How do you calculate FOC on arrows?

How do you calculate FOC on arrows?

If you're new to archery, you've probably heard a number of confusing acronyms and terms when it comes to the flight of an arrow. One of the terms you'll hear in archery is FOC or Forward of Center, which on an arrow can greatly impact the trajectory of your arrow on longer shots. If you're always shooting short distances with high powered bows, the Forward of Center on an arrow may not impact the shot, but once you start shooting further out, it does affect where your arrow will hit. So, let's look at FOC and what it is.

Calculating FOC on an arrow

What Exactly is Forward of Center or FOC?

The Forward of Center or FOC on an arrow is the percentage of the arrow's weight in the front half of the arrow. The FOC will cause the center of balance of the arrow to shift. (The center of balance being the point on the arrow where if you balance the arrow on a finger it stays perfectly level.) If the front half of the arrow is heavier than the back half, it causes the center of balance on the arrow to shift towards the front. Arrows with high FOC will be more stable in flight, but they will drop quicker and will nose dive, creating a shorter trajectory. They also have more penetrating power. Arrows with low FOC will keep a more flat trajectory, but will be less stable in flight and will veer off course more often. They also have less penetrating power than arrows with high FOC. Obviously, you want to minimize the problems that both high and low FOC present while maximizing the benefits. So, let's look at how to calculate FOC.

How to Calculate Forward of Center or FOC

The first step to understanding the FOC of your arrow is to calculate it. The equation for FOC is as follows: FOC percentage = (100 x ( A - L / 2) ) / L 

Where: 

  • A is the distance from the center of balance (often called center of mass) on the arrow to the bottom of the nock groove on the arrow's shaft. 
  • L is the overall length of the arrow shaft (not including field point or broadhead) to the bottom of the nock groove on the arrow's shaft. 
  1. You'll need to figure out your arrow's center of gravity first. To do so, you will need your field point or broadhead on the arrow and try to balance the arrow on the back of your index finger until the arrow is level. You'll need to mark that point. 
  2. Get a tape measurer and measure the arrow from the center of gravity to the bottom of the nock groove on the arrow's shaft. That is A. 
  3. Then, measure your arrow shaft from where the field point or broadhead attaches to the shaft to the bottom of the nock groove on the arrow's shaft. That is L. 
  4. Divide L by two.

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