Patent Publication Number: US-6698413-B1

Title: Compression spring powered, rigid limb bow

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application relates to and claims priority from Provisional U.S. Patent Application Serial No. 60/380,358 filed May 14, 2002. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a compression spring powered, archery bow which is adjustable for draw weight, draw length and draw weight let-off at full draw length in a rigid frame that is integrated to form one continuous loop in the shape of a contemporary archery bow. 
     BACKGROUND OF THE INVENTION 
     Many attempts have been made in the development of the archery bow to make it more adjustable in order to provide a single design which can be used universally by archers of all ages. The present designs generally all lack the ability to perform this task while being compact, easily and independently adjusted, simplistic, cost effective and aesthetically pleasing. Prior art in this filed shows designs that are overly complex or designs that do not fill the above stated criteria. 
     U.S. Pat. No. 4,903,677 issued to Colley et al. on Feb. 27, 1990 describes an archery bow in which at least one flat wound coil spring is mounted on a rigid limb frame and is connected through an eccentric wheel or cam, a drive sprocket and a drive wheel or storage spool to a bowstring so that the action of drawing back the bowstring causes the flat wound coil spring to be wound up storing energy. A let-off arrangement for reducing the force string movement of the string between an at-rest position and a fully drawn position is a function of the flat wound coil spring, the eccentric wheel or cam and the relative dimensions of the drive sprocket and storage spool. The size of the storage spool determines the amount of draw length by functioning to store more or less bowstring in the at-rest position of the bow. Draw weight adjustment is limited to altering the pre-load on the flat wound coil spring at the time that the spring is installed on the bow. In this design, let-off action is interdependent upon larger and smaller diameter drive and driven wheels, their mounting shafts, and the eccentric wheel or cam. In addition, draw weight and draw length adjustment are not easily accomplished without an involved interchanging of the storage spool and flat wound coil spring, respectively. 
     U.S. Pat. No. 5,503,135 issued to Bunk on Apr. 2, 1996 discloses an archery apparatus which includes a main sheave rotatably mounted on the lower limb of a rigid main frame and engaged with a bowstring extending between guide sheaves on opposing ends of the frame. A pair of tensioning mechanisms are located on opposite sides of the lower limb of the frame for selectively applying torque to the main sheave responsive to drawing of the bowstring. Each tensioning mechanism includes a coil spring, a cable attached at one end to the spring and at another end to a connection member on the sheave, and a cam sheave engaged by the cable and rotatably mounted on the frame in spaced relationship from the main sheave. Any desired residual level of draw weight let-off is provided as the periphery of the cam sheaves come into alignment with the connection members of the tensioning cables and the rotational axis of the main sheave. A pair of stop members is removably attached to the main sheave so as to abut the lower limb of the frame at the point of maximum desired draw length. The design of this archery bow does not allow a full range of onboard adjustability of draw weight and the tensioning springs are attached to the lower limb using various closely spaced holes which only allows for predetermined changes in spring pressure. In addition, constant elongation of the tensioning springs will result in eventual fatigue and declining torque capability. The Bunk design also does not allow for a more complete adjustability of draw weight let-off since the let-off system is integrated or tied into the tensioning springs through a set of cables and cams which again only permits predetermined changes. 
     Accordingly, it is desirable to offer a differently styled spring powered, rigid limb bow which overcomes the shortcomings of the prior art, and provides a contemporary, more simplistic design having independently and more fully and easily adjustable draw weight, draw length and let-off features which enable an arrow to be accurately shot with a high level of substantially vibrationless, high energy. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a high efficiency archery bow which is universal in usage. 
     It is one object of the present invention to provide a compact, rugged and easily adjustable rigid limb bow. 
     It is also an object of the present invention to provide a spring powered, rigid limb bow having independently adjustable draw weight, draw length and draw weight let-off adjustments. 
