easton-archery easton tune User Manual

The term F.O.C. (Front-of-Center) describes the percentage
of the arrow’s total weight that is located in the front half
of the arrow. The more weight that is located in the front
half of the arrow, the more forward is the center-of-balance
of the arrow. Why is this important? It is generally believed
that the F.O.C. balance position of the arrow is one of the
more important elements affecting the shape of the arrow’s
trajectory curve.

Unfortunately, there are too many variables affecting the
path of an arrow to make it possible to precalculate the
trajectory of an arrow simply by controlling its F.O.C..
Some archers, though, find it valuable to determine and
record the F.O.C. of their arrows once they have achieved
the flight characteristics they desire.

The importance of F.O.C. is greatest in archery events that
emphasize long range shooting. FITA archery (Olympic
style) which requires archers to shoot up to 90 meters is the
best example of this.

In short range events such as indoor archery, the effect of
F.O.C. is not significant. Other factors affecting arrow
flight are more important to these archers.

Changes in the trajectory of an arrow, for whatever reason,
are quite apparent in archery events or bowhunting
conditions that call for shots to be made at both short and
long ranges. Bows for these situations usually include the
use of multiple sight pins or multiple premarked sight
settings. Since changing the F.O.C. of an arrow may
change the shape of its trajectory, it may also change the
preset sight locations on the bow sight.

The balance of an arrow can be modified by adding weight
to either the front or the rear of the shaft. Heavier vanes,
for example, increase the weight of the rear portion of the

100 x (A-L/2)

      L

F.O.C. % =

L = Correct Arrow Length—Distance from
bottom of nock groove to end of shaft
A = Distance from bottom of nock groove
to finished arrow balance position (includes
weight of point [+ insert], nock system and
fletching)

1/2 Length = L/2

Front-of-Center balance position

of the finished arrow

Center of

Correct Arrow Length

F.O.C.

Distance

L = Correct Arrow Length

arrow. Replacing vanes with feathers lightens the rear of
the arrow. Various points, inserts, and even added weights
are used to modify the weight of the front of the arrow.

Because of the differences in archers and types of shooting
there is not a perfect F.O.C. for each setup. To provide a
good starting point when constructing a set of arrows or
when analyzing an arrow’s flight characteristics, it may be
helpful to look at the range of F.O.C.s most common for
each style of archery.

Typical F.O.C. Ranges

FITA (Olympic-Style)

11-16 %

3-D Archery

6-12 %

Field Archery

10-15 %

Hunting

10-15 %

Use the following formula to calculate the exact F.O.C. of
an arrow.