F.O.C.'s the percentage of an arrow's weight situated forward of it's center. To determine this number take a hunt-ready arrow, head installed, placing it on a sharp edge to determine it's balance point and marking it with pencil. Next measure that same arrow from cut-off point to bottom of nock throat and mark mathematical center. Measure distance between these points and divide by total arrow length. Move decimal point two spaces right to create percentage.
To better grasp how F.O.C. affects arrow performance, understand that as you release an arrow, energy transfered by the bowstring into the nock quickly travels up the arrow. By the time the nock passes the bow riser 100 percent of that energy's carried by the point. Take a moment to allow that to sink in. This essentially means the higher your F.O.C. the more efficiently your arrow carries and tranfers that energy.
Eight to 10 percent F.O.C. is considered a good minimum for a combination of broadhead/arrow stability and flat trajectory. Yet, further boosting F.O.C. offers still more stability (added forgiveness and less influance from light deflections and wind) and greater penetration potential (heavier tips actually "dragging" arrows through animals and leveraging through bone). This is why I now strive for 12 to 15 percent F.O.C. with compound arrows; 17 to 20 percent with less-effcient traditional bows (also automatically boosting overall mass).
Granted, increasing F.O.C. by increasing front-end mass erodes arrow speed by about 5 to 7 fps per 25 grains added. But I'll happily sacrifice 10 to 15 fps in trade for deeper penetration, added forgiveness and reliability. F.O.C.'s also a direct function of balance. Adding weight to the rear of an arrow automatically lowers F.O.C. Rear-end weight comes via lighted nocks (15 to 25 grains), arrow wraps (10 to 12 grains), or 4-inch plastic vanes (28 grains/3) verses 2-inch Blazer-style vanes (20 grains/3) verses 4-inch feathers (8 grains/3). Adding front-end mass also directly affects arrow deflection, which may require a stiffer (and heavier) arrow. This is all about balancing needs and desires.
Instant jumps in F.O.C. are achieved by installing heavier heads (shooting "old-fashioned" 125-grain broadheads instead of "standard" 100-grain) or using heavier insert materials or systems. Heavier broadheads sometimes equal more reliability, more bulk making it less susceptible to failure. Replacing standard aluminum inserts with heavier brass (or steel, like Easton Injexion G-Series H.I.T. Inserts) is another easy solution, allowing use of 100-grain broadheads you already own. Another slick option is Precision Designed Products' adjustable weight system; steel 5-, 10-, 20-, 30- and 50-grain weights with Allen heads screwed into the rear of the company's precision aluminum inserts.
Finally, tapered carbon arrows - like Alaska Bowhunting Supply's Momentum U-FOC, Carbon Tech Panther, or Quest Outdoor's Power Punch (offered with brass inserts), provide instant F.O.C. increases, placing more material up front and less at the rear. Carbon Express' Dual-Spine, Built-In Weight Forward construction, found in shafts like the Maxima Hunter, also serves essentially the same purpose.
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