Abstract:
The invention is an improved sports racket comprising streamlined transversal strings and streamlined longitudinal strings with surface indentations that permit the strings to lock into other at the string crossings so the strings do not move with respect to each other and also stay oriented so as to minimize wind resistance during the swinging motion of the sports racket before and after ball contact. The invention includes a spacer for assembly of the strings in the sports racket incorporating a flexible body between the string and the racket frame for the absorption and restitution of part of the ball&#39;s kinetic energy during impact with the string to beneficially alter the feel of the strings.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This is a Continuation-In-Part of application Ser. No. 09/566,666, filed May 8, 2000 now abandoned, which is a division of non-provisional U.S. application Ser. No. 09/088,602, filed Jun. 1, 1998, now U.S. Pat. No. 6,132,325, which, in turn, claims priority benefits of provisional application Serial No. 60/050,678, filed Jun. 25, 1997. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER 
     Federally sponsored research and development: 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     This invention relates to a stringing for a sports racket such as, but not limited to, a tennis racket or a racket-ball racket. 
     2) Background Information 
     Conventional tennis rackets are strung with strings passed above and below each other to produce a woven string network. Since the strings are not bonded at their crossover points, the pattern of the string network may deform when the ball is struck by a racket with an upwards or downwards component of motion, such as that used by players wishing to place a spin on the ball. The movement of the strings relative to one another produces wear through attrition and leads to premature string failure. This effect is accentuated when the tennis game is played on clay, where clay micro particles are brought to the racket by the tennis ball and, upon reaching the location of string attrition, accelerate the process of wear. 
     U.S. Pat. No. 4,741,531 by Szedressy and U.S. Pat. No. 4,949,968 by Korte-Jungermann permit replacing a single broken string without rebuilding the entire string network. These designs share the basic idea of building the string network with individual string segments that traverse the string network only once and are attached to the racket frame at oppositely disposed locations via a fixing means. In both cases, extra tension must be applied in the process of stringing the racket in order to compensate for a string shortening caused by a yield of the fixing means in the string axial direction following the release of the string by the string tensioning means. The axial yield alters the string tension and complicates the stringing process. 
     As will be seen from the subsequent description of the preferred embodiment of the present invention, these and other shortcomings of the prior art are overcome. 
     SUMMARY OF THE INVENTION 
     The preferred embodiment of the present invention has a new string design wherein strings interlock with one another at the string crossings. The interlocking is achieved through matching surface indentations on the strings at the location of the string crossings, the matching surface indentations allowing lateral forces to be transmitted between the crossing strings. 
     The interlocking strings are woven in the usual interlaced pattern and placed under axial tension, wherein the combination of tension and the matching surface indentations allows the crossing strings to transmit both lateral forces and bending moments to one another, thus maintaining the strings in their relative position and orientation during, and after, the transient motion of the string network resulting from the impact between the string network and a sports ball. Consequently, strings with aerodynamic cross-sectional area can maintain the correct orientation of least aerodynamic drag. 
     The string attachment to the frame is accomplished by a fixing means wherein the desired radial string compression is produced via a wedging action between a string clamping member and a pressing means, with the string clamping member being also prevented from moving in the string axial direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described in greater detail by means of embodiments as illustrated in the drawings in which: 
     FIG. 1 is a plan view of a portion of a racket according to the first embodiment of the invention; 
     FIG. 2 is a plan view of the interlocking string network  2  and the fixing means  30 ,  31  and  32  shown separate from the racket frame. 
