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.

Gold Tip 100-grain brass insert

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.

Strickland’s Archery 200-grain broadhead

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 bear.

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.

Saunders 250-grain field point

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.

What’s the difference between momentum and kinetic energy?

When it comes to bowhunting, kinetic energy gets all the love. You can find scads of articles extolling its virtues and charts describing its professed impact.

Momentum, by comparison, is the genius, recluse cousin few people talk about, but who has a cult following. It’s punk rock before that brand of music became mainstream.

broadhead momentum

In another blog entry, we explored how kinetic energy relates to bowhunting. Kinetic energy is described as the energy of motion. The faster and heavier an object is, the more KE it carries.

Lots of hunters swear by KE, and they do whatever they can to maximize it with their bowhunting rigs.

But there are others who could care less about KE, and instead focus on boosting their arrow’s momentum. Both account for the weight and speed of a moving object, but they are not the same.

One of the best explanations I’ve heard about how to differentiate KE from momentum comes from the Quality Deer Management Association, which is a true believer in momentum.

QDMA explains that, with KE, speed is emphasized over weight. An arrow’s KE is calculated by the equation: velocity squared times weight divided by the constant 450,240. That results in a unit of energy in foot-pounds. That’s the amount of energy needed to exert a 1-pound force for a distance of 1 foot.

Momentum, QDMA says, emphasizes weight over speed. An arrow’s momentum is calculated by taking weight times velocity and dividing by the constant 225,400. (The constant accounts for the arrow’s weight in grains and factors in the pull of gravity.) This gives you a unit of force measured in slug feet per second. A slug is a unit of mass that accelerates by 1 foot per second per second when acted upon by a 1-pound force.

So my Easton FMJ that weighs 485 grains and flies at 292 feet per second has .63 slug ft./sec. worth of momentum.

(And therein lies the likely reason momentum takes a backseat to KE. Who knows what .63 slug ft./sec. means other than a physicist?)

Momentum is a measurement of the force of the forward movement of an object. KE is a measurement of the energy a moving object possesses, but it has no direction.

Momentum advocates say KE tells you how hard an arrow will hit, but that has nothing to do with penetration. Momentum is what allows an arrow to blast through tissue, hide, bone, etc.

QDMA suggests that momentum is particularly more important than KE when archers are working with low-poundage bows – say, below 60 pounds. Archers can increase their KE by switching to lighter arrows, which will boost the arrow’s speed. But that lighter arrow isn’t going to have the same momentum as a heavier one, and so it takes less resistance to stop its forward progress.

Generally speaking, it’s often recommended that bowhunters using compounds shoot arrows that weigh 6-8 grains per pound of draw weight. That would require a hunter using a 60-pound bow to shoot an arrow weighing 360-480 grains. The bigger the game, the heavier you’ll want to go.

(If you really want to push the momentum envelope, check out the super heavy shafts that weigh more than 9 grains per inch for a 400-spine arrow. The Easton FMJ weighs 10.2 GPI ; Gold Tip Kinetic Hunter weighs 9.5 GPI; and Carbon Express PileDriver weighs 10.4 GPI.)

To keep your arrow’s momentum as high as possible when shooting compounds that draw less than 60 pounds, stick to the high end of the 6-8-grains-per-pound weight range. So if you’re shooting a 40-pound bow, go for an arrow that’s in the neighborhood of 320 grains.

Traditional bowhunters tend to stick to an average of about 10 grains of arrow weight per pound of draw weight in the interest of generating lethal momentum from a bow that produces relatively slow arrow speeds.

(Of course, arrow selection always starts with matching the spine to your bowhunting setup. Don’t deviate from that just to gain arrow weight.)

As QDMA notes, no one apparently has done any tests to devise a chart that suggests how much arrow momentum is needed to punch through various game animals. So there don’t seem to be any definitive recommendations for momentum.

The good news is, modern bow and broadhead designs are making bowhunters more effective at achieving the lethality we owe the animals we pursue. The discussion continues, and our understanding will too.

What is Kinetic Energy and Why does it matter?

What better time for a physics lesson than the start of the fall bowhunting seasons?

If you’ve never heard the term mentioned, you should familiarize yourself with it before you head out into the field with a bow and arrow in pursuit of wild game.

Kinetic energy.

What is it?

According to physicsclassroom.com, kinetic energy is “the energy of motion”  ‑ or, the energy possessed by an object in motion.

The amount of kinetic energy a moving object possesses depends on two factors – its weight and its speed.

The heavier an object is, and the faster it’s moving, the greater its kinetic energy.

In the bowhunting world, archers should be concerned about the amount of kinetic energy (KE) carried by their arrows as they hit game animals.

An arrow needs sufficient KE to punch through hide, tissue and possibly bone. And generally speaking, the bigger the animal, the greater an arrow’s KE needs to be to carry it through the vitals.

KE, incidentally, is measured in foot-pounds.

You can think about KE this way. Let’s say a ping-pong ball and a golf ball both traveling at 30 mph hit you in the head. They’re both about the same size traveling at the same speed, but the golf ball is heavier, and is obviously going to hurt more. Its KE is much higher than the ping-pong ball’s.

To calculate your arrow’s KE, you will need to weigh it as you’d shoot it. That is, weigh your exact hunting arrow, with the broadhead attached.

You want to come up with your arrow’s weight in grains, so adjust your scale accordingly.

Next, shoot your arrow through a chronograph to come up with its speed.

Once you have the weight in grains and the speed you can complete the following equation:

Velocity squared times weight divided by the constant 450,240.

(The constant number is derived from the conversion of grains to pounds, while factoring in the effect of gravity on mass.)

So let’s say you have an arrow that weighs 400 grains that flies at a speed of 290 feet per second.

Your equation would look like this:

290 x 290 x 400 / 450,240 = 74.72 foot-pounds of KE.

What do you need to get the job done in the field?

There are a number of charts that recommend a certain amount of KE for game animals of different size.

Generally, the recommendations are as follows:

Kinetic_Graph

 

What can you do to boost your arrow’s KE?

For starters, you want to make sure you are maximizing the available KE by tuning your bow so your arrows fly straight. A fishtailing arrow won’t fly as fast as it could. And make sure you’re using razor-sharp broadheads. The more energy it takes to push an arrow through an animal, the faster it’s going to slow down.

You can shoot a heavier broadhead. That’s going to slow down your arrow a bit, so you’ve got to repeat the KE calculation to make sure you’re getting a benefit from the switch.

You can switch to a heavier arrow shaft. It’s also going to fly slower, so be sure you recalculate your KE.

And make sure you don’t change the arrow’s spine when you switch to a heavier shaft.

(Click here for a discussion about arrow spine vs. arrow weight.)

Compound bow shooters can increase the draw weight of their bows by turning the limb bolts clockwise, provided they’re not already at the maximum weight limit.

Be sure to turn the top and bottom bolts exactly the same. That is, if you put half a turn into the top bolt, do the same to the bottom.

Recurve archers who shoot takedown bows might want to switch to limbs with a heavier draw weight.

Increasing draw weight increases arrow speed regardless of the bow.

Of course, that can make your bow difficult  – if not impossible – for you to draw, so proceed with caution.

Know your KE before you hit the woods this season. It can mean the difference between success and failure.