Rod blank diameter

[PBBrandon]

My 7’3” HF Daiwa Rebellion came in and the first thing I noticed is how thick the blank is. Thicker than any of my other rods. Does this affect anything? It still feels nice in my hand and is light but does that make it any less sensitive? I plan on using the rod for jigs and sensitivity is important lol

[MickD]

Butt diameter usually signifies an extra fast action with a powerful butt which does not, in my opinion,  adversely affect sensitivity.  In fact if sensitivity is proportional to True Natural Frequency, the chances are that rods designed this way will be more sensitive rather than less sensitive.  From my testing, extra fast action blanks tend to have a higher true natural frequency.

 

To really know what's going on with one's blanks/rods performance-wise it's a good idea to learn to measure CCS numbers, then one has the power and action objective values, and if one adds TNF, then another significant objective attribute is added.  With length, weight, CCS power, CCS action, and TNF one has most of what it takes to accurately describe a blank/rod and to predict how it will perform.

[bulldog1935]

It goes back as long as there have been rods, both wood and bamboo, to mixed linear and helical graphite. 

Wood and bamboo are both natural linear-fiber composites. 

Stiffness goes up with diameter, so to have a rod taper, the diameter changes from smaller at the top to larger at the bottom (I'll keep the math out of it - for now). 

 

In 1881, Doc Henshall called it "giving him the butt" when you fight a fish with the rod low.

Vintage cane fly rods typically have a flared butt, that kills the taper for casting any deeper into the rod (no farther, no more fly line), and the purpose of the butt flare is fish-turning power. 

 

You definitely won't lose sensitivity in the rod - if anything, you'll gain it compared to a deep-flexing rod, because that part of the rod won't bend working the lure, but will transmit vibration into your hand. 

Because of the amount of material used in the rod, it will be slightly heavier than a more expensive rod that uses helical cloth layers to get the same stiffness in a smaller diameter. 

[PBBrandon]

Thanks y’all ? I suppose the fact that despite the thickness it is still really light, means there are no downsides to it being this way. If anything it helps with giving it a strong backbone and equal if not slightly better sensitivity than if it were thinner. Perfect for jigging in some cover like I plan on doing with it. Thanks guys I appreciate the help 

[LCG]

I have the 6'10" MH-R rebellion rod and I think it is very sensitive. Caught a few using a weightless senko and had no trouble feeling the bite. They are nice rods. 

[MickD]

4 hours ago, PBBrandon said:

despite the thickness it is still really light,

How light is it?  Small, accurate, digital scales on Amazon can be had for less than $15.00.  I'm curious.

[LCG]

34 minutes ago, MickD said:

How light is it?  Small, accurate, digital scales on Amazon can be had for less than $15.00.  I'm curious.

Not sure on the 7'3, but the 6'10" MH is 3.9oz.

[MickD]

Thanks, sounds about right, would really like to know its CCS power number, ERN.  Some may think I'm being picky, but I've had rods of ERN 19.8 called both MH and ML.  And a 25 called M.   Easy to see the problem when discussing what is "light" when the power descriptions vary this much.

[Delaware Valley Tackle]

There are two basic philosophies in blank manufacturing. Thin wall / large diameter and thick wall / small diameter. Both work equally well just two ways to skin a cat. 

[bulldog1935]

The math doesn't work that way. 

Thicker wall reduces stiffness (a is smaller below).  Thinner wall Always increases geometric stiffness, but the wall has to be thick enough not to collapse or break under load. 

Smaller diameter blanks get equivalent stiffness and strength from Specific Modulus increase, which allows them to work with lower geometric MOI (and lighter weight).  

[Tennessee Boy]

@bulldog1935 It's been a long time since I studied this but I think when you calculate stiffness you use the area moment of inertia which would increase when inner diameter is smaller.  All things being equal,  when area moment of inertia increases,  so does stiffness.

 

Here's an article which explains the concept

 

https://www.fictiv.com/articles/how-to-design-for-stiffness-using-a-geometric-approach

[WRB]

 Trying to discuss hoop to yield math formulas to members who consider a diameter thickness is a difficult task.

