Sloppy Rods

I’m sure this statement is going to add to the debate, however if you’re going to prove me wrong you’re going to need stress/strain data from the mechanical loading of a rod, not just hand-wavy descriptions of how you bugger up a cast with a specific rod as soon as the carry gets beyond 60ft and you’re certain it’s the rod’s fault and not yours.

Most rods are tapered tubes; the taper and the mechanical properties of the material it is fabricated from (modulus etc.) largely determine the action of the rod.  Hopefully it’s obvious that the flexural modulus (stiffness) of a rod changes along its length due to its tapered construction, i.e. the tip will flex deeply before any significant deflection in the section above the handle is noticed.  In this respect the flexural modulus of the rod (or section of the rod) is very different from the carbon or glass fibre that it is made from, i.e. the ultra-high modulus figures used in the marketing of some rods relate to the individual carbon fibres in tension, not the actual bending of a fly rod.  However, by using high modulus materials the rod designer can achieve the level of stiffness they desire in the composite tapered tube using less material (although less material can also mean fragility).

What people are implying by saying a fishing rod goes ‘sloppy’ or ‘it folds’ is that the flexural modulus of some part of the rod’s construction suddenly drops.  As previously mentioned, the rods tapered construction means that as the load at the tip is increased the stiffness of the rod effectively increases.  You can prove this easily for yourself – clamp a rod by the handle horizontally and then add a weight and measure the deflection.  Then double the weight – in this case the deflection will be less than double.  If you double the weight again and again the deflection measured reduces each time as the tip of the rod is pulled in-line with the mass and the lower (thicker) sections of the rod start to strain.  But you don’t actually need to do this because it’s common sense right?

For the ‘sloppy rod’ hypothesis to be correct at some point during the loading of the rod the defection will have to go up(or level off as a minimum).  Now ask yourself – how can this actually happen?  Apart from ‘pixies’ (again) the only suggestion forthcoming is ovalisation of the rods cross-sectional geometry that could potentially result in a lowering of the flexural modulus.  

In order to have a quick look at whether rods softened under extreme loading, this week I tested a section of a Cortland #4 weight rod in an Instron mechanical tester.  I need to thank Vince for donating the rod – I assumed it was one that he’d broken on a monster fish or crawling around an overgrown stream, but it turns out it was recovered from a recycling centre where someone was disposing of it – a neighbour thought Vince could do something with the rather nice reel seat and spacer.  Anyway, I cut 20cm from the section above the butt and mounted it in a 3-point bend test jig (note: 3 point bend tests of ‘tubes’ promotes localised deformation i.e. ovalisation).  The machine was set to 10mm/min deflection and the resultant load/deflection response was as follows.

Note, this test shows the movement of the jig from the starting position. I set up the moving crosshead so that the middle point of the 3 was just above the rod, therefore the first ~1/4mm of deflection with no load is simply the crosshead moving downwards before it makes contact with the test piece.

In such a load-deflection plot the flexural moduli of the rod section can be calculated from the gradient of the load/deflection plot i.e. the steeper the gradient, the stiffer the test piece.  In perfectly elastic materials the load/deflection plot will be linear, thus the deflection will be directly proportional to the load.  I think I’m satisfied that, up to 100N, the response of the Cortland rod was perfectly elastic –then I heard an audible ‘crack’.  In reality at this point the rod is broken, there’s been some permanent damage inflicted, however the Instron doesn’t care for rods and it ploughed on.  From this point onwards there was a constant ‘crunching’ noise emanating from the rod as it squeezed flat and folded.  What is quite interesting though is that during this ‘catastrophic failure’ region the modulus is actually higher than the initial, undamaged, rod (as denoted by the gradient line extrapolated from the 125-180N region, compared to the 0-100N line).

So in conclusion we have a one off test that shows a section of a Cortland rod being tested to destruction.  Before failure there is no sign of the rod going sloppy, in fact during failure it actually stiffens.  Now I know this is a one off test but I’m more than happy to add to it if people want to donate rod sections – 20cm will do fine.

So what do I think ‘sloppiness’ is?  I suspect it’s a feeling of a certain type of rod not being as stiff in the lower sections as the caster would like.  When they decide to apply the power, such a rod will still result in too much additional bend for the caster rather than direct drive.  It’s just a rod that doesn’t suit or they simply don’t like, as such no fanciful physics is required.

Have a great week,

James