     In one aspect of the invention, an archery bow includes a rigid frame having an upper limb, an opposite lower limb and a handle therebetween. An upper wheel is rotatably mounted to a free end of the upper limb. A lower wheel is rotatably mounted to a free end of the lower limb, the lower wheel having a sprocket attached thereto. The sprocket has a diameter which is smaller than a diameter of the lower wheel. A bowstring has one end attached to the upper wheel and another end attached to the lower wheel. The bowstring is windable about respective peripheries of the upper wheel and the lower wheel and is moveable between an at-rest position and a drawn position. The lower limb has a cavity defined by wall structure threaded to receive a cooperably threaded, rotatable spring housing containing a compressible power coil spring having one end engaged against a bottom surface of the spring housing and an opposite end engaged by a holding member. A cable passes centrally through the power coil spring and has one end fastened to the holding member and an opposite end passing through the bottom surface of the spring housing and fixedly secured to the sprocket on the lower wheel. A cam is rotatably mounted to the lower limb between the lower wheel and the spring housing, the cam having a periphery engaged by the cable. With this structure, rotation of the spring housing relative to the lower limb will selectively change the compression rate of the power coil spring without the need for disassembly of the spring housing or requiring an adjustment tool to apply an adjustable torque transmitted through the cable to the lower wheel and thereby establish a variable draw weight required to move a bowstring to the drawn position. 
     The archery bow includes an elongated straight cross brace extending between the free ends of the upper limb and the lower limb. The opposite ends of the cross brace are connected at rotational axes of the upper wheel and the lower wheel. A synchronizing string has one end attached to the upper wheel, another end attached to the lower wheel and a midportion passing through a retainer fixed to the cross brace. A synchronizing string is windable about respective peripheries of the upper wheel and the lower wheel. The periphery of the upper wheel is formed with first and second grooved tracks and the periphery of the lower wheel is formed with first and second grooved tracks. The bowstring is wound on the first tracks of the upper wheel and the lower wheel, and the synchronizing string is wound on the second tracks of the upper wheel and the lower wheel. The lower wheel has a peripheral area formed with a plurality of threaded holes for facilitating adjustment of draw length of the bowstring. One of the holes receives a removably threaded, let-off pin engageable with a stop member located on the lower limb as the bowstring is drawn and the lower wheel is rotated against resistance of the power coil spring. The upper wheel and the lower wheel are located behind the handle of the frame. An independently adjustable, draw weight let-off mechanism is mounted on the lower limb and is engageable with the let-off pin as the bowstring is drawn and the lower wheel is rotated against resistance of the power coil spring. The adjustable let-off mechanism is independent of the power coil spring, the cable and the cam. The let-off mechanism includes a housing removably secured to the lower limb in a hole formed therethrough. The housing has an interior passageway formed by a cylindrical internal wall provided with a retainer ledge. The passageway extends from an open threaded top end through a threaded bottom end formed with a slot therein. The passageway receives a plunger held therein by the retainer ledge. The plunger has a top end formed with a chamber for retaining a let-off coil spring therein, and a bottom end having a horizontal surface and an upwardly ramped surface. A recess is defined between the bottom end of the housing and the bottom end of the plunger. The recess receives a biased spring normally urging an interference member out of the recess and partially through the slot. The open ended top end of the housing receives a threaded adjustment screw engaged against the let-off spring in the chamber of the plunger. Rotation of the adjustment screw will change the compression force exerted by the let-off coil spring and selectively control the amount of draw weight let-off desired. The let-off pin is engageable with the biased interference member as the bowstring approaches its draw weight. 
     In another aspect of the invention, an archery bow includes a rigid frame having an upper limb, an opposite lower limb and a handle therebetween. An upper wheel is rotatably mounted to a free end of the upper limb. A lower wheel is rotatably mounted to a free end of the lower limb, the lower wheel having a sprocket attached thereto. The sprocket has a diameter which is smaller than the diameter of the lower wheel. A bowstring has one end attached to the upper wheel and another end attached to the lower wheel. The bowstring is windable about respective peripheries of the lower wheel and the upper wheel, and is moveable between an at-rest position and a drawn position. The lower limb screwthreadedly receives an adjustable, rotatable, self-contained tensioning unit having a variably compressible power coil spring therein. The tensioning unit provides a resilient pull to establish a draw weight required to move the bowstring from the at-rest position to the drawn position. A cable passes centrally through the coil spring and has one end attached to a holding member at one end of the coil spring and an opposite end passing through the tensioning unit and fixedly secured to the sprocket on the lower wheel. A cam is rotatably mounted to the lower limb between the lower wheel and the tensioning unit and is engaged by the cable. A pin is variably positioned on the lower wheel along a peripheral area thereof for movement toward an engagement with a stop member fixed on the lower limb. A spacing between the pin and the stop member corresponds to a desired draw length for the bow. An independently adjustable draw weight let-off mechanism is mounted on the lower limb separate from the coil spring, the cable and the cam. The let-off mechanism is engageable with the pin for selectively reducing the draw weight on the bowstring once the bowstring reaches the drawn position. 
