It was some years after I started fly fishing when I first heard the term “swing weight”. It wasn’t until more recently that I began to actually pay attention to what it meant. For a long while I figured it was just some smooth-talk coming from the tackle industry to try and justify the purchase of yet another high-end fly rod that was otherwise indistinguishable from the armfuls I already owned. Fish didn’t seem to mind, anyway.
Swing weight is well entrenched in the language of the modern fly shop, in gear reviews and company tech bulletins. It’s probably time to start paying attention to it if you haven’t already. Truth is that swing weight is a legitimate means of measuring rod performance that takes into account some math-y stuff (more on that in a moment). But how much should swing weight influence the purchase of new (or any) rod?
I was curious to see how my current stable of rods compared in terms of their swing weight, and if I could correlate swing weight to their actual performance in the field. Could I somehow quantify my perception of the rod’s overall performance on the water? And is swing weight the way to do it, or is it an overrated metric in a world of tech-hungry fly rodders?
But let’s back up a bit first. What exactly is swing weight, and why should you care?
Swing weight is part of what makes a rod feel “good” in your hands (the other two major factors in my mind being 1) actual physical weight and 2) flex profile). When you pick up a new rod off the rod wall and wave it around like your Obi-Wan in Revenge of the Sith, you’re sensing– no, not the Force– the apparent weight of the rod in your hand. Sometimes a rod can feel light and crisp– and hopefully a brand spanking new rod will feel this way– or it make feel heavy and sluggish (like a fiberglass rod from days-gone-by might be). This intuitive feeling is linked to swing weight.
The scientificky name for swing weight is “moment of inertia” (MOI), which describes the amount of resistance an object has to being rotated along a predetermined axis. In most cases with fly rods, the axis is the butt of the rod itself. The resistant needs to be overcome by force; thus, you need to put actually work into casting a fly rod to cause it to move, or “swing”, during the cast. In general the higher the MOI the more energy needs to be put into the cast, and the apparent weight of the rod (swing weight) increases. Keep in mind that swing weight need not necessarily correlate to physical weight, and it is possible to build two fly rods that have the exact same physical weight but have different swing weights.
Higher swing weights will cause an angler to fatigue quicker, making the day on the water that much longer. Thus, in theory, you want a rod with a low swing weight to maximize your time and potential on the water.
Unlike other physical measurements commonly used to describe fly rods (physical weight, line weight, length, flex) swing weights are rarely specifically advertised with rods, and I have never seen a rod labeled with it’s swing weight. That’s not to say that the numbers aren’t out there, but they’re hard to come by. Hence, I decided to do the maths myself.
Measuring Swing Weight
Sexy Loops has a great rundown on swing weight and offers a great paper by Grunde Lovoll and Magnus Angus regarding how to calculate swing weight. It is the method I use here to analyze my rods. I won’t get into the details here, buut you can refer to their paper for specifics. They also offer an online MOI calculator, but for ease of use with multiple calculations I used an Excel file to input the equations.
I also decided to use the method described in the Yellowstone Angler shootouts for comparison. This method is extremely quick and easy, although I feel the results aren’t exactly representative of MOI of a rod during the cast: unlike the Sexy Loops equations, which place the axis of rotation right at the butt of the rod (which is a nice approximately for the axis of motion of a rod in a proper fly cast), the Yellowstone Angler method (YAM) puts the axis of rotation in the middle of the grip (which would make sense only if the rod was being cast with a broken wrist).
The rods I used for this study run the gamut in age, type, maker, and pretty much everything else: an Orvis Recon 865-4 (2015), Orvis Clearwaters 967-4 and 909-4 (2014), St. Croix Premier 905-4 (2006), Cortland Pro Crest 8′ 7/8 (c. 70s), and a Berkley 8′ 6/7 (c.70s). The first four are four-piece graphite and the last two are two-piece fiberglass. Measurements of the rods were done as a whole and as their individual components for length (meters) and mass (grams). A kitchen scale was used to measure mass. A few additional measurement needed to be made: 1) the balance point (BP) for each rod section, defined as distance from the butt (thicker) end of the section to the point on the section that allowed it to be balanced on a hard edge (in this case, the limb of my fly vise); and 2) the length d from each BP to the butt end of the entire rod (i.e. the bottom of the reel seat or fighting butt). All these measurements were then plugged into Equation (1) described by Lovoll and Angus for each section, and then summed up to describe the entire rod.
