Zradix,
Don’t feel alone! Most folks are confused about the different FOC measurements different folks use. Here’s a link to the FOC discussion in the Study Updates. It gives a complete discussion of FOC and its effects.
http://www.tradgang.com/ashby/2007prologueupdates.pdf Here are a few excerpts from it that might help answer your initial question.
In archery, FOC is specified as a ratio of the location of the arrow's balance point to the shaft’s (or arrow’s) mid-point, expressed as a percentage.
The above definition is sufficiently correct for the common uses archers apply FOC to. However, for the discussions that follow we also need to state the true, precise definition of FOC: For projectiles in flight, FOC represents what percent a projectile’s gravitational balance point is forward of the projectile’s center of pressure (CP). What the heck is the CP? The CP is that exact point where the maximum 'bending force' is exerted upon a projectile during its flight. Note that this true FOC definition relates only to a projectile in flight, and expresses a relationship between the gravitational balance point and the resultant center of pressure of all forces acting on the projectile as it flies through any given medium. Also note that there's no mention of any projectile 'length' in this definition. Additionally, keep in mind that your arrow is still 'flying' during penetration; all that's changed is the density of the medium(s) it's 'flying through'.
The CP of an object in flight is dynamic, and is constantly changing as propulsion forces, resistance forces and forces exerted by moving air currents change. For convenience, the 'practical purpose' formula(s) we archers use merely assumes the CP to be at the shaft's (or arrow's) mid-point. Why we do that is coming up shortly. Note very carefully that the CP (and the genuine amount of FOC) does not - in any manner whatsoever - reflect the point of greatest projectile flex. It indicates the point upon which the greatest flexional force is exerted, not the point of greatest flex. The point of greatest flex depends not only upon the forces encountered in flight (and launch) but also on the projectile's structural design and the material(s) from which the projectile is made.
Is precise FOC measurement critical? Well, yes; if you're trying to calculate a trajectory to guide a missile to a pinpoint target from 2000 miles away, or design an F22 Raptor that can change directions on a dime and darned near fly laterally! For archers, no, precise measurement is not all that critical. All we require is a relative reference point. However, in order to understand Extreme FOC's effects on both arrow flight and arrow penetration you do need to know and understand that an arrow's true FOC is not the same as the 'relative FOC' archers normally discuss; and you need to know what the difference between the two is.
What's the “correct way" to measure my arrow's FOC? The AMO Standard FOC measurement uses shaft-length; ignoring insert, taper and tip (broadhead) length. The other commonly used formula employs the arrow's overall length; including the insert, taper and tip. Which is “correct”? Neither. True FOC is based on the center of pressure. We merely simulate the CP location in both formulas. The AMO formula was adopted as 'standard' merely because, between the two commonly used formulas it uses a simulation point nearer the actual CP location for most commonly used target arrows during flight through air. Just as it is with static spine, the FOC 'number' we use is definitive of absolutely nothing about our arrow's flight.
The commonly used static spine and FOC 'numbers' merely allow us to make a relative comparison of one arrow to another; nothing more. For example, static spine measures relative stiffness of a shaft; how much it flexes when a weight of specified mass is suspended mid-way between two shafts supporting points; which are located a specified distance apart. Everything about the measurement is relative, not absolute. Static spine tells you nothing at all about an arrow’s dynamic spine – how it will react when you shoot it off your individual bow. If you doubt that, perfectly tune an arrow from a true center-shot bow and then measure its static spine on your spine tester. Now take that same arrow and shoot it from a non-center-shot bow (one with a peg rest – no arrow shelf at all) of equal draw weight. What happens? The arrow will shoot massively strong-spine. The arrow hasn't changed; the launch force and power stroke are the same; and the shaft's static spine hasn't changed. However, the shaft's dynamic spine is now no longer anywhere close to correct, and it no longer shoots where you're aiming.
All static-spine indicates is the relative stiffness of the shaft. What it does do is provide you a reference point. This helps whenever you need to find a stiffer or softer spine in order to get your arrow to shoot well from your bow. This is all it does; nothing else. It merely allows you to compare shafts relative to each other, so you can tell which one is 'stiffer' and which one is 'weaker'. Static spine's 'relativity' is precisely why it's necessary to tune your arrow to your bow in order to get correct arrow flight. No static measurement or calculation contends with the myriad variables encountered when you shoot an arrow from your bow. This is why, besides charts, Easton publishes 35 instructional pages on selecting 'the right arrow' after you’ve used their 'static-spine' charts to find a 'starting place'. No chart provides a magic number saying, “Pick me. I’m the right one!”
Commonly used FOC measurements are exactly the same; they are relative. Neither formula is “correct”, nor is either “wrong”. Each serves its purpose equally well; providing a reference point. As long as you know which formula was applied to a given arrow to determine its 'relative FOC', you can duplicate results. If you prefer, you can re-measure and state the arrow's FOC in the other format; that’s perfectly alright. It still provides you a 'relative reference'. For practical applications, either commonly used FOC formula works equally well. Just remain aware that neither genuinely tells you anything at all that's 'precise' about an arrow's true FOC.
However, for a given arrow design, when our 'commonly measured' FOC goes up the true FOC also goes up; but the amount we've 'measured' won't indicate the actual amount of change in true FOC. The single most important thing to remember is that the 'relative measurement' method you use should always be stated, so everyone is “reading off the same page” when making comparisons, or trying to duplicate results.Hope that helps,
Ed
Topic: FOC Calculator differences..which is right? (Read 378 times)
