Physics FOR Fly Casting – The Einstein Series

What the Rod Is and Does – Really

Short Version

  1. Fundamentally, the flyrod acts as a flexible lever. It is not, fundamentally, a spring which we load and unload to propel a fly line like a bow fires an arrow.
  2. Rod loading was perhaps a useful fallacy but as a description of what we do with the rod it is just plain wrong.
  3. Rod levering is what we should be thinking about. A fly rod is a third class lever which extends our reach – like a spear thrower does. It amplifies the distance moved by our casting arm and thus the speed at which it moves.
  4. Every cast has an energy budget. The energy comes from the caster. For a longish cast about 80% comes from the rod as lever and about 20% comes from the spring effect of a flexible rod.
  5. The real advantages of using a flexible rod are that it helps smooth the application of power provided by the caster. There is a lag or delay (at both the beginning and end of a cast) between what the rod butt is doing and what the rod tip is doing. These lags stretch the distance over which the rod tip tows the line and the time during which it is towed. This is kind to our bodies and provides additional energy bang for the casting Force buck.
  6. How much delay is too much and too little is the Goldilocks choice each angler makes for themselves. It influences rod preferences. A stiff rod has less delay and a soft rod has more.
  7. Good casters also have personal rod preferences but what sets them apart is their ability to adapt their casting action to any rod. You can buy rods you prefer but you can’t buy sound casting technique.


Why bother? Everyone knows what a fly rod is, how it works and what we do with it right? Well, no, actually. It is very commonly misdescribed and misunderstood and not just by we humble fly casters. The common description is that a fly rod is a spring which stores energy when we load/bend it and releases that energy when we unload/unbend it whereupon the fly line is propelled as a bow fires an arrow. That’s why “rod loading” is (supposedly) so essential and such a vital task for any fly caster. And this kind of “thinking” is not just a quaint traditional belief. In casting circles it is taken almost literally as gospel and woe betide any heretic who says otherwise. They will be cast out if not burned at the stake. So gather the wood and break out the matches, here goes me.

Rod Loading

I suspect that rod loading was a useful fallacy in at least two senses. Rod loading is supposed to be something that we should both do and then feel when we do it.

Yes, it does make some sense to talk about “feeling the rod load” but what we probably mean is feeling resistance that helps bend the rod. That feeling provides sensory feedback on our casting movements which is absolutely essential for controlling those movements – or any others for that matter.

Bending the rod helps in another really important way. It shortens the rod temporarily and that makes it easier to stay on the Straight Line Path. Imagine your task is to lightly run the rod tip along the underside of a shelf. The shorter the rod the easier it will be to stay in touch with the shelf. Too easy with just your index finger; much harder with a 3 metre long broom handle.

When it comes to casting mechanics, however, the rod/spring/loading stuff is just plain wrong. So what? Well, because what we do is greatly influenced by what we think we are doing and if I think I am using a spring that boinks the fly line I am likely to make casting boinks instead of casting strokes. Very bad idea. At the very least it is inconsistent and slightly confusing to be told it’s a spring but you have to accelerate smoothly with it – which is actually what you need to do with a flexible lever if you are going to keep its tip moving in a straight line underneath that imaginary shelf.

Rod Levering

At the heart of the rod loading belief system is a terrible misunderstanding about the fundamental nature of a fly rod. Yes, ok, it’s partly a spring but far more importantly and fundamentally it is a lever which extends our reach like a spear thrower does or one of those tennis ball launchers people use to entertain their dogs in the park.

Let’s talk levers. Archimedes told us he could move the world if we gave him a long enough lever. Good concept if not entirely practical. Say you want to lift a log but it’s too heavy. Hmm. Have a crowbar and a house brick handy? Put the brick down nice and close to the log. Shove the tip of the crowbar over the brick and under the log. Go to the other end of the bar and push down. Bingo. The log is lifted. The brick is a fulcrum and the crowbar is a first class lever. You have just demonstrated mechanical advantage. You amplified your effort.

