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Rod Design - Rod Bending

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When a rod with the fishing length of line is moved, which part of the rod start bending first?  From tip side, from butt side or both simultaneously?  (Max Satoh)

I don't think we could cast rods that actually began bending first in the butt section.  Though  they may feel like the butt begins to bend before the tip, in that case the rod would not be able to support its own weight.  Would it?  (Harry Boyd)

My impression based on observation has always been that they bend progressively from the tip to the midsection and then into the butt.  (Larry Puckett)

I should be more strict in cases.

Let's assume a rod moves translated: If the rod receive force by movement, it's inertia force. Then, entire part of the rod will receive the same acceleration, as a result, entire part of the rod will start bending thought the degree of the bend is different to each part.

Let's assume the tip top has stronger load while rod stays unmoved: It must start bending at tip top and the load would be passed toward butt side.  This would be more typical if the rod is soft enough.

Let's assume the rod (steel for instance) is hard enough which does not bend. Probably butt portion would slightly bent, but within the rod each part still receiving  moment force and bend very very slightly.

How about this?  (Max Satoh)

Here you are talking about something slightly different than the loading of the line.  You're talking about the mass of the rod accelerating, and the inertia of the rod tending to keep it at rest.  You'll notice that even without the mass of the line attached.  You will notice it more with the line attached, since the mass of the line is then added to the mass of the rod, and as you accelerate the rod, you accelerate the line, increasing the inertial moment.  Acceleration does interesting things to a body at rest.  (Mark Wendt)

Since energy, like water, flows in the path of least resistance, I would think that it starts at the tip, until the tip is parallel to the energy path, then as the energy increases, and more resistance is met by the taper, the rod bends further down the shaft.  (Mark Wendt)

Pardon my inexperience. I'll probably sound really stupid here, but what the heck. I'd love to do as you specified. But I'm not sure I understand. I'm not sure about 1) 'sweet spot' or 2) butt bending, or 3) tip 'giving out'. Of the three the butt bending seems the most understandable, but not sure I can see it in action. By the way, was that where the butt section starts bending in a significant way? or where the butt section flexes under the grip? I may be doing it, just not know what it is I'm doing. 

• To find the line length at the 'sweet spot' I presume that the rod is in its own zone, all cylinders firing. I've cast a lot while fishing, but I rarely am thinking stress, flex, sweet spot, and such like. I'm usually thinking- is that where the fish is, and is what fly I've got on correct? or BITE YOU ORNERY FISH!  I get the idea of a sweet spot; just not sure I can identify it. Perhaps, like in picking up poetry writing again where you need to write 10,000 lines carefully, and then throw them away, then write for real. So I probably need to make 10,000 casts, not on the water, to start beating a 'feel' into my brain. My normal rod is a 7wt, 9ft (probably too heavy) and I've fished a lot of streamers (chuck and duck), and size 16 dries when I can. I have felt at a certain point when the rod has hit its limit of 'helping' me get the line out, or where the line is so short the rod isn't really helping. Is that remotely close? Or those are the outside limits between which I need to pay more attention? (I need more time with lighter rods, I can see. Hence, my 4ft and 5ft quads I'm building from leftover strips.) 
• Butt bending. As I've been building my first couple bamboo rods, I've taken to 'shaking' each section automatically when I pick them up, to get them to flex so I can feel and convince myself that this bamboo rod building is actually working in my hands. (Probably a bad habit.) So would this 'feel' be in sensing the difference of how the tip section flexes and the butt section flexes? Or the effort to get them to flex? (Not the quantitative, but the qualitative? It would be different than on the full rod, of course.) 
• Tip 'giving out’. I'm completely lost here, not even sure what this would mean. Stressing the tip to where it completely gives out and only the butt section flexing is functional? (akin to fighting a fish with the butt?) Or where I'm putting enough energy into the rod that the tip just can flip out the line instead of dapping? I think I am confusing myself here. And I am unanimous in that. 

I am no engineer/mathematician either, but I almost was, some decades ago. I can rise to the level of moments, inertia and etc., only under 'stress' now, when needed. (or I can ask my Physics professor brother to help me.) This bit of getting the graphical, the numbers, etc. into the realm of tactical under my fingers, has me lost. I think I can get one or the other, but not the crossover between the two, yet. 

