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Estimating the Effective Mass of a Fly Rod - What is the rod mass divisor factor?

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Bobinmich
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Post by Bobinmich »

Merlin,

I understand. I have thought a great deal about design. But there are two levels to that. I think what is being discussed now is what can you measure in a blank and know what the rod will be. The next level of detail is how do you design the actual rod section and taper to achieve some desired result. I have played, unsuccessfully, with FEA and I have also tried piecwise linear for the latter.

But keeping the design aspect in mind, you still have to have metrics for the completed design. So you must design towards whatever tool the public will use to make measure of your result. If that measure is ERN and MOI then you must design towards that. If it is something else, it must be something that Joe Blow can use. Right now, he has nothing but the manufacturers recommendation. And if you go into the CCS database (the best database we have at present) and plot ERN vs manufacturer's recommendation, you can see they are all over the map. In short, there are stiff 6 weights and there are whimpy ones.

Despite what you say, the manufacturer's rating is his expert opinion of what works best on the rod he has designed for the purpose he thinks you will use it. If that is not subjective, I don't know what is. I think that was Bill's original intent for CCS. If you choose to develop a better metric, you need to establish a better database than ERN. And at present, MOI isn't accepted so the database is woefully inadequate. If frequency is your preferred choice, it needs to be standardized and run through a test population of anglers the same as you would have to do for MOI. And if you like Mo, you would need to do the same also.

In short, you cannot develop a design procedure without having design metrics that are consumer oriented. So you see, we are joined at the hip in our efforts.

Now the whole HO thing is aimed, amoung other things, at understanding motion and energy flow in a cast. It is complex and not nearly mature enough to be universally useful, but I hope it will get there. Efforts here are nothing short of heroic. Maybe it will help rod design. But the design metric problem remains and will not go away.

Bob
Bill Hanneman
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Post by Bill Hanneman »

Bob,
A plot of ERN vs MOI would tell you almost everything you want to know to compare different rods. Naturally not everything, but just about. Certainly more than the manufacturers subjective rating for you using the rod how he subjectively thinks you should. And it is one hell of a lot less complicated than all this other stuff.

If you want real simplicity, and a visual record, why not consider the BIG Picture?. Anyone can produce it easily. I would also add a measurement of CCF to complete the comparison.

Since the BIG relates power to length and power is a function of diameter and wall thickness or mass, shouldn’t one be able to relate this somehow to MOI without having to actually go to all the trouble of measuring it?

Bill
crunch
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Post by crunch »

With this simple model I can explain (to myself) the difference <---> of line speeds between different ERNs, MOIs, Line Weights, etc.

When two rods on left have their handles aligned and their starting position on right was the same so caster did use similar casting stroke.

Of course in real life a handle of rod with higher ERN would not have reached the same position and the rod handle of lighter MOI and lighter Line Weight would have traveled farther but the basic idea is this:

Image
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Bobinmich
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Post by Bobinmich »

Thinking about this overnight I have to say I am sorry Gordon, I think my comments helped push your thread off topic. I'll start a new thread on design metrics when I get back from grouse hunting next week. Bird season opens in Michigan.

Bob
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Merlin
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Post by Merlin »

Bob

You said it right, I agree with you.

ERN is a stiffness characteristic that Joe can use, but as an engineer, I need N/m scales, which I cannot find in the Rosetta Stone.

Same for the BIG picture which looks like Don Phillips measurements of stiffness along a rod shaft.

I look forward for your new thread.

Merlin
Fly rods are like women, they wont´play if they're maltreated.
Charles Ritz, A Flyfisher's Life
gordonjudd
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Post by gordonjudd »

I shall be able sending you the data for a Garrison rod during this week end I think.

Merlin,
Thank you for sending me your estimate for the mass of discrete sections along one of Garrison's bamboo rods.

It was much different than I expected since I would have thought the mass density of bamboo would be much higher than graphite, and its Young's modulus would be much lower than graphite. As a result I expected the mass density of the bamboo rod would have been much larger than what shows up in your data assuming I computed the linear mass density correctly (and that is always a big if).

Here is the linear mass density I computed from your data.
Image

You can see it was only .052 kg/m at the butt and went down to around .003 kg/m at the tip.

Using the one size fits all deflection curve for the first mode that was shown earlier I get this relative K.E. values to come up with the divisor for the effective mass term.
Image

You can see that the expected divisor was 14.2 for this rod design. That value is much larger than I was expecting for what I expected in a full flex rod. Maybe Garrison had the wherewithal to produce tip flex rods with his bamboo designs.

It would be interesting to measure one to see, but I would expect his rods are displayed on a collector's wall somewhere, and rarely if ever fished.

Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
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Merlin
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Post by Merlin »

Gordy

The result will change for another deflection profle. If we change for a static one, which may be better than the first approximation I sent you, then the figure comes closer to the expected one from design: we get something like 18.2 instead of 17.3. So the methodology is good.
Ay the end of the day, the deflection / action parameter could well be the dimensionless ratio of the effective mass (mo) and the mass of the blank. So simple again.

Merlin
Fly rods are like women, they wont´play if they're maltreated.
Charles Ritz, A Flyfisher's Life
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