I made the subject as ugly as possible so nobody would think I was going for anything too fancy. :)

First post, new member, and based on the level of expertise I'm seeing in the posts and replies here, I won't mind a pile-on for this idea, if it's deserved.

On a recent backpacking trip, we lucked into a giant flush of porcini mushrooms, all throughout the mountains (the Wallowas, of Eastern Oregon) and ate like kings all week...porcini and wild onion stew! But i couldn't help but think how much better it would have been with some trout, and none of us came prepared to catch fish. Which got me thinking about a lightweight fishing kit for backpacking, searches for which, of course, quickly turned up tenkara.

In the weeks since that trip, my wife and I have caught many small trout on lakes in the California Sierra (around Mammoth), small rivers in Tuolumne Meadows, and closer to home in the many tiny-to-small streams of the San Bernardino Mountains. In the process, we've discovered that we really love tenkara fishing for fishing's sake; catch and release with barbless hook tenkara flies. I've begun tinkering with rod modifications, while my wife has designed and sewn some very nice tenkara fishing packs. :) The rods we have mostly used are a pair of super-cheap $13 telescoping "travel" rods we got on amazon that I've measured at 9 ft. and a CCS rating of 35 pennies. This is extremely stiff for a tenkara rod, which seem to fall mostly between 15 and 25 pennies, based on Chris Stewart's database: [www.tenkarabum.com] They catch a lot of fish, but I understand why a more properly-built tenkara rod would be better.

I have never even handled a "real" tenkara rod, though between the CCS rating and the flexibility ratios (5:5, 6:4, etc.) I feel like the fundamentals of tenkara rod performance can be measured, at least relatively, which is good enough for me.

I'm generally cheap and pretty handy, and I recently bought a few rods on fleabay for about $10 each, with the intention of measuring and modifying them. These are "Ultralight Fishing Tackle Carp Feeder Stream Hand Pole Telescopic Fishing Rods" Haha. Anyway, they're standard design telescoping rods with 6-10 carbon fiber sections that are light and compact, but generally very stiff, and pretty roughly finished. I've measured the CCS of each, dissembled them and, using a clamped hand drill as a "lathe," slowly and carefully sanded the sections down to reduce the stiffness of the rod. I have been really surprised at the results. For example, I've taken one of the 300cm 9-section rods down from 38 pennies to 26 pennies, and changed the rod's flexibility profile to close to 5:5, while reducing the rod's overall weight by 6g from 65g to 59g. It feels great and I'm going to continue working it, thinning the sections a little at a time toward a goal of 20 pennies.

I searched the forum here, and the wider internet, and was surprised to not find anyone else having done this. What are some of the potential pitfalls of this approach? I have a basic understanding of carbon fiber, building blanks on mandrels (though I've never done it myself) and, in a past life, was a sculptor, so I understand additive and subtractive shaping of materials pretty well. I do not, however, know what sort of problems can be created by sanding down a thick blank to get to a certain level of flexibility. I assume that blanks are built up on mandrels with specific goals that don't require any reductive sanding. Also, I really like the appearance of the bare carbon rod sections, once they've been polished with 1000 grit. Is there any risk in leaving the sections bare when I'm done?

Looking forward to feedback on this! Thanks in advance.

The power in a rod blank is mostly in the outer diameter. By sanding the outside walls you may easily compromise the structure of the rod blank. I'm not surprised that its power was reduced. The question now is - will it hold up for what you want it to do?

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That's interesting, and I suppose that only time and testing will tell. That said, we're mostly catching trout in the six to twelve inch range, so not putting any big demands on the rods. I could be wrong, but from what I've read, most of the considerations given to tenkara rod construction are for improved casting, operating on the assumption that great strength isn't necessary for the intended application (small mountain stream fishing).

Makes me wonder though... Are there factors that contribute to the strength and flexibility of blanks beyond the blank's diameter and wall thickness? I should probably look into that before getting too deep into it. It wouldn't be hard to build a little tool that tests each section for change in flexibility after each sanding, and as cheap as these rods are, it might make sense to test each section of one to failure. Hmm...

The Common Cents "Big Picture" will do what you ask.

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This discussion about the power of a rod blank being mostly in the outer diameter of the blank really got me thinking and wondering why that would be. I looked around (including reading an interesting PDF by you, Tom, on NFC's website about surface damage leading to rod failure) and found lots of other people saying the same thing about the strength of a blank being mostly in the outer diameter and cautioning against sanding blanks (or really any carbon fiber, for that matter) beyond where you're just sanding the surface resin.

