Bamboo Tips - Tips Area
Rod Design - Modulus of Elasticity

< Home < Tips Area < Rod Design < Modulus of Elasticity


Now that there's been considerable discussion about rod stresses I wonder if anyone has actually performed tests to determine the modulus of elasticity ( ratio of stress to strain within the elastic limit) for Tonkin cane? My guess is that perhaps Frank Paul may have done so. True? Here's why I ask this question: Garrison claims the modulus to be between 12,000,00 and 15,000,000 PSI and determined that the allowable working stress is 13,750 PSI (220,000 oz./inch squared). Yet I have found literature published by Orvis that says the modulus is 6,400,000 PSI and that the breaking strength is 165,000 PSI. That's an awfully big variation (between Garrison and Orvis) so I'm wondering if other tests can verify. At the same time it's important to recognize that "allowable working stress" has a safety factor built in to allow for such things as dynamic loadings Vs static loadings. It looks like Garrison used a factor of safety of about 3.4.

Any data out there?   (Ray Gould)

    We did some two point bending tests at Clemson when I had students working on designing ovens a couple of years ago.  I am having trouble finding the data in my pile of stuff, but if I remember correctly, the Modulus of Elasticity was measured between 2 and 6 million PSI - depending on the samples of bamboo used. Some were heat treated per Harry Boyd’s recommendations as I remember and some were not. My guess is the HT MoE will be towards the higher value and the non-HT towards the lower value. I will try and look for the reference. Clearly, Garrisons MoE is a factor of 2 or 3 times larger than I would expect it to be, but his allowable working stress is probably not too far off for bamboo rod design based on the Milward tensile test data - see below. There could be some published book errors on the Garrison MoE.  I think his MoE values are more like what one finds in Graphite rods today - not the carbon fiber MoE but the effective rod material MoE. I think Don Phillips has something in his Technology book.

    I took Milward’s tensile test data. Assumed that the bamboo behaves as a brittle material - it does not show much ductile behavior. His book gives failure stresses (Milward calls it Rupture Stress) that I took the median of 50,000 PSI in tension for a rectangular specimen that is 0.1 inches wide and .015 inches thick (area = .0015 in 2).  One can calculate the failure loading force for this of 75 pounds - seems reasonable.

    Using a middle value of MoE from Milward of 5x106 PSI and the failure stress, the engineering strain for the test specimen is 1%  or  .01 inch per inch. The actual elongation in tension of the 1/2 inch bamboo specimen at failure would be .005 inches. This also seems reasonable.

    My guess is the Orvis number for MoE is more correct and agrees with Milward and what we did at Clemson. It seems that using a working stress of 13,750 PSI is fairly conservative for Garrison, including the dynamics of the casting process with his "fudge factor". I would not know or guess how Garrison arrived at his working stress number given the number he suggests for bamboo MoE - seems way off. (Frank Paul)

      Not being an engineer or a physicist myself, could someone answer some questions for me?

      Does it matter if different people measuring stress and modulus use different dimensions of bamboo samples?

      Does it matter if different people use bamboo samples that have different density of power fibers?

      Does it make a difference to measure sticks or bamboo splits versus glued-up rod sections?  (Steve Weiss)

        1.  In theory, different dimensions won't effect the outcome if the sample under test is not stressed excessively.  The formulae for calculating stress or MOE take into account cross section, length and load.

        2.  "strength of material testing" assumes a homogeneous material.  Power fiber density will dramatically effect the outcome since the fibers are responsible for the greatest portion of strength.  You'll find significant differences in strength between successive node from the same culm

        3.  There will be a difference in results depending on the orientation of the stick during the test, IE power fibers up, down or perpendicular to the plane of testing since the power fibers are most concentrated toward the outer skin.  Further, if you compare a stick that is shaped to the same cross section as a glued up piece, the glued up piece will be stronger since it is balanced, having power fibers on the two opposing faces, one in tension and the other in compression, IE: power fibers in the areas of greatest stress.  The single stick will have power fibers on one side and pith on the other, in a typical test setup.  (Al Baldauski)

          Thanks for the cogent reply. From what you write, it seems that different people, testing different samples, can determine stress data that are not comparable. If one type of calculation of stress is applied to all tapers, the assumption, for our purposes has to be that all bamboo is alike so we can make rough comparisons. Are the discussions of the differences in stress values among different writers more academic than useful? (Steve Weiss)

            Not being an engineer or a physicist myself, could someone answer some questions for me?

            Does it matter if different people measuring stress and modulus use different dimensions of bamboo samples?

            It should not matter, but it probably does because of the material.

            Does it matter if different people use bamboo samples that have different density of power fibers?

            Yes. Take a look at the Milward book.

            Does it make a difference to  measure sticks or  bamboo splits versus glued-up rod sections?

            Yes. The adhesives and geometry will have an influence.

            Now having said that, there should be some agreement on magnitudes of the fracture or bending stress of bamboo and the MoE that can be measured. Milwards book has good data in this regard. (Frank Paul)

            I don't think the differences in stress values are academic if one doesn't know which set of values was used to generate a curve.  Two different sets of values yield two different curves for the same rod.

