Patent Publication Number: US-2019178603-A1

Title: Fixed axle compound crossbow and method for operating a crossbow

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation-in-Part of co-pending U.S. patent application Ser. No. 15/816,904, entitled “FIXED AXLE COMPOUND CROSSBOW,” filed Nov. 17, 2017 (docket number 3033-002-03). U.S. patent application No. 15/816,904 claims priority benefit from U.S. Provisional Patent Application No. 62/423,922, entitled “FIXED-AXIS AXLE CROSSBOWS,” filed Nov. 18, 2016 (docket number 3033-002-02), now expired. Each of the foregoing applications, to the extent not inconsistent with the disclosure herein, is incorporated by reference. 
    
    
     SUMMARY 
     According to an embodiment, a compound crossbow includes a stock assembly having a front end, back end, left, right, top, and bottom sides; left and right flexible limbs extending laterally respectively from the left and right sides of the stock assembly to respective limb tips; and a rigid cam support structure disposed at or near the front end of the stock assembly. Left and right cam assemblies are rotationally coupled to the rigid cam support structure on axles such that rotational axes of the left and right cam assemblies are fixed relative to the stock assembly. Left and right power cables are respectively directly coupled between the left limb and a cam surface of the left cam assembly and between the right limb and a cam surface of the right cam assembly. A bowstring operatively coupled to the left and right cam assemblies and a tension force applied to the bowstring causes transmission of corresponding strain to the left and right limbs by way of the left and right cam assemblies. 
     A synchronizing cable may optionally be operatively coupled to the left and right cam assemblies to cause the left and right cam assemblies to counter rotate synchronously. 
     According to an embodiment, a method for operation of a crossbow, includes the steps of supporting; from a stock assembly having front, back, left, right, top, and bottom sides; left and right flexible limbs to extend laterally respectively from the left and right sides of the stock assembly to respective limb tips. The method includes supporting, from the stock assembly, a rigid cam support structure at or near the front of the stock assembly and supporting, from the rigid cam support structure, left and right cam assemblies for rotational motion around respective positionally fixed axles. The method includes coupling the left limb tip to the left cam assembly with a left power cable and coupling the right limb tip to the right cam assembly with a right power cable. The method includes receiving tensile forces from a bowstring through the left and right cam assemblies and through the power cables to cause a reactive spring force in the respective left and right limb tips, storing the spring force in the limbs, and receiving a user input to release the bowstring. Upon release of the bowstring, the method includes driving the bowstring in tension, through the cam assemblies and the power cables, by releasing the stored spring force in the limbs. Driving the bowstring in tension can cause the bowstring to propel an arrow or bolt in flight forward from the front of the stock. 
     According to an embodiment, a crossbow includes a stock assembly defining an arrow track configured to be in contact with an arrow prior to release, the stock assembly having a front end and a back end, the stock assembly being characterized by a length L between the front end and back end. Left and right flexible limbs extend laterally from the stock assembly respectively to a left limb tip and a right limb tip. A left cam assembly and a mirror-image right cam assembly are each rotatably mounted on a respective end of a rigid cam support structure. Each one of the left and right cam assemblies includes a respective bowstring cam, a synchronizing wheel, and a power cable cam. The rigid cam support structure is disposed adjacent and perpendicular to the front end of the stock assembly and fixed rigidly thereto. The cam support structure can be disposed to place a top surface in a first horizontal plane . The first and second limbs may be disposed to have respective limb centerlines lying along a common second horizontal plane parallel to the first horizontal plane and extending left and right from the stock. A left bearing and a right bearing are disposed in the cam support structure and configured to rotatably support the left and right cam assemblies on respective fixed axes of rotation. A left limb cable is coupled between the left limb tip and the left cam assembly. A right limb cable is coupled between the right limb tip and the right cam assembly. A synchronizing cable is disposed between a left synchronizing wheel and the right synchronizing wheel, and is configured to cross itself in a central medial portion for synchronizing motion of the left and right cam assemblies in opposition to one another. A bowstring extends between a left bowstring cam or wheel and a right bowstring cam or wheel, the bowstring being anchored to both. The bowstring, limb cables, and synchronizing cable are arranged so as to exert force only on an outer circumferential portion or cam surface of any cam or wheel. The synchronizing cable extends freely in space between the left and right cam/wheel units. Force is exerted between any cable or string and any other cable or string only by way of axial torque exerted by the cam/wheel units. 
