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Spiral Rods

With some help from Tom Smithwick and Letcher Lambuth I just took my first Spiral Rod out of the fixture. I glued it up a week ago and wanted to give it plenty of time to cure. On Tom's suggestion I put it in the back seat of the Lumina yesterday for about five hours. I am sure it was well over 100.

This whole week I have been contemplating the straightening that will be required when I took it out of the form. How bad are the bends going to be? (For once I wasn't worried about the twist). I went to sleep every night mentally straightening bends without removing the spiral.

I sighted down the two pieces as soon as I took them out of the forms this morning. All they needed were sights on the ends , seriously it was like looking down a gun barrel. These were the straightest strips that I have produced. I think I might use this form to dry sections on my next straight rod, whenever that might be. I am as excited about this spiral as I was about my first rod.

Thanks for the help Tom. I will keep you posted on the progress. It looks as if scraping the blank is really going to be a chore.  (Steve Trauthwein)

Recently, I acquired a copy of Lecher Lambuth's book, The Angler's Workshop, and I have a question for the list gurus.  I know that some of you (Ralph at least) have built one or more spiral rods and that, as I recall, Ralph had good results.

My question is what taper did you use or, if you would rather not disclose that, what characteristics would lend themselves to spiral building?  Since the technique makes a more powerful rod, do you intentionally start with a taper that is sort of on the slow side?  (Steve Dunn)

    I had no idea of how to calculate a taper for a spiral rod when I first started.  What I actually did on my one piece rod, was take a 7.5' 4 wt conventional rod, and simply twisted it.  It comes out to be about a six weight rod, but I did hear one poor soul who thought it might even take an eight.  Having good results, I have on my other rods simply taken a conventional taper about 2 line sizes less than what I hope to attain.  It is not exact, but then it works.  Tom Smithwick might have some additional information.  (Ralph Moon)

    I have built a dozen or so spiral rods. I think it works with any type of taper, and my advice would be to build something you have built before and like. Don't change the taper for a first attempt. Maybe you will get different results than me, but I seem to gain only about half a line weight in initial stiffness.  The real kick comes when a lot of load is applied, and the rod keeps on casting, sort of like a quad. Don't use a glue that allows any creep, or the spiral will unwind a bit. Titebond and epoxy are no good, URAC and resorcinol will work. If you decide to try a spiral rod, contact me before you glue up for some more tips.  (Tom Smithwick)

I have built about a dozen spiral rods. I wrote an article for The Planing Form a while back detailing the method. It also wound up in the "Best of the Planing Form", if you can get a copy. The recently mentioned "Anglers Workshop" by Lambuth is the best source of information, if you are so fortunate as to find a copy.

Briefly, build the rod the way you normally would. The spiraling is done to the glued and bound strips. You build a form with clamps at every guide station, and turn the rod 1/6 of a turn between each clamp.

Some tips:

  • Wet straighten as well as you can before clamping into the form.  If you have to straighten the cured blank, use the minimum of heat, or just massage with your hands, if you can get away with it. The blank will unwind a bit with heat.
  • Use a powerful glue. Resorcinol is fine, but URAC seems to hold a tighter spiral.
  • Don't use a "soft" glue like PU or Titebond. I'm not sure about the epoxies.

In use, a spiral rod is a more powerful caster than the same rod not spiraled. Bob Milward says a spiral rod feels a bit stiffer when flexed. That is true, but not the whole story. The spiral rod will hold more line in the air when false casting, and seems to oscillate a bit faster. If you like to fish a long line, with wet flies, streamers, and the like, you will like the spiral rod.

My favorite way of building them is with one spiral tip, and one straight tip. The spiral tip drives the butt deeper, and produces a wet fly & streamer action. The straight tip flexes easier, producing a more tip oriented dry fly action. Thus you end up with a nifty, dual purpose rod, without changing the tapers of the tips.

If you want to try this without a big commitment, make a spiral butt for an existing rod. I think you will find the results interesting.  (Tom Smithwick)

I was wondering if anyone can furnish me with information about the history of spiral rods?  I have the Lambuth book and the Planing Forum with Tom Smithwick's article.

I would like to find out about Fred Divine and  his patent on spiral rods. Is there any information that predates Divine?  (Steve Trauthwein)

    There isn't much information around on Divine. He definitely invented the idea, and patented it. There is a little bit about it in Marty Keane's book, and one of the versions of Herter's manual is supposed to have a picture of the Divine twisting device. I don't have that edition. All Keane says is that the Divine rods were popular, but that the idea was abandoned because of production problems. My guess is that he couldn't make a straight section reliably.

