What is F.O.C.? And how does it affect my arrows?
F.O.C. is a hot topic in arrow-building discussions today.
What is F.O.C.?
It’s the acronym for “front of center.” What it refers to is
the percentage of an arrow’s total weight – including the point - that is
concentrated forward of the center of the arrow.
F.O.C. is something that mainly bowhunters are concerned with, and there’s no question that having a solid F.O.C. number is key to getting good arrow penetration on a big game animal.
But some bowhunters think F.O.C. is the only factor they
should be concerned with in preparing hunting arrows, and they don’t understand
the consequences of simply beefing up the front end of their arrows.
Let’s start with a minimum. Easton Archery recommends arrows
have a minimum F.O.C. of 10-15 percent. That’s going to allow an arrow to fly
accurately, especially at longer distances. If you go less than 10 percent, the
arrow’s trajectory will be flatter, but its flight will be more erratic.
That 10-15 percent is what Easton recommends for target
arrows and for hunting arrows. The amount of weight needed up front to hit that
range will be sufficient for hunting, according to Easton.
A lot of bowhunters today try to get their F.O.C. to 20 percent and even a little higher. They can do that by adding weight to their inserts. A standard aluminum insert might weigh about 16 grains, where there are brass inserts that can weigh 100 grains. Also, some insert manufacturers allow weights to be screwed into the backs of their inserts, which is another way to add weight to the inserts.
Bowhunters also can add weight by shooting heavier broadheads. A standard broadhead weighs 100 grains. But there are common options for 125 and 150 grains. And there are special broadheads aimed primarily at the heavy F.O.C. fans that weigh 200 grains.
So if a bowhunter swaps out that 16-grain aluminum insert
for a 100-grain brass insert, and trades a 100-grain broadhead for a 150-grain
model, that hunter just increased the front-end weight of that arrow by 134 grains.
That’s sure to boost the arrow’s F.O.C. considerably.
No question that arrow now will have improved penetration
capabilities. But it also could cause problems for the bowhunter.
For starters, with all that weight added to the front of the
arrow, the arrow’s spine is considerably weakened, and accuracy problems are
likely. According to Easton’s hunting arrow shaft selection chart, an archer
shooting a 29-inch arrow from a 62-pound bow should choose an arrow with a 340
spine while using a 100-grain broadhead. If the archer only increases point
weight by 50 grains, that archer should be shooting a 300-spine arrow. The more
weight you add to the front of an arrow, the stiffer that arrow needs to be to
support that extra weight.
A second issue could be trajectory. When you add weight to
an arrow, you slow it down, which adds more curve to its trajectory arc. For
the Eastern tree stand hunter who expects most shots to be under 20 yards,
that’s probably not an issue. But it could be for the Western hunter who is
spotting and stalking and might have to shoot out to 60 yards. With that much
weight added, a 2-yard miscalculation in shooting distance could easily result
in a miss.
No question there are benefits to boosting an arrow’s F.O.C.
to increase its capability of punching through an animal. Some animal hides are
notoriously tough, and if the arrow hits a bone, it would be nice if the arrow
could punch through that bone.
But as with many things in archery, balance is important.
Kinetic energy is the amount of energy a body has in motion. It’s calculated by
a formula that relies on the weight and speed of a moving object.
To calculate KE in foot pounds you would take the arrow weight and multiple it by the velocity squared, and divide that number by 450,800. For hunting game animals like antelope and deer, Easton recommends an arrow have KE values of 25-41 foot pounds. For elk, black bear and boar, Easton recommends 42-65 foot pounds.
To illustrate what an arrow build would be to meet those
minimums, let’s look at the popular Easton Axis 5mm. A 29-inch, 340-spine arrow
weighing 9.5 grains per inch, with a standard insert and fletchings would weigh
about 315 grains. Add a 100-grain point and you get a 415-grain arrow. Shoot
that arrow from a 70-pound bow drawn to 29 inches, and a speed of about 290
feet-per-second is likely.
The KE value for that arrow is 77 foot pounds. That’s well
above Easton’s recommendation for any of those animals. So it’s safe to say
that arrow is sufficient for bowhunting all of them.
The F.O.C. for that arrow is 12 percent, which is also
within Easton’s recommended range. If I add a bunch of weight to the front of
that arrow to try to get to 20 percent F.O.C., I am increasing the penetration
capability of an arrow that already is capable to killing a deer, elk or black
That’s not necessarily a bad thing. But remember to consider
arrow spine and performance, along with your hunting expectations as you are
building arrows with an eye toward boosting F.O.C.
A simple, inexpensive way to test arrow performance with different F.O.C. values is to get screw-in field points of varying weights. Saunders makes field points as heavy as 250 grains. Shoot several arrows with points of different weights at whatever you consider to be your maximum effective range. By doing this, you should be able to determine what gives you the tightest, most consistent groups.
Don’t just look for the tightest groups. You also want to
consider forgiveness. That is, which arrows hit closest to your aiming point
when you make a bad shot. If you have an arrow setup that produces 2-inch
groups at 50 yards, but a slight bobble on your part throws the arrow off 8
inches, versus an arrow setup that produces 4-inch groups, with imperfect shots
only missing by 3 inches, you should consider going with the latter setup.