Move the brick close to the other end of the crowbar (the head) and try pushing down on it again. Hopeless. Now try pushing it down onto the brick with one hand while pulling up with other hand gripping the crowbar a bit further down. Lots more effort is required to move the log – far more than if you tried to lift it again without any lever involved. The weight of the log has been amplified. In tech speak the load has been amplified. Now you have the concept of mechanical “disadvantage”. The crowbar is now a third class lever and it works against you instead of for you if you are trying to lift a log.

Don’t be too hard on third class levers. They too are useful and common. Your casting arm, for example, is a series of third class levers and the fly rod is a long one. What it amplifies is the distance over which and thus the speed at which you move your casting arm. Your advantage is now speed instead of effort.

Lever v. Spring

Let’s now consider the relative contributions of the rod as a lever and the rod as a spring to casting a fly line. A few years back a real physicist named Grunde Løvoll ran the numbers. What he found was that by using a flexible fly rod instead of an inflexible fly rod a good caster managed to get about 20% more tip speed. In other words the spring effect was good for an extra 20% of tip/line speed over a broomstick rod that didn’t bend. Line speed is what we use to beat gravity. Speed comes from the Force we apply which puts kinetic energy into the fly line. It follows that more speed implies more Force.

It is a bit more complicated than this but, simply stated, on a longish cast about 80% of we get from a fly rod is due to leverage and only about 20% is down to energy stored when the rod is bent and then released when it unbends. The exact proportions of lever effect and spring effect don’t really matter to us. What matters is that leverage is by far the major contributor.

Each cast has an energy “budget” to do the job of propelling the line to reach our target. In making budget, the proportion of leverage to spring storage and release of energy changes depending on the type of cast we are making. However, for most people and most of their casting, the rod as a lever accounts for most of the energy put into a fly line. Allow me to explain that a bit more.

The bow and arrow cast is exceptional in that we use spring energy more than leverage but of course that’s a very different beast from a standard overhead cast. With short overhead casts, just the leader and a metre or two of fly line, the rod may not bend very much so unbending contributes very little to line propulsion. As we lengthen the cast the rod will bend more and so contribute more spring energy when it unbends. However, it’s probably not until we go seriously long that spring energy will make a meaningful contribution and even then it’s only about one fifth of the total energy budget. Given this it makes no sense to me to talk about rod loading as the engine and therefore our primary objective when casting a fly line. Leverage is the primary engine of a cast and our objective is using it to best advantage.

What Does the Work?

When you pick up your fly rod you are holding a long flexible stick, a lever, you use to accelerate the paltry mass of the fly line and ultimately the fly tied to the end of your leader. Your flexible stick is towing a string and if you pretend your fly rod is a spear thrower that pulls the spear instead of pushing it, things might Work out surprisingly well.

Remember F=ma? Here is another bit of useful maths and mechanics. The movement of the fly line produced by a caster puts kinetic energy into the fly line. In the physics trade that movement is a form of Work. The Work done by the caster is equal to the Force applied multiplied by the distance over which it is applied. The equation? W=Fd. Yep, the same F which equals mass by acceleration so it’s still a net Force and still headed in a single direction.

Like a lot of other work, getting the casting job done requires sustained and focussed effort. Force is always a net force in a single direction so to do our Work we need to stay on track. Anything else is goofing off.

What Grunde went on to show us was something else very nifty about our springy lever. When your hand stops and the casting stroke is done the lever bit is finished but then the spring part does its Work and delivers its contribution to the kinetic energy put into the line. Wonderfully, as the rod spring unloads it continues to accelerate the line. In a long cast it’s only twenty cents worth of bang for the casting force buck but it’s very nice to have. When the rod straightens fully the acceleration party is over. The line now overtakes the rod tip and a loop is formed. Hold that thought. You will need it for the next episode of the series.

