math ew soutback

MATHEWS OUTBACK
by Norb Mullaney
82
Bowhunting World
April 2005
Mathews
Outback
to be that a compound bow that measured 36 inches from
Iweretaxleusedgenerally
to axle was considered a short bow. Longer compounds
over 40 inches axle to axle, and some were as long
as 48 inches. Today, with continued development following
demand, a short compound has an axle-to-axle length under 32
inches or it doesn’t fit in the category. With an axle-to-axle length
of 31 1/2-inches, Mathews Outback is certainly a short bow, but
it has been engineered to have shooting qualities that belie its
short length. In addition to parallel limb technology, it incorporates all of the tried and proven Mathews innovations that
make these bows so universally popular and such outstanding performers.
The Outback features the High Performance Straightline cam
with anti-friction bearings on the cam and idler, the V-Lock limb
cup system, String Suppressors, a ball bearing roller guard, integral Harmonic Damping System, Perimeter Weighting on the
cam, and Mathews’ exclusive Zebra ZS Twist string. Any way you
want to look at it, that’s a fistful of special features.
The Outback is built on a fully machined aluminum alloy handle that measures approximately 24 inches from end to end. The
sight window has a usable length of 6 inches. In the Outback’s
catalog description, there is a section that states that the contrasting wood inlay in the face of the grip establishes the centerline (vertical) of the bow. Measuring from this designated centerline, the window is cut 1⁄2 inch past center, and there is an additional relief of 1⁄4 inch at the arrow pass for a total of l inch
clearance from the plane of the string to the surface of the
cutout. Mathews built this centerline indicator into the grip as
an aid to setup, and it
does help considerably in that regard.
As near as I could
determine, the offset
in the cam’s main
track and the offset
in the upper limb
pocket contrive to
keep the string plane
parallel to the bow’s
vertical centerline.
This is frequently not
the case with singlecam compounds,
which have a centered idler and a cam
with the main track
offset.
The cable guard
strut is bolted into a
Comparative data from the static and slot machined into
dynamic tests of the Mathews Outback. the offside of the
upper riser. A clevis at the end of the
strut is angled inward to align the two
ball-bearing sheaves that it mounts at a
proper angle to carry the yoke cable
and the return stretch of the string. The
standard ATA two-hole pattern for
attaching a sight is found above the
cable guard. A pair of brass bushings
(tapped 5/16-24 UNF) is located, one
on the back and one on the face, below
the grip section on the thickest section
of the lower riser. The laminated, onepiece, black walnut grip incorporates
an extension that provides a wide and
flat shelf.
The Mathews Harmonic Damping
System is incorporated in the riser ends
inboard of the limb pockets. This system
consists of a pair of weights suspended
in circular elastomeric units that permit
the weights to oscillate independently
and thus absorb energy that might normally be confined within the handle.
Mathews provides two sets of weights for
the damping system. One set is anodized
aluminum alloy while the other set is
made of brass. The weights are readily
interchanged, permitting the archer to
tune the feel to individual preference.
The V-Lock limb cups are pivoted on
socket-head bolts that fit into projections at the risers’ ends. The pockets
are precision machined from aluminum
alloy. The lower limb pocket is symmetrical from side to side while the
upper pocket is eccentric, establishing
an offset to the bow hand side to achieve
the alignment condition previously
described. The pocket itself is V-shaped
at the base to accept a matching “V”
shape on the butt of the limb. This
insures the limb’s snug fit into the
resilient lining element in the pocket,
thus resulting in a zero gap condition.
The forward-directed force component
on the limbs keeps them securely seated in the pockets.
The Outback limbs are machined
from solid, filament-reinforced, epoxymatrix material. Each limb is just 123⁄4
inches long with a maximum width of
11⁄2 inches. The limb’s principal working
section is only about 31⁄2-inches long.
Considering that at 30 inches draw each
axle moves only about 13⁄4 inches, this
type of limb design is quite appropriate.