    It would be interesting to locate the patent. My understanding is that he grasped both ends in a lathe like device and turned the twist in. This would put most of the twist in the thin end. Lambuth, as you know, controlled the twist with a locking form.  (Tom Smithwick)

    Here is some more info, and I would recommend the Sinclair book about Divine (but I do not have that one).

    The original patent that I found is patent #476,370.  (Bob Maulucci)

I was  thinking  of  approaching  the  twisted  concept  a  little  differently:  Glue up with Epon and mount in a Gould  "tensioner" (p. 21 "...Tips & Tapers) and then twist it 1/6 of a turn for each guide that needs to be put on that rod section, leaving it under tension until the glue sets. That might put most of the twists in the tip of the taper and could require more twits than the # of guides, but that way the tension would be evenly distributed rather than be changed at every guide so that the spacing would be right. Comments?  (Henry Mitchell)

    I know not how all others do it, but I have always followed Letcher Lambuth's procedures.  That is that the guide placement determines the twist.  i.e.:  Lock the butt and twist 1 flat to the first guide , then 1 flat twist to the second guide. etc.  You have to have a form built for the twisting all ready for the gluing.  If Tony and Henry do it differently.  More power to them.  I like my way, though  I think that is the way Tom Smithwick does it too.  (Ralph Moon)

    I think that is how Fred Divine did it, But I also think it  is necessary to take too many twists, and there is anecdotal evidence  (read: old rodmakers tales), to suggest that too much twist causes  the rod to fatigue out earlier. I don't know for sure if it's true,  but I have only used the Lambuth method, and have had no failures.  Check out this picture:


    I have seen Tony's Twisted Miss rod, and he got it pretty good using  his method. I don't have the nerve to try it. Further, I am only  comfortable with URAC, Weldwood, or Resorcinol to hold a twisted rod  permanently.

    Anyone reading the archives should be aware of how to prevent  failures with these glues. Do not allow the glue to set up at  temperature less than 70, don't glue freshly heat treated cane  without moisturizing it, and don't use old glue unless you glue up  some test sticks a day or so earlier, and bash them apart to make  sure the glue is stronger than the wood.  (Tom Smithwick)

    Thanks for your replies. I've seen the pictures but need to set up a dipping tank and maybe insulate the shop before I make any more tools. Not to mention fish and finish some rods. A twisted rod is next winter at the earliest. I might try my idea on a short one-tipper just to see what happens.

    Does anyone have suggestions for taper types that might be suitable for twisting? I was thinking Garrisons would still be smooth but quicker whereas Dickersons are fast enough already.  (Henry Mitchell)

      Your thoughts are on the money. Garrison is a good place to  start. I have only tried one parabolic, a Driggs, and it was nice  too, after I got used to it.  (Tom Smithwick)

      What are the advantages of a twisted rod? wouldn't they be much stiffer?  (Wayne Caron)

        My experience has been that they only feel slightly stiffer  initially. Where you see a difference is when the rod is heavily  loaded. The spiral rods seem to hang in there longer.  Sort of like  quads, but they do feel stiffer than a comparable quad.  (Tom Smithwick)

          Interesting to note too that Divine states in his initial patent that "the angle of the spiral will of course be varied according to the quality of the bamboo and the degree of stiffness and resiliency required."   As well he states "by my invention the strain and wear are much  more uniformly  distributed  among  the sections."   Essentially making the rod more parabolic if you think about it as the twisting would have to effect the initial taper itself by smoothing it out and removing subtleties or points of weakness.  (Rob Smith)

      I would think you could twist almost any taper with the exception as to those that have a big "Swell in the Butt" as that extra stiffness may make twisting in the butt section down by the handle more difficult.  (Ray Gould)

I noted that several members are making "twisted" rods and wondering about guide placement.  Here is what I do:

This works with "heat set" glue. I use Epon on the ones I make.

Make a blank in the normal way.

Heat set the glue.

Sand the blank and apply several coats of Formby's Tung Varnish, then steel wool the blank.

Wipe down with denatured alcohol.

Set the guides at normal spacing on adjoining flats.

The guides will spiral around the blank. I use rubber rings cut from surgical tubing to hold the guides in place.

Heat the section between the guides over a heat gun and twist that section to line up the guides. Twist all in one direction. It takes a bit of heat on the butt section so some care needs to be taken there.