The Real Advantages of Bendy Rods

Now that we have slammed dunked rod loading I hope it is safe to say a few more words in praise of the real advantages of the spring thing. I hesitate because I fear the return of the rod loading zombies but I’d hate for my readers to rush out and swap their beautiful fly rods for a matched set of broom handles with runners and reel seats attached. Don’t do it. Here’s why.

When we apply force, via the rod handle, the bend we put in the rod helps smooths out the acceleration of the line. Power on. There’s a slight lag or delay between our movement and the rod tip/line movement. Power off. When we finish our casting stroke there is also a slight delay between when we stop our hand and when the rod fully unbends. This second delay is much kinder to our body, especially to our wrist and forearm, than stopping the movement of a broom handle.

Let’s revisit the vehicle towing example used earlier only this time we will use a snatch strap instead of a tow rope to connect the two vehicles. A snatch strap is somewhat elastic, far more so than a normal tow rope or steel cable. It is often used to recover a bogged vehicle. The tow vehicle is connected up and starts to drive off (accelerate). The strap comes tight but then stretches until it reaches its maximum stretch and about then (hopefully) the bogged vehicle starts to move. This delay avoids a sudden jolt and the towing vehicle can effectively apply more Force, without damaging jolts, than it could with a relatively inelastic tow rope.

When the recovery of the bogged vehicle is completed the tow vehicle slows. At this point the snatch strap will contract again so there is a slight delay before the Force being applied to the towed vehicle reduces to the same level as it would had we been using a normal tow rope. The delays are due to storing and releasing energy in the snatch strap and are similar to what happens when we bend and unbend the fly rod during a cast.

A quick look at the technical stuff before we get to another topic of much debate among fly fishers namely, rod preference. Earlier we saw that the equation for the Work we do to the line was the Force we apply multiplied by the distance over which we apply that Force. W=Fd.

When a delay is introduced we start to consider a time difference instead of distance travelled (which, of course, involves a difference in spacial location). Am I about to make Einstein happy by referring to the space time continuum? No, but we are sort of headed that way because our fly rods move both in space and in time. From a time perspective we multiply the Force applied by the time during which it is applied. Now we are calculating the Impulse. It’s like Work, only seen from a different dimension – time, instead of space. The bending of the flexible rod stretches the distance over which and the time during which the rod tip tows the fly line – as compared with using a broom handle. This is very handy.

Rod Preference

As you will know different anglers have different preferences in fly rods. Some have different rod preferences for different fishing scenarios. What’s that got to do with bendy rods? The stiffness of a fly rod will affect how easily it absorbs the shocks of a take or a run by a fish. A broom handle won’t help very much with that.

Rod stiffness or modulus has another really important influence on our casting and therefore on our fishing. A relatively stiff rod will bend less than a relatively soft rod in the same line class. That means that the delays in power on and power off will be different – shorter for stiff rods and longer for soft rods. Each angler faces the Goldilocks choice – how much is too much and too little, delay. Different folks, different strokes, different preferences.

No-one wants a tomato stake and very few like a noodle rod to cast with but there’s a lot of room between those extremes. It would be pointless exploring all the variables involved in making an ultimately subjective choice – things like caster smoothness, stroke length for cast length, rate of acceleration and deceleration. However, I can’t finish without a word on casting technique and gear choice. Nothing to do with physics but it needs to be said anyway.

Good casters have varying preferences in fly rods. What sets them apart, however, is that they can readily adapt their casting action to get the most from rods of greatly varying stiffness. They can do that because their casting technique is sound. That is, they have such control over their movements, timing and effort, that they can vary both pretty much at will and without duffing a cast. You can buy rods you like better than others. No problem. Go for it. You cannot, however, buy a rod to fix problems with your casting technique. You can’t buy casting technique. Marketers are very happy to take the money of casters willing to believe otherwise.

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