For those who are interested in the resonance frequency of a rod,
I think an easy way to estimate it is to wiggle it.

Keep the same strength in wiggling but vary the periodicity from
slow to fast. Try to hold the grip as stiff as possible, and make sure
the rod is not deflecting with a "node" -- this is the next harmonic.
At certain speed of wiggling, the rod tip will go through the
largest deflection. That's the resonance frequency.
It feels like that's the wiggling cycling to break the rod, and you may
really break it!

Start counting, and timing. Do it over, say a minute, like measuring
your pulse.

Resonance frequency = No. of deflections (at max amp)/time interval.

Am I talking nonsense?

-Sing

No it's not nonsense cause it's what we've all been doing for the past 100 years. Shaking rod blanks is the old fashioned way and we've all done it to compare or get a feel for different blanks. It's not new, just an old idea come around again.

My guess is that if you have the blanks in hand, you won't need to do any counting. You can handle each one and find out about what you need to know by how they feel.

Now if you don't have the blanks in hand and could instead read a chart or graph of the resonant frequency and knew how to interpret it, then you would have access to pertinent information.

Frankly I don't see the blank makers adopting any such thing. It would be prudent of them to make a move in adopting the Common Cents System at the very minimum. I've not seen them do that but hold out hope when I get the 2004 catalogs. Maybe if they go that far they'll add resonant frequency the following year.But I'm not holding my breath for either.

How will knowing the resonate frequency of a rod blank help you? Nothing you will do while fishing could produce a wavefront as long as the resonate frequency of a blank. The resonate frequency of a rod blank will never come into play. The average must be 10 hz. Humans can't hear below 35 hz or 30hz. The resonate frequency of a blank won't tell you any thing about the action. You might could tell a higher modulas graphite from a lower one. But, there are better ways to get that info. in a more accurate form. Am I missing something? I'm not trying to challenge anybody's opinion. I really would like to know how this will help me build a rod.

Bill Doherty

Actually it can. By understanding how to read the graph left behind you can get a decent idea of where the material in the blank is distributed, how fast the action is and how stiff the blank is. It would make an excellent means for comparing blanks against each other.

The only drawback is what it would take to graph all the rods and blanks. Then you'd have to learn to interpret the graph or data. It's not as simple as just looking at a scale of relative numbers.

I think between the Common Cents System AA and ERN, and the length and weight already given, we can tell most of what we as rod builders really need to know about a blank. But we're still a ways off in getting the manufacturers to adopt the Common Cents System. First things first, and then move on to the next step.

Aurthur:

I'm not trying to start an argument. I still don't see how you can detect the action? The resonate frquency of anything is one number that is an average of the whole vibrating mass. The whole surface of a violin vibrates at different rates. Yet every violin has ONE resonate frquency. If I tell you one violin has a resonate frequency of 400 hz and another has a resonate frequency of 415 hz, can you tell me who made each? No way. Every blank manufactured will have it's own resonate frequency, so will every single violin. You can have two blanks made by different people, two different lengths, two different actions, that do have the same resonate frequency. How can that one number tell you anything about the action of either. Please don't take my comments as an attack of your intelligence. I find this topic very interesting.

Bill Doherty

Bill,
Everything about the rod not just modulus of elasticity will show up in the resonant frequency including the weight, action, power, length etc.

Sing,
I am not sure that I understand what you are saying but if you are suggesting that the rate that one shakes a rod should be increased from slow to fast I don't agree. What will happen as you increase the rate that you shake or vibrate the rod is it will go from the resonant frequency or first harmonic to multiples of this or the second or even the third harmonic.
You are correct that the rod will deflect the maximum amount at it's resonant frequency though. But once it starts resonating at multiples of this the tip amplitude will drop significantly.
As far as counting the number of cycles is concerned: you are right this can be done with long, slow action, rods like many fly rods, however with most other rods the vibrations will unfortunately be too fast to count by eye.
There are a number of approaches that can be taken to get around this problem including:
1. Video taping the vibrating rod while simultaneously video taping a stop watch and then playing the tape back in slow motion so that you can now count the number of cycles and also see the elapsed time.
2. Add several different weights to the tip of the rod, which will slow down the resonance and make it possible to count the cycles by eye, then plotting these frequencies versis weight. Put weight on the vertical axis and frequency on the horizitonal axis and extrapolate the curve you get back to zero on the vertical axis which will be the resonant frequency.
3. Use either an oscilloscope or frequency counter and photo detector.
Sorry for the long winded response, sometimes I get carried away.

Emory:

I'm missing something. What if I tell you I just measured the resonate frequency of a blank, and it was 12 hertz. Tell me about it's action, weight, power and anything else you can.

Bill Doherty

Bill,
I am not sure that your analogy with a violin is appropriate. A violin has many different resonances. The volume of the air in the violin, the mechanical resonance of the basic structure of the violin, and then of course the resonance of each string as the violin is being played. Then other strings resonante in sympathy with the string that the bow is causing to vibrate.
A note played on a violin has a fundamental and many harmonics of varying amplitude. A rod that is excited properly will have a single frequency of resonance.

Emory:

Yes. that is my point. If you take a picture of a vibrating string or in our case a rod blank, you are looking at the harmonic vibrations between nodes. You don't actually see the resonate frequency, unless you can photograph the wavefront propagating off of the surface, into the air. As a matter of physics, I know two different bodys of disimilar mass can have the same resonate frequency. One might be a 11 ft surf rod, the other a 8ft 5 wt fly rod.

