Danny, this is a topic that will draw a heated debate from both sides.
Before going much further, take a minute and read this article, from the Library on this site, related to a spiral wrap demo device:
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www.rodbuilding.org]
At the heart of the spiral wrap demo is essentially the same piece of equipment that Mud Hole sells as a spine finder. So, for a little cash, you can get one to tinker with without having to source parts and build one.
Now, let's perform, as Einstein called them, a gedanken.
You have a tubular composite shaft that you intend to flex. By the nature of the way that these composites are made by wrapping a cloth around a mandrel, there are going to be variations in material thickness, resulting in a stiffest axis, and a softest axis (side opposite of the 'spine'). When flexing these shafts by loading the tip alone, the shaft will seek the orientation resulting in the least amount of energy stored in the shaft, i.e. the configuration of minimum potential. When you test for the spine, you are looking for this axis, and it is easy to locate, as the rod will 'snap' into that orientation as your rotate it under load. This is the evidence that gets cited by those who build relative to the spine. However, there are debates among this group of builders about whether the weakest axis should go on the bottom of the rod or the top. Many settle on the spine being on top and the softest axis on the bottom.
Pause for a minute. How often do you load the rod this way while fishing?
Unless you are using a rod with the line attached directly to the tip top, i.e. a cane pole or a tenkara rod, a golf club, or a cat toy the answer is: Never! You load the rod by using a line that passes through a set of guides attached to the blank. This is fundamentally different than loading the rod only by contacting two points on the rod, i.e. the tip and the butt. Now we will take our rod blank, and tape a fishing reel to it, as well as a set of guides. It doesn't matter which reel you choose, it can be a spinning reel, casting reel, or a fly reel. Next, tape up a set of guides. On our first test we will place the guides on the softest axis, pass the line through the guides and tie the line to a weight. Now, place the blank in your spine finder, what happens? The rod flexes along the axis where you found the spine with the spine of top. Now you might say, "Hey, maybe there is something to that spine thing!"
In the words of Lee Corso, "Not so fast, my friend!" Our next experiment is going to be place those guides and the reel on an axis that is, say 45 degrees away from the spine. This is an arbitrary angle not tied to the spine, weakest axis, or stiffest axis (which typically doesn't correspond to the spine). Put it in the spine finder, and what happens? The rod will rotate and flex in a curve that has the line at the bottom of the curve. The reason that this happens is that the line is interacting with the blank by using the guides. The guides will apply a torque to the rod, as the line seeks to get to the bottom of the rod, as that is that is where the load is located.
Let's perform a third variation of this experiment. This time, you are going to set up a casting rod in a traditional guides on top configuration. You can even place them on the spine, as many will tell you that the softest axis of the rod should go on the bottom. Again, run your line through the guides and tie on a weight. Have a friend hold the rod while you inspect it. What happens here, well the line passes through the first couple of guides closest to the reel with them on top of the rod, but as you get closer to the tip, you notice that the rod twists with the guides seeking to get where they 'want' to be, on the bottom of the rod, as this will be the configuration of least potential when the rod is loaded at multiple points with the line passing through the guides.
You can perform a final experiment with a rod built with the stiffest axis on the bottom of the rod. Being the hardest axis to flex against, it should be nearly impossible to make this axis "want" to be on the bottom of the rod. Set up your casting rod, but let's do a spiral wrap. Since we aren't fishing with this rod, we'll go extreme. Place the first guide from the reel on top, and the rest on the bottom. Run the line through the guides and tie on a weight. What happens? In this configuration, even fighting the stiffest axis on the rod, your rod will remain upright with the casting reel sitting on top.
Edited 1 time(s). Last edit at 12/11/2020 06:27PM by Joe Vanfossen.