     The let-off mechanism is positioned between the cam and a rotational axis of the lower wheel. The let-off mechanism includes a housing removably secured to the lower limb in a hole formed therethrough. The housing has an interior passageway formed by a cylindrical internal wall provided with a retainer ledge. The passageway extends from an open threaded top end through a threaded bottom end formed with a slot therein. The passageway receives a plunger held therein by the retainer ledge. The plunger has a top end formed with a chamber for retaining a let-off coil spring therein, and a bottom end having a horizontal surface and an upwardly ramped surface. A recess is defined between the bottom end of the housing and the bottom end of the plunger. The recess receives a biased spring normally urging an interference member out of the recess and partially into the slot. The open threaded top end of the housing receives a threaded adjustment screw engaged against the let-off spring in the chamber of the plunger. Rotation of the adjustment screw will change the compression force exerted by the let-off coil spring and selectively control the amount of draw weight let-off desired. The rotational axis of the lower wheel is substantially parallel to a longitudinal axis of the housing in the let-off mechanism. An elongated straight cross brace extends between the free ends of the upper limb and the lower limb. The cross brace is aligned with rotational axes of the upper wheel and the lower wheel. A synchronizing string has one end attached to the upper wheel, and another end attached to the lower wheel and a midportion passing through a retainer fixed on the cross brace. The synchronizing string is windable about respective peripheries of the upper wheel and the lower wheel. The stop member is located beneath the let-off mechanism and partially overlaps the slot formed in the bottom end of the housing. The cam has a first circular periphery, and a second eccentric periphery adjacent to the circular periphery. 
     In yet another aspect of the invention, an archery bow has a frame with an upper limb, an opposite lower limb and a handle therebetween, a bowstring extending between the upper limb and the lower limb and wound about a wheel rotatably mounted on one of the limbs and a spring operably connected to the wheel for establishing a desired draw weight for moving the bowstring from an at-rest position. The wheel is rotated as the bowstring is drawn. The invention is improved by means of a let-off pin variably positioned on the rotatable wheel, and an independently adjustable let-off mechanism located on the wheel mounting limb in a hole formed therein and selectively engaged by the let-off pin for reducing the draw weight required to move the bowstring towards the drawn position. The let-off mechanism includes a biased spring for exerting a first force to normally urge an interference member to a first position, and a let-off spring for exerting a second force larger than the first force and defining a draw weight reduction force. The let-off pin engages the interference member overcoming the first force of the biased spring in a first position, and the let-off pin engages the interference member overcoming the second force of the let-off spring in a second position. 
    
    
     Various other objects, features and advantages of the invention will be made apparent from the following description taken together with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate the best mode presently contemplated of carrying out the invention. 
     In the drawings: 
     FIG. 1 is a left side elevational view of the archery bow embodying the present invention; 
     FIG. 2 is a right side elevational view of the archery bow in FIG. 1; 
     FIG. 3 is an enlarged, fragmentary elevational view of a lower portion of the archery bow as the bowstring is drawn; 
     FIG. 4 is a perspective representation of certain components of the archery bow with the bowstring in an at-rest position; 
     FIG. 5 is a representation like FIG. 4 showing the bowstring in a drawn position; 
     FIG. 6 is cross sectional view taken on line  6 — 6  of FIG. 1; 
     FIG. 7 is an enlarged, cross sectional view taken on line  7 — 7  of FIG. 3 showing initial contact of a let-off pin with a let-off mechanism; 
     FIGS. 8 through 10 are sequential views of the action of the let-off mechanism of FIG. 7 as the bowstring is drawn; 
     FIG. 11 is an enlarged detail view taken on line  11 — 11  of FIG. 2; 
     FIG. 12 is a fragmentary, sectional view taken on line  12 — 12  of FIG. 7; 
     FIG. 13 is cross-sectional view taken on line  13 — 13  of FIG. 12; 
     FIG. 14A is a view like FIG. 3 showing the cam initially aligned with the lower limb; 
     FIG. 14B is a graphical portrayal of the draw force over the distance the bowstring is drawn for the configuration shown in FIG. 14A; 
     FIG. 15A is a view like FIG. 14A but showing the cam initially non-aligned with the lower limb; and 
     FIG. 15B is a graphical portrayal of the draw force over the distance the bowstring is drawn for the configuration shown in FIG.  15 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now the drawings, FIGS. 1 and 2 generally illustrate an archery bow  10  embodying the present invention. At the outset, a brief overview of the structure and function of the archery bow  10  will be presented to be followed by a more elaborative description. 