Bill Doherty

Bill,
OK, I'll take your challenge. 12 Hertz is very fast. It tells me that the blank is probably short, probably fast action, probably medium to high modulus, and probably a fairly powerful rod. In fact, I will bet you a swift kick in the backside that it is not a long, medium action, low modulus, light action blank.
But, what the resonant frequency is most useful for is comparing blanks, particularly simular blanks. I will argue that what action Angle and Power in the CC System are most useful for is comparing simular blanks. The absolute numbers are not vary useful by themselves and the same thing is true of resonant frequency.

Emory:

Your a good man. I've read your posts here for a long time. You know your rodbuilding. But... I still can't believe the resonate of a blank will help you in any way. I don't have the knowledge to prove or disprove your analysis of a 12 hz rod blank, so we'll put that butt kickin' on hold, if thats OK with you. Thanks for putting up with my questions.

Bill Doherty

Bill,
You are correct that two completely different mechanical structures can have the same resonant frequency. However, you are not correct that we can not measure the fundamental or first harmonic. It depends upon how fast we put energy into the rod and I will agree that if we shake it very rapidly it will try to vibrate at a higher harmonic. But the rod will try, like most mechanical structures, to vibrate at it's fundamental or first harmonic, if the energy that is used to stimulate it (your arm shaking the rod) is even close to this fundamental.

Bill -

Correct - you can have the same natural frequency for an 11' rod and 8 ft rod.

So if you have 2 rods of similar IP (aka. common cents intrinsic power or stiffness) and length and same freq they will perform similarly.

If you have 2 rods of the similar IP and length and a freq of 3 htz and one at 2.5htz the one that is of higher freq will most likely perform better for you (given the same casting stoke).

The work I am doing is trying to make this meaningful across different IP (stiffness). Hopefully I should have information soon.

Christian

Emory:

Yes, everything in a higher state will try to settle into a lower state. In this case the lower state being the resonant frequency. But again, this is represented by one number. This one number will not help us because different blanks could have the same number. Example: What if the bend of a fast action rod falls on a node of a slow action rod. Depending on the mass of the graphite in each, they could have the same resonant frequency. Does that make sense? I am talking at the limits of my physics understanding.

Bill Doherty

Christian:

Your post came up while I was trying to write my last one. What you are using the resonant frequency for, is to compare rods of like performance, or of known specs. I buy that. If two blanks of the same length and similar action resonate differently, you could deduce some things from that. In your example you are comparing TWO numbers. That I understand. Knowing the resonant frequency of ONE blank tells me nothing.

Bill Doherty

Bill,
Yes, you are absolutely correct. Two rods could have the same resonant frequency, for example a faster action rod that was a little longer could have the same resonant frequency as a slower action rod that was a little shorter, or a slower action rod could have the same frequency as a faster action rod that did not have as much power.
I think that everything shows up in resonant frequency and it is therefore the most useful overall indicator of a rods potential performance. However, it can be difficult to measure and it most useful as a relative indication of a rods potential performance or for comparing two rods that are simular. Just like power or action angle, resonant frequency is most useful for comparing one rod or blank to another.

I think that the concept of a resonant frequency for a particular blank is similar to that of a spine. The effective spine is the combination of a number of individual effects created during the manufacture of a blank. The location of the effective spine for a particular blank gives you some useful information about that blank. The problem with resonant frequency is that it depends on the complex detailed makeup of a blank - the length, material, taper, crossection, and thickness. It also depends on the end conditions - that is, how you shake it. I think that very different kinds of rods could have similar resonant frequencies. For example a short fiberglass rod and a longer graphite rod. I'm sure that the measurements described above give some information about the feel of the rod, but I don't think that anybody really knows how to interpret it.

During previous discussions on this topic, I have tried to stress that for any such thing to be useful, it must be converted or translated into some type of simple relative scale.

With a known length, weight, power rating (ERN) and action (AA), we have most of the pieces of the puzzle. However, we're still up against what I like to call efficiency or what others may describe as feel. We have no relative scale to measure that - you have to get ahold of the blank or blanks in question and use your human sense of touch to gain this last tidbit of information. It might be helpful if you didn't have to do that.

Often we see blanks of the same action, power and length, but offered in different fibers. If you were to look in an old Loomis catalog and pick out, say, an MB843, and then notice that it's offered in GL2, GL3 and IMX series. Each is made from a different modulus graphite (and perhaps resins, scrims, etc.). An educated guess would tell you that the highest modulus model, the IMX, should be more efficient and feel more "crisp" than the former two. It should recover faster as well. But you're still guessing and because the designs may not be identical, you may even guess wrong or at the least, not get exactly what you were expecting.

It's in this area that a resonant frequency scale would be most helpful. It could complete the final piece of the puzzle, but again, it would have to be reduced or translated in some way to a simple relative scale in order to be widely understood and accepted. And of course, you'd have to get the manufacturers to adopt it. That would be hardest part.

.......................

Tom,
I think that you are correct about the need for simplification and also about resonance being used as an indication of "feel". But I think that it could be useful for much more than as an indication of feel. All of the measurements that we are presently using are very useful but they are static measurements that do not tell us enough about the dynamic characteristics of a rod/blank. Our use of a fishing rod, when casting or landing a fish, is dynamic not static, the rod is in motion.