Loop Control Paradox (divertimento for curious casters)

Loop width control is a recurrent topic in casting instruction. Several aspects govern loop width, but “matching casting arc to rod bend” is what instructors use the most; so “if your loops are too wide narrow your casting arc” is the usual fix we offer.

Let’s say that I am bass fishing with a popper knotted to a short, stout leader, followed by a bass taper #7 weight line. I want to cover a very promising spot, just a small hole among low hanging branches, which asks for a side cast with a narrow loop.
First try… my line crashes against a branch; I need to narrow the loop considerably. So keeping everything equal I decrease my stroke angle… and the loop fails to straighten. What has happened?

Think of you starting your car. You accelerate by pushing the gas pedal and your car increases its speed. After covering a distance of, say, 20 meters, the car has gained some speed. If you keep pushing the gas pedal the speed attained after covering a distance of 50 meters will be even higher. That is, for the same force the longer the distance over which that force is applied the higher the attained speed.

So back to rod and line in the example above, if you decrease your stroke angle you have decreased the total distance your rod tip has travelled; now you are applying the same force –pushing the pedal equally– along a 20 meters long distance instead of a 50 long one; the result, obviously, is less line speed.

Well, if by decreasing stroke angle I fail to unroll my line completely I obviously need more rod tip speed to fix that.

So —with the narrow loop I need in mind— I keep that same reduced stroke angle but increase the acceleration applied to the rod to gain that lost line speed due to having decreased the distance covered by the rod tip. But, hold on one second! In that way I will get a tailing loop!

More acceleration applied to the same mass results in more force –F = m.a–, and more force results in more rod bend, and a basic principle of casting mechanics says that we have to match casting arc to rod bend! So although I started that cast thinking of decreasing the casting arc, actually I am forced to increase it to get a good loop!

So on the next try I increase the force and also increase the stroke angle to accommodate a bigger bend in the rod. The result of increasing the force and widening the angle is similar to pushing the gas pedal further down and covering even more distance: much higher line speed… and my popper curves to the left hooking a branch, as a result of an overpowered cast!

Fellow fly casting instructor Walter Simbirski has summarized it all on a recent comment to this article:
We use more arc to generate more line speed but we need to use more force in order to maintain slp over the larger arc but with the extra force we need less arc to generate the line speed but if we use less arc we get a tailing loop so we need to increase our casting arc but now we have a large loop so we need to increase force to flatten the rod tip.
Seems pretty simple to me. Why are you guys struggling with this?

Reducing casting stroke angle to narrow your loops doesn’t work in isolation. No wonder that beginners have a hard time in controlling loop width, as it is a question of very fine-tuned adjustments –very subtle indeed– more than just varying casting arc.

Playing with loops by changing the casting stroke angle is fine; but play with force as well. And specially, with stroke length too, as this element is key and, however, it is the less understood of all them. But this is stuff for another article.

19 comments on “Loop Control Paradox (divertimento for curious casters)

  1. mark says:

    Couldn’t agree more…had this very discussion just this morning.


    • Aitor says:

      Glad to see that we are on the same page. Probably we modify stroke angle and applied force to get the needed line speed, then adjust loop width via stroke length.


  2. stoatstail50 says:

    Couldn’t agree more…had this conversation just this morning.

    I don’t think I have ever seen anyone casting or instructing who actively matches Arc to Bend to achieve a particular tip path…(although quite a lot of people believe that they do). Tip paths and tip velocities are not produced by a strictly binary relationship between Arc and Bend.

    Rotation, Force and Translation, in the form of the Hand Path, all contribute in varying measure.

    For me, Arc and Stroke Length are just terms for the measurements of the angle through which the rod has been rotated and the rectilinear distance the rod has been translated. Translation can be curvilinear as well as rectilinear, hand paths are frequently curvilinear.

    Bend is a byproduct of the material properties of the rod and the net forces applied to it.

    There is definitely a relationship between bend and arc but they are not the only controllable inputs that mediate the behaviour of the tip. We have both cast a rod that doesn’t bend and achieved close to SLP with a very tight loop…this isn’t done by matching arc to bend because there’s no bend, changing something else made this tip path possible…. or maybe it just happened by magic.


    Liked by 1 person

  3. Staffan Dahlbom says:

    Interesting stuff, I had a couple of “soon to be instructors” over this weekend and as we were working us trough the performance test we had a long discussion about task 19.

    [Task19: Teach an intermediate caster the adjustments needed to maintain loop control when
    changing distance from 30 feet to 50 feet, and the reasons for those adjustments.]

    The intention, I guess, is to explain the ajustments needed in force, as well as in casting arc. (Covered in expectations A and D).

    The relationship between mass (or acceleration) and arc is quite simple (As you write Aitor)
    The relationship between mass (or acceleration) and force is also simple.

    More mass the wider the arc, more mass the more force you need.

    But to study this in real casting situations (as the complex phenomenon it is) is something else.

    I’d say Open up your casting arc will give you a longer force over distance (longer SLP). It will also give you “shorter” rod and less speed with the same but rotation. Same but rotation with a heavier line calls for more force. More force and longer tip travel is more work onto the line. //I could go on loosing myself//

    Could opening the casting arc (and hence gaining distance, and work done to the line) be enough, no added force that is?

    Could lengthen the translational motion (not opening up the arc) and not adding force be enough?

    My conclusion is, the adjustments that has to be done as you lenghten the line, may vary (from one cast to another, and from one caster to another). Most likely it is a successful mix of both arc, force and stroke length. And the differences are so tiny (in terms of centimeters or degrees or newtons), that they become hard to notice for the human eye.



    • Aitor says:

      Hi Staffan,

      I agree with your conclusion. In my view we adjust stroke length and angle to give the line the necessary speed to reach the target, the farther we want our fly to go the mire we increase the elements of our casting stroke.
      I find that traditionally stroke length has been underestimated, for it is as important as angle regarding line speed. Mots of the force we apply goes into rotating the rod, but since work is F x D, line speed depends on torque as much as on the distance over which we apply that torque. In this regard it is possible to increase the casting distance without increasing the force; from a different standpoint it is possible to cast longer without increasing rod bend.


  4. I think that enlarging the movement is useful as the mass increases, the more mass less speed, to recover the second it widenes the movement.


  5. why does my car run more and more, consumes more?🤔😳😁


  6. 🤔I translated 30 times, only now I understand the meaning of speech 😳, that the longer the run is the faster it is, that’s for sure!🙌👍😬


    • Aitor says:

      Of course. It is about the supposedly easy relationship between rod bend and “casting arc”. Walter did put it nicely on Facebook yesterday:

      “We use more arc to generate more line speed but we need to use more force in order to maintain slp over the larger arc but with the extra force we need less arc to generate the line speed but if we use less arc we get a tailing loop so we need to increase our casting arc but now we have a large loop so we need to increase force to flatten the rod tip.”

      As I told you very difficult to explain if we don’t share a common language.

      Liked by 1 person

  7. Aitor says:

    Grazie Ivan.
    You should learn English: all the relevant info about fly casting mechanics is in that language. The irrelevant also 😈


  8. flyqcca says:

    Again, an interesting regard.

    I would say that « So keeping everything equal I decrease my stroke angle… and the loop fails to straighten. What has happened » is not something we see in reality because most/all of us put more line speed than required… so the observed result would probably be a more compact loop and a better result because most have a too prononced arc, especially in the last forward motion…

    That being said, your point is completely valid and made me ré-questionned myself again and again… which is good!!

    Best regards,

    Liked by 1 person

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