     The archery bow  10  includes a rigid main frame  12  having a midportion  14 , an upper limb  16  extending from an upper end of the midportion  14  and a lower limb  18  extending from a lower end of the midportion  14  and upon which a handle  20  is mounted. The upper limb  16  terminates in a bifurcated free end  22  upon which an upper wheel  24  is rotatably mounted. Similarly, the lower limb  18  has a bifurcated free end  26  upon which a lower wheel  28  is rotatably mounted. A frame cross brace  30 , which is shown as being cylindrical, is rigidly connected to the respective free ends  22 ,  26  of the upper limb  16  and the lower limb  18 , and carries a fixed guide retainer  32  at generally the midpoint thereof. The cross brace  30  has opposite ends  34 ,  36  connected at rotational axes of the upper and lower wheels  24 ,  28 . A bowstring  38  equipped with a nocking device  39  spans the upper and lower wheels  24 ,  28 , and opposite ends  40 ,  42  thereof wrap around and are connected to peripheral locations on the upper and lower wheels  24 ,  28 . A synchronizing string  44  passes through the guide retainer  32  on the cross brace  30 , and has opposite ends  46 ,  48  which wind about and are secured to peripheral locations other than those used for connecting ends  40 ,  42  of the bowstring  38 . 
     The archery bow  10  also includes a self-contained tensioning unit  50  having a compressible power coil spring  52  incorporated in the lower limb  18  for tensioning the bowstring  38  without flexing the rigid frame  12 . An idler roller  54  and a cam  56  are each rotatably mounted to the lower limb  18  on axes generally parallel to one another. A cable  58  is connected at one end  60  to an upper end of the spring  52 , passes over the idler roller  54 , winds around the cam  56  and is fastened at an opposite end  62  to a central sprocket  64  formed integrally on the lower wheel  28 . A let-off pin  66  is variably positioned in one of a plurality of holes  68  formed around the periphery of the lower wheel  28 . A stop member  70  is stationarily secured to the lower limb  18  next to an adjustable let-off mechanism  72  used to reduce the amount of draw weight or draw force required to hold the bowstring  38  at its drawn position. The let-off mechanism  72  is designed to selectively apply a small biasing force as well as a larger let-off force. 
     In operation, pulling of the bowstring  38  with an arrow nocked therein from an at-rest position shown in solid lines to a drawn position shown in phantom lines will cause the upper and lower wheels  24 ,  28  to rotate. Rotation of the lower wheel  28  causes the cable  58  attached to the sprocket  64  to compress the power coil spring  52  and, at the same time, rotate the cam  56  off center. This action results in a tensioning pull or torque applied to the lower wheel  28  which determines the draw weight or draw force required to draw the bowstring  38 . As the bowstring  38  moves towards maximum draw, the lower wheel  28  will have rotated so that the let-off pin  66  thereon approaches the let-off mechanism  72  and the stop member  70 . The let-off mechanism  72  allows the moving let-off pin  66  to overcome the small biasing force and contact the stop member  70  at which point the bowstring reaches its maximum draw length. Substantially instantaneously thereafter, the let-off pin  66  encounters and overcomes the larger let-off force which offsets and reduces the draw weight or draw force so that the bowstring  38  may be more easily held without tiring effort. When it is desired to propel the arrow, the bowstring  38  is released, the let-off pin  66  will have disengaged from the let-off mechanism  72  and the draw weight or draw force will drive the arrow forward smoothly and with high velocity and accuracy. 
     A more detailed description of the structure and function of the archery bow  10  follows with further reference being made to FIGS. 3 through 15B. 
     The rigid main frame  12  is integrally fabricated in one piece, such as by casting or molding, or from multiple pieces joined together as desired, such as by fasteners. The frame is constructed preferably from aluminum or a carbon fiber composite, but may also be made of other inflexible, rugged and durable materials. The frame  12  is formed with a series of material relieving voids  74  for reducing the weight of the frame  12  without sacrificing its rigidity. Although not illustrated, the midportion  14  of frame  12  is desirably offset from the plane of the upper and lower limbs  16 ,  18  so as to accommodate the placement of an arrow in alignment with the nocking device  39  on bowstring  38 . The midportion  14  also facilitates the attachment of commercially available aiming and arrow supporting devices (not shown) which do not form the present invention. 