     According to an embodiment, for a crossbow including a trigger actuating a bowstring hold-and-release mechanism to release a bowstring after the crossbow has been cocked, and a stock, the stock further including a projectile track, a method of stiffening the stock of the crossbow includes deploying a strut substantially parallel to the stock and at distance below the stock and fastening the stock to the strut to the stock at selected places along the length of the stock with fasteners. 
     According to embodiments, a compound crossbow achieves rigidity and low mechanical play for precision arrow flight. 
     According to an embodiment, in a compound crossbow, all cams, wheels, and other rotating parts are rigid and fixed, except for their ability to rotate. None of the rotating parts are translatable, nor do their axes of rotation change direction. 
     According to another embodiment, in a compound crossbow, the rotating parts are rotatably coupled to a rigid cam support structure, which in turn is rigidly coupled to the front end of the stock. This creates a framework with minimal play or looseness. In an embodiment, braces or trusses prevent the cam support structure from rotating relative to the stock, especially in the horizontal plane that is parallel to the bowstring and the stock. 
     The cam support structure can be made additionally rigid, in relation to the stock, by connecting these two frame parts with tensile stays, braces, trusses, etc. 
     According to an embodiment, a compound crossbow keeps arrow fletches or vanes out of contact with other parts of the crossbow, especially tensile members such as the bowstring and limb cables. An arrow is fletched with vanes to keep it flying straight. Typically, the vanes protrude farther from the arrow shaft in the case of hunting arrows as compared to target arrows. If the vanes hit any part of the crossbow, including a limb cable or a figure-8 synchronizing cable, the arrow or bolt will be deflected somewhat. 
     In an embodiment, a strut may be disposed above or below the stock on which the bolt slides. The strut may be, along with the stock, part of the crossbow frame. The strut can have a similar layout as the stock and its rigid cam support structure. The strut can include a second bearing for each cam/wheel axle. 
     In an embodiment, a synchronizing cable extends between left and right cam/wheel units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a plan view of a crossbow stock, according to an embodiment. 
         FIG. 1B  is a side view of the crossbow stock with cams, according to an embodiment. 
         FIG. 2A  is a plan view of a crossbow, according to an embodiment. 
         FIG. 2B  is a side view of the crossbow of  FIG. 2A , according to an embodiment. 
         FIG. 3  is a detailed elevational view of the crossbow of  FIGS. 2A and 2B , according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure. 
     As used herein, the terms bolt and arrow are used interchangeably and are considered synonymous. As used herein the terms limb cable and power cable are used interchangeably and are considered synonymous, referring to a cable that couples a limb or other spring device to a cam assembly. 
     A compound crossbow includes fixed axles and bearings configured to support cams. The fixed axles are coupled by a synchronization mechanism configured to equalize motion between the cams. According to embodiments, the bearings, axles, and cams are disposed at or near a front end of a crossbow stock to maximize length of travel of the crossbow string while in contact with the knock of the arrow. 
     An object of embodiments is perfect horizontal and vertical nock travel, which requires a rigid crossbow structure. Rigidity, in the stock especially, is important for the following reason: at rest, during the draw stroke and at full draw, tensions in the bowstring and other tensile members are exerted between points that are offset from the centerline of the stock, which tends to bend the stock; when the bolt is shot, these bending forces dissipate, changing the bending moment on the stock during the launch of the bolt, and causing it to change shape as the bowstring and other tensile members relax. Thus, the stock will change shape during discharge of the arrow, the direction of the bolt will change, and the aim of the shot will be thrown off. The shot can also be thrown off by looseness between parts. 
       FIG. 1A  is a simplified depiction of a crossbow assembly  100 , according to an embodiment. The crossbow assembly includes a stock  110 , a cam support  140 , and a truss  130 , all of which are fastened together or made as a unit so as to achieve a high rigidity. According to embodiments, a separate truss  130  may be omitted if the cam support  140  is sufficiently stiff. The illustrated left and right trusses  130  triangulate the frame and increase the resistance to relative rotations of the stock  110  and the cam support  140 . The trusses  130  are optional and may have different forms, such as triangulated beams, tensile members, etc. 
