by Jeff G. Phillips
FastestBows.com

Fine tuning arrow flight by using FOC. (Forward/Front of Center).  This method of fine tuning is something that not everyone knows about, but should.  By determining your arrows FOC percentage, you will be able to make changes/adjustments to help increase arrow flight stability and range.

In order for an arrow to fly correctly, with the tip in the lead and the fletching following in the rear, the center of mass must be located somewhere between the tip and the middle of the arrow shaft.  If the center of mass is located close to the tip, the arrow will have good stability but will drop quicker because of the heavy nose.  However, if the center of mass is located close to the center of the shaft, the arrow will have good range, but arrow flight may be unstable.  As you can see the trade offs here are stable arrow flight vs. arrow range.  The object is to find a happy medium that will allow you to have the best of both worlds.

How to determine the FOC manually.  Take one of your arrows, fully equipped including tip, that you will be using.  First you must find the balance point on the arrow's shaft.  To do this try to balance the arrow on a sharp edge.  Once you have successfully balanced the arrow, place some sort of mark at that spot on the arrow's shaft.  Next, measure (inches) from the bottom of the nock grove to the balance point.  Then measure (inches) the length of the arrow from the nock grove to the edge of the arrow shaft, not the insert (This is called the arrows cut length.)  Divide the arrow length by two, this will give you the physical center of the shaft.  Now subtract the physical center number from the balance point value and divide by the arrow length value.  Multiple this number by 100 to get a percentage (%).

A FOC value range of 7-10 percent is widely used as the best for a good balance between arrow range and arrow flight stability.  If your calculated FOC doesn't fall with this range, don't fret.  You can still have good arrow flight with an FOC as high as 18 percent, but your range will not be as good.  Try not to go below 7 percent.

In this scenario, the tip of the arrow is a bit heavy for the arrow but the FOC is within the 7% - 18% range (use the lower end for aluminum arrows and the upper end for carbon arrows).  If I wanted to lower the FOC for this arrow I could change to a lighter point or I could add weight to the end of the arrow.  Lead tape works pretty well, it can be installed near the nock by completely wrapping the arrow's shaft.  Lead tape can be found at most sporting good stores.  Once you have the FOC calculated, use some trial and error to get the FOC percent you want.

Here are some other alternatives for calculating the FOC.

FOC gauge — Check with you local archery pro shop.  They cost around $10 and work pretty well without doing any math.

Jackson's Archery Web - This is a great web site with many archery related tools including an online FOC calculator.