     The upper limb  16  has a first vertically extending segment  76  and a second upwardly and rearwardly extending angular segment  78  to locate upper wheel  24  behind handle  20  such that the bowstring  38  attached thereto will be spaced a suitable distance relative to the midportion  14  in accordance with the length of the arrow being used. The upper wheel  24  is rotatably mounted to the free end  22  of the upper limb  16  upon a bearing-supported shaft  80  of an elongated bolt  82  passing through the center of the upper wheel  24  and defining a rotational axis thereof. The upper wheel  24  is formed with a plurality of material relieving holes  84  for weight reduction purposes, and a number of peripheral apertures  86  for receiving respective retaining pegs  88 ,  90  at the respective upper ends  40 ,  46  of the bowstring  38  and the synchronizing string  44 . 
     As seen best in FIG. 4, the upper wheel  24  is formed with first and second grooved tracks  92 ,  94 . An upper portion of the bowstring  38  is wound about the entire periphery of the upper wheel  24  in the first grooved track  92  when the bowstring is at-rest, and terminates in the retaining peg  88 . An upper end  46  of the synchronizing string  44  terminates in the retaining peg  90  located approximately 180 degrees from the bowstring retaining peg  88 . As depicted in FIG. 5, an upper portion of the synchronizing string  44  is entrained or wrapped around a portion of the upper wheel periphery in the second grooved track  94  when the bowstring  38  is drawn and the upper wheel  24  is rotated. Simultaneously, the upper portion of the bowstring  38  partially unwinds from the first grooved track  92  as the bowstring  38  is drawn and the upper wheel  24  rotates against the pull of the compressible spring  52  as transmitted by the synchronizing string  44 . 
     Referring now to FIGS. 1,  2  and  6 , the lower limb  18  has a first vertically extending segment  96  for mounting handle  20 , and a second downwardly and rearwardly extending angular segment  98  to locate the lower wheel  28  behind handle  20  in alignment with the upper wheel  24 . The lower wheel  28  is rotatably mounted to the bifurcated free end  26  of the lower limb  18  upon a shaft  100  of an elongated bolt  102  passing through the center of the internal sprocket  64  on lower wheel  28 , a pair of legs  104 ,  106  on the bifurcated free end  26  and a set of bearings  108 ,  110  surrounding the shaft  100  and positioned between each leg  104 ,  106  and each side of the sprocket  64 . The bolt  102  defines a rotational axis upon which the lower wheel  28  turns. The bearings  108 ,  110  help to minimize friction and vibration as the bowstring  38  is drawn. The end of the shaft  100  is externally threaded into an internally threaded lower end  36  of the cross brace  30 . It should be understood that the structure set forth for the bolt  102 , the bifurcated legs  104 ,  106 , the internal sprocket  64 , the bearings  108 ,  110  and the cross brace end connection at  36  are identical for the upper wheel  24  previously described herein. 
     The lower wheel  28  is formed with a plurality of material relieving holes  112  for weight reduction purposes, and a number of peripheral apertures  114  for receiving respective retaining pegs  116 ,  118  (FIG. 11) at the lower ends  42 ,  48  of the bowstring  38  and the synchronizing string  44 . As shown in FIGS. 4,  5 , and  6 , the lower wheel  28  is formed with first, second and third grooved tracks  120 ,  122 ,  124 . A lower portion of the bowstring  38  is wound around substantially the entire periphery of the lower wheel  28  and the first grooved track  120  when the bowstring  38  is at-rest, and terminates in the retaining peg  116 . A lower portion of the synchronizing string  44  is wound about the entire periphery of the lower wheel  28  in the second grooved track  122  when the bowstring  38  is at-rest, and terminates in the retaining peg  118 . The third grooved track  124  is formed on the sprocket  64  and is used to engage the cable  58  as will be understood more fully below. As depicted in FIG. 5, lower portions of the bowstring  38  and synchronizing string  44  are partially unwound from the respective first and second grooved tracks  120 ,  122  as the bowstring  38  is drawn and the lower wheel  28  rotates against the pull of the compressible spring  52 . The lower wheel  28  further has a group of the threaded holes  68  along its periphery to facilitate adjustment of the draw length of the bowstring  38 . One of the threaded holes  68  threadably receives a shank  128  of the let-off pin  66 . As seen in FIGS. 7 through 10, the let-off pin  66  has a domed head  130  which is selectively engaged with an edge of the stop member  70  lying along the inside surface of leg  104  when the lower wheel  28  rotates as the bowstring  38  is drawn. The stop member  70  has a threaded shaft  132  which is cooperably received in a threaded hole  134  formed through the leg  104 . 
