October 5, 2018

Swing Weight

If you have been a long time reader of Teton Tenkara you will recall that I like taking various physical measurements of rods. I don't take advertised specifications of rods at face value, being more inclined to take the measurements myself. One measurement that I have talked about in the past is rotational moment. Rotational moment is a simple estimation of the tip heaviness of a rod. As I have stated in other articles, this measurement is used by Gamakatsu to measure their rods.  Here are some measurements of rotational moment for specific 380-400 cm tenkara rods:

Gamakatsu Ryokei 390 (seiryu) _____3.8
Oni type I ______________________ 5.2
Nissin Air Stage Honryu 380 _______ 5.3
Shimotsuke Ten _________________ 5.4
TenkaraBum 40 _________________ 5.5
Tanuki XL-1 ____________________ 5.6
Nissin Zerosum Oni Honryu 395 ____ 6
Gamakatsu Multiflex Suimu 4.0 _____6.5
Allfishingbuy Hirame-ML-3909 _____ 6.8
Nissin Royal Stage 400 7:3 ________6.9
Nissin Royal Stage 400 7:3 ________ 7
Discover Tenkara Karasu 400 ______ 7
Daiwa LT39SF _________________ 7.1
DRAGONtail Hydra ZX (@390) ____ 7.5
TUSA Ito (@390 cm) ____________ 7.8
TUSA Ayu II ___________________ 8.5
TUSA Amago __________________ 10



Another way to think of tip heaviness is what is called the swing weight. Swing weight is commonly used in the golf and tennis world to evaluate clubs and rackets. Here is how one person defines swing weight: "Swingweight is a balance measurement and is the degree to which the club balances toward the clubhead." If Club A has a balance point closer to the clubhead than Club B, then Club A will feel heavier in the swing (regardless of how many total grams Club A and Club B actually weigh)."

So how is swing weight measured, at least for golf clubs? Swing weight is measured using a specially designed swing weight scale invented by clubmaker Robert Adams in the early 1930's, which contains a fulcrum and a sliding weight. The golf club is placed on top of the scale and the sliding counter weight adjusted until the club is in balance. The position of this sliding weight against the swing weight scale indicates the swingweight of the club (ref).

(reference)


So what does swing weight have to do with tenkara rods? A lot! If the weight is towards the tip then the rod will feel tip heavy.Duh! Rotational moment is easier to measure and is reproducible, but swing weight measurements (as described below) can vary depending on where you choose the fulcrum. Because of this, rotational moment and swing weight values don't always match up, but they are pretty close (taking into account my error in measurement).

from here


I've measured rotational moments for a lot of tenkara rods, but I wanted to measure swing weights as well. I found that George Anderson of Yellowstone Angler has measured swing weight for 5wt fly rods. Here is how he measured swing weight: "Swing weight refers to the weight you feel out ahead of your hand when the rod is held in a horizontal position.  Measuring swing weight is easy to do yourself with a little practice. First, use a good digital postage scale (we now use a Brecknell scale, with a max of 11 lbs in .1 oz. increments).  Zero out the scale.  Then place a foam-packing pellet that will act as the fulcrum, on the center of the scale.  The reel seat rings and band should be placed at the bottom of the reel seat.  Position the grip of the rod precisely on the pellet while the rod is held in a horizontal position with fingernail pressure.   Position the grip over the foam pellet so that the pellet is about half way up the grip – but more importantly over the center of the swell, where your hand will rest when you grip the rod.  For most rods this is close to the center of the grip, and at the center of the swell. Use pressure from your index finger to hold down the butt cap of the rod and keep it horizontal.   Then when you have the rod parallel to the floor, read the pressure of the weight in oz. and you have the swing weight."  I thought this method was as good as any, so I wanted to try it on tenkara rods. I modified Anderson's method slightly by canceling out the weight of the rod first, before measuring the amount of weight to bring the rod up to level. That way I just measured the swing weight and not the rod weight as well. Rod weight measurements are easy to find, anyway.

I'll reiterate a statement by Anderson as well: "Yes, for all you engineers out there, this may not seem precisely accurate, but as a practical way to measure swing weight it works fine.  The most important thing here is not the actual figures, but how the rods compare with each other."

As I measured the rods that I had on hand, I noticed something interesting. That is, the swing weight for tenkara rods is, on average, higher than that of 9 foot 5 weight western rods. I think this should be intuitive because tenkara rods are generally a lot longer than 9 feet. After all, 9 feet is only 274 cm. And we all know that a 274 cm tenkara rod is a short rod!

For the fulcrum, I chose the point where my thumb was on the handle when I grasp the handle in my usual grip (be aware that you may normally hold the handle at a different location than I do).



