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MATHEWS OUTBACK

by Norb Mullaney

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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

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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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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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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

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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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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 With the fixed-position roller cable guard, the Outback does not offer

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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.