     It is one distinctive feature of the invention that the let-off pin  66  is selectively positioned in any one of the threaded holes  68  so as to vary the distance between the stop member  70  and the let-off pin  66 . This variation of spacing equates to an independent, convenient and quick adjustment of the draw length of the bowstring  38 . 
     The angular segment  98  of the lower limb  18  includes a cylindrical section  136  having a cavity  138  formed by internal, cylindrical wall structure which is threaded at a lower end thereof as illustrated in FIG.  6 . The cylindrical section  136  has an open top end  139  and an open bottom end  142 . The cavity  138  is designed to threadably receive a threaded, rotationally adjustable spring housing  140  for holding the compressible power coil spring  52  such that a lower end of the spring  52  rests against a bottom end  141  of the housing  140  formed with a hole  144  therethrough. An upper end of the spring  52  is fitted with a plug  146  having a radially extending top section  148  supported on the top of the spring  52 , and a depending lower section  150  which projects partially into a space into the center of the spring coils. The plug  146  is formed with an internal pocket  152 ,for a purpose to be appreciated hereafter. A removable end cap  154  is threaded into an open top end  156  of the housing  140  and provides access thereto. 
     It is another distinctive feature of the invention that the self-contained tensioning unit  50  comprised of the spring  52 , and the spring housing  50  is rotatable either clockwise or counterclockwise as shown by the arrows in FIG. 6 at the top end of spring housing  140  so as to move the spring housing  140  up and down as depicted by the arrows at the side of the cylindrical section  136 . This motion has the effect of selectively and independently varying the compression rate of the spring  52  in order to infinitely adjust the desired draw weight or draw force required to draw the bowstring  38  without the absolute necessity of replacing the spring  52 . 
     Below the lower end of the spring housing  140 , a pin  158  is passed through the idler roller  54  rotatably and centrally mounted thereon. The pin  158  has opposite ends  160 ,  161  which are fixedly retained in the upper ends of the spaced apart legs  104 ,  106 . 
     Beneath the pin  158  on the lower limb  18 , the cam  56  is rotatably mounted on a shaft  162  of an elongated bolt  163  having an end  164  which is threaded into the leg  106 . The cam  56  includes a circular portion  166  having a hub  168  projecting to one side thereof, and an eccentric portion  168  which, in the preferred embodiment, is centrally aligned with a longitudinal axis of the lower limb  18  so that it lies in a recess  170  between the legs  104 ,  106  when the bowstring  38  is at-rest as shown in FIG.  6 . It should be understood, however, that the initial position of the cam  56  may be adjusted to obtain different draw characteristics as will be explained later. A bearing  172  is mounted on the shaft  162  between the eccentric portion  168  and the inside surface of leg  106  to minimize friction and hold the cam  56  at a desired axial position on the shaft  162 . 
     The cable  58  is used to transmit the pull of the compressible coil spring  52  to apply a torque to the lower wheel  28  which torque is transferred in a balanced manner at the same rate to the lower wheel  24  by the synchronizing string  44 . An upper end  60  of the cable  58  carries an end holder  174  which is lodged against a base of the plugged pocket  152 . The cable  58  then extends through the center space of the coil spring  52  and the hole  144  in the bottom end  52  of housing  140 , passes over the idler roller  54 , is wound about a peripheral track on the cam circular portion  166 , crosses over to wind about a peripheral track on the cam eccentric portion  168  and terminates in a retaining peg  176 . The peg  176  is received in an opening on the periphery of the sprocket  64  at a 6 o&#39;clock position when the bowstring  38  is at-rest as shown in FIG.  4 . 
     It is yet another distinctive feature of the invention that the lower limb  18  is provided with the let-off mechanism  72  which is independently adjustable to infinitely vary the reduction and draw weight or draw force required to draw the bowstring  38 . 