Here’s an explanation of some of the terms:
– Next to the name of each rod is it’s actual, physical weight as measured by me (in ounces).
– Lengths are given for each individual rod section and the rod as a whole, as measured by me (keep in mind that, due to the presence of ferrules, the sum of all rod sections is longer than the whole rod assembled)
– MOI is measured in grams-meter-squared, and TMOI is the total (overall) MOI for the entire rod
Here are the MOI determined from the Yellowstone angler method:
Recon: 6.8oz; Clearwater 909: 9.8; Clearwater 967: 9.1; St Croix: 12.7; Cortland: 13.5; Berkley: 10.9
What Does It All Mean?
It’s not surprising that swing weight correlates rather well with my own perceptions of each of these rods. The Recon, being the newest rod of the lot, is extremely light and nimble. I’m continually impressed by how it feels in hand. Both Clearwaters are nicely responsive considering their size. The Cortland rod feels a bit unwieldy, especially at the tip. The Berkley is a nice caster, even though the tip is a wet noodle. And the St. Croix, after all these years, feels like swinging a broadsword compared to the more modern graphite rods.
One of the most interesting things to come out of doing these calculations is the realization of where most of the rod’s swing weight derives from. Unlike physical weight, which is mostly concentration toward the butt end of the rod, MOI is the result of physical weight being carried at or near the tip, It makes sense: when a rod is being cast along an axis at the butt, the tip travels in a much greater arc than the remainder of the rod.
A little less clear here but also likely is the fact that shorter rods generally have lower swing weighs than longer rods. A more comprehensive set of rods, using the same line weight but different lengths, would likely make this correlation more obvious.
Even this revelation isn’t a real a-ha moment. I stared at this data for a while trying to come up with something more meaningful, but the truth is that in such a varied line up of rods that it doesn’t provide much context to understanding swing weight. I mean sure, rods with a lighter swing weight feel better in hand; but it is just as clear that physical weight is a good proxy of swing weight when comparable rods are a few tenths of a ounce different in physical weight. It doesn’t take a rocket scientist to intuit that a 2 3/8 oz 5-weight will probably cast a lot nicer than one that weighs nearly 4 oz.
So When Does Swing Weight Matter?
I’ll return now to my previous comment: it is my perception that swing weight has become a bigger deal to fly rod manufacturers in recent years. What happens when we take rods with similar physical weights but with different weight distributions?
For this thought experiment I’ve created two hypothetical rods based upon the real-world data taken from the Recon 865-4. The first I’m calling R9054, which is a Recon-type rod built on a 9′ blank. I’ve extrapolated the likely lengths, mass, and balance points of such a rod based upon the measurements from the real Recon 865-4. The second I’m calling H8654, a rod which has the exact same physical weight but has it’s mass slightly redistributed toward the butt section by 20%. Here are the swing weights for the three rods: Recon 865-4, R9054, and H8654:
In this case the difference in physical weight is minimal (0.1 ounce). Still, we see that the effect of a slight increase in weight and length with rod R9054 results in a 19% increase in swing weight, certainly enough to be noticeable in hand. At the time time redistributing 20% of a rod’s weight toward the butt end of rod H8654 resulted in a 17% decrease in swing weight, again a significant difference in hand without any change in physical weight.
The effect of this means that even small amounts of unnecessary weight carried near the tip will have a pronounced effect on swing weight. It’s no wonder why certain rods, such as the Helios 2, feels so light in hand even though it’s physical weight isn’t drastically less than competing rods: it relies on weight-reducing technology in the tip (in the H2’s case, via the use of lightweight thermoplastic resins) to reduce it’s swing weight. This is probably (in my opinion) why swing weight has become such a major buzzword in the rod industry in recent years: physical rod weights have been drastically reduced over the past decade-plus to the point where the difference between various models is a fraction of an ounce. Redistributing rod weight along a blank allows manufacturers to produce “light in hand” rods even when physical weights are nearly indistinguishable from another.
The Final Word
So what does swing weight matter? When you’re talking rods of vastly different physical weights, it might not mean much. However, when dealing with a series of very similar, lightweight rods– the kind of rods you expect to see side-by-side at your nearest fly shop– slight differences in how weight is distributed could vastly effect how “good” or “bad” a rod feels when in hand. At a time when physical weight between competing rod series can seem inconsequential, the way a rod is built can make all the difference.