When all the guides are lined up, I wrap the guides.

Final varnish is done with wrapped guides.

Hope this helps some "wanna make a twisted rod" list member.

Barber Poles can be done with alternating flamed and unflamed strips.  (Tony Spezio)

I have been finding time to make a spiral rod for the last 15 years. I have one finished.  A Smithwick 5.6 ft hex.

Tomorrow I am going to attempt a another Smithwick only this time spiral. If any rodmakers out there can pass some tips on to me before the big day. regarding getting this nice and straight. I will be very interested to hear from you.  I will let you all know if there is any great improvements to way the blank performed. Both rods are constructed from  the same culm and will be identical.  (Gary Nicholson)

    Lay out string guides the length of your twisting form. It also helps if you can shim up the stations a little so that the rod is parallel with the surface of the form.  The string guides are essential as far as  am concerned.  I did a 7.5 one piece rod with this method and did not have to do any additional straightening.  (Ralph Moon)

    Is the blank made or are you going to make the blank. I have made a couple of spiral rods a bit different than the conventional spiral way. I make a hex blank, glue with Epon. Glue grip and ferrules. Space out the guides on the blank on adjoining flats using dental rubber bands. The guides are on the blank in a spiral. Heat each section in between the guides and twist each section to line up the guides. I end up with one flat twist between each guide. It may not be the right way to do it but it works for me. I have only done this with the Sir D taper and the Twisted Miss. I find the rod is a bit faster, not much though.  (Tony Spezio)

      Thanks for the post on spirals. I have done it the tradition way, but your methods sounds interesting. Did you find any difference in casting comparing  to ordinary Hex’s  with your method of spiral construction. I am interested how the rod handles load when carrying over 10 yards of fly line. Its too early to comment on mine at present its still in the locks. In will leave it there for a week then put the guides on.  (Gary Nicholson)

Update on the spiral rod I expressed an interest in making:

Well, job done both rods completed.  Do I find any difference in them?  Emphatically NO.

OK, so I have only made one so its nothing conclusive.  Well, in my quest for performance maybe I should look else were maybe a quad?  (Gary Nicholson)

    Can you tell us more about your construction techniques?  Seems unusual that you don't feel a difference.  (Scott Grady)

      Yes, both rods were constructed from the same culm same taper very thing was duplicated.  (Gary Nicholson)

      Yes, it seems a little weird.  My one piece 7'6" was designed as a #4 and I have had reports that it is as high as a #8.  I think it is about a #6,  but it  sure is not like the #4 that was its progenitor.  (Ralph Moon)

    The difference in stiffness of a spiral rod over a normal hexagon is I think quite small.  Bearing in mind practicalities of construction probably the most efficient structural section is a hollow built rectangle.  You might find the writings of Mike Montagne on this subject illuminating but I no longer have copies of the extensive e mails that were going around some time ago.

    Interestingly Mike had little time for static analysis including consideration of the section modulus being more concerned with the dynamics of energy transfer.  However his solution still ended up using the structural form that is also the most efficient under static stress.  (Gary Marshall)

      I think I read somewhere the difference was approximately 10%.  I don't even think it's that, but its just one rod.  (Gary Nicholson)

This is primarily to satisfy my own curiosity.  I know the Divine was granted a patent for spiral rods, but I have never seen any information on his output, nor have I even heard of anyone who has seen one.  I am curious to know why the dearth of information.  Why did he stop making them, How many did he make, How did he spiral etc. 

It is my understanding that Lambuth (alone or with Stinson) only built about 35 rods.  True?  How many other builders than Tom S and I have built Spiral and why would we want to?  (Ralph Moon)

    I know Tony has made some - hard to imagine making them freehand.  Last year I made twin Garrison 206's and twisted one of them 60 degrees at each guide station.  True to what Lambuth says the spiraling  stiffened the twisted one, as measured by Common Cents.  Straight one deflected 1/3 of it's length with 18 cents - the spiral one 20 cents.

