While there are other factors to consider when setting up a compound bow for hunting, arrow speed is certainly at the top of the list for many bowhunters. A fast arrow has a flattest trajectory possible, and that’s a good thing for several reasons, a big one being the leeway in aiming at a distant target at an unknown distance. There are several factors that go into the arrow speed equation. All things being equal, they are as follows:

1) Draw weight: The more draw weight you pull, the faster the arrow will fly. The rule of thumb is for every 10 pounds of draw weight your bow is reduced, arrow speed is reduced 15-20 fps.

2) Draw length: A longer draw length creates a longer power stroke than a shorter draw length, allowing the bow to store more energy. The rule of thumb is for every added inch of draw length, you increase arrow speed approximately 10-15 fps. And remember, the draw length of your compound bow must be set exactly to your own body morph. If your draw length is 28 inches, then your compound must be set at 28 inches — not 27.5 or 28.5, but 28 inches. 

3) Arrow weight: The rule of thumb is for every 5 grains of added arrow weight, you will drop arrow speed by 1-2 fps. Keep that fact in mind when choosing broadheads.

4) Bowstring weight: Adding weight to a bowstring (peep sight, string silencers, string loop) will affect arrow speed, but not by much. Generally speaking, adding 20 grains of overall weight to the string will deduct only about 5 fps from arrow speed.

Advertised Bow Speeds

Many bowhunters don’t quite understand the complexity behind advertised bow speeds. They see a speed rating listed on a website and assume they’ll get that arrow speed if they buy that bow. But this rarely happens, and here’s why: There are two standard methods of estimating – and selling -- the arrow speed of a compound bow. The IBO method sets up a bow, then uses an arrow weighing 5 grains per pound of draw weight. Another common speed rating is the ATA rating, which uses a 30-inch draw length, 70-pound draw weight, and 350 grain arrow.

Thus, when a manufacturer touts its bow as having a speed of, say, 350 fps, the truth is a hunting weight arrow of 400-450 grains from that same bow will leave the bow much slower – especially if the draw weight and draw length are significantly reduced from 70 pounds and 30 inches. You have to shoot your hunting weight arrow through a chronograph to know exactly how fast it is flying. And remember, in bowhunting speed is good – but it is far from everything.

Shaft Selection

Choosing the right arrow shaft comes next. You must use a shaft with the correct arrow spine for your draw length/draw weight combination. Spine is a measure of stiffness, gauged by how much the arrow flexes when weight is applied to the center of the shaft. To determine static spine, a 28-inch arrow is supported at both ends, then a 1.94-pound weight is hung from the middle. The amount of flex that is induced on the arrow shaft by the force of the weight is then measured, giving the “static” spine rating.  For example, if an arrow bends .340-inch at the center, then the shaft has a static spine deflection of 340, and indicated by a number on the shaft and charts created by arrow manufacturers. The higher the number, the easier the shaft to bend. For example, an arrow with spine number 340 is stiffer than a 400 spine arrow.

Arrow shafts come in varying weights, measured in grains/inch, in the same spine size. Thus, you can choose how heavy an arrow you want to shoot and still use the proper spine. All the elements of arrow flight are controlled by your bow’s draw weight/draw length combination, the weight of your finished arrow shaft, and the front of center (FOC) balance of the shaft. This, in turn, will determine how much kinetic energy and momentum the shaft has when it reaches the target.

For most bowhunting, balance means settling somewhere in the middle of the extremes between ultralight and super-heavy arrow shafts. An ultralight shaft will leave the bow faster and initially have a flatter trajectory than a heavier shaft, but will shed its velocity faster at longer ranges and not have as much penetrating power. The heavier shaft will not fly nearly as fast, but it will produce more penetrating power downrange. A medium-weight shaft will create a balance between arrow speed, trajectory and penetrating power.

Here’s an example, using my own 340 spine, 28.5-inch shafts with both 100- and 125-grain broadheads shot from a 70-pound-draw compound. With the 100-grain heads, a Victory Archery VAP has a total arrow weight of 379.1 grains, while an Easton 4mm FMJ weighs 480.1 grains. The VAP leaves my bow at 283 fps, while the FMJ leaves my bow at 263 fps. Switching to 125-grain heads, arrow speed is 273 fps for the VAP, and 253 fps for the FMJ. While none of these shafts fall in the ultralight category (which I would never use when bowhunting big game), all these combinations produce enough kinetic energy (KE) to ethically bowhunt any animal in North America.

The lightest VAP with 100-grain head generates 67.42 foot/pounds of KE, while the FMJ with 100-grain head generates 73.74 foot/pounds. The heavier FMJs fly slower, but they will definitely out-penetrate the medium-weight VAPs. However, the FMJ’s trajectory arc will be larger, making precise knowledge of the distance to the target more critical.

Kinetic Energy vs. Momentum

Let’s talk kinetic energy (KE) and momentum. The term kinetic energy is derived from the Greek words for motion, kinesis, and active work, energeia. Therefore, the term kinetic energy means “through motion, do active work.”