Do you hire out for casting coaching? Come out when the salmon flies are creeping everyone out on the Henry's of the Snake. (Sam Nielson)

Well Sam, I'm probably the second to last person you want to give you casting lessons. You need someone like Harry Boyd, or some Certified Casting Instructor, if you really feel the need for lessons. I've been to the Fork many times and I love it.

• The best place, as strangely as it seems, to do these tests is not over water. Fishing is the true test of a rod, but here you just want to concentrate on the rod. The sweet spot to me is the point at which the rod is working and I seem to have total control. It may be over 10' to 20' range. Most 5wts, for instance, seem to work really well for me between 30 and 50 ft. What you are looking for is the pt where you feel you and the rod are in sync. Others should chip in here and give you a hand. This is all a feel thing. Its the fishing distance where you know you can put the fly on target without shooting line or overworking yourself.
• This is the tough part. What you are looking for here is the point just as the rod starts working, or giving you feedback. This is just a number to temporarily use to see where the stress graph is showing you the rod is not moving enough to feel. It's not as important as (1) but it will give you a little more insight.
• Again just cast out line normally until you feel you loose control, the line doesn't turn over, or the rod just can't be pushed any more by adding more force. On most rods it is the tip that is failing, I don't mean fracturing, I just mean loss of line control. Put that number in the program and you will see that the tip stress is probably way up there somewhere. So this is a number you want to stay under when designing rods. For-instance, an 8 wt rod with a .030 tip would probably display this weakness right away. It wouldn't be able to turn 30' of line over.

As I said, others can help here. (Jerry Foster)

Stress curves and the programs that create them help to understand the "sweet spot" and design them into rods at whatever casting distance you choose. For me, using Hexrod, I have come to learn that the sweet spot is where the stresses are in the 180-190k range. I can look at the inputted line weight, length, and peak stresses, and decide if for me the feel is correct (including the amount of loading I want to feel, the average distance I want to fish with this rod, and the pattern of flex). 

If I have a rod and I want change it fish at longer distances, I'll modify fundamentals to increase the length of line cast, but maintain the same stress curve. If I want a rod to flex deeper, or be more tip action, I'll modify the stress curve visually, point by point, to flex where I want, then back plot the dimensions.  

For you, take a rod you like, cast until the rod shows its sweet spot in the zone, then measure the amount of line out. Plot the dimensions, length of line, and see what the peak stresses are. This will give you your own personal yardstick to evaluate other rods with, and with which to design your own. (Chris Obuchowski)

Thanks Jerry and Chris and others for the additional description about casting tests. I think I do ok with casting, (I catch fish) but I'm just not in the habit of watching myself to know what is going on while casting. So the extra help in your descriptions should be useful. I just need to get the rods out on the snow and try to make sense of it for figuring out these stress curves, or my stress curves. See if I can figure out the feel with length of line out. Not sure why it never occurred to me to look at stress curves by pushing them in my hands while casting. Just makes so much sense to do that. Thanks for bringing it up. I just hoping I won't be dreaming about fishing too much to actually decide on some numbers.

But I'd never turn down casting lessons!

Now, I feel like I've dampened the discussion here on tapers and stress curves. I apologize for that. I've been tempted to start taking the little comments as you guys mention them and start compiling them so I don't forget when I'm in the middle of a taper fiddling. (Things like the .006 to change line weights, Garrison's stress limits, hinges, etc.) I figure at some point there will be enough of them that I can figure out what I need when I need it.

Between, the flu and cutting more wood, I haven't been watching so carefully lately. I did get some nice black walnut feathering on some pieces, and I've starting thinking about stabilizing some other woods I've got, notably root ball black walnut and sagebrush. Not sure yet how I want to approach getting a vacuum drawn on it yet. Depends on what equipment turns up. Maybe with my big mouth and a straw, I could probably get 10 in Hg?

Oh! For those interested, the tree (Utah juniper, red cedar) that gave me the forehead scar (too many decades ago) has been located! So when I make it back to visit home stomping grounds, that tree will pay a little for its impertinence and also make a couple reel seats. Just gotta see what embarrassed reddish it is, or deathly gray-brown scared colors it shows! Heh, heh. Wouldn't want anyone I've annoyed chasing me with a roaring chainsaw and an evil glint in their eyes! (Can a fish hold a chainsaw?) (Sam Nielson)