Test Curves and Vectors; Any Mathematicians Out There?

robint

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Aug 6, 2020
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This is probably not the best place for my first post, but it's bugged me for a long, long time so here goes! Has anyone out there successfully managed to measure the test curve of a rod? It's almost mandatory to state a value for course rods and I can remember from long ago reading that (for cane rods, anyhow, which may provide an inkling of my antiquity) the optimum line b/s is five times the test curve and a rod casts one ounce for every pound test curve. The method that's always given is to fix the rod handle to a horizontal surface and add weight to a line attached to the tip, or run through the rings, until the rod bends through 90° - I.e. the tip points vertically down. However, there is, to me, a logical flaw in this: to bend the rod, a force must be applied such that a vector of the force acts perpendicularly to it. However, as the tip approaches the vertical, the vector acting perpendicular to it approaches zero and it follows that the weight required would be infinite to achieve a true 90° bend. That would result in a good many broken rods, not to mention backs, were anyone to try.
So: my question is, what is the correct method - or is a nonsense that originated back in the mists of time and has just been perpetuated since?
 

kingf000

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Jun 13, 2016
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This is probably not the best place for my first post, but it's bugged me for a long, long time so here goes! Has anyone out there successfully managed to measure the test curve of a rod? It's almost mandatory to state a value for course rods and I can remember from long ago reading that (for cane rods, anyhow, which may provide an inkling of my antiquity) the optimum line b/s is five times the test curve and a rod casts one ounce for every pound test curve. The method that's always given is to fix the rod handle to a horizontal surface and add weight to a line attached to the tip, or run through the rings, until the rod bends through 90° - I.e. the tip points vertically down. However, there is, to me, a logical flaw in this: to bend the rod, a force must be applied such that a vector of the force acts perpendicularly to it. However, as the tip approaches the vertical, the vector acting perpendicular to it approaches zero and it follows that the weight required would be infinite to achieve a true 90° bend. That would result in a good many broken rods, not to mention backs, were anyone to try.
So: my question is, what is the correct method - or is a nonsense that originated back in the mists of time and has just been perpetuated since?
I've not seen test curve data for fly rods and https://www.harrisonrods.co.uk/Testcurve.htm describes the problem you have. For a fly rod it probably isn't relevant and we use the #wt instead. In simple terms the bigger the #wt value of the rod, the bigger the test curve would be. What may be more important is the deflection curve, as that is one description if the action of the rod. For that they hold the rod at a 45o angle from horizontal, suspend a weight and see how much the rod bends and how far down the rod the bend goes (for example, see https://www.yellowstoneangler.com/gear-review/2020-five-weight-shootout/).
 

three rivers

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Apr 13, 2017
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Your assessment is entirely correct; it is impossible to produce a true 90 degree angle between rod tip and butt when pulling on the tip perpendicular to the butt. The closer you get to 90 degrees, the less effect a given additional pull makes to the angle, so it is quite easy to over-rate the test curve. Moreover, the actual power rating of a rod is dependant on action as well as test curve. A through-action rod will, when pulled by a tensioned line perpendicular to the butt, approach 90 degrees with most of the rod flexed; a tippier rod will reach somewhere near 90 degrees long before the lower part of the rod reaches maximum flexion. Drennan realised this when they introduced their legendary Tench Float rod, which was rated at a 12oz test curve and a 2lb power curve.

However, for all practical purposes it is possible to pull the tip close to 90 degrees and read the pull on a spring balance. It will give a rough and ready indication, subject to the caveats above.
 

Banksie

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Just a suggestion. Maybe, just go to the park with a variety of weights, (either shot, or Arlsey bombs), and cast until it feels right for you and the rod. That way, it’s a ’real life’ result.