     With reference to FIGS. 7 through 10, the let-off mechanism  72  includes a cylindrical, tubular housing  178  removably secured to an outer surface of lower limb leg  104  in a threaded hole  180  formed therethrough. The let-off housing  170  has a longitudinal axis which is parallel to the rotational axis of the lower wheel  28 . The housing  178  is constructed with a reduced diameter bottom end  182  which is threaded so that it can be turnably fastened in the threaded hole  180 . The housing  178  defines an interior passageway  184  that extends from an open threaded top end  186  through the bottom end  182  which is formed with an oval slot  188  as shown in FIG.  12 . An internal cylindrical wall of the housing  178  is provided with a retainer ledge  190  for retaining a vertically moveable plunger  192  in the passageway  184 . A top end of the plunger  192  has a chamber  194  formed therein for holding a let-off coil spring  196  having a lower end resting upon a base  198  for the bottom of the chamber  194 , and an upper end which is engaged by the bottom of an adjustment screw  200  threadably received in the top end of the housing  178 . A bottom end  201  of the plunger  192  is configured with a horizontal surface  202  and an upwardly ramped surface  204 , and a recess  206  is defined between the bottom end  201  of the plunger  192  and the bottom end  182  of the housing  178 . Disposed within the recess  206  is a small bias spring  208  located beneath the ramped surface  204  for normally urging an interference member  210 , such as a ball bearing, out of the recess  206  and partially through the slot  188  in the bottom end  182  of the housing  178 . As seen in FIG. 12, the slot  188  is appropriately sized so that the ball bearing  210  is otherwise retained in the bottom end  182  of the housing  178 . It can also be seen that the stop member  70  partially overlaps the slot  188  at one end thereof. 
     It is to be noted that the let-off coil spring  196  has a much greater applied force then the bias spring  208  and is chosen to exert a range of force which is less then the rotational force or torque applied to the lower wheel  28 . That is, the let-off coil spring force is designed to offset or reduce the draw weight or draw force so that the bowstring  38  may be more easily held upon reaching the maximum draw and before release of an arrow nocked in the bowstring  38 . The adjustment screw  200  of the let-off mechanism  72  may be turned clockwise or counterclockwise so as to selectively vary the compression rate of the let-off coil spring  196  and thereby infinitely adjust the let-off to a desired level. 
     A detailed operation of the archery bow  10  now follows with attention being directed particularly to FIGS. 1,  3 ,  4 ,  5 , and  7  through  10 . 
     With the bowstring  38  in the at-rest position of FIG. 4, the upper and lower wheels  24 ,  28 , respectively, are stationary, and a pre-load force is applied to the lower wheel  28  via sprocket  64  by means of cable  58  which is wound about the circular periphery  166  and eccentric periphery  168  of the cam  56  and is attached to the upper end of the power coil spring  52 . As seen in FIG. 1, let-off pin  66  is positioned in lower wheel  28  so as to define a predetermined draw length or maximum draw defined by the spacing along the periphery of the lower wheel  28  between the pin  66  and stop number  70 . When the bowstring  38  is drawn as shown in FIGS. 3,  5  and  15 A, the upper wheel  24  rotates in a clockwise direction and the lower wheel rotates in a counterclockwise direction. Rotation of the upper wheel  24  causes unwinding of an upper portion of bowstring  38  and winding of an upper portion of synchronizing string  44 . Rotation of the lower wheel causes unwinding of both the bowstring  38  and synchronizing string  44 . Rotation of the lower wheel also causes simultaneous rotation of integral sprocket  64  which, in turn, results in winding of cable  58  around the sprocket  64  as shown in FIG.  3 . Winding of cable  58  around sprocket  64  further causes counterclockwise rotation of cam  56  and compression of power coil spring  52  (as depicted in FIG. 5) to store a quantity of draw energy therein and simultaneously apply a torque to the lower wheel  28 . The torque on the lower wheel  28  is distributed to the upper wheel  24  in a balanced manner by the synchronizing string  44  which also enables the upper and lower wheels  24 ,  28  to rotate at the same rate. 
     FIG. 3 illustrates that as the bowstring  38  is pulled towards maximum draw, lower wheel  28  continues to rotate such that the let-off pin  66  approaches the let-off mechanism  72  and the stop member  70 . As seen in FIG. 7, the domed head  130  of the let-off pin  66  on the torqued, rotating lower wheel  28  eventually engages the ball bearing  210  and forces the ball bearing  210  along the ramped surface  210  into the recess  206  against only the small force of bias spring  208  (FIG.  8 ). As the center of the domed head  130  of let-off pin  66  moves past the bottom of ball bearing  210  and contacts the stop member  70  (signifying the maximum draw), the ball bearing  210  is again forced into its initial position by bias spring  208  (FIG.  9 ). Substantially instantaneously after reaching maximum draw, the bowstring  38  is advanced a fraction of an inch so that the domed head  130  of the let-off pin  66  moves away from the stop member  70  and the torque applied to the domed head  130  overcomes the force of the let-off spring  196  acting downwardly upon the top of the ball bearing  210 . This causes the plunger  192  to move slightly upwardly (FIG. 10) so that the ball bearing  210  will move correspondingly upwardly. At this point, the torque applied to the lower wheel  28  and thereby the draw weight or draw force is “let-off” or reduced by the force of the let-off spring  196  so that the bowstring  38  may be much more easily held until it is desired to release the bowstring  38  and propel an arrow nocked therein. 
