Arrow FOC Basics
by Michael Larsen of Archery Report
Arrow FOC (front of center) is an important concept that all archers should understand. Whether bowhunting, target or 3d shooting, the FOC of an arrow will affect the flight of the arrow. FOC becomes especially important the further downrange the target is, and also if a broadhead (most notably a fixed blade broadhead) is used. The basic definition of FOC is the percent difference between the physical midpoint of the arrow and the center of gravity (balance point) of the arrow as compared to the total length.
When an arrow is shot, the fletchings immediately begin working to correct the flight of the arrow. Any little imperfection in the initial launch of the arrow, either from shooting with fingers, imperfect release, improperly tuned bow, torquing the handle of the bow, etc. will cause the arrow to flex as it leaves the bow and/or come out crooked. The job of the fletchings is to correct this imperfect flight by straightening the arrow as air flows over the fletchings during flight. In order for the fletchings to correct the flight of the arrow, they must move the arrow into a straighter flight path by rotating the shaft about the center of pressure. The center of pressure is the point along the shaft where the force that flexes the shaft is greatest (FOC formulas assume that this point is the midpoint of the shaft, when in reality it is dynamic for a moving arrow). This is more easily done with a longer distance from the fletchings to the center of gravity. Therefore the further the center of gravity is from the back of the arrow, the easier it is for the fletchings to correct the arrow flight.
For penetration purposes, the opposite phenomenon occurs. A larger FOC will prevent and arrow from flexing as much when coming into contact with the intended target. Hunters will want a higher FOC so that as the broadhead enters the the game, the distance from the front of the arrow to the center of gravity is lower, making it more difficult for the arrow to flex. Any flex or deflection of the shaft upon entry means that energy is being lost somewhere other than directly along the center of the shaft, lessening the overall penetration.
A larger FOC is necessary for shooting long distances where crosswinds tend to have a large effect on arrow flight. The same applies to shooting broadheads over field points because the larger surface area of a broadhead is more greatly affected by launch imperfections as well as crosswinds. Broadheads tend to steer an arrow which can often have a negative affect that the fletchings must overcome.
There are two main ways of calculating FOC. Both use similar formulas; the difference being that one takes into account the length of the point and insert while the other does not.
1) Measure the length of the shaft from the throat of the nock to the end of the shaft, excluding the insert; this is length “L”
2) Using a sharp edge, balance the arrow (including the point) and mark the balance point
3) Measure the distance from the throat of the nock to the balance point; this is length “B”
4) Input B and L into the following formula:
The second method replaces the total length of the arrow, including the point, for the “L” value.
Arrow FOC calculator
Comparing arrow FOC values
When comparing FOC values, it is important that both values be calculated with the same formula. The second method will generally result in a lower FOC value.
Recommended values for FOC (using the AMO method) vary depending on the application, target arrows (field points) should generally be in the 8-11% range and broadhead tipped arrows in the 10-15% range. FITA and longer distance shooters will often go for a 12%+ FOC, even though they are using field points, because of the long distances involved. Some traditional hunters have used FOCs of over 30% because with the right setup it can enhance penetration on animals.
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Originally posted 2010-10-23 10:34:40. Republished by Blog Post Promoter