    One reason to spiral might be to gain stiffness without extra cane weight, short of hollow building.  It was a fun experiment, though I have to say I don't care much for scraping a spiral blank.  I like to went bug-eyed making sure I was on the right flat.  One neat thing about having a spiraling form now is that it's great for taking twists out of blanks.  (Darrol Groth)

    I have made three of them. I know Steve Trauthwein has made at least one.  (Tony Spezio)

    The only reason I can see for building a spiral rod would be to test your rodmaking skills, to see if you can do it, so to speak. With only 9 rods under my belt and getting ready to start #10, my skills aren't ready to be tested like that yet. Heck, it's a struggle to keep twists OUT of the damn thing sometimes LOL. I don't see me building one(at least not intentionally) anytime soon.  (Will Price)

      If you really want to test yourself try out the first spiral I built.  a 7'6" one piece,  It came out straight and is a magnificent rod.  (Ralph Moon)

    I made a jig and have built six rods. They are a little stiffer. I don't know if it is worth the effort but it is a lot of fun when someone looks down the barrel, so to speak.  (Steve Trauthwein)

      When you say they were stiffer how did you measure that and what were the figures?  (Robin Haywood)

        I have no figures. I did a presentation were I had made four rods, two identical pairs as near as possible, given the material). It was the subjective opinion of those who cast them.  (Steve Trauthwein)

          The reason I asked is because there is no preferred plane of bending in any regular polygon (including, obviously, a circle) so there is no reason at all for them to be stiffer.  (Robin Haywood)

            My understanding is that the feeling of stiffness comes from the splines being set under tension as they are glued.  I don't know if this is actually the case and I can't recall where I picked up that idea, so maybe I am way off-base.

            I do remember some speculation by (I believe) Bob Milward in The Best of the Planing Form Vol. 1.  Something about how the same effect could possibly be realized by heat treating the strips along a convex (enamel side down) jig.  If memory serves the way I am hoping it should, he had not tried this, only theorized.

            I will say I did truly enjoy one of Tom Smithwick's spirals that I had the pleasure of casting for whatever reason. (Carl DiNardo)

            I'm no engineer (though I play one on TV. . .)  But I'll posit a reason for a spiral rod to be stiffer.  Assuming that the bamboo exhibits a greater resistance to bending in tension the more it is put in tension, then by putting the spiral into the rod you are already pre-loading the power fibers in tension.  Therefore, the spiral should be "prestressed" and stiffer.  (Chris Obuchowski)

              I'm not an engineer either, but this is my conjecture of how a spiral rod could be stiffer.

              First I'll venture a hypothesis that a hex rod is stiffer if the bending plane is apex to apex compared to flat to flat. The apexes are further away from the center of the rod, giving an effective larger diameter if flexed in the apex to apex plane.

              When a rod is spiraled it is blending in the apexes with the flats in the flexing plane, combining the effective larger diameter of the apexes with the flat to flat diameter, giving the same effect as if the rod was flexed partially over the apexes.

              The reason I don't think it's a prestressed bamboo fibers effect is because as in the reason bamboo rods will take a set, the prestressing will eventually relax, and so far I haven't heard that spiraled rods lose their extra stiffness over time.  (Darryl Hayashida)

                Your hypothesis may appear right intuitively but in actual fact it is not.  The stiffness of a hexagon (or any regular polygon as Robin has noted) is the same about any axis though the centerline.  The two obvious axes being though the center of two flats or through corners.

                Those wishing to verify this can try this site. Input the same across flats dimension for each of the two cases in turn.

                The property that is directly relevant is the moment of inertia and this will be seen to be the same.  Those looking more closely will see that the section modulus for each axes is not the same so the extreme fibre stress is different.

                I hope this is helpful although you will note that I have ducked the original question!  (Gary Marshall)

            We are looking for reason?  They are stiffer Robin.  My 7'6" was designed as a four weight.  I use a six on it and one perverse soul claimed it needed a #8.  I suspect that it is because prestressing the segments does it.  (Ralph Moon)

            Rods can't read...  (Larry Blan)

            It was I that measured two rods identical except one was spiraled.  The rods were measured, in effect, by deflection according to Hanneman's Common Cents system for your perusal.  I don't necessarily advocate Hanneman's system - it's just that it's about the only objective measure we have and I use it on occasion - so I won't argue it.

            According to Lambuth twisting a rod in this fashion, i.e. 60 degrees every guide station produces a rod that is 10% stiffer.  My straight rod deflected 1/3 of it's length with 18 pennies (694.98 grains).  The spiral rod deflected 1/3 of it's length with 20 pennies (772.20 grains).  That's pretty close to 10%.  Like others I suppose it's because of some inner tension.  I don't know why.  I don't care why.  It's just stiffer.  (Darrol Groth)

Fred Divine had a patent on spiral rods, but his spiral was calculated to 1 and 1/2 turns to a given length.

