FOC stands for Forward of Center balance point. This measurement results from the relative weights of the components used in the arrow: shaft, insert, head, Fletching and nock. A properly blanced measurements of 7 to 10% There's an archery term that's used to describe an arrow's balance point. It's called Forward of Center, or FOC. You arrive at FOC by making a few measurements and then running the numbers through a simple formula. Here's the process:
Overall length: Measure the total length of the arrow from the nock groove to the tip of the arrow with the point you plan on using.
Balance point: Install the tip you will be shooting. If you are testing stability for 3-D shooting put your field point or nibb into the arrow. Of course, for hunting install your broadhead. Find the arrow's balance point by sliding it back and forth along a fairly sharp edge. You'll find the spot where the arrow just balances. Mark it carefully. Now measure from the point of the arrow to the balance point and enter the distan ce above.
Determine FOC: To find the FOC (which is always expressed as a percentage) divide the overall length by two. This should produce the physical center of the shaft. Now subtract this number from the balance point and divide by the overall length. Multiply by 100 to express the fractional value as a percentage.
If you've ever played a friendly game of darts, you've surely noticed that the dart is designed so that it's heavy in the front, and light in the back. If the dart were weighted the opposite way, with the tail being heavier than the tip, it would literally spin around and hit the target tail-first. Obviously the ballistics of a dart and an arrow are a bit different, but the underlying concept is similar. A projectile's flight is most stable when most of the projectile's mass is positioned Front of Center (FOC). As such, an arrow should be heavier in the front than in the back. But how much? Where's the "perfect" balance point?
This is another hotly debated issue among archery enthusiasts. Some claim that FOC makes little or no difference, others swear that FOC has a profound effect on accuracy. Even the industry experts don't seem to agree, as the ballistic physics for FOC include some rather elastic variables that make finding an "mathematically optimal" FOC very difficult to declare and prove. To make matters worse, we even see a variation in how FOC itself is calculated, depending upon which "expert" you ask. So while we have no interest in the fine points of the debate, we will agree that the tricky issue of FOC is at least worth considering when purchasing a new set of arrows.
With all that said, it is generally believed that an arrow with a high FOC will fly well, but with premature loss of trajectory (nose-diving). While an arrow with a low FOC will hold it's trajectory better, but it will fly erratically. So again, another trade-off for you to consider.
What am I looking for? While there doesn't seem to be a magic number to aim for, it is generally agreed that the optimal FOC balance for an arrow is somewhere between 7% and 15%. In the example on the above, the 30" long arrow has balance point that is 3" forward of the arrow's actual center (15"). So it's FOC is 3/30 or 10% - a reasonable FOC balance.