What I would like to know, and have been unable to find, is the science behind this assertion. Is it simply true that the outer diameter provides the most strength *because* it's the OD? Or is there something specific to carbon fiber that makes this true for the material? Intuition is often wrong, mine more than most, but intuitively, one would assume that the wall of a carbon fiber rod blank would be materially identical from the ID to the OD, so any weakening of the blank would be simply proportional to the amount of material removed (much the same as would be the case with a tube made of any material). These cheap rods i have purport to be made from 45 degree cross-woven fabric, if that matters.

In some posts on this forum (like this one:[www.rodbuilding.org] ) seemingly opposite statements are made about the subject, unchallenged: "...reducing the diameter does lower the resistance to bending, but the fiber strength does not vary from outermost to inner layers."

Is there a discussion somewhere that could be linked to here that quantifies the weakening of a carbon fiber tube when it's surface is reduced?

I think the consensus on this forum is don't do it.

One reason that the most strength in a rod blank is in the outer fibers is due to the fact that as the diameter gets larger the tube becomes more stiff. As it gets smaller, the tube becomes less stiff. So as you cut through those outer fibers and reduce the diameter at any particular point, you have an effectively smaller diameter tube. This becomes particularly critical the closer you get to the tip area as there is so much less material and diameter there to begin with.

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I know nothing about Tenkara rods but the conversation about sanding and strength ignited my inner nerdiness... The OP asked,

"What I would like to know, and have been unable to find, is the science behind this assertion. Is it simply true that the outer diameter provides the most strength *because* it's the OD? Or is there something specific to carbon fiber that makes this true for the material?"

The simple answer is that the science behind all of that is Mechanics of Materials. It has been many moons since I took Mechanics of Materials in College and I'll admit I only got a C on it but, like Tom mentioned above, the basics are the greater the distance from the centroid of a cross section the stronger the cross section of the material when looking at a single material.

One practical experiment you can do:
1. take a yard stick and clamp the end to a bench or table so it is oriented flat with the majority of the yard stick unsupported. Apply some load straight down on the unsupported end of that yard stick. It will bend quite easily.

2. take the same yard stick and rotate it 90 degrees and re-clamp it to your bench with the same amount unsupported. Apply some load straight down on unsupported end. It will be much harder to bend the yard stick. Reason is because much farther distance to extreme end of cross section when oriented that way.

You could do the same type experiment with say a 3 or 4' length of 1" vs 2" SCH 40 PVC pipe. Going to be much harder to bend the 2". Just because you have more material further away from the axis of bending. Same concept with steel beams and why most are "I" shaped with the vertical portion of I oriented in the same direction as the load.

Gets more complicated when you are talking composite materials (like a carbon fiber rod) but you can theoretically calculate bending strength at any length along the tube (rod) if you knew all the material properties and at what distance from the center of your tube they are placed. You aren't going to have that info on your project rod but it is theoretically possible. With a fishing rod, the diameter is generally changing along the length so that makes it more complicated than constant diameter tube as well.

If you really wanted to dive deep google you up some textbooks or other resources on Carbon Fiber mechanics.

For a simple test, I found this online calculator that calculates basic beam deflection and stress for a tube beam. It allows the use of generic carbon fiber as material type and it is using constant diameter beam but you can get the idea... Keep everything constant except for your wall thickness. As the wall thickness reduces your deflection and stress increase.

[www2.engineering.com]

Going to work the same way as you sand down your rod. As you reduce your wall thickness the more stress and deflection you will have for the same amount of applied load. May be OK for the application you are describing. Only way to know for sure is to test it out and see how it performs. Good luck and thanks for the trip down memory lane.

Thank you for the link! I can't say for sure what the numbers all mean, but that calculator is exactly the sort of thing i was looking for! I've been fiddling with it and using measurements i've taken from the blanks with a micrometer, plugging them in and comparing the outcomes from different wall thicknesses and it give me a much better idea of what's happening. Oddly, it stands up to intuition! Ha!

So far, I've managed to snap one blank completely in half (don't try polishing a spinning tube with steel wool...eventually, it'll catch and send the blank into a death-spiral) and also have cracked the butt-end of one section in the drill chuck. Definitely not a precise operation here, but I feel like I'm learning a bunch about the materials, and at almost no cost. I'll report back once I've actually fished one or two if these franken-rods!

Thanks again!