     The cam support  140  may define bearing holes  141  and  142  on the left and right sides respectively, which accept optional bearings (see  180 ,  182  in  FIG. 3 ) and axles and support the axles against translational and inclination movements, as discussed below. 
     The stock  110  defines an arrow guide or projectile track  112 , which may be of conventional design, and a space  117  that can accept a bowstring hold-and-release mechanism (not shown in  FIG. 1A ), which may also be of conventional design. 
       FIG. 1B  is a side view of the embodiment of  FIG. 1A , with additional elements shown: a strut  120 , a limb  150 , and a cam assembly  160 . The strut  120  may parallel the stock  110  over at least a part of the length L of the stock  110 , and if fastened to the stock  110  in selected locations, will greatly stiffen the stock  110  against bending. One place where the stock  110  and strut  120  may be fastened together is around the limb  150 , to which both may be fastened (limb  150  is depicted in cross section in  FIG. 1B ). The strut  120  and stock  110  may be fastened at other places to stiffen the frame, and in general the more places they are fastened the stiffer the structure will be. According to an embodiment, the entire space between the stock  110  and the strut  120  may be filled with a layer of crush-resistant material (not shown) adhered to the stock  110  and strut  120  to create a “stressed-skin” structure. 
     On the right side of  FIG. 1B  is shown a cam assembly  160 , which is illustrated in detail in  FIG. 3 . 
       FIGS. 2A and 2B  are plan and elevation views of a crossbow including the frame of  FIG. 1A , and showing the limbs  150 . The tips of the illustrated limbs  150 , which are resilient and bendable, are attached to respective left and right power cables  191  and  192 , for example by direct attachment or by way of a rotatable pin, eye, etc. The power cables do not exert any substantial rotational moment on the ends of the limbs  150 , and convey only tension force. The distal end of the left power cable  191  is wrapped around the outer circumference of a left power cable cam  163  (obscured in  FIG. 2A ), and the right power cable  192  is wrapped around outer circumference of a right power cable cam  164  (obscured  FIG. 2A ).  FIG. 3  provides a better elevation view of the right cam assembly  160 . 
     In  FIGS. 2A and 2B , the space  117  houses a bowstring hold-and-release mechanism  118  which is actuated by a trigger  116 . Pulling the trigger  116  releases a bowstring  196  and an arrow (not shown). The bowstring  196  is wrapped around bowstring wheels  165 ,  166 , which are the uppermost cam or wheel on the cam units  160 ; although shown as wheels (circular, round, or arc-segment cams), they can comprise cams of any shape needed to adjust the force function. 
     A synchronizing cable  194 , seen in  FIG. 2A , is wrapped around in figure-8 configuration around the circumference of a synchronizing wheel or wheels  162 , as seen in  FIG. 3  (left-hand synchronizing wheel(s)  161  is behind wheel  162  in  FIG. 3 ). This arrangement reduces any unevenness in the forces or motions of the two sides of the crossbow. 
       FIG. 3  also illustrates an upper bearing  180  and a lower bearing  182 , mounted respectively in the stock  110  and the strut  120 , which hold the axle  169  of the cam unit  160  on either side. The strut  120  and the lower bearing  182  are optional. According to many embodiments, the cam units  160  are supported on a single bearing  180 . The bearing(s)  180 ,  182  can be of various types, including simple holes in the frame, but should be precise enough to prevent the axis of the cam unit  160  from changing direction relative to the frame. According to embodiments, the bearings  180 ,  182  are bronze, sintered, and/or polymer, such as polytetrafluoroethylene. The strut bearing  182  can further immobilize the rotation axis of each cam unit axle, as compared to the use of just an upper bearing, which will more firmly fix the axes of the cam units  160 . 
     In the illustrated embodiments, the strut  120  is on the lower side of the stock  110 , leaving the upper side of the stock  110  open for emplacing an arrow (not shown) in the arrow guide  112 . Alternatively, the strut  120  can be placed above the stock  110 , with a slot or other arrangement to permit an arrow to be placed into arrow guide  112  from above. Furthermore, the limbs can be doubled (with one strut above the stock and another below) if double cams are used to transmit the limb forces; this would result in a more symmetrical frame and could lead to reduced bending. 