The limbs’ action is primarily vertical
due to the decidedly parallel limb orientation. This not only minimizes limb
motion but also results in opposing limb
action that substantially
reduces forward-directed
shock when the limbs return
to brace position. Reducing
limb motion increases
dynamic efficiency since less
energy is required to recover the limbs, leaving more
energy available to propel
the arrow. It must be kept
in mind that parallel limb
action does have some disadvantages, so the overall
design must be such that the
advantages outweigh the dis- Tabulations of bow or dynamic efficiency and initial
advantages. This happy sit- arrow velocity for a wide range of arrow weights for
uation is apparent in the the three draw weights tested.
This approach to cam design is readily
Outback.
understood since reducing the draw
Matt McPherson designed a whole
length with the draw stop arrests the
new family of cams for the Outback
draw short of the bottom of the valley,
covering a draw length range from 26 to
thus reducing the percent of letoff.
30 inches, with a different cam for every
Consistent with the Outback’s par1/2-inch increment of draw length. In
allel limb design, the cam and idler
addition, each draw length increment
have large diameters in order to provide
can be set up for either 65 or 80 percent
for the longer draw lengths. Since the
ATA (AMO) letoff. Some cams serve
rearward limb action is reduced during
double duty, offering draw stop adjustthe draw, more string travel is necessary
ment that can provide draw length
in order to meet the draw length requireand/or letoff options. For example, the
ments. This means a larger cam and
“OUTB-AR” that was installed for
idler. The maximum diameter of the
these tests could be set up for either 30
main track of the cam is 51⁄4 inches while
inches draw length with 80 percent
the diameter of the idler is 33⁄4 inches.
nominal letoff or 29.5 inches draw
The bowstring’s shooting section is
length at 65 percent nominal letoff by
aligned in the cam’s main track on the
changing the position of the draw stop.
bow hand side, and the return stretch is
recovered on the track on the far (string
hand) side of the cam. The yoke cable
is wound on the center track. Pegs on
which to anchor the two ends of the
bowstring and the yoke cable are positioned in line with the track each serves.
Perimeter weighting of the cam is
accomplished by inserting a carbide disk
near the cam’s periphery in the lobe at
the outboard end of the main track.
Both the cam and the idler are equipped
with anti-friction bearings.
The adjustable draw stop consists of
a short cylinder of resilient material surrounding a metal tube. It is fastened to
the bow hand side of the cam’s main
track by a through bolt that bottoms on
the metal cylinder when tightened. Two
tapped holes are provided to offer an
optional draw length accompanied by a
change in the percent of letoff. In operA comparison of force-draw curves derived ation the stop rotates with the cam as the
for levels of 50, 55 and 60 pounds peak bow is drawn until it reaches the point
draw force.
where it contacts the string suppressor
April 2005
Bowhunting World
83
Initial arrow velocity plotted versus arrow weight for the three
draw weights taken from the values given in Table 2.
bracket attached to the lower limb tip.
The yielding of the resilient stop material softens the stopping action a bit.
The String Suppressors, a Mathews
innovation, are mounted at the ends
of arced aluminum brackets that are
bolted to the limb tips on the string
hand side. These elastomeric units con-
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Bowhunting World
April 2005
Initial kinetic energy plotted versus arrow weight for draw
weights of 50, 55 and 60 pounds.
tact the shooting string just inboard of
the points where the string leaves the
cam and idler tracks. They are in contact with the string at brace height and
arrest it to dampen vibration but not the
over travel when the string returns to
brace height just before the arrow leaves
the string. This approach to damping
string vibration is
more dynamically
efficient than stringmounted damping
devices that add
weight to the bowstring.
Adding
weight to the bowstring acts in a manner similar to
increasing
the
weight of the arrow,
however the actual
effect varies with the
position of the
weight on the bowstring. The closer the
weight is to the
nocking point, the
greater is its effect on
reducing performance.
