Understanding F.O.C. Balance Point - June 2009

Question: How do you calculate F.O.C. (Forward of Center) balance to determine the ideal weight of a broadhead to give my carbon arrows the most accurate flight? -- Kris Folland, Halma, Minn.

UNDERSTANDING F.O.C. BALANCE POINT
Arrow balance point presents us with some trade-offs. If you keep the arrow's nose light, it will remain a little more level in the air and actually plane or sail along a flatter trajectory than it would if the nose were heavier and it flew pointing more nose downward. On the other side of the tradeoff is stability. The closer the center of gravity gets to the physical center of the arrow the more unstable the arrow becomes while in flight. Take it to the extreme. If the center of gravity were behind the physical center of the arrow it would actually flip around as soon as it left the bow and try to fly tail first. It would be like trying to throw a dart tail first. The closer you get to a tail-heavy arrow the more unstable it becomes.


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:



Balance point: Install the tip you will be shooting. If you are testing stability for 3-D shooting put your field point or nib 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 bottom of the nock groove to the balance point and write this number down for later.

Overall length: There are different conventions for measuring overall arrow length depending upon the type of point you are using.


Arrows that include inserts: Measure from the bottom of the nock groove to the end of the arrow not including the insert. This is often referred to as the arrow's cut length.


Shafts with swaged tips: The overall length is measured from the bottom of the nock groove to the most forward extension of the full diameter of the shaft, just behind the swage.

Shafts that include outserts: Measure from the nock groove to a point three-quarter inches forward of the rearward end of the outsert.

Shafts with glue-on heads: Measure from the nock groove to the most rearward portion of the glue-on point.

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.

HOW FOC RELATES TO STABILITY
Most expert archers agree that a FOC value between 7-10 percent will produce the best compromise between stability and a flat trajectory. The American Society for Testing and Materials, in their specification for measuring balance point, state that a value of 9 percent is typical. But, they also state that the range can be as wide as 7-18 percent while still producing good arrow flight characteristics.

The best way to achieve your desired FOC (to play it safe, let's shoot for around 9 percent) is to try several different weight field points until you hit the right balance. However, if you are sold on a particular broadhead that's too heavy to permit the arrow to fall into the desired FOC range, you can change from feathers to vanes. You can change from aluminum inserts to lighter composite inserts or you can even play some games with the weight of the nock end. For example, you can experiment with adding weight by placing a narrow strip of lead tape around the shaft just behind the fletching. Make sure it fully circles the shaft so you don't introduce a wobble. You can get lead tape from any full-service golf supply shop.

MATCHING SHAFT AND HEAD
The shafts I was hunting with the year of my New Mexico elk hunt were 2514's. That's a moderately heavy shaft and when coupled with a lightweight 100-grain broadhead it's no wonder they floated through the air a little. I was on the low end of the acceptable range for FOC. The only benefit being that I enjoyed a significantly flatter trajectory without having to increase arrow speed. But, I also ran the risk that the arrows wouldn't track as true or recover as quickly when affected by external forces such as wind, a rough release or poor bow tuning.

A good basic rule of thumb for shaft and head selection suggests that your bare shaft (without any components) should weigh roughly three times as much as your broadhead or field point. Given average length arrows (28 to 30 inches) and typical fletching (four or five inch vanes) your finished arrow will balance fairly well using this general criteria. (I stress that this is just a rough guideline to keep you from putting together grossly unbalanced arrows.)

To determine your exact FOC and fine-tune it for the kind of arrow flight you desire, you still need to take the measurements and use the formula. Arrow balance is an important part of field accuracy. By spending a few minutes this summer you can make sure your outfit is properly balanced for stable, accurate arrow flight.

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