     Whether one strut is used, or two or more, an external strut will increase the stiffness of the frame and resist any bending of the stock which would throw off the aim. Considering a cross section of the frame taken perpendicular to the arrow, the stock itself will have a certain moment of inertia, I (a measure of resistance to bending, or stiffness), which is proportional to the square of the thickness of the stock. If a strut is added the stock effectively becomes thicker, the moment of inertia is greatly increased because of the factor of the square of the thickness; this is true even if the strut is fastened to the stock only at certain points, such as near the ends. (If there were no connection between the two, the stiffness of the stock would not be increased at all.) 
     The structure on the left and right sides of a center plane can be symmetrical and the motions synchronized, so that any forces tending to bend the stock left or right are incidental. In contrast, the forces exerted on the stock that tend to bend it in a vertical plane are generally not as symmetrical. The vertical plane is substantially perpendicular to the horizontal plane (or planes). 
     In an embodiment, the limb or limbs  150  are fastened between the stock  110  and a strut  120 , and can act as one stiffener connecting the stock to the lower strut. As noted above, such connections make the stock stiffer in the vertical direction. 
     If the limb or limbs  150  are centered between the stock and a strut, and so are the limb cables  191 ,  192 , and the limb cables are symmetrical in the vertical direction (illustrated in  FIG. 3 ), then the force transmitted from the limbs by the limb cables will be exerted along a line halfway between the stock and the strut, assuming that the cams to which they attach are also centered. If so, then the limb cables will not exert any bending force on the combination of the stock and strut. The height of the cables  191 ,  192  can alternatively be adjusted downward to compensate for the tension in the bowstring  196 , and by proper design known to those skilled in the mechanical arts, can result in a negligible bending force on the strut  120 . 
     Referring to  FIGS. 1A, 1B, 2A, 2B, and 3 , according to an embodiment, a compound crossbow  100  includes a stock assembly  110  having a front end, back end, left, right, top, and bottom sides. Left and right flexible limbs  150  extend laterally respectively from the left and right sides of the stock assembly  110  to respective limb tips. A rigid cam support structure  140  is disposed at or near the front end of the stock assembly  110 . Left and right cam assemblies  160  are rotationally coupled to the rigid cam support structure  140  such that rotational axes of the left and right cam assemblies  160  are fixed relative to the stock assembly  110 . Left and right power cables  191 ,  192  are respectively directly coupled between the left limb  150  and a cam surface of the left cam assembly  160  and between the right limb  150  and a cam surface of the right cam assembly  160 . A bowstring  196  is operatively coupled to the left and right cam assemblies  160 , wherein a tension force applied to the bowstring  196  causes transmission of corresponding strain to the left and right limbs  150  by way of the left and right cam assemblies  160 . 
     According to an embodiment, the left and right power cables  191 ,  192  do not pass over any intermediate guide between the respective limb  150  and cam assembly  160 . 
     One or more synchronizing devices, referred to herein as a synchronizing cable, may optionally be provided to help to maintain equal rotation and equal tensile stress across the two cam assemblies  160 , through the power cable  191 ,  192 , and through each respective limb  150  to the stock  110 . Among other utilities, the synchronizing cable may be used to accommodate tolerance variations between left and right sides of the crossbow  100  (e.g., to accommodate tolerances between the left and right limbs). 
     According to an embodiment, the compound crossbow  100  further includes a synchronizing cable  194  operatively coupled to the left and right cam assemblies  160  to cause the left and right cam assemblies  160  to counter rotate synchronously. In one embodiment, the synchronizing cable  194  includes two synchronizing cables. In another embodiment, the synchronizing cable  194  includes a belt and/or a cogged belt. A synchronizing cable  194  path in each of the cam assemblies  160  may include a complementary and meshing cog pattern with the cogged belt. 
