Bamboo Rod Design
1. ‘FAST’ ACTION
2. ‘SLOW’ ACTION
I am fascinated by the process of rod design. To me the design of a rod is an intriguing combination of art and science. One of the main attractions of building bamboo rods for me is being able to play around with a rod design to fine tune it. The numerous classic tapers that have been published are an invalauble source of base material although it is clearly necessary to build and fish each taper in order to assess its merits. For this reason I initially made a range of rods that explore various types of rod actions based on classic tapers and from these have deduced the type of rods I like. I then fine tune these deduced designs using a combination of trial and error ‘Art’ and the physics of bending ‘Science.’
There are a number of factors that need to be considered in the design of a rod. Some of these are as follows:
Action – This is a concept that is notoriously difficult to describe in words and there is much debate about the various words used to describe it. Essentially there are types of action i.e. progressive, parabolic etc. and there is rod ‘speed‘
Basically when I refer to a fast action rod I mean a rod that bends (when cast) in a tight radius in the tip and a much greater radius in the butt -refer the dark blue line of max bending in Figure 1 alongside. This represents a fast action rod. A slow action rod is depicted in Figure 2. Here the radius of bending is pretty even or consistent throughout the rod resulting in a quarter cirle bent form. A medium action falls somewhere between these two.
All other things being equal a fast action rod results more readily in tighter loop formation and higher line speed, but the faster it becomes the more it loses a feeling of smooth transition of power into the hand and the more it approaches a broomstick with a wheel on the top of it ( which would theoretically be the fastest rod possible). The casting stroke also shortens considerably and takes place in a shorter space of time, which is probably why it is referred to as a ‘fast’ or ‘quick’ action.
Compare the difference in distance the rod travels and the size of the angle between the rod start position (light blue line) and the rod recoil position (pale tan line) in Figures 1 and 2. Graphite rods are generally made as fast action rods. I like my stream rods to be medium to medium fast and stillwater rods to be faster. Preferably stream rods should have have fine flexible tips (for casting short and with tight loops if reqd), but strong enough butts to be able to cast a longish line when necessary BUT without compromising on a smooth transition of power into the hand.
Types of action
Within the range of fast (or medium and slow) rods there are different ways in which the power is transferred from tip to butt and back during the casting sequence. There are essentially 2 types of action, Progressive and Parabolic (with numerous hybrids of the two).
Progressive action rods transfer power smoothly and incrementally up and down the rod.
Parabolics on the other hand transfer power at different rates during the cast.
These differences are a direct consequence of the specific taper of the rod. Progressive rods have tapers that tend to move at fairly consistent rates of change from tip to butt. Parabolics on the other hand tend to vary the rate of change so that the rod is generally thicker in the middle section and thinner in the butt than a progressive would be (for the same lineweight rod).
Parabolics tend to have something of a slingshot feel to them once the cast has increased in length such that the loading has moved into the butt of the rod. This can be a difficult feel to get used to at first but, once mastered, parabolics are capable of great length casts per weight of rod. That said, some people hate parabolics ands others love them. I tend to prefer a progressive action but also thoroughly enjoy a parabolic on occasion.
There are a number of other factors that have to be taken into consideration in the design of a rod which I will not go into at this stage. These are factors such as weight (the weight of the rod itself), centre of gravity (essentially the ‘balancing point’ resulting from where the weight is located) swing (the momentum of the rod resulting from rod weight and acceleration) design for ferrules (factoring in the weight and stiffening properties of the ferrules) the position and number of guides relative to rod bending etc.
3. GARRISON STRESS GRAPH
Approaches to rod design
There is no substitute for physically casting a variety of rods and really thinking about what each one is doing and where in the rod it is being done. Trial and error, making small changes to a design and observing the differences in feel is a valuable process in and of itself. I spend a lot of time fishing and casting rods that I build. This is design by feel or ‘art’
There are a number of scientific processes that can be applied to rod design that augment the ‘art’ aspect.
Everett Garrison introduced the bamboo rodmaking world to the concept of designing rods by examining the Stresses that are introduced into a rod under loading. Figures 3 and 4 are examples of stress graphs of rods. Figure 3 is a Garrison progressive rod and Figure 4 is a Paul Young parabolic. The overall shape of the graph and the detail of where the maximum and minimum stresses occur are good indicators of what the rod is likely to feel like PROVIDED one can link this with an understanding derived from actually casting a variety of rods and examing their stress graphs.
Another approach is to model the actual bending or deflection shape that a rod assumes under loading. Again this provides a good idea of what to expect in terms of the feel of the rod. Figures 5 and 6 show a rod will bend under different loads.
Straight Line Taper (SLT) comparisons
A good approach to examining a rod design is to compare it to a straight line taper SLT (i.e. a taper where the rate of change is consistent from tip to butt). This provides an understanding of where the rod’s design varies from a datum. The effect of these variances can be felt when casting the built rod. An SLT graph also provides a datum for comparing different rod tapers simultaneously in a spreadsheet refer Figure 7.
Combination of Stress, Deflection and SLT analysis
A combination of all of the above provides the best of all worlds and I design all my rods using all of these approaches. In this respect I am fortunate to have a pretty good understanding of the essential mathematics and physics involved in the determination of the deflection and stresses induced in a rod and how to interpret these. I am also fortunate to have been assisted by Winston Jones, a gifted engineer and software writer who enjoyed a challenge and was excited to tackle an interactive rod design programme. He developed the software shown in Figures 5 and 6.
I now work in an excel spreadsheet that provides all the various graphing tools listed above and generates my workshop sheet.