Energy can exist in many forms, and each form of energy can often be converted to other forms. When it comes to bows, energy is stored in the limbs and cams when the bow is drawn, then transferred to the arrow shaft at the shot in the form of kinetic energy, or KE. Because the speed of the shaft is reduced due to both gravity and air resistance, the KE also changes along the flight path. That means the KE you measure a few feet in front of the bow as the arrow is released is nowhere near the KE delivered downrange on the target. That’s why when hunting large animals such as elk and bears, experienced bowhunters try to “juice up” their bows to deliver more initial KE, and thus deliver more energy on the target. 

This is the formula for KE as measured in foot/pounds: KE = mass x velocity-squared divided by 450,240, with mass the total arrow weight and velocity the arrow speed. (Click here for an online kinetic energy calculator from Victory Archery.)

So let’s say you have an arrow/point combo that weighs 400 grains and flies at a speed of 290 fps. Your equation would look like this: 400 x 290 x 290 / 450,240 = 74.72 foot/pounds of KE.

For decades, the following have been the generally accepted minimum initial KE values for bowhunting game of all sizes: small game (rabbits, squirrels, etc.), up to 25 foot/pounds; medium-sized game (deer, pronghorn, sheep, etc.), 25-41 foot/pounds; large-sized game (elk, black bear, wild boar, etc.), 42-65 foot/pounds; toughest game (brown bear, grizzly bear, large African game), 66 foot/pounds or more.

Momentum can be defined as “mass in motion.” All objects have mass, so, if an object is moving, then it has momentum, or mass in motion. This is the formula for calculating momentum: p (momentum) = m (mass) x v (velocity). (Click here for an online kinetic energy vs. momentum calculator from GrizzlyStik.)

Because both involve calculations of mass and weight, KE and momentum are often interchanged, but that’s not right. Simply stated, KE can be thought of as a big hammer and the arrow shaft as a nail, with KE driving the nail through the animal. KE values can be adjusted by altering either arrow speed, arrow weight, or both. On the other hand, think of momentum as downrange energy retention, or penetration potential.

Another way to look at it is that, because velocity is squared, KE favors arrow speed, while momentum, which gives both mass and velocity equal bearing, favors weight. And, all things being equal, the heavier arrow will penetrate better than the lighter arrow.

Final Thoughts

The key is finding the sweet spot. You can go to a lighter arrow to increase your speed and flatten trajectory, which might give you a KE value increase. But that might actually reduce momentum — and the depth to which your arrow and broadhead will penetrate larger, tougher animal. A good example of the difference: Which would hurt you more, being hit in the ribs by a ping-pong ball traveling at 300 fps, or a golf ball traveling at 200 fps?

I’ve never seen a chart showing recommended momentum values for various big game animals. However, when I bowhunted Cape buffalo in South Africa, the minimums required by law were an 80-pound draw weight on a compound bow, an arrow that weighed a minimum of 750 grains, and a two-blade, cutting tip broadhead. My bow launched my 816-grain arrow (with broadhead; photo below) at 218 fps, and it penetrated the 1,700-pound buffalo to the fletching. That was all about momentum.

In 2021, I shot a bull elk at 15 yards with an arrow weighing 410 grains (this included a 125-grain SEVR mechanical broadhead) flying at 285 fps, and it penetrated the entire chest cavity. And that’s what you’re striving for, a balance between KE and momentum.

Sidebar: A Lesson in FOC

An often misunderstood and misused term in the world of arrows is FOC, an acronym for front-of-center balance that describes the percentage of the arrow’s total weight (field point or broadhead included) located in the front half of the arrow. The more weight that is located in the front half of the arrow, the more forward is the arrow’s center-of-balance.

Why is this important? Simply stated, the FOC position on the arrow shaft is a key factor in determining the shape of the arrow’s trajectory curve. The right FOC is especially important in long-range shooting. The accepted standard for the correct FOC for a hunting arrow is 10-15 percent FOC for optimum accuracy potential — especially at long distances.

The FOC formula is FOC % = 100 x [B – (A/2)] / A, where “A” is the arrow length, and “B” is the distance from the nock groove to the arrow balance position, including the arrow point/insert weight, nock and fletching. Note: Arrow length “A” is the distance from the bottom of nock groove to end of shaft, not including broadhead or field point.

To get started, build your hunting shaft using all of the components – broadhead, insert, vanes, nock and cresting if desired. Then, follow these five steps.

1. Divide the arrow’s overall length “A” by 2.

2. Find the balance point. This is where the arrow balances perfectly. Mark the balance point, and measure from the throat of the nock. This is the letter “B” in the above formula.

3. Subtract center of the arrow measurement (calculated in step 1) from the balance point (calculated in step 2).

4. Multiply the resulting number in Step 3 by 100.

5. Divide the resulting number from Step 4 by the arrow’s overall length. Boom! This number is the FOC % of your arrow.

Of course, if you don’t want to do the math yourself, there are many FOC calculators available online. You’ll still need to measure the “A” and “B” for your arrow. Click here for one from Easton Archery. 

You can change the balance point of an arrow shaft by simply adding or subtracting weight to either the front or the rear of the shaft. Heavier vanes and a lighted nock, for example, will add weight to the back end of the shaft, decreasing FOC. Trading out a 100-grain broadhead for a 125-grain head is a simple way to increase the FOC. You can also use a heavier arrow point insert or add a metal weight insert.

Photos by Bob Robb