Here’s about the best I can explain what we are measuring. I hope it clarifies the question.
No matter which measuring method is used it is imperative to remember one is only getting a relative number. What we measure is not the true FOC. The true FOC depends on the location of the Center of Pressure (CP) on the arrow in flight, in relation to the center of mass (the Gravitational Center). It has nothing at all to do with to do with projectile length. Our main need in measuring “relative FOC” is a method to be able to duplicate an arrow setup.
A good analogy is with the static spine. Why do we measure it at 26” or 28” between centers? Why not measure it at the full length of each shaft? The answer is simply that it doesn’t make any difference what method we measure it by, because it only measures relative stiffness, not the actual dynamic spine the arrow will show when shot. All we need to know to duplicate an arrow is the relative relationship, one shaft to the next. It’s the same for FOC.
We generally do the initial development of our EFOC arrows with something other than a broadhead, then changing over to a broadhead of the same weight. If we measure the FOC using the overall arrow length we are going to have two arrows that shoot the same but have different measured, “relative FOC’s”. Going back to our static spine example it would be like using the traditional 26” or 28” on-center static spine measurement while developing our arrow then, once finished, going back and measuring the static spine using the actual length of the arrow and then stating that as the measured static spine. What actual purpose does it serve? None. All it does is add a layer of confusion.
Similarly, I think measuring the FOC using the overall length merely adds a layer of confusion, where the same arrow setup, with the same point weight shows two different “relative FOC’s” depending on the length of the point. For purposes of duplicating the arrow we don’t really need that information.
It really makes no practical difference which method is used but if one states the FOC of their arrows with broadheads in place, using the overall length, then someone trying to develop an identical arrow, having the same FOC, they will have to ether do the development with a broadhead or periodically take their field point off and stick a broadhead on the shaft to see what the FOC measures. Just seems like unnecessary complication to me.
Though not directly related to how we measure our ‘realative FOC’ I will add (and I HOPE this does not add to any confusion) what MAY help with understanding FOC. The Center of Pressure (CP) is that point along a projectile in flight where the air pressure forward of and posterior to it are equal. The farther the CP is behind the Center of Gravity (GC) the more stable the flight. By moving weight forward on the arrow we are trying to increase the distance between the GC and the CP. The other way we can increase true FOC is by increasing fletching size; but there are downsides to increasing fletching size.
It’s important to realize that the ‘true FOC’ is dynamic; it changes during flight. Our goal is to maintain the highest possible level of true FOC throughout the arrow’s flight; we want the highest ‘AVERAGE TRUE FOC’ from the instant of arrow launch until the arrow stops all forward movement – even during penetration. The big advantage of increasing FOC by moving the weight forward, rather than increasing fletching size, is that the effect remains more of a constant during flight. Fletching’s effect on FOC is nearly nonexistent at the instant of launch, at its maximum at maximum arrow velocity, and diminishes as the arrow slows.
Increasing the size of fletching on an arrow increases it’s stability in flight because it increases the air pressure on the arrow’s rear, which also increases the distance between the GC and CP. The downsides of using increased fletching to get increased ‘true FOC; in flight are: (1) it slows the arrow; (2) the effect is not present at the instant of arrow launch; it comes into effect as the arrow begins to move forward and increases as the arrow accelerates (and decreases as the arrow slows down); (3) it takes longer for it to overcome arrow paradox than does having a higher FOC resulting from increased weight forward and, (4) because the arrow has slowed dramatically it has virtually no beneficial effect on on ‘true FOC’ during tissue penetration.