The Outback is
offered at draw
weights of 40 to 70
pounds in 10-pound
increments. Draw
weight is adjustable
downward by 10
pounds. Draw length
is available from 26
to 30 inches, with
half-inch sizes from 261⁄2 to 291⁄2 inches.
Brace height is specified as approximately
75⁄8-inches. I measured the test bow at 7
7/16 inches for the 50- and 60-pound settings. The Mathews catalog lists the
physical weight at approximately 4.3
pounds. With a NAP QuikTune rest
installed, the test bow weighed 4 pounds,
61⁄4-ounces, or 4.39-pounds. The handle
and limbs are finished in High Definition
Realtree Hardwoods pattern.
The Tests
The Outback I had for testing was rated
for 60 pounds peak draw force and was
equipped with an OUTBAR cam that
provided 30 inches draw length with a
nominal letoff of 80 percent. With draw
stop adjustment, the draw length could
be shortened to 291⁄2 inches with a concurrent nominal letoff of 65 percent.
When considering a compound with
adjustable draw weight, I like to vary the
draw weight in order to determine the
effect on the force-draw characteristic,
the stored energy and the performance
factors. Therefore I elected to test the
Outback at the standard ASTM draw
weight of 60 pounds and also at draw
weights of 55 and 50 pounds. Often
this approach yields very interesting
results
Static testing is performed using a
force-draw machine equipped with a
Mark-10 digital force gauge capable of
reading to the nearest 0.1 pound. This
type of force gauge is necessary to obtain
credible letoff characteristics for the
high-performance bows of today, particularly when the letoff is precipitous.
Spring gauges will not respond fast
enough to monitor steep letoff, nor are
they sufficiently accurate to adequately define the very short valleys that typically accompany the hard walls used for
draw stops. Force readings are recorded
at one-inch increments from brace
height to just past full draw in order to
define the bow’s force-draw characteristics and permit determination of the
stored energy. The procedure includes
recording forces during letdown as well.
This allows calculation of the static
hysteresis. Other static measurements
taken include brace height, axle-to-axle
distance, weight- in-hand, tiller and
cable clearance.
The first nine lines of Table 1 list the
data obtained from the static tests.
Observe that there is only a 0.3-pound
difference in the holding weights listed
for the three draw weights even though
they range over 10 pounds. This occurs
because the bottom of the valley moved
to a longer draw length as draw weight
was lowered. At 60 pounds peak draw
force, it bottomed at 301⁄8 inches; at 55
pounds it bottomed at 301⁄2 inches; and
at 50 pounds it moved to 305⁄8 inches.
Because of this, the holding weight at
precisely 30 inches fell further up the letdown curve, resulting in similar readings
for all three draw weights. This condition is also reflected in the values
derived for percent of letoff. The following table contrasts the holding
weights and percent of letoff at 30 inches draw length and at the bottom of
the valley:
P. D.F.
DRAW LENGTH
60 lbs.
55 lbs.
50 lbs.
30 in.
30 in.
30 in.
shoot. The results of the dynamic tests
confirm that there is another way other
than an aggressive cam to achieve excellent performance.
Static hysteresis is a measure of friction in the system. It is obtained by
subtracting the energy represented by
the letdown curve from that represented by the force-draw curve. From testing many bows, I have found that it
usually ranges from about 5 to 12 percent of the stored energy. The Outback
@ TEST DRAW LENGTH
Hold Force % Letoff
18.5 lbs.
69.2
18.4 lbs.
66.6
18.2 lbs.
63.6
The levels of stored energy achieved
and the ratios of stored energy to peak
draw force (S.E./P.D.F.) demonstrate
that the Outback cam is not particularly
aggressive. Instead it is tempered somewhat to make the bow more pleasant to
@ BOTTOM OF VALLEY
Hold Force % Letoff
17.2 lbs.
71.3
12.5 lbs.
77.3
11.8 lbs.