     In an embodiment, the crossbow  100  may include only a single flexible limb  150  configured to provide force to both cam assemblies  160 . For example, two power cables  191 ,  192  may respectively operatively couple the single limb  150  to both cam assemblies  160 . Alternatively, one or more limbs  150  may provide power, via power cables  191 ,  192 , to a single powered cam assembly  160 . A synchronizing cable or cables may then transmit power and rotational motion between the powered cam assembly and a driven cam assembly. The use of a cogged synchronizing cable  194 , in addition to conventional uses, may help to maintain predictable power transmission from the powered cam assembly  160  to the driven and opposing cam assembly  160 . In an embodiment, a cam in at least one of the power cam assembly  160  or driven cam assembly  160  may be characterized by a diameter selected to compensate for mechanical strain between the powered cam assembly  160 , across the stock  110 , and through the driven cam assembly  160  in the crossbow  100 . 
     According to an embodiment, the left and right flexible limbs  150  are each characterized by a spring constant having a value within 5% of its complement flexible limb  150 . In one embodiment, the left and right flexible limbs  150  are each characterized by a spring constant having a value within 1% of its complement flexible limb  150 . In another embodiment, the left and right flexible limbs  150  are each characterized by a spring constant having a value within 0.3% of its complement flexible limb  150 . 
     According to an embodiment, the left and right flexible limbs  150  extend laterally at an angle between 15 degrees and 165 degrees from the stock assembly  110  respectively to the left limb tip and a right limb tip. In one embodiment, the left and right flexible limbs  150  extend laterally at an angle between 30 degrees and 150 degrees from the stock assembly  110  respectively to the left limb tip and a right limb tip. In another embodiment, the left and right flexible limbs  150  extend laterally at an angle between 70 degrees and 110 degrees from the stock assembly  110  respectively to the left limb tip and a right limb tip. The stock may optionally include respective limb support structures (not shown) disposed to support the limbs  150  away from the stock  110 . 
     The cam assemblies  160 , the synchronizing cable  194 , and the power cables  191 ,  192  are configured to cooperate to apply a respective selected tension from the limbs  150  to the bowstring  196  at all respective points along a bowstring  196  path of travel. 
     The left and right limbs  150  may extend laterally from locations rearward from the front end of the stock  110 . 
     According to an embodiment, the compound crossbow  100  further includes a force transfer bracket operatively coupled to the front of the stock  110  and operatively coupled to or continuous with the rigid cam support structures  140 . In an embodiment, the left and right limbs  150  are directly coupled to or continuous with the force transfer bracket, and the left and right limbs  150  extend laterally from the force transfer bracket at respective rearward angles away from the stock  110 . 
     According to an embodiment, the stock  110  has a length L between the front end and the back end, and the left and right limbs  150  extend from locations at least 25% of L rearward from the front end of the stock  110 . For example, the left and right limbs  150  may extend from locations about half way (L/2) between the front end and the back end of the stock  110 . As depicted in  FIGS. 1B, 2A and 2B , the left and right limbs  150  can extend from locations greater than 50% of L rearward from the front end of the stock  110 . 
     When the bowstring  196  is pulled rearward to cock the crossbow  100 , the power cables  191 ,  192  pull the limbs  150  toward the front end of the stock  110  into respective stored energy positions. The power cables  191 ,  192  may each remain within a 30 degree angle of parallel to the long axis of the stock  110  throughout the range of the bowstring pull. According to an embodiment, the power cables  191 ,  192  each remain within a 15 degree angle of parallel to the long axis of the stock  110  throughout the range of the bowstring pull. According to an embodiment, the power cables  191 ,  192  each remain within a 5 degree angle of parallel to the long axis of the stock  110  throughout the range of the bowstring pull. According to embodiments, each of the power cables  191 ,  192  remains at an equal and opposite angle to the stock  110  compared to the other power cable, to a precision of within 1 degree of angle. 
     The synchronizing cable  194  can be disposed on wheels below the bottom of the stock  110 , the synchronizing cable wheels being coupled to axles  169  of the left and right cam assemblies  160  to rotate synchronously with other portions of the cam assemblies. The top surface of the stock  110  may define an arrow groove  112  configured to hold an arrow before release of the bowstring  196 . The arrow groove  112  is formed coincident with the longitudinal axis of the stock  110  on or adjacent to the top surface of the stock. In an embodiment, the left and right limbs  150  join to the stock  110  sufficiently far below the top surface of the stock  110  and the arrow groove  112  to prevent vanes of the arrow (not shown) from contacting the limbs  150  during propulsion of the arrow (i.e., release of the bowstring  196 ). Similarly, the rigid cam support structure  140  may be positioned below the top surface of the stock  110  sufficiently to prevent vanes of the arrow from contacting the rigid cam support structure  140  during propulsion of the arrow. 
