Recently while making some measurements of sensitivity something occurred to me that I have not seen discussed before that should add some food for thought to the spine and how the guides should be positioned relative to the spine of a blank.
The sensitivity of a rod is a function of the rods stiffness. If all other things are equal the sensitivity will increase or decrease directly with the increase or decrease in the stiffness. In making the measurements that I was making I first measured the power of one of the blanks I was using. The blanks power using the Common Sense system is just the stiffness at a deflection of 1/3 of the blanks length. It turned out that this blank had a power of 175 pennies on the spine or softer axis and 195 pennies at 90 degrees out from the spine or on the stiffest axis.
What is the point? The point is that if you choose to mount your guides on the spine or softest axis you are reducing the sensitivity of the rod by an amount roughly equal to the difference between the spine and the stiffest axis. In the the case of the blank that I was testing mounting the guides on the spine would reduce the sensitivity of the finished rod by roughly 10%.

Would the blank you have happen to have a curve? Could you take this measurement and use it to compare a blank with a curve vs the spine?

I have no doubt that building on the spine reduces the sensitivity to some degree, but I would question the 10% figure. There are so many variables that all factor into the sensitivity equation that I can't believe that building on the spine alone would ever reduce the sensitivity by that much. But I'd have to run it through my head a bit. You just might be right.

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Interesting. I'm surprised you found 20 pennies difference, which is about 10 percent in your example. On all the rods I've tested, I've never found a difference bigger than 2 pennies, and that was for a blank that took about 80 to deflect, which is only 2.5% difference. Based on my experience, I basically stopped locating spine since it seemed to make little or no difference in power. If I found a rod with 10 percent difference, I might give it some more thought. Thanks for sharing your thoughts.

Billy,
No this blank was relatively straight. However, depending upon how you measure spine any curvature in the blank can strongly influence the affect we call spine.

Tom and Steve,
I agree that 10% is a surprisingly large difference. This blank did have a strong spine which is why measuring it occurred to me.
But the important point is that mounting the guides on the spine will reduce sensitivity. The amount that sensitivity is reduced is directly a function of the difference between the softest axis that we call spine and the stiffest axis and those that feel strongly about spine should at least consider this affect.

Tom,
I agree that there are many other variables that affect a blanks sensitivity but when measuring the difference between the softest axis and the stiffest axis on the same blank all of the other variables are constant.

Right. I just wasn't sure if the relationship between softest and stiffest axis led to a purely linear increase or decrease in sensitivity.

Not that I don't believe you, but this thing about rod movement still bothers me a little - the rod doesn't necessarily move when a fish hits or stops the lure. The fisherman's hand prevents movement at that point, although the arrested movement would translate to force. Which is the reason I believe a longer rod is more sensitive than a shorter rod, all else being equal. The fish's movement is harder to arrest on the longer rod, therefore it will move farther and faster on the longer rod simply because it is able to do so.

It will be interesting to see the numbers as things progress.

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Very interesting. Thank you for the effort and info.
Maybe a dumb question, but you measured at the spine and 90 degrees off the spine. What does 180 degrees off the spine yield?

I can help answer that - 180 degrees off the spine won't be the stiffest axis, but in most cases it would be stiffer than the spine will be. So positioning that on the plane used to fight the fish would result in greater sensitivity than if you placed the spine in that same plane or axis.


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So my thought or question is?
If your measurement of lose is taken at the tip were the rod is (just for example) say 5/64” and it equates to 10%
Would it be 10% at the handle or butt were the rods diameter is 7/8”?

Would make sense that the percent of lose would increase the further is had to travel the diameter of the blank at the larger end.

Then the over all lose would be different depending on the taper of a particular blank, could be greater or less.

Also could be more or less depending on, if the splines strength remains true over the length of the entire blank or tapers off or increases as you get closer to either end.
Just some thoughts

I don't align the spine for sensitivity but rather, for lures/line intended. For example, with UL rigs, I want to protect 2-4 lb test but cast lures that may be heavier than UL use. (1/3 oz spoons for example) So, I align the spine so that it is stiffest during casting to handle heavier lures, and softest when fighting fish to flex with UL lines. The differences are small but it's there....:)

UL

Tom,
I think that what we sense with our hand is both force and displacement or movement. But because force is mass times acceleration I am not sure that we can have one without the other. For the follow on article I am measuring force though instead of displacement but mainly because it is easier to measure force.

