December 14, 2013

Peek-a-boo, I see you!

There has been some talk recently regarding the tip heaviness of tenkara rods and how some rods seem to defy physics, while other rods don't. One of the rods that seem to defy the laws of physics is the Oni rod. I reviewed this rod last summer.

What's amazing about this rod is that while it is pretty heavy (101 g), it doesn't feel heavy. Also, despite it being a long rod at 396 cm it doesn't feel tip heavy at all. I find this quite fascinating. All of my other tenkara rods that are 380 cm or longer have some degree of tip heaviness, yet I do have a few seiryu rods that don't.

It has been rumored that one way the Oni rod creates the illusion of lightness is that additional weight has been added to correct its center of balance. This balance point can be seen in the rod's moment value. Moment equals force or weight (kg) times radius (distance from the butt end to the balance point of the fully extended rod). I recently posted about using moment of inertia (MOI) as a way to show a rods tip heaviness, but after many reader replies it appears that moment and not moment of inertia would be a better formula to use. Moment is calculated by Gamakatsu for their rods, so there is industry precedent. For tenkara rods, a moment less than 5.5 will make the rod feel without tip heaviness. I gave some examples of the moment of popular rods in my last post. These examples were: the Iwana 12' moment is 6.14; Ito at 390 cm is 7.84 and at 450 cm is 10.93.

Here are the moment values for the two rods of this post:
Oni rod moment: 5.2 kgcm

Hirame-ML-3909 moment: 7.51 (with handle wrap)

Anyway, to see if Team Oni really has added weight to the rod near the butt I decided to use some x-ray vision. The other night I was called to the hospital to perform a procedure on a very ill man. This procedure required me to use an endoscope along with fluoroscopy to place a drainage tube to relieve the man's pain. Fluoroscopy is real time x-ray and allows you to see inside a person or object. When I went to the hospital I just happened to have two rods in my car: Oni and Hirame-ML-3909. I was planning on using them the next day on the river.

(Not my actual patient) C-arm flouroscopy unit similar to mine.

Before the patient was sequestered for the procedure I had a thought, "why not x-ray the Oni rod and see if there really is any added weight". So I ran to the car and retrieved the rods. I used the Hirame -ML-3909 as my control as it is about the same length as the Oni rod (it was the only other tenkara rod I had with me anyways). I quickly x-rayed the rods looking for anything that would look like added weight. Nothing unusual was seen except in the butt end of the Oni rod. There was a dark, stepped down cylindrical object (or objects, one inside another) in the handle of the Oni rod that was not present in the Hirame.  On fluoroscopy metal appears black, while other materials that allow x-rays to penetrate at varying degrees will have various shades of grey. The darker, the more radiopaque (and usually denser) the material. Thick or dense metal stops the x-rays and that's why it looks black.  Here are the x-rays of the butt ends of the rod handles (ignore the pink -- it's the reflection of another monitor that was behind me):

Hirame-ML-3909 handle x-ray (for reference, the butt cap is stainless steel)

Oni rod handle x-ray

Here is what the handles look like to our eyes:

Hirame-ML-3909 handle

Oni rod handle

So, it appears that something has been added inside the butt of the Oni rod. Exactly what it is I'm not sure, but it appears to be radiopaque and therefore is likely metal. "It" probably is two metal sleeves, one inside another. However, I'm not about to dissect my Oni rod to see what it really is! If it is metal, this would add weight to the butt end of the Oni rod and potentially change the dynamics of the rod.

Hirame-ML-3909 handle butt parts

Oni rod handle butt parts

I'm not saying that tenkara rods should have extra weight added to their handle butt. All I'm saying is something is in the handle of the Oni rod that is not in the Hirame (and likely not in my others rods). What it is, and how it affects the rod is only a guess. For now I'm satisfied with the mystery. I'll let others play around with this discovery and draw their own conclusions. For now, I'll get back to fishing!


  1. Tom, i see moment as a real plus to
    one's buyng decision.

    About balance and tip heaviness i really don't mind if the rod is heavy but i do mind a lot if it "feels" heavy.