Lambuth's spiral varies according to the length of the rod.  1/6 twist at each guide,  Lambuth says:

"Our theory to explain the apparent increase in the stamina of the spiral rod, is that the corners act as a buttress which increases the acting diameter of the rod.  Also the splines being locked in a balanced distortion, are more resistant to bending strains,.  Also the constant action of the rod is more evenly distributed in every direction over the cross sections of the individual splines.  In fact our reasoning is to us so plausible that we wonder that the spiral rod has not displaced the straight rod commercially."

Has anyone ever seen a Divine spiral?  (Ralph Moon)

    Many of you know that I have been playing around with these things for at least 20 years. Not that I have built a lot of spiral rods, it's probably about a dozen. I built the first one as a novelty, believing as Robin does, that there would be no difference. I had  previously built the same taper as a straight rod, and was astonished to find a real difference. I let a number of experienced people cast the rods, and all narrowed their eyes and accused me of various forms  of nefarious deception. Being Irish, I would have gladly denied it  even if I was guilty.  But being innocent, it ignited a serious enough outrage that I built a few truly matched rods, same cane, same  glue, heat treated at the same time, etc.  In each case, the spiral  rod was significantly superior in distance casting. I built a rod for the Grand Experiment, someone else did, too. (Scott Grady?).  John Long concluded that spiral rods did cast farther. A number of people  have also experimented and concluded the same.

    I don't believe in something for nothing either, but think it's a question of degree. If you really twist them, I believe they would fail sooner. I have not seen a Devine, but if he did them the way illustrated in Herter's, he would have had to put in a lot more than 1.5 twists to get the guides to line up. I'll stick with the Lambuth method. I have seen some Lambuth's, and they appear to have been built with very even spirals. None of my rods have failed.

    I have never really offered an explanation of why the spiral rods cast better. That's because I don't really know.  I love a good BS story, and love to tell them, too, as long as everyone knows I'm BS'ing.  I suppose I ought to develop a good BS story, as people seem to expect that from rodmakers.  :-)

    I think the most interesting recent experiment is that recently performed by Gary Nicholson.  Gary built two rods, one straight and one spiral,  and was disappointed at first to find no difference.  Several of us wrote to try to figure out what happened. It turned out he had glued with epoxy. After a few weeks, the epoxy fully hardened, and he began to feel a difference. I have long felt that harder drying glues like URAC produce the most difference between straight and spiral rods. I think the difference has nothing to do with geometry, or pre stressing. I think it has to do with locking the fibers in the twisted configuration, and preventing any internal slippage. It is just a theory.  (Tom Smithwick)

      Yes it was me who built the other spiral as part of the Grand Experiment.  I did not do any heat treating before or any straightening with heat after glue up.  The rod has not changed since I built it, still is a cannon. I agree with you about the twist preventing slippage.  If you take three pieces of light copper wire and hold them parallel to each other, they bend easy.  If you twist them together, they become much stiffer.  The wire is not stressed as it does not want to unravel.  (Scott Grady)

    Divine's patent was 1892.  if you would like a look at the patent drawing go here.  (Ralph Moon)

      This link will take you to the US patent office.  If you click on "images" it will take you to the drawings (same as Ralph's link plus a second drawing) as well as the documentation that was submitted with the patent.

      USPTO uses TIFF files for all of their patent scans, so if you don't have the capability to view them, you would need to download a viewer.  I think they have a link buried in a FAQ somewhere to some that are available.  (Carl DiNardo)

Hi all, I have just asked a good friend of mine about the stiffness issue in regards to spiral rods.  This guy teaches engineering at a higher education college this is what he said.

I would need to look at a rod.... BUT, if the hexagon is slightly offset to the previous one, or if one of the 'points' of the hexagon is along the bending plane it would be stiffer. This is because you are trying to bend the rod over a thicker area.... if you look at normal rods, the flats of the hexagon are always inline with the reel seat. Does that make any sense coz I am on nights this week and not thinking properly! Try it with a single section. flat then angled so the point is underneath.  (Gary Nicholson)

    I don't know if any of you guys have any spare time on there hands to try a little experiment  to get to the bottom of this issue. At the moment I don't.  I have too many orders to fill. Anyway, can I make a suggestion, make two identical tip sections.  Twist one after glue up with a heat gun.  The other one twist when you glue up as normal. 

    This should show if there is any difference in resistance to bending.  We know the twisted one which is made when we glue up will be stiffer than a normal hex rod that's concluded.  But what about the heated and twisted section will that be comparable to it in stiffness?

    The glued up and spiral tip section will be under tension.