76.4
shines in this area, yielding values of
static hysteresis ranging from 4.57 to
5.09 percent of stored energy. Minimizing system friction is a singularly
effective method of increasing bow performance by improving dynamic effi-
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86
Bowhunting World
April 2005
ciency. It is my observation that Mathews does an outstanding job in this area.
Dynamic tests are conducted using a
shooting machine and a double chronograph arrangement. The standard
chronograph, a Custom Chronograph
Model 1000, is positioned three feet
downrange from the back of the bow at
the arrow pass. The checking chronograph, a Custom Chronograph Speed
Tach, is located immediately adjacent
(downrange) to the standard unit.
Seven test arrows, ranging in weight
from 360 to 650 grains in approximate
50-grain increments, are each shot and
chronographed a minimum of five times
to establish a credible value of average
initial velocity for the individual arrows.
The arrow weights and velocities are
used to calculate experimental values
of virtual mass. A curve of virtual mass
is determined by linear regression from
the experimental values. This permits
the calculation of initial arrow velocity
and dynamic efficiency for any desired
arrow weight.
Bow or dynamic efficiency is the initial kinetic energy of the arrow expressed
as a percentage of the stored energy of
the bow. In other words, it is the energy obtained (initial arrow kinetic energy) expressed as a percentage of the
energy or work applied to draw the bow
(stored energy). Kinetic energy is the
energy the arrow possesses as a result of
its mass and velocity. Table 2 presents
values of bow or dynamic efficiency and
initial arrow velocity for the Outback for
each of the test conditions. Values are
given in 25-grain increments of arrow
weight for the wide range of arrow
weights tested. The curves of initial arrow
velocity shown in Figure 2 were plotted
from the data tabulated in Table 2.
The table of bow or dynamic efficiency heading Table 2 shows the high
levels of efficiency foretold by the very
low values of static hysteresis cited previously. While other factors also influence
dynamic efficiency, low hysteresis and
good dynamic efficiency generally go
hand in hand. Other contributing factors
are minimized limb translational inertia
and cam translational and rotational
inertia. These are factors that can be
calculated, but it is not a simple matter.
The kinetic energy carried by an
arrow is related to the penetration
potential of that arrow when it strikes
a target medium. The actual penetration
is a function of the target medium as
well as the kinetic energy. Of course, the
form characteristics of the specific arrow
also affect the actual penetration. To
properly evaluate the actual penetration effect as related to kinetic energy,
the target medium must be consistent
and the arrow must be identical. To
recognize the effect of changing target
medium, I use the term “penetration
potential” rather than “penetration”
because the target medium can be a
April 2005
Bowhunting World
87
highly variable factor, and it can be
eliminated by using the term “penetration potential.” The kinetic energy,
which is a function of the mass of the
arrow and its velocity, is the primary
determinant that concerns us. The
arrow is assumed constant in all cases.
Figure 3 presents curves of initial kinetic energy plotted versus arrow weight.
The average virtual mass listed in
Table 1 is the arithmetical average of the
experimental virtual mass values
obtained for the seven test arrows. It
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corresponds to the virtual mass of the
bow when shooting an arrow weighing
about 500 grains. This arrow weight has
no particular significance. It is just the
midpoint of the range of arrow weights
used for these dynamic tests. My experience indicates that the average virtual mass for single-cam bows tested at 60
pounds draw weight under these conditions will range from about 109 to 165
grains. The lower the virtual mass, the
more efficient is the bow. Again, the
very low values of virtual mass obtained
for the Outback (90.57 to 97.87) are a
tribute to the efficiency of the bow.
High dynamic efficiency is something
that the manufacturer must design and
build into a bow. It’s a performance bonus
that comes with the bow and never stops
functioning in favor of the archer.
