As new Bowhunters, we are often bombarded by bow hunting terms and ideas and TLA’s (three letter acronyms) that has no real meaning to us and when a salesman tells us it has a lott of thingamajiggies… we don’t know if that is good OR bad…
Here we will clarify some terms for (and if there is another term you are looking for, please contact us, and we will add it to the list!)
Back Wall
(double wall, solid wall, spongy wall)
The Back Wall refers to the end of your draw cycle. Once you have reached maximum draw (on a compound bow) – that is the “Back Wall”
Sometimes archers will refer to a soft back wall or a spongy back wall. This refers to the amount of movement there is left after you have hit the maximum extension. Lower poundage bows will often be able to pull the arrow back past the “wall” because the cable stop can press against a soft string.
A solid back wall is when the draw stops dead against a hard cable or string (or a limb) and a double back wall is when your top and bottom cams are out of sync, and the cable stop on the top or bottom hits before the other one, giving two distinct “walls”
Bow Hand (Bow Arm)
The front hand, the hand/arm holding the bow
FOC – Front of Centre
The FOC of your arrow is important because it allows your arrow to track behind the point. 12-19% FOC is considered “High” and is becoming the new standard for longer shots.
How is FOC Calculated?
- Divide the arrow’s overall length (distance to the bottom of nock groove to end of shaft by 2).
- Find and mark the balance point. Measure from the throat of the nock to the mark
- Subtract center of the arrow measurement (calculated in step 1) from the balance point (calculated in step 2).
- Multiply the result by 100.
- Divide the result by the arrow’s overall length.
That is the FOC percentage of your arrow
Example:
30.3” / 2 = 15.15”
(22.44” – 15.15” = 7.29 x 100) / 30.3”
= 24.05%
| Normal FOC | <12% |
| High FOC | 12%-19% |
| Extreme FOC | 19%-30% |
| Ultra Extreme FOC | >30% |
FPS – Feet Per Second
The speed of an arrow
ft-lbs or ft/lbs – Foot Pounds
ft-lbs is used as the measurement for Kinetic Energy and some places in the world has specific set minimums for hunting with a bow or hunting specific animals.
For instance, in South Africa, to legally hunt a Cape Buffalo, you need by law (Act 10 of 2004 – Hunting Norms and Standards) a minimum of 80 ft-lbs of KE and a minimum of a 750 grain arrow
GAP Shooting
This refers to shooting od distances with a multi-pin sight. For instance, If you need to shoot at 36yds, you will aim with the gap between the 30yd pin and the 40yd pin, instead of holding the 30yd pin high (yes, I know its the same thing, but we tend to overestimate holding high when we focus on a single pin)
GPI – Grains per Inch
In reference to your arrows, every arrow has a weight in grains per inch of carbon rod (that makes up your arrow), so you can calculate the FOC and total arrow weight (and therefore approximate speed and Kenetic Energy) – So if your arrow is labeled as 8.9gpi, then it means that an inch of arrow shaft weighs 8.9grains (or just over half a gram)
I.B.O. Speed (International Bowhunters Association) – Measured
Under this standard for testing the speed of an arrow from the bow being tested, The bow will have a maximum pull weight of 70lbs. The arrow will have a grain weight of 350(5 grains of arrow weight per pound of bow weight). The draw length will be set at 30 inches. The chronograph used for measuring the speed will be placed at point blank range for testing.
KE – Kinetic Energy
Kinetic Energy is essentially the bow’s ability to transfer it’s energy into the arrow and we can use it as a fair measure on our setup to see if there is enough energy to be able to make an ethical kill.
We calculate KE with the Arrow Weight (in grains) and the Speed of the Arrow (in feet per seconds) as follows:
(FPS x FPS x Weight of the Arrow) / 450,240 = Kinetic Energy of the Arrow
example:
274 x 274 x 475 / 450,240 = 79,03 ft-lbs of KE
Momentum
Momentum is the measurement of concentrated force that is moving in one specific direction over a period of time.
Momentum is calculated:
Momentum = mass multiplied by velocity but for an arrow, it is
Momentum(arrow) = (mass multiplied by velocity)/225400
or p = ((m)x(v)) / 225400
(Just FYI: the symbol P for momentum comes from the original term used to describe persistence of the object’s motion)
example:
(472gr arrow x 280fps ) / 225400 = 0,586 slug ft/sec
Release Hand (Release Arm/Release Elbow)
The hand holding the trigger or release