Fair enough - I was not about to enter the debate on heavy verus light – in fact I never mentioned spines etc - those are all different subjects.
However, should one want to venture down this path ...........
How much FOC does one need?
The range of FOC recommended for different forms of archery varies. In their charts,Easton shows the following recommended FOC ranges, with calculations based on the AMO Standard formula:
o FITA (Olympic Style) 11% to 16%
o 3-D Archery 6% to 12%
o Field Archery 10% to 15%
o Hunting 10% to 15%
FITA shooters, who compete at the longest ranges, use the highest average FOC’s; 3-D shooters the lowest; with field archers and hunters in-between.
Why do FITA shooters prefer a high amount of FOC?
They are seeking precision long range accuracy. To achieve this, the arrows must be very stable in flight. High FOC permits stabilization from relative smaller fletching. Smaller fletching offers a lower drag factor and is less subject to the effects of cross-winds than larger fletching. These factors become important at the extreme ranges at which FITA shooters compete.
What is the lowest FOC usable?
Hunters need fairly high FOC. Broadheads exert a steering effect upon the arrow, due to wind-shear. Fletching must to overcome these "wind-plane" forces. High FOC means fletching has a longer "lever", and more steering control. The shorter the arrow one shoots, the higher the FOC should be. Shorter arrows are inherently less stable in flight. The longer rear lever helps fletching overcome this. A finger release also adds to arrow instability, especially ininitial flight. Here to, high FOC is beneficial.
Spine selection
Static spine measures relative stiffness of a shaft; how much it flexes when a weight of
specified mass is suspended mid-way between two shaft-supporting points; which are located a
specified distance apart.
Everything about the measurement is
relative, not
absolute. Static spine tells nothing of an arrow’s dynamic spine. From it one gleans only an
indication of
relative stiffness. What it does do is provide a
reference point. This helps when one needs to move to a stiffer or softer spine. It allows comparison of shafts;
relative to each other. This is why tuning arrow to bow is important. No static measurement or calculation contends with the myriad variables encountered when one shoots an arrow from a bow. That’s why, besides charts, Easton publishes 35 instructional pages on attaining "the right arrow". Charts provide no magic number saying; "Pick me. I’m the right one!"
Why does Extreme FOC give more tissue penetration?
They encounter lower resistance. The reduced resistance results from less shaft-flex on impact. Prior testing has shown shaft flex increases shaft-drag, and shaft-drag is a major influencing factor on penetration.
How and why do Extreme FOC arrows achieve this reduction in shaft flex?
Reduced shaft flex is
related to CP location, relative to the arrow’s center of mass. Extreme FOC means the
predominate arrow mass has a very short lever arm. The shorter this lever arm, the less the flexion when obliquely acting force is applied at the arrow’s tip. It is
suspected that, for a given shaft, the effect
may be
proportional to the ratio of the lever arms; when all else is equal.
Should this prove true, one would want as high a FOC as possible when maximizing penetration.
Extreme FOC arrows have at least two design features which reduce shaft flex on impact. These are:
1. Less arrow mass is towards the rear, reducing the force with which the arrow’s rear "pushes" on the shaft.
This is easier to understand if one thinks of super-gluing a brick to one end of a slender shaft. Now place the other end of the shaft on the floor. Unless one keeps the shaft absolutely perpendicular to the floor, the shaft flexes. Next, bump the shaft against the floor. Even when perpendicular, the shaft flexes at impact. Collision forces are
required to go somewhere. The resultant force vector; between floor-impact and "push" from the rear; must either compress the shaft linearly, or be redirected, causing shaft flex.
Now reverse the shaft, placing the brick on the floor. The shaft does not flex. Bump it up and down. Shaft flex is scarcely visible. This is a drastic example of this Extreme FOC effect, but clearly demonstrates what happens.
2. Extreme FOC arrows concentrate arrow mass far forward.
The
forward lever arm is short. This means the
dynamic center of pressure at impact is also far forward. To understand how this short lever with forward mass affects shaft-flex, think of the distance from arrow front to balance point as being a short section of shaft. The shorter the section, the stiffer it is. The stiffer it is, the less it flexes.
Here one may wish to use the slender shaft and brick again. Hold the shaft near mid-point and turn it such that the shaft is not vertical. Note the "bending". Without changing the angle at which the shaft is held, shorten the "forward lever" by holding closer to the brick. The shaft flexes less. The closer one’s hand is to the brick, the less the brick’s "given force" flexes the shaft.
It is hoped the forgoing will help clarify FOC, how it is used, and its many effects. but as I have stated before - the older, experianced guys probably know all this stuff already - this might be interesting for the new guy's though.
Originally Posted by Kudu
Fair enough - I was not about to enter the debate on heavy verus light – in fact I never mentioned spines etc - those are all different subjects.
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