The Rating Velocity is a performance parameter developed by ATA
(AMO) to permit standardized comparison of the performance of various
bows. Simply stated, it is the initial
velocity of an arrow of specific weight
shot from a bow set at 60 pounds peak
draw force and 30 inches AMO draw
length. ASTM standard F 1544-04 was
created to detail and control the testing
procedure necessary to determine the
Rating Velocity. It establishes two different test arrow weights, 360 and 540
grains, because some bows that yield
similar Rating Velocities with the 540grain arrow demonstrate substantially
different Rating Velocities when tested
with the 360-grain arrow. In other
words, some bows gain arrow velocity at
a greater rate than other bows when
arrow weight is reduced. The method for
obtaining the Rating Velocity set forth
in ASTM standard F 1544-04 uses the
average of five shots of the specified
arrows to establish the value. The
method I have used for the Bow Reports
involves 35 or more shots to establish a
performance profile for the bow. The Rating Velocity is calculated from the velocity curve that is part of the profile. The
results of the two methods seldom differ
by as much as one foot per second. The
Bow Report method actually includes the
F 1544-04 procedure, hence it is possible
to provide both values as follows:
METHOD
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Bowhunting World
April 2005
ASTM F
1544-04
Bow Report
RATING VELOCITY – FPS
360 GRAINS 540 GRAINS
281.4
235.0
281.4
234.7
These results show that the Outback
is unquestionably a high-performance
bow. My tests reveal that it almost equals
the Mathews LX for Rating Velocity,
certainly a notable feat – all this in a 311⁄2inch bow with vertical limb action and
a relatively high brace height.
General Commentary
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Bowhunting World
April 2005
adjustable cable clearance. I measured cable
clearance as 5⁄8 inch
from the inside of a 5⁄16inch diameter shaft
set on centerline to
the nearest cable.
The cable guard
bracket is quite rigid,
so there is virtually no
deflection during the
draw; hence the clearance remains essentially constant.
At 30 inches draw, I
measured the included
string angle at 72 degrees.
This is not too bad for so
short a bow, although I
believe that this bow is
intended to be shot with a
release aid rather than fingers. The large-diameter cam
and idler can be credited with
a major contribution to this
condition.
For me to hand shoot the Outback
it was necessary to change the cam from
the OUTB-AR used for the shooting
machine tests to a OUTB-D.5R that
provided draw lengths of 271⁄2 inches at
a nominal 80 percent letoff, and 27
inches at nominal 65 percent letoff. I set
the draw stop for the former setup and
found it to be very acceptable when I
used a release aid. There is no need to
change either the bowstring or the yoke
cable when switching cams. Upon
checking the timing of the cam after the
change-out, I found that the cam was
still properly timed and that the nocking point had retained its original position. This was a pleasant surprise.
Changing the cam took all of 10 minutes – it was that easy.
I found the Outback very pleasant to
shoot and exceptionally quiet. It draws
smoothly and evenly without an abrupt
letoff. It sits quietly in the hand upon
discharge, yet sends the arrow downrange with decided authority. For this
hand shooting, I used two different carbon composite shafts weighing 400 and
511 grains, respectively.
Based on the fact that I found that
the location of the bottom of the valley
changed when I reduced the draw
weight on the test bow, I suggest that a
prospective purchaser select his cam
size carefully if he is picky about percent
of letoff. Frankly I don’t feel that a
pound or two of holding weight is anything to be concerned about, but some
archers are very particular about it. For
the record, I run the tests with the draw
stop removed to facilitate obtaining an
exact draw length, but I always measure
the precise location of the bottom of the
valley. With a draw stop of the type
used on the Outback, a simple modification can yield precise draw length,
but it does not affect the location of
the bottom of the valley where the holding weight is minimal.
I think that the Outback is an exceptionally fine bow. I might lean toward
the LX because of its longer length, but
for shooting qualities it would be a hard
choice. The short length of the Outback
makes it ideal for treestand, blind and
brush hunting. With the high level of
performance that it offers, the inherent sweet shooting characteristics and
the readily apparent quality of construction and finish, in my opinion the
Outback is a standout in the field of
hunting bows.