Designing Rod Angle

The relationship among Casting Style, Rod Deflection and the Line Loop

It is a good dicipline to start designing a bamboo rod after knowing the style of casting of the client fisher.
It is the objective for a rod maker to design a rod which is best match to the fly fisher.

There is a clear theoritical background  which can explain the relationship among Casting Style, Rod Deflection and the Line loop.   I hope the readers of this page to know why Max Satoh is proposing the rod design method by Rod Deflection.

At first, let's understand the relationship between Rod Deflection and the Line Loop generation.

The graph below is drawn by using DynaRod.  The rod is transferred from Phase 1, Phase 2, Phase 3 and finally stopped at Phase 4.  On the graph, at the end of Phase 4, its rod deflection is shown as the curve of AO.
The rod, by deflected during Phase 4, will release a whole of the stress which has been accumulated by stopping the movement.  The rod will become straight with the power which is equal to the aggregate moment force, and throw off the fly line which is connected to the top of the rod. 

The line AE is showing this direction.  This is drawn by extending the line AB as the tip top moves on this line.
The rod BO is the shape at the moment when the rod is streatched out.

Well, how long could the fly line be thrown away after being pushed out by the tip top of the rod.  To understand this, we will assume a time duration like below.

From the time when the rod received the deepest deflection AO, once stretched out, To the time when the rod stops after reflected up to its maximum.  In other words, the time duration from the rod shape AO to the rod shape CO on the graph below.  During this time frame, the shape and the direction of the fly line is determined.

The deflected rod AO is stretched and becomes a straight rod BO by the power of recoil.  The load remained on the rod becomes to the weight of the rod itself at this point of  time.  As the fly line was accelerated within AB movement, from the rod's view point, it is not the load to carry.  Further the rod recoils toward the shape CO by the force remained on the rod itself.  This recoil is done by the total of the force of inertia which is generated by the rod movement from AO to BO, and the remained force which were generated by the moment.  General formula of this could be written in pseudo physics manner.

(1): at AO, the accumulated force on the rod = the moment generated by fly line weight + the moment generated by rod weight

(2): at BO, the spent force = the force which is needed to straighten the rod = the moment generated by the rod weight itself + the force which is needed to accelerate the fly line between point A and B. This acceleration requires less force than the moment generated by the fly line weight since the fly line was already accelerated between Phase 3 and Phase 4 to the effects.

By subtracting the force (2) from (1), there is some remained amount of the force on the rod. Plus, by moving from AO to BO and stop at BO, the force of inertia is newly generated to bend the rod to opposite sitde. (It is another story in the case of such casting that loosen the rod hand at BO and abosrb or kill the recoiling power.)

Anyway, we assume that the rod is stopping at CO.  At CO point of time, the point of fly line that was with top guide of the rod at AO or BO, is pushed off by the rod end and flies the distance of BE.  This length becomes the longer when the verocity between A and B is the faster and when the time duration is the longer while the rod reflexes from B to C.  That is , BE = time BC multiplied by verociry AB.

Here, let's see the forces applied on the fly line.  During the rod tip movement from B to C, the fly line which is once took off toward the distance BE, is pulled back toward the direction EC due to the rod is bent up to CO.

We are remembering the class of physics now on the subject of addition of forces or addition of vector.  On the graph below, the force BE and the force BC is combined and it will generate the force BD (magnitude and direction).  On the other hand, as the fly line length BE is actually connected with the tip top C, as far as we hold the fly line by line hand, point E of the fly line is pulled toward the point C.  As the result, fly line length BE is turned at the point G.  As the GEC is an isosceles triangle, the length BE = BG + GC.

On the other hand, we saw that the fly line point B was accelerated towad BD direction, the resulted fly line is basically shaped as BFC.  The reason why the fly line loop front shapes round is because of the hardness of the fly line.  If the fly line is coated the harder, the line loop front becomes the more round and the wider and if the fly line coating is the softer, the loop shape becomes the sharper at the up front.

Next, we will see the relationship between the Rod Angle(the depth of the rod deflection) and the casting style (casting behavior), the position where the rod stops.

The depth of rod deflection, in other words, Rod Angle, is presented on the graph as the angle between the straight line AO and BO.   In the case of a larger Rod Angle, that is, in the case of a softer rod, what will happen if we do not change the casting style, especially the position where the rod stops.

The rod shown on the graph below has the Rod Angle=27 degrees, alittle softer than the case of above where the Rod Angle=21 degrees.  In the case of softer rod with the same casting style, the difference of the Rod Angle by 6 degrees results in the difference of the take off angle of fly line.  And as the recoiled CO's bend angle also becomes larger, the line loop shapes wider.  The line loop would be go forward and toward the direction a little above the horizon.

The last graph below is the case of Rod Angle=29 degrees.  In this case, the rod is stopped at the position where it rotated 10 degrees more than two cases above.  The take off angle of the fly line becomes a bit lower than horizon.  Though the line loop is wider than above, the line loop go forward and a bit lower than horizon.

As shown above, the relationship among the Rod Angle (Rod Deflection), stop position (casting style) and the resutled line loop and direction, got clear, I hope.

In rod design, I will consider in the opposite way.  First I should know the caster's behavior how he/she stops the rod, and assume the range of stop positions, then consider the appropriate rod angle (rod deflection) for the caster.   Thus rod deflection requirements for the fly fisher becomes visible.

Though there are other considerations when I design bamboo rod,  let's discuss about those in another place.

Max Satoh
May 11, 2007