Been meaning to do this myself on a 12oz test light ledger rod, as it’s a lot easier and quicker than faffing with vices, Workmates, etc. Gonna use a snaplink and bombs.
edit: and welcome to the forum. 😎
 

kingf000

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This is probably not the best place for my first post, but it's bugged me for a long, long time so here goes! Has anyone out there successfully managed to measure the test curve of a rod? It's almost mandatory to state a value for course rods and I can remember from long ago reading that (for cane rods, anyhow, which may provide an inkling of my antiquity) the optimum line b/s is five times the test curve and a rod casts one ounce for every pound test curve. The method that's always given is to fix the rod handle to a horizontal surface and add weight to a line attached to the tip, or run through the rings, until the rod bends through 90° - I.e. the tip points vertically down. However, there is, to me, a logical flaw in this: to bend the rod, a force must be applied such that a vector of the force acts perpendicularly to it. However, as the tip approaches the vertical, the vector acting perpendicular to it approaches zero and it follows that the weight required would be infinite to achieve a true 90° bend. That would result in a good many broken rods, not to mention backs, were anyone to try.
So: my question is, what is the correct method - or is a nonsense that originated back in the mists of time and has just been perpetuated since?
A much better assessment would be to fix the rod at a 45o angle above horizontal, as it is done for the deflection curve of the fly rod, then add weights on a vertically, until the tip is 45o below horizontal. This could be marked out on a deflection chart behind the rod. That would have a much smaller loss in the vector, as at the 90o flex, there would still be a 45o angle.
 

Banksie

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I can imagine the scenario where there’s a slip or a break, the tip shatters on the ceiling while the spring balance flies straight through a window killing a neighbours cat or something. 😳
Seriously, even just getting a rod at a suitable height would be problematic. A Workmate on a table, for instance? Or carting everything outside? Bugger that. 😎
 

kingf000

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With respect to fly rods, why does it matter in the first instance? :unsure:
I agree, the # value of the rod tells me what I want to know from that point of view. Even those rod reviews that measure and quote deflection curves don't know what they mean!
 

tangled

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As with most things in fly fishing, it's better to have a near-subjective, mystical approach to equipment that are essentially engineering components, than produce anything that could provide a useful and objective comparison. If you haven't already seen it, you might find this interesting:

 

three rivers

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Apr 13, 2017
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I agree, the # value of the rod tells me what I want to know from that point of view. Even those rod reviews that measure and quote deflection curves don't know what they mean!
It's worse than that. Many companies produce carp rods of increasingly high stated test curve, but some are deliberately over-rated to sell; most carp anglers want a high test curve rod (coz that means it casts further and catches bigger fish right?) but they don't actually like a stiff broom handle, so the rod that says it's 3lb tc but feels like 2lb tc sells. The fact it's probably nearer a 2lb tc anyway is ignored. Besides which, the stiffer a rod is, the less pressure you can apply to a hooked fish for any given amount of effort, and stiff rods actually work in favour of the fish by applying more leverage to the angler's arm. Most fly anglers know how much pressure you can apply to a fish going away from you with a typical nine foot fly rod, which is much softer than even the lightest carp rod. The problems arise only when the fish is diving towards you or towards marginal snags, where having some stiffness in the lower part of the rod is beneficial.

The upshot of it all is that a test curve rating was valid when rods first began to be designed on engineering lines instead of by empirical development, more to indicate their suited line strengths and casting weights as their ability to stop a big fish. Nowadays, with compound taper carbon fibre blanks, rods can be made in a variety of actions and it is quite common to combine a softish, progressive top with a more resilient, powerful butt to produce a rod that is far more versatile. Modern match rods can land big carp or barbel quite easily if paired with a strong enough line and hook; the reason they are not used routinely for this is because the soft tip prevents effective setting of big hooks, and limits casting weight.
 

Ged

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Jun 7, 2008
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They also used to under rate carp rods too. The Daiwa Amorphous 2.75lb felt alot more powerful than say a harrison 2.75, the Daiwa would probably have easily cast 3.5oz.
I'm not sure that test curves are measured with a straight pull at 90 degrees to the butt. watch this video of a rod on test
amazing what a blank can take.
 
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