Monday, January 19, 2009

Posted by Lee Koch

The Tri-Hex Rod

As I mentioned in a previous post, I’m not much for repetition as it pertains to rodmaking. Since I started rodmaking, I’ve been intrigued by alternative geometries, pentas and quads, but lacking the specialized forms or a Morgan Handmill, I’ve stuck with hexes – until now. So when I saw an article on a 13-strip rod with an unusual geometry (but which used only equilateral triangles), I wondered if I could simplify the shape into 6 strips, and work up the math for converting a regular hex taper to the new semi-triangular geometry. Actually, the rod in cross-section looks like a triangle, but with the corners cut off. It still has 6 sides though, hence the name “Tri-Hex.”

Here’s a diagram of the geometry:

As you can see, the rod consists of 3 large triangles and 3 smaller triangles, which are exactly half the size of the large ones. Then the apex is shaved off the large triangles, to half their original height.

That’s the basics of the design. Ron Barch is going to publish a more detailed article in The Planing Form this year, the Jan/Feb issue, which explains the math behind the design. The Planing Form, in case you don’t know, is a wonderful newsletter for the craft. David has a link to it on the right-hand side of this page.

I’ve made one rod like this so far, (based on the Dickerson 8013), I’m working on a second (a Driggs), and Ross Smith, a Utah rodmaker who saw my rod at CRR last July, has made 2 or 3 since, based on other tapers.

I didn’t know what kind of rod I might end up with, I was just doing it to see what might happen. Imagine my surprise when the rod turned out to be really nice – light, smooth, powerful, equally comfortable at 15 feet and 50 feet.

Ross’s rods, which I cast at the Denver flyfishing show, are very different from mine. I consider that to be mostly a function of differing source tapers. They are more delicate, with markedly less bamboo in the rod – the butt of my 8013 5-wt tri-hex seemed massive compared to his 5-wt, and it was, as he put it, “more muscular.” However, his rods and mine seemed to share some important qualities. They’re very smooth casters, short or long, with no noticeable hinges in the action. They are lighter than you would expect for their line weight. My 8013 weighs 4.3 ounces finished out, which is light for a Dickerson 8013. Ross’s rods must have weighed a good bit less. The Tri-Hex construction seems to require less bamboo than a regular hex, to get the same action. They seem to track very straight, in the way quads are reputed to track “linearly.” One could imagine that is due to one of the big strips acting as the “Backbone” of the rod. With the 2 big trapezoids opposing the “backbone”, you might have expected some torque as the rod casts – we haven’t seen it at all. All of them were great casting tools.

There are some construction peculiarities involved with building a Tri-hex. You have to set your forms twice, of course, once for the small triangles, and again for the large ones. The tips of the small triangles get to be very small, say 21 or 22 thousandths, so you need to find a way to get your forms that tight, or otherwise cope with the taper; with a Morgan handmill, you may have to dedicate an anvil specifically to the set of small strips. The apexes of the large triangles need to be shaved off with some precision, because if you leave them too big, your strips won’t fit together and you will have gaps in your glue lines. Finally, speaking of glue lines, many makers have noticed that glue lines and node chips tend to occur on the fat butt strips of a regular rod – that challenge is accentuated with the very large strips of a Tri-hex, and you really have to work at straightening these strips.

The trade-off for this extra attention is a light, smooth, powerful rod … and don’t we all want one of those?

If you’re interested in the math behind the Tri-Hex, or the conversion factors needed to convert any regular hex taper to a Tri-hex taper, check out the January/February Planing Form (coming soon, from what I hear from Ron.)


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