     In one embodiment, the rigid cam support structure  140  supports axles  169  of the left and right cam assemblies  160  forward of the front end of the stock  110 . In another embodiment, the rigid cam support structure  140  supports bearings that rotatably support cam axles for rotation about respective rotational axes in the left and right cam assemblies  160 . In another embodiment, the rigid cam support structure  140  supports the axles of the left and right cam assemblies  160  at locations perpendicular to and within the length of the stock assembly  110 , such as one to two inches rearward of the front end of the stock  110 . 
     In an embodiment, the rigid cam support structure  140  is formed integrally with the stock  110 . 
     Referring to the preceding  FIGS. 1A, 1B, 2A, 2B, and 3 , a method for operation of a crossbow  100  may include the steps of supporting; from a stock assembly  110  having front, back, left, right, top, and bottom sides; left and right flexible limbs  150  to extend laterally from the left and right sides of the stock assembly  110  to respective limb tips. The stock assembly  110  supports a rigid cam support structure  140  at or near the front of the stock assembly  110 . The rigid cam support structure  140  supports left and right cam assemblies  160  for rotational motion around respective positionally fixed axles  169 . The left limb tip is coupled to the left cam assembly  160  with a left power cable  191 . The right limb tip is coupled to the right cam assembly  160  with a right power cable  192 . 
     In use, tensile forces are received from a bowstring  196  (when cocking the crossbow) through the left and right cam assemblies  160  and through the power cables  191 ,  192  to cause a reactive spring force in the respective left and right limb tips. Spring force is stored in the limbs  150  while the crossbow is cocked. When the crossbow is fired, user input is received to release the bowstring  196 . The bowstring  196  is driven in tension, through the cam assemblies  160  and the power cables  191 ,  192  by releasing the stored spring force in the limbs  150 . Driving the bowstring  196  in tension causes the bowstring  196  to propel an arrow or bolt in flight forward from the front of the stock  110 . 
     According to an embodiment, the left and right power cables  191 ,  192  do not pass over any intermediate guide between the respective limb  150  and the cam assembly  160 . 
     According to an embodiment, the method for operation of a crossbow  100  may further include the step of synchronizing rotation of the cam assemblies  160  through at least one synchronizing cable  194  coupled between the left and right cam assemblies  160 . In one embodiment, the step of synchronizing rotation of the cam assemblies  160  through at least one synchronizing cable  194  coupled between the left and right cam assemblies  160  includes synchronizing the rotation through two synchronizing cables. In another embodiment, the step of synchronizing rotation of the cam assemblies  160  through at least one synchronizing cable  194  coupled between the left and right cam assemblies  160  includes synchronizing the rotation through a belt or with a cogged belt to a cam surface having a complementary and meshing cog pattern with the cogged belt. 
     According to an embodiment, the step of driving the bowstring  196  in tension includes applying force from the limbs  150  to the cam assemblies  160  in pure tension along straight power cables  191 ,  192 . In one embodiment, the step of driving the bowstring  196  in tension includes applying force from the limbs  150 , wherein each limb  150  is characterized by a spring constant having a value within 5% of its complement (opposing) flexible limb  150 . In another embodiment, the step of driving the bowstring  196  in tension includes applying force from the limbs  150 , wherein each limb  150  is characterized by a spring constant having a value within 1% of its complement flexible limb  150 . Additionally or alternatively, the step of driving the bowstring  196  in tension includes applying force from the limbs  150 , wherein each limb  150  is characterized by a spring constant having a value within 0.3% of its complement flexible limb  150 . 
     According to an embodiment, the step of supporting the left and right flexible limbs  150  includes supporting limbs  150  that extend laterally at an angle between 30 degrees and 150 degrees from the stock assembly  110  respectively to the left limb tip and a right limb tip. In one embodiment, the step of supporting the left and right flexible limbs  150  includes supporting limbs  150  that extend laterally at an angle between 70 degrees and 110 degrees from the stock assembly  110  respectively to the left limb tip and a right limb tip. 