Emory,

I was hoping that was what you'd do.

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Mo,

Only problem with that is that the stiffest and softest axii aren't opposite each other.

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Ah Tom, in that case I'm trying to get as much of the effect as possible, though not ideal.

Mo

I understand.

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The two stiffest axis are 180 degrees apart and the two softest axis are also 180 degrees apart and the stiffest and softest are 90 degrees apart. The reason that does not usually appear to be the case when checking the spine is that where they cross each other is not in the center of the blank but is offset. In other words the stiffest axis is offset from the center of the blank and the softest axis is also offset from the center of the blank. I hope that I said that clearly.

I've not found that to be the case. The softest and stiffest seem to fall anywhere from about 45 to 170 degrees apart. I've noticed this when hand spining and confirmed it with the CCS ERN measurement. I don't think there is a cut and dried rule as to where they fall in relation to each other - my experience is that it varies from blank to blank.

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Tom,
Yes, I agree that they may appear to be 45 to 170 degrees apart. But think of a cross section of the blank with a stiff plane running through the blanks cross section and a softer plane running through the blanks cross section at 90 degrees to the stiff plane and the two planes crossing in the exact center of the blank. Now think about offsetting where they cross each other so that they do not in the center of the blank or the cross over point is to one side or the other and the result will be the effect that you suggest.

I know bloody well I should stay out of this and just read and learn. But what the heck:

I'm going to interject a couple of variables for you to chew on:

1) I'll posit that a stiffer rod might have an apparent sensitivity greater than a more flexible rod (all else equal). Let's say we have two rods, asame mfgr, same model number, same tip and butt diameters, etc. But one has a very fast action and one has a more moderate action. Let's assume the faster rod is by definition, stiffer along the total of its length. But the faster tip can "feel" more sensitive because it is,indeed, more sensitive at the tip.

If, as many fishermen do, you keep a finger on the line just ahead of the reel, which will "feel" more sensitive? The question isn't as simple as it sounds because the hit will be felt on finger on the line, not through the rod, and that seems to me to depend on the sensitivity of the tip, which is the first interference the line has on it's way to the finger.

2) There's spine, there's soft and stiff, there's warpage along the blank, there's multiple spines and stiff spots along the blank, there's variances in wall thickness between otherwise identical blanks, and that's just on one-piece rods. There's the spine of the butt section /v/ the spine of the tip section of two-piece rods. Placement of the guides can "push" the tip (or not). The Cents system can provide relative and comparable set of measurements between rods but I'll ask, what about a "set" of test results that account for all of the above when we take the measurements of a rod (and maybe some I haven't thought of) instead of a single number?

Uncle Russ
Calico Creek Rods

Emory: That evil word called "spline" again, Eh? Taking you back to personal communications and the link you supplied to the N1/N2 plane effect in tubular composite shafts. My concern was not stiffness or sensitivithy but fiber concentration on the bottom where failure in compression could be expected when more that one modulus is used in the layup. I did something that resulted in some interesting results. Take one of the regular 843 popping rod blanks. Spine it any way you choose - start choping off tip in one inch increments - spine blank.after each cut - record. I wish I had the money and the time to take each one of these blank sections to failure after each trimming. Do the same thing with a composite lay up, Lami LMB and note the difference. Some how all this stuff is related and if you can sort it all out where it makes sense to Dummies should really be a challenge that will keep you up late at night for a long time!

Russ,
You are right there are a number of other variables affecting sensitivity and as you suggest there are a number that you do not have on your list. I think that Tom has plans to do a series of short articles in RodMaker on the different variables because an article explaining them all and the measurements of them just gets too long. But the variables that have the largest effect are the rods stiffness, and mass or weight both of which are determined mainly by the modulus of elasticity and density of the materials in the rod and of coarse the rods design mainly the diameter and wall thickness.

Bill,
I did not want to get into another discussion on spine but in looking into sensitivity I realized that I had never seen anything written on the affect of spine on sensitivity and thought that I would mention it especially since it tends to fly in the face of conventional thought on where guides should be relative to the spine.