    1. Just for the sake of curiosity, have you calculated the moment for the Kasugo?

    2. Sorry Carlos, I sold the Kasugo a while ago so I don't have the numbers. Sorry.

  2. From my experiments, moment changes little when adding weight to the butt. Thus if a rod casts better with weight added to the butt, the rod's moment number would largely fail to indicate that fact.

    If we are using Moment to gauge tip-heaviness then either: a) adding weight to the butt does little to help tip-heaviness or b) or the Moment number is failing to tell the whole story.

    Also from my experiments I found that there are limits to how much the perceived tip-heaviness of a rod can be affected by adding weight. (To be honest, I found little obvious change I tip heaviness no matter how much I weight I added.) The simplified MOI calculation changes significantly as weight is added to the butt, so I would say it exaggerates the effect of that added weight (at least with respect to predicting the tip-heaviness of a rod)

    So we can surmise:

    if added weight changes tip heaviness: then Moment isn't going to tell you it is, and MOI is likely to exaggerate it. (Suggesting we need a better formula to predict tip-heaviness)


    If added weight doesn't change tip-heaviness: This strengthens the case for using Moment to predict feel. It also implies that there's probably only one way to making a rod feel less tip heavy, and that's by using a lighter or shorter blank (the weight of the tip sections having the largest impact on feel). It doesn't explain the purpose of the added weight in the Oni rod.

    Finally: Has anyone put the question of added weight to Masami Sakakibara directly? (Why are we guessing?)

    1. Possibly they are not for added weight but for structural support? I don't personally know Mr. Sakakibara but it would be fun to hear what they are and their purpose. I agree that it is most likely the rod blank that gives this rod is properties. I was just curious is all.

  3. Tom, you are better than the Discovery Channel and PBS. I learn something every time I read your blog. This is like a forensic tenkara-topsy!

  4. Gone fishing, you can study physics.

  5. Aw, come on... how about one more way we could measure our rods. (ha, ha)

    I came across this in an infamous fly rod "shootout". One of their measures is "swing weight":

    "You have heard this term if you are a golfer. Every pro shop has a simple scale that measures swing weight – the weight of the head of the club in relation to the shaft when you waggle the club. Since a scale like this won’t work with a fly rod, we had to come up with a better way to measure swing weight – that weight you feel out ahead of your hand when you hold the rod in a horizontal position. The best way that we have found to measure this is by placing a foam fulcrum in the middle of our scale, position the handle of the rod so that the fulcrum sits slightly forward of the middle of the handle, then position the rod horizontally, put some finger pressure on the very butt of the rod to hold it level and read off the weight in ounces."

  6. The shunt in the oni is tungsten impregnated dragen bone that has been kissed by unicorns. It purpose is to convert a cut down seriu rods into money by adding a cheap foam handle getting Americans to believe it the magic of all Tenkara rods.

  7. Tom, the weight added to the butt changes only the feeling of the rod from heavy tip to light tip. From the other side, it adds the total mass and the total rod inertia. It helps your wristle, but adds the load to the elbow. I've tested this trick with butt added weight and do not use it for The Tenkara Times rods. For me It is better to focus on real tip and rod lightness and rod balance.

    1. Your attention to your The Tenkara Times rods is very evident. They are wonderfully light weight and balanced.

  8. The Moment value of the rod appears to be set by the design of the rod blank. By adding 30g to butt end of several rods I was surprised to see the Moment value did not change. The rod weight increased by 30 grams and the CoG decreased thus making the rod feel lighter, more balanced, less tip heavy. The lower the Moment value the better the balance built into the rod blank. CoG(cm) = Moment (kg-cm) / rod mass (kg) . From this formula you can see there will be a sweet spot for ratio of Moment and mass. M < 3.3 rod feels very light. M < 5 to 6 rod feels light. M > 7,rod begins feeling more tip heavy. btw Tom I was an field engineer for Philips for 34 years primarily taking care of cardio labs & lesser extent vascular labs. Thinking about making my sign - Will Service Your Angio Lab for Rod Money. ; - ) DW