     When the bowstring  38  is released, the stored energy in the power coil spring  52  is expended to retract the cable  58 , rotate the cam  56  and upper and lower wheels  24  and  28  in opposite directions and pull the bowstring  38  forward in a smooth, high velocity manner without any vibration which would affect the accuracy of a propelled arrow. 
     It should be emphasized that the particular design of the cam  56  with its circular periphery  166  and eccentric periphery  168  engaged by the cable  58  between the power coil spring  52  and the lower wheel  28  rectifies the problem of non-linear force characteristics obtained by using a compression coil spring by itself. It can be said that the cam  56  acts as a pressure or force compensator so that a linear force characteristic may be realized. 
     It should also be understood that the initial position of the cam  56  determines a particular linear force profile. Referring to FIG. 15A, the cam is shown as described above in its initial position in solid lines with the bowstring  38  at-rest, and its rotated position in phantom lines as the bowstring  38  is drawn. Using the cam  56  in this setting will result in a constant draw weight or draw force for a distance of bowstring pull short of maximum draw. However, setting the initial solid line position of the cam  56  as shown in FIG. 15A will provide a slightly increasing draw force in the early stage of bowstring pull which draw force becomes constant for the remainder of the pull short of maximum draw. Other variations and cam positions will yield different linear force profiles. 
     In a specific example of the present invention, the archery bow  10  has a length of 28 inches from the center of the upper wheel  24  to the center of the lower wheel  28 . The distance from the back of the handle  20  to the bowstring  38  is 7 inches. The combined length of the frame midportion  14  and the frame vertical extending segments  76 ,  96  is 15 inches. The length of the upper limb angular segment  78  is 9 inches, while the length of the lower limb angular segment  98  is 12 inches. The diameter of the spring housing  140  is 1⅜ inches. The diameter of the upper and lower wheels  24 ,  28  are 4.75 inches and their sprockets  64  have diameters of 1.2 inches so that the ratio of the wheel to sprocket diameter is 4 to 1. This particular ratio is chosen so that using a power coil spring  52  designed to deliver 240 to 480 inch-pounds of force connected in combination with the unique cam  56  will provide a wheel torque and a draw weight or draw force of about 60 inch-pounds. Advantageously, this torque value can be infinitely adjusted by simply rotating the spring housing  140  so as to change the compression rate of the power coil spring  52 . If desired, the end cap  154  on spring housing  140  can be removed, the cable  58  can be disassembled and a replacement power coil spring  52  may be installed. The let-off coil spring  196  is designed to produce 45 inch-pounds of force which offsets the 60 inch-pounds of draw force so that at let-off, the required force to hold the bowstring  38  is decreased to about 15 inch-pounds. Rotating the adjustment screw  200  on the let-off mechanism  72  will selectively alter the let-off spring force so as to infinitely vary the let-off. The adjustment screw  200  can be removed as desired to replace the let-off coil spring  196 . The peripheral spacing between the threaded holes  68  for retaining the let-off pin  66  and the stop member  70  translates to providing a variation of about 20 to 31 inches in draw length. 
     It should now be appreciated that the present invention provides a rugged, compact, rigid limb bow  10  which is universally adaptable to archers of all ages by providing independent, infinite and easy adjustments for the draw weight or draw force, the draw length and the draw weight let-off without affecting each other. The extreme rigidity provided by the frame  12  and the cross brace  30  enables significant lower vibration. The cam  56  is particularly shaped so that the cable  58  when entrained about the peripheries  166 ,  168  will not contact itself. The cam  58  also ensures that the force transmitted from the power coil spring  52  to the lower wheel  28  will remain constant over a distance of bowstring pull. The synchronizing string  44  serves to equally distribute torque between the upper and lower wheels  24 ,  28  and provide that the wheels rotate at the same rate. The archery bow  10  is more simplified, cost-effective and aesthetically pleasing than previously known bows. In contrast with the prior art, the let-off feature is not dependent on the spring, cam or cable as in other known devices. The rotatable self-contained tensioning unit  50  permits power coil spring  52  to deliver a range of different forces without disassembling the unit or requiring an adjustment tool. 
     While the invention has been described with reference to a preferred embodiment, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made without departing from the spirit thereof. Accordingly, the foregoing description is meant to be exemplary only and should not be deemed limitative on the scope of the invention set forth with the following claims.