is about 2.4314e-10/2.898e-10=.838. Thus the area moment for the Reuleaux annulus is about 16% less than the moment for the circular annulus.

That ratio can be improved by using the more rounded double extended shape shown below. That double extended shape gets rid of the sharp internal apexes in the extended Reuleaux shape.
cir_extended_results.jpg
cir_extended_results.jpg (47.85 KiB) Viewed 15 times
The more rounded Reuleaux annulus shape has an Ix that is about 5% less than the circular one.

Gordy

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