Nice illustration?

Tom

[Tennessee Boy]

50 minutes ago, WRB said:

 Trying to discuss hoop to yield math formulas to members who consider a diameter thickness is a difficult task.

Nice illustration?

Tom

[GReb]

[QED]

1 minute ago, GReb said:

 

Sacrilege! If you can't realize optimized mathematical solutions in the real world, then that's just an engineering problem.  ?

[Tennessee Boy]

5 minutes ago, QED said:

 

Sacrilege! If you can't realize optimized mathematical solutions in the real world, then that's just an engineering problem.  ?

You're a mathematician,  does a thinner wall increase stiffness or decrease stiffness in the blank? 

[MickD]

Let's take the thinner wall towards zero and see what the answer is.   As the wall approaches zero, what happens to the stiffness?  

 

 

[QED]

1 hour ago, Tennessee Boy said:

You're a mathematician,  does a thinner wall increase stiffness or decrease stiffness in the blank? 

 

Assuming equivalent materials (especially mass per unit volume) and external diameters, then an increase in wall thickness should lead to increased stiffness for the latter case.  One problematic variable is the mass calculation in some of the cited equations - assuming equivalent mass, then a solid blank would have a smaller diameter than a hollow blank so that screws up some of the cited anaylysis.  But again, I'm a THEORETICAL MATHEMATICIAN so despite the fact that I passed the EIT exam, I'm not an engineer and I don't play one on TV.

 

Cf. https://extrudesign.com/why-hollow-shaft-is-better-than-a-solid-shaft/#:~:text=is 0.9375%3A 1-,In simple words%2C the hollow shaft is almost has a,made of the same material.

Comparison of stiffness of Hollow shaft with solid shaft of same diameter

We know that stiffness relation from the torsion equation

The Stiffness of a hollow shaft will be

S_H = G/L × (π/32) [ (d_o)⁴– (d_i)⁴]

The stiffness of a solid shaft

S_S = G/L × (π/32) × d⁴

Since we are comparing the stiffness of the same diameter of the hollow shaft and solid shaft, and also we assumed that both shafts were made of the same material and have the same length.

Therefore let us divide the stiffness of the hollow shaft with the solid shaft stiffness and put d_o = d, we get

= 1 – k⁴

= 1 – (0.5)⁴

= 0.9375

[bulldog1935]

No, yield and stiffness are two different properties.  Yield is a strength measurement and denotes Permanent change in shape. 

 

@MickD, the surfaces have a stiffness effect.  As the wall approaches minimum, see Thin Wall Hoop in the formulas I posted.  Thin Wall Hoop has twice the geometric stiffness factor of the solid - intermediate wall thickness fits progressively in between - the closer to solid, the less stiff, the closer to thin wall hoop, the greater the stiffness. . They do not represent strength, but the geometric effect on stiffness. 

If you can't see the math of zero mass in the equation, further discussion is without purpose.  When the rod designer gives up geometric stiffness by increasing wall thickness, it has to be made up in other ways, such as increasing specific modulus. 

 

Nor are we discussing rotating steel shafts here.  The math of bulk rod modulus (=resistance to bending, or stiffness), the change of which defines the taper along the rod length, isn't guessing, it's known. 

Anyone who can't see the thickness is the difference in radius a and b is really lost. 

Speaking of permanent, this is where I bow out, because everything that needs to be discussed here has been discussed. 

The next thing that will happen is more pointless memes. 

 

Since we're talking about fishing rods here, I'll add this.  In practice, most rods with greater thickness get there using more resin, not more high-grade graphite.  Anyone who has messed with a solid-tip rod should immediately recognize the bulk modulus difference from a tubular rod.  Anyone who has ever handled Airex, Shakespeare and Fisher fiberglass rods will also understand this completely.  They all use the same modulus fiberglass.  Airex is solid with a lot of resin, and totally flimsy.  Thick-wall Shakespeare is heavy as the dickens and feels totally without backbone.  Thin-wall Fisher is light in hand, fast and crisp.  The trick is building the rod strong enough to get away with the thin wall. 

