I stumbled upon the quote above the other day and thought that it was worth talking a little bit about “rod load” or, as I prefer to say, “rod bend”.
There is only one thing able of bending a rod: force.
Force is related to mass and acceleration as per the formula: F = m.a.
Mass is the amount of matter of a body; on planet Earth it is equivalent to weight —and, to my knowledge, there aren’t any fly rods available in the Moon. Acceleration is an increase of velocity.
What this formula means is that if a force is applied to an object, that object will increase its velocity.
Back to our fly rod, we need a force to bend it. The needed acceleration comes from the caster’s arm. Mass comes from the weight of the rod itself and of the line being cast. The bigger the mass the bigger the resultant force, if the acceleration applied is the same.
So what is the requirement in a line for it to bend the rod more, for the same acceleration applied to the rod butt? Obviously to increase it mass, i.e. its weight. You don’t need much technology to put more plastic material in a line, do you? In fact if you were a line manufacturer you wouldn’t need to change your production process at all, just put the lines formerly labeled as #6 weight —or even #7— in a spool and box with a #5 on them.
Some time ago, line manufacturers started increasing the weight of their lines, while keeping the same AFFTA number on them —throwing that standard out of the window in the process. It doesn’t have anything to do with technology, but, maybe, with the inability of the average fly angler to properly use the ever increasing rigidity of modern fly rods. And, IMHO, it isn’t a good idea whatsoever; but that would be a topic for another article.