     According to an embodiment, the step of driving the bowstring  196  in tension, through the cam assemblies  160  and the power cables  191 ,  192  by releasing the stored spring force in the limbs  150  includes providing motion in the cam assemblies  160  and the power cables  191 ,  192  to apply a respective selected tension to the bowstring  196  at all respective points along a bowstring  196  path of travel to cause acceleration of the arrow or bolt according to a smoothly changing third derivative of position. Additionally or alternatively, the step of driving the bowstring  196  in tension, through the cam assemblies  160  and the power cables  191 ,  192  by releasing the stored spring force in the limbs  150  includes providing motion in the cam assemblies  160  and the power cables  191 ,  192  to apply a respective selected tension to the bowstring  196  at all respective points along a bowstring  196  path of travel to cause acceleration of the arrow or bolt according to a linear third derivative of position. 
     According to an embodiment, the step of supporting the left and right flexible limbs  150  includes supporting the left and right flexible limbs  150  from the rigid cam support structures  140  or from a force transfer bracket operatively coupled to the front of the stock  110  and operatively coupled to or continuous with the rigid cam support structures  140 . In one embodiment, the step of supporting the left and right flexible limbs  150  includes supporting the left and right limbs  150  extending from locations at least 25% of L rearward from the front end of the stock  110 . In another embodiment, the step of supporting the left and right flexible limbs  150  includes supporting the left and right limbs  150  extending from locations about half way L/2 between the front end and the back end of the stock  110 . Additionally or alternatively, the step of supporting the left and right flexible limbs  150  includes supporting the left and right limbs  150  extending from locations greater than 50% of L rearward from the front end of the stock  110 . 
     According to an embodiment, the step of receiving tensile forces from a bowstring  196  through the left and right cam assemblies  160  and through the power cables  191 ,  192  to cause a reactive spring force in the respective left and right limb tips causes the power cables  191 ,  192  to pull the limb tips in a forward direction relative to the stock  110  when the bowstring  196  is pulled rearward. 
     According to an embodiment, the method for operation of a crossbow  100  may further include the step of defining an arrow groove  112  in the top surface of the stock  110  to hold an arrow before release of the bowstring  196 . 
     According to an embodiment, the step of supporting, from the stock assembly  110 , the rigid cam support structure  140  at or near the front of the stock assembly  110 , includes providing the rigid cam support structure  140  formed integrally with the stock  110 . 
     The resilient limb  150  on either side of the stock  110  may be half of a single limb passing between the stock and strut, which can double as a fastener (as shown), or may be an individual piece mounted on either side, for example on a plate attached to the side of the strut and stock. 
     The cam units  160 , which can alternatively be referred to as “cam/wheel units,” “cam assemblies,” “cam/wheel assemblies,” and the like, need not be integral in the sense that they are made from a single piece of material, or are otherwise inseparable into parts, or have no parts; they can be integrally constructed, for example with different planar cams or wheels riveted, welded, adhered, or otherwise fastened together, but such assemblies are referred to in the claims as “cam/wheel units.” Assemblies are suitable for the invention if they are rigid enough to act as a unit, or as one piece. 
     The phrase “cam/wheel” is redundant in the sense that a “wheel” is merely a circular “cam,” and thus “cam” is broader than “wheel.” Here, “wheel” means a cam having a substantially circular shape and/or a constant radius through at least some angle around the axis of the cam. The phrase “cam/wheel,” though redundant, is used for clarity. 
     Above, and in the following claims, “substantially” means a factor of 0.9; 0.99; 0.999; and so on. 
     It will be understood that terms such as “lower” and “above” are used for convenience and refer to the usual shooting position for a crossbow. They do not limit the crossbow to any orientation. 
     In the following claims, “limb” covers any arm-or leg-like extension such as the traditional bow, but also covers other devices for storing mechanical potential energy, including but not limited to coil springs, compressible gases, elastomers, etc. that lie at least partially outside the stock on the left or right. For example, the traditional elastic bow can be modified to include an elastic tensile member (coil spring, elastic cable, etc.) and a more-rigid arm (limb). 
     As used herein, the term “synchronizing cable” can consist of one continuous cable fixed to a left and right cam assembly, a cogged cable, a cogged belt, or a chain, etc., or two cables with ends fixed in their respective tracks. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.