 

A lot of words have been used on this thread, most of which don't apply to the subject matter. 

[Tennessee Boy]

1 hour ago, MickD said:

Let's take the thinner wall towards zero and see what the answer is.   As the wall approaches zero, what happens to the stiffness?  

 

 

We may all be trying to say the same thing but we have probably confused a lot of people in the process.  As @QED has shown a solid shaft would be stiffer than a hollow shaft if they are the same diameter and made of the same material.   A rod with a thick wall will always be stiffer than one with a thin wall if they are the same diameter and made from the same material.  The thick walled one will weigh more.   The thin walled one will be more efficient in providing stiffness for the amount of material used.  There is a reason rod blanks are hollow.

[Chris Catignani]

[Deleted account]

11 hours ago, bulldog1935 said:

Thicker wall reduces stiffness

No.

[WRB]

 Very complex to calculate tubular flexibility when the tube is changing diameters tapering larger to smaller and made up of layers of materials and scrim or binding resins. It’s a lot easier to evaluate the finished product.

Tom

[MickD]

I still maintain that a cylinder with a wall thickness of zero will not be as stiff as one with a wall thickness that is not zero.  And this makes the trend obvious.

 

But, why do we care?  We care because we are curious.  Curiosity is good.

 

We also care because we are striving for the highest stiffness to weight ratio where it counts.  For many years experts have maintained that the sensitivity of a rod is proportional to its true natural frequency, which is proportional to the stiffness to weight ratio and is affected by many design parameters.  The problem was that expensive , sophisticated equipment was required to measure the true natural frequency.  (TNF is not the CCF that Hanneman developed in his CCS work.  CCF is not TNF but a frequency derived by adding weight to the tip of the blank to get its resonant frequency into a range that could be measured with a stop watch.)

 

As has been pointed out, it's easier to evaluate the finished product.  Because of the capability of the inexpensive technology that we now have it is possible to quickly and easily measure the true natural frequency of blanks and rods.  The effect of adding guides and wraps to a blank can be measured, and the differences between titanium and SS guides and guide sizes (weights)  quantified.  Differences between high and low modulus blanks can be checked.  

 

If anyone wants to know how to do this, send me a message with your email address and I'll send you a pdf file with instructions.

 

 

[Tennessee Boy]

2 hours ago, MickD said:

But, why do we care?  We care because we are curious.  Curiosity is good.

Very well said.  Curiosity makes life worth living.  It's the reason I fish.

 

9 hours ago, WRB said:

 Very complex to calculate tubular flexibility when the tube is changing diameters tapering larger to smaller and made up of layers of materials and scrim or binding resins.

It's darn near impossible but we're not trying to do that.   The issue at hand is how does the geometry of the rod (the diameter of the rod compared to the diameter of the hole that goes through the rod) affect the stiffness of the rod.  Calculating the actual stiffness of a rod is very complicated.  Calculating the role that geometry plays is not.   

 

We got here when someone said "Thinner wall Always increases geometric stiffness" and posted a formula to prove it.  That seems to go against conventional wisdom and doesn't match with what one observes when walking down the plumbing isle at Home Depot.  The problem with the posted formula is that it is the formula for mass moment of inertia.  It includes mass in the calculation.  There is no mass in geometry.  

 

If you use area moment of inertia then you can calculate the roll that rod geometry plays in the stiffness of the rod.  Put in the diameter of the rod OD and the diameter of the hole in the middle of the rod ID into this formula for area moment of inertia and you can calculate the geometric effect on the stiffness of the rod.

 

It doesn't tell us how stiff an actual rod is.  That depends on many factors.  It does allow us to compare geometric effect of a thick wall to that of a thin wall.

 

Outer diameter of 5 and inner diameter of 3 (thick wall) = 26.7

Outer diameter of 5 and inner diameter of 4 (thin wall) = 18.1

 

The rod with the thicker wall will be stiffer.  Just like we all thought before this thread started.

 

I'm not an expert on rod blank design.  Someone correct me if I made any mistakes in my calculations or if you disagree.  It won't hurt may feelings.  I welcome the opportunity to learn more about this kind of stuff because I'm curious.