    The other tip section  twisted with the heat gun will not be. If you heat treat the bamboo you destroy the lignin cellulose bond and will in fact destroy any pre stressing in the twisted rod.

    I think this experiment will give us the answer.

    I can get around to this but not for a few month anyone willing to try it.  (Gary Nicholson)

    Well, if I'm understanding what your engineer friend said, that would explain(to me at least) why a penta feels stiffer than a hex of the same length & line weight. One of the apexes in the bending plane. With the spiral rod you are putting the apex in the casting (bending) plane at several points. It would be interesting to see a penta put into the test and see how the stiffness of a spiral rod feels in relation to it as well as how it feels to the hex rod.  (Will Price)

      When we spiral a rod  we are putting the power fibers on the bias at an angle to the centerline just like in glass and graphite rods. I think that this should change the modulus of elasticity of the rod because tension and compression on the power fibers would tend to deform the matrix that binds them together. Wouldn't this increase the amount of force required bend the rod a given amount?  (Doug Easton)

    I did some research last night and found some good  reference  material In Dick walkers Rod building for amateurs.

    This is what he says:

    “There has however been a great deal of discussion about the best number of sides for a polygonal rod and some notions exist on this matter, especially in the U_ _ (forgive me for that but that's what it says) which are mistaken. These misconceptions are due to a lack of mathematical knowledge. The first error is in supposing that a polygonal rod has what is known as a preferred plane of bending. It is quite commonly supposed that a six sided rod bends more readily across the flats than at the corners. This is not true. It is equally stiff in either condition, its deflection for a given load is identically stiff  in no matter which plane it is bent. This is true of all polygonal rods having four or more sides.

    Of course, if you take a piece of hexagonal rod and bend it in your hands, it will feel stiffer when bent across the corners because your hands will be against an edge, and the pressure will be greater in pounds per square inch against your hand but the load for a given bend will be the same.

    I tested this with a 5 mm 2 foot long piece of beech there is no difference in resistance to bending on any flat or corner.”

    So it looks like Gary was right.

    So I think it has to be pre stressing which is making for a stiffer rod.

    we should also be able to tighten the spiral and increases the pre stressing.  (Gary Nicholson)

    Here is my take on this ongoing discussion:

    Here are the formulas directly from a “strength of materials” text:

    Deflection ~W / MOI  (i.e.  Deflection is proportional to load applied divided by the Moment of Inertia)

    MOI for various shapes:

            For a rod 1 inch (flat to flat)

    Square (flat to flat)          = 0.0833 8 d4 = 0.0833
    Square (corner to corner)   = 0.0833 8 d4 = 0.0833
    Hex (flat to flat)               = 0.06 * d4 = 0.06
    Hex (corner to corner)        = 0.06 * d4 = 0.06
    Octagon (flat to flat)         = 0.055 * d4 = 0.055

    Where “d” is the dimension flat to flat in all cases

    Since MOI is in the denominator or the Deflection equation, the larger the value the less deflection.  If you assume the amount of deflection for a constant load is a measure of stiffness then the following is true:

    If ALL rods compared have THE SAME flat to flat dimensions the square rod is stiffer than a hex by 38.8% whether flat to flat or corner to corner (at the expense of 15% more weight).  The octagonal rod will be 8.3% less stiff than the hex.

    There are so many variable in making a rod that a subjective evaluation of a couple of rods is chancy at best.  Even a carefully structured experiment is fraught with difficulties.  For instance: if you evaluated a hex rod of 0.100 diameter versus a hex of 0.102 diameter, the larger section is 8% stiffer!  Imagine what the difference in bamboo from culm to culm will do!

    Regarding spiral rods:

    Since the stiffness is the same flat to flat or corner to corner then the spiraling will have no  effect due to orientation of apices. If any one can MEASURE a difference, then it will be because of a change in cross section. (Al Baldauski)

      The RELATIVE stiffness figures are 0.791 for a Hex and 0.812 for a Quad.

      Bigger is gooder.

      A Quad, weight for weight, is thus 2.65% stiffer than a Hex. Peak stress is up 10.79%, however, and smaller is gooder!  (Robin Haywood)

      Is it possible that under stress the spiral rod wants to untwist, if even slightly, and give it a bit of a spring action when the tension is released? I have no proof.....just a thought.  (Tom Kurtis)

        I would speculate that once the rod is assembled, while you may have built in some stress while twisting, it will not tend to untwist during use significantly to notice otherwise you’d be throwing curved casts!  (Al Baldauski)


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