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Rod Design - Inside-Out Rods

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I was planing along pondering life. Was thinking about a comment I made to another builder a couple of days ago.  The comment was: There is more resistance to bending on a single strip when the pith is rotated to each side rather than with the pith rotated to the center.

Drew a bunch of pictures and tried to figure out how to maximize this effect in a finished rod. In a 6'er, didn't see how it mattered much. In a fiver, the same. How about a rectangle with the top narrower than the sides. Or a laminate with all the power fiber laying on edge.

Claude, you got any ideas how this could be calculated?  (Don Anderson)

If I understand what you're describing it can be calculated by figuring out the moment of inertia for each cross section. It's the same reason why structural steel shapes such as I-Beams or channels are stiffer in one direction of load compared to another ninety degrees to it.  (Ray Gould)

You are correct about the resistance to flexing.  I've built rods with rectangular cross-section, they are much stiffer with the guides on the narrow side.  The problem is that they tend to twist when casting, sloughing the bending onto the more flexible plane.  When the guides are mounted on the wider flat they tend to cast a very straight line.  I've also built hexagonal rods with wider strips (trapezoidal in cross-section) under the guides.  These are a delight to cast, they are probably pretty resistant to stress, but don't have the greatest power:weight.  I have a friend who casts with the reel turned at about a 45 degree angle to the casting plane.  He can't cast either one of these designs worth a damn because both  tend to twist the rod back into plane.  (Bill Lamberson)

A triangle strip will bend more easily with the enamel on the outside of the bend because the pith has little resistance to compression. When you rotate the strip, the resistance to bending becomes greater because you are adding compression resistance to the bottom.

Fortunately for bamboo rods the resistance to stretching/compression is equal on all faces.  (Ron Grantham)

I spent an hour yesterday in my shop.  It was too cold to hold a plane, so I ended up breaking lengths of bamboo. Thick ones thin ones, long ones short ones.  I broke a lot of bamboo and I ended up with a couple of observations.  Now for those scientific types, this test is a nonsequiture.  I just took a strip one end in each hand and bent until it broke.

1.  Non of the splinters (power fibers?) exceeded 5" by more than a fraction.

2.  The longest ones came from the outside of the strip. There seemed to be two planes of breakage.  The first was the sudden explosive rupturing of the long initial fibers, and the second required shorter axis of bend to further the break into the shorter middle fibers.  In almost every case the pith never did separate

3.  Never could I force the break at the node

I have absolutely no idea what conclusions can be reached, but a few observations.  The initial break took a lot of force.  In some cases the two ends of the strip met before the break occurred.  Bamboo is tough.  The depth of the initial fibers on the first break was often almost half the thickness of the strip.  The power fibers are deeper than most of us think them.

In no case were the fibers contiguous the whole length of the strip between nodes.  (Ralph Moon)

Exact calculation would be difficult, but a good approximation wouldn't be too difficult - just a bit time consuming. My spreadsheet version of Hexrod can calculate stress curves for rectangular rods.  For an individual strip, it could be done by pretending the strip was laminated, with several different "materials", each having a different MOE and so forth.  My stress and strain text considers this a common calculation...

Bill and Ron and Ralph are absolutely correct with their comments, in my opinion (I agree with them; therefore they are correct! <G>)

I've played around with the stress curves a bit using rectangular quads and my conclusion is that a really fast rod (acts most like a Sage graphite) would be one that has a rectangular taper that changes.  Starting at the butt, the taper would have a 5/4 aspect ratio - meaning guides would be on the narrow flat, if any guides were used under the cork, and it would change the ratio gradually until it was 4/4 at the ferrule.  The tip section would begin with a 4/4 ratio at the ferrule and continue this about half way to the tiptop, where the ratio would gradually change until it hit a 4/5 ratio at the tip top (meaning the guides would be on the wide side.

My conclusions are that a rod of this type should have a very stiff butt section, fairly stiff midsection and a very sensitive tip. Because the tip is wider than tall, so to speak, it would tend to cast in a very straight line.  The taller than wide butt section would allow a relatively small and lightweight butt for the power (carrying this to extremes would result in a butt section that was like a yardstick on edge - very stiff in one direction, but quite limp in the other, and it would twist, as Bill pointed out - the idea is that with an aspect ratio of 5/4, you get the stiffening effect without a lot of the twisting effect...(hopefully).

I'd love to make a rod with a taper such as this to try it out and see if the theory is anywhere close to the practical, but I'm still unable to build rods for various reasons.  Anyone with time on their hands who wants to build such a quad rod, let me know and I'll send you a suggested taper with it's stress curve - you can build the rod and tell us all how it casts.  (Claude Freaner)