Here are my measurements (tenkara, seiryu and keiryu rods):

350-360 cm rods:

Gamakatsu Ryokei 360__________260 g
TenkaraBum 36________________299
TenkaraBum Traveler (@360)_____299
Nissin Air Stage Fujiryu 360 5:5___300
DRAGONtail Hydra zx (@360)____302
Discover Tenkara Karasu 360_____307
Nissin Zerosum 360 7:3__________309
Gamakatsu Multiflex (@350)______330
Tenryu TF39TA (@360)__________369
Sakura Seki Rei________________409
Tenkara USA Ebisu_____________475


390-405 cm rods:

Oni Type-I___________________263 g
Gamakatsu Ryokei 390_________278
TenkaraBum 40_______________306
Nissin Oni Honryu 395__________317
Daiwa Sagiri MC39 (@390)______323
Tanuki XL-1 (405 cm)___________326
Discover Tenkara Karasu 400____346
Shimano Mainstream ZE (@400)__390
Gamakatsu Multiflex (@400)_____402
TenkaraBum Traveler (@390)____423
Tenryu TF39TA (@390)_________436
DRAGONtail Hydra 390 (@390)__440




Some things to point out: 
1) This method of measuring swing weight reaffirms that the Gamakatsu Ryokei seiryu rods are amazingly light in the hand, only challenged by the Oni Type-I. They cast really well too.
2) The Oni Type-I really does have an amazing rotational moment and swing weight. Yes, it really is that good!
3) Remember that these are swing weights and do NOT take into account the overall weight of the rod. A rod may have a low swing weight but may also have a relatively heavy overall weight. This will change how it feels in during the cast. If they both are light, it will feel like a wand. If they are heavy, it will feel like a club!
4) If you change the fulcrum point by even an inch the swing weight changes dramatically. So, for rods like the DT Karasu 400, which has an overall heavy weight of 98 g and has a moderately heavy swing weight of 346 g, if you hold the rod further up its long handle the swing weight will drop dramatically and the rod feels much less tip heavy. This is when a longer handle is beneficial, and the data points it out, going from 346 to 270 g. Holding the Karasu further up the handle will cause much less stress on your extensor muscles of your forearm than lower down the handle. This concept, of course, works with all rods, not just the Karasu.
5) I still think that rotational moment is a better estimate of tip heaviness as it's easier to measure and more reproducible, but this method of swing weight measurement gives interesting and useful data as well.
6) Again, I don't put this data out there as scientific data for any given rod, but comparing rod to rod can be enlightening.

Anyway, I thought this was an interesting experiment. I love my Ryokei's, Oni, and TenkaraBum rods. Now I have some more data to know why!





2 comments:

  1. Way cool test Tom! I like your curiosity and the way you think about such things. Of course the ultimate final test is how you like the feel of a rod as you cast it. How it feels in the hand.

    But still I think it is a useful thing to figure out ways to measure and analyze rod characteristics, using a simple method, as a way to understand why different rods feel the way they do in motion in your hand. Which should guide you to hold the rod in a different place to increase or decrease the swing weight feel of the rod. Something that also would be useful to understand if one becomes interested in modifying rods by for example replacing the grip of the rods, as I see being done on many Japanese tenkara websites. An activity that may increase here as tenkara matures here in the USA. And thanks for a few of the measured numbers for rod models that I own. D

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  2. Tom you may find this kind of interesting. Today I tried to reproduce your swing weight measurements using an Ito rod that I modified a few years ago. Using an AWS-600 digital scale. First using your method, but I had trouble getting a stable reading on the scale as I attempted to hold the rod horizontal while balanced on a fulcrum on the scale. (I used a piece of triangular chop stick as the fulcrum). Using the point where my thumb would usually be as the balance point.

    Then I used an alternate method. I hung a drawstring bag filled with weights hung on the butt cap, filled with lead sinkers + coins for fine adjustment until the rod remained stable balanced horizontally. (the fulcrum was a piece of triangular wood I have long used to measure rod moment) Then I measured the weight of the bag and added counter weight. The scale delivered a stable reading with that method. And the weights measured were near the swing weight the scale was oscillating around using the first method.

    I modified the Ito a couple of years ago by winding several feet of leaded fishing line onto the butt end of the grip over a length of about 53mm. That became about 2.25 layers of close spaced line. That added 29g of additional weight to the rod. I did that to see what effect it would have on rod moment. Which effectively did not change. (Rod moment seems to be built into (controlled by) the rob blank itself). However, the radius distance to the balance point decreased quite a bit: Before radius: @3.9m = 68.2cm, @4.5m = 95cm. After radius: @3.9m = 58.6cm (~ - 10cm), @4.5m = 73.2cm (~ - 20cm). Thus a decrease of about 4 inches and 7.9 inches.

    While the rod weight had increased by about 1 oz, and the rod moment (~ torque) did not change, (only slightly within ability to reproduce the measurement).

    I think the improved balance feel was probably due to the decreased swing weight. The rod feels less tip heavy when held steady in horizontal position.

    The swing weights I measured today using the second method were: 3.9m = 248g, 4.5m = 378g. If it wasn't a tedious job to remove the leaded line counterweight I added a couple of years ago and put it back on - I would remove it and repeat the swing weight measurement on the unmodified rod.

    Most surprising to me was the s.w. of 248g at 3.9m was lower than for the Oni 1 (263g). (making me wonder if I was measuring s.w. incorrectly). But the Ito s.w. of ~379g at 4.5m, was slightly higher than for the Karasu 400 (346g). Which did not surprise me, and I judged it about correct knowing the feel of both of those rods.

    The point being that adding weight to the butt end of the rod will not change rod moment, but it can change the swing weight. Perhaps the function the reel fulfills on a western fly rod. D

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