            The wide discrepancy in MOE reported leaves a lot of room for big differences.  If everyone who has translated Garrisons calculations to computer programs has used his MOE, his 4G factor, etc, then all curves generated are relative to each other and good for comparison.

            A lot of suggestions have been made to STANDARDIZE these value so that a consistent comparison can be made.  Even if G's method is not reality it does yield consistency according to a lot of Listers.

            But the consistency is only valid if ALL bamboo were the same.  Clearly it is not, neither from culm to culm, nor node to node, nor oven batch to oven batch.  These variables can lead to big differences amongst rods by different makers even if they each faithfully copy the same taper.  (Al Baldauski)


I found some data that was shared on this list in 2004 by a gentleman named Howard Bryan who used a sound meter to measure velocity in bamboo.  He measured the velocity of sound in Tonkin bamboo and found it was about 6000 meters per second (did not seem to matter whether it was heat treated or not). Knowing the density of bamboo from Garrison of 0.67 ounces per inch cubed and the equation from physics of:

Velocity = SquareRoot ( Modulus of Elasticity x Gravity  /  Bamboo Density)

If you get the units right and do all the math, one calculates a MoE ~= 6.1 x 106 PSI.  This is close to what Milward measures and the upper end of what my students measured at Clemson a year or so ago.  Garrisons number on MoE is incorrect by at least a factor of 2 or more in my opinion. If you have the Book you should probably make a note to correct this number.  (Frank Paul)

    Most of the new Hexrod programs allow you to change the line weight value. And come with modern weights as default values. So if you don't like them change them.

    Robin I would like to ask you one more time, what is the QC thing that is stuck in your craw. Please go here, down the page you see an actual flex, it is glass, but from there could you explain it to me. Thank You

    I get the impression after much of this discussion, that few of us actually USE a Hexrod program. Much of the fault finding has been about what G did incorrectly. (and we are all in agreement that he was not perfect, Oh Dear).

    This singular fact remains, if you use Hexrod or input a rod from measured values, It will show exactly the prediction of the rod that the maker had in mind.

    And although the value range could be improved almost all of the rods we covet fall within his ranges. It is a tool, not perfect, but it will yield a working rod without 25 years of feel behind your effort.

    All other methods proposed are after the fact.

    We all must hope that Max's effort is the next forward step in this effort.  (Jerry Foster)

      Its a constant deflection, that's all, very simple.

      Without it how else are you going to relate loading to line weights?

      When casting, strictly speaking, the tangent to the tip must not fall below the extended line. The curve of the rod could be more or less than a quarter circle, except that for experimental purposes it usually coincides with the QS pull.

      Perhaps its the terrifying simplicity of all this that worries people.  (Robin Haywood)

      Well, I use Hexrod regularly to store tapers (thanks Frank for setting up the private library's along with the rest of the great program!) and to look at both taper graphs and stress curve graphs, though I will admit to using the taper graphs much more. I also am a firm believer in it for making changes to tapers. Every rod I've made by changing length or line weight or both has turned out just as I hoped. I don't know if the math is flawed or not, but it sure has worked fine for me. I like that it changes line weights by what seems to be a percentage, I never could understand how just adding or subtracting the same amount for the whole length of the rod would work. I have also done some work with the changing stress values part of the program, but I haven't had time to make any of them, when I run comparisons to existing tapers it looks like I've just spent a lot of computer time reinventing the wheel. I have a few tapers I like and I know how they work when I change the fundamentals, so I mostly build those.  (John Channer)


After searching the web I have found very widely differing values for the Modulus of Elasticity  for the bamboo we use for rod making. Can some of you folks that are using deflection programs post the value of MOE you are using in your programs?  (Jerry Drake)

    If anyone is interested,  I'm using 5,000,000 PSI in FlexRod.  Also, see "Bamboo in the Laboratory" beginning on page 13 for Wolfram Schott's MOE research.  (David Bolin)

    I’ve done some significant testing which in a general way agrees with Wolfram Schott.  I got an average for about 4.5 x 10^6 for a section 0.150 thick (typical of a butt strip)

    and a value of 5.5 x 10^6 for strips 0.100 thick (typical of a mid strip).  I haven’t measured the MOE on a strip of about 0.04 (a tip strip) but when I use my deflection program to compare theory versus actual then a value of 6 x 10^6 gives a better match.  That is, when I put a rod on a deflection board and place a load on the tip and measure deflection it compares favorably with the deflection my program predicts if I use a value of 6.0.  The reason, I propose, is that most of the deflection in most rods occurs in the top half where the sections are thinning out and the average MOE is increasing.  Schott found values in strips 0.03 thick (from the outside fibers) of greater than 6.5. So an average value around 6.0 is reasonable.  (Al Baldauski)


Site Design by: Talsma Web Creations

Tips Home - What's New - Tips - Articles - Tutorials - Contraptions - Contributors - Search Site - Contact Us - Taper Archives
Christmas Missives - Chat Room - Photo Galleries - Line Conversions - The Journey - Extreme Rodmaking - Rodmaker's Pictures - Donate - Store