     FIG. 3 is a plan view of a portion of a racket with fixing means including a conventional hole-loop arrangement  25 ; embodiment of the invention; 
     FIG. 4 is a close-up view, in perspective, of the string network showing the string crossings; 
     FIG. 5 is a close-up view, shown in perspective, of a string crossing in which the upper crossing string and the lower crossing string are separated from one another to reveal the preferred shape of the interlocking surface indentations; 
     FIG. 6 is a close-up view, shown in perspective, of a string crossing in which the upper crossing string and the lower crossing string are separated from one another to reveal the shape of the interlocking surface indentations in another embodiment of the invention; 
     FIG. 7 a  is a side view of a single isolated string; 
     FIG. 7 b  is a magnified view, shown in perspective, of the end section of the string in FIG. 5, the magnified view showing the grooves on the string surface; 
     FIG. 8 is an enlarged cross-section taken along line  8 — 8  in FIG. 1, showing the first fixing means; 
     FIG. 9 is an enlarged cross-section taken along line  8 — 8  in FIG. 1, showing the first fixing means with the flexible spacer; 
     FIG. 10 is an enlarged cross-section taken along line  10 — 10  in FIG. 1, showing in full perspective the components of the second fixing means in an exploded view; 
     FIG. 11 a  is an enlarged section taken along line  10 — 10  in FIG. 1, showing the components of the second fixing means in the locked position; 
     FIG. 11 b  is an enlargement of the area encircled by the dashed line in FIG. 11 a,  showing the grooves on the string clamping unit; 
     FIG. 12 is an enlarged section taken along line  12 — 12  in FIG. 1, showing the components of the modified second fixing means in an exploded view; 
     FIG. 13 is an exploded view of a flexible spacer that embodies features of the invention; and 
     FIG. 14 is a perspective view, in full section, of a second fixing means that characterizes another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Within the specification and the claims, the following words carry the meaning assigned below: 
     String network: given a set of crossing strings touching at the locations of string crossings, the string network is that portion of the set of strings that lies within the periphery described by the outermost string crossings. 
     Substantially planar string network: a string network whose strings lie within two bounding parallel planes with the minimum distance between the two bounding planes being essentially equal to twice the maximum thickness of any one string. The mid-plane of the substantially planar string network is the plane lying parallel to the bounding planes and dividing the string network into two essentially equal parts. 
     Surface indentation (in a string): a change in surface geometry to form a recess in a string, the recess having a maximum width less than or substantially equal to the maximum string width within the string network. 
     No rotational symmetry: a property of a body, whereby only a portion of rotation of 360 degrees, or an integer multiple thereof, about a specified axis brings the body into an orientation that is indistringuishable from the original orientation. 
     Unique angle: the value in degrees after integer multiples of 360 degrees are added or subtracted to a given angle to make the value equal or greater than zero degrees, and less than 360 degrees. 
     String enlarged section: a portion of the string having an enlarged cross-sectional area, with the enlargement extending over a section of the length of the string. 
     Streamlined cross-section: a cross-section characterized by a major axis and a minor axis perpendicular thereto, the body formed by extruding the cross-section having lower aerodynamic drag than the drag of a cylinder having equal cross-sectional area; 
     Surface normal: the direction obtained by averaging the point-wise perpendicular direction to the surface over all points on the surface or over all points in a specified region of the surface. 
     First fixing means: a means for fixing a string to the frame of a sports racket, the means employing a fixing procedure necessitating substantially the entire length of the string to be threaded through the means. 
     Second fixing means: a means for fixing a string to the frame of a sports racket, the means employing a fixing procedure not necessitating the entire length of the string to be threaded through the means. 
     The drawings represent only the preferred form of the invention and are only to be considered as examples. 
     FIGS. 1 and 2 show the preferred embodiment of the sport racket of this invention. The sport racket has a frame  1  having a handle  100 , usually partially shown, a throat  110  and a head  12 . The head has an outer head surface  14  and an inner head surface  13  that defines a central opening spanned by a plurality of transversal strings  21  running essentially parallel to each other and a plurality of longitudinal strings  22  running substantially orthogonal to, and being interwoven with, the transversal strings to form a substantially planar string network  2 . The transversal strings and the longitudinal strings are secured to the frame by means for securing strings to the frame, such as a first fixing means  30 , or a second fixing means  31 , or a conventional hole and loop combination  25  (FIG.  3 ); 
     Each point of contact between transversal and longitudinal strings defines a string crossing. One such string crossing is indicated at  23 . In the neighborhood of each string crossing we identify an upper crossing string  26  and a lower crossing string  27  (see FIG.  4 ). 
     At each string crossing, the upper crossing string has a first surface indentation  40  (FIG. 5) opening in the downward direction, and the lower crossing string has a second surface indentation  41  opening in the upward direction. The first and second surface indentations are made to match with and mate with each other to form a common contact surface. 
     An example of the preferred form of the first surface indentation and of the second surface indentation is shown in FIG.  5 . In this figure a grid is presented on both the first and second surface indentations to help communicate the surface shape. 
     Upon mutual string contact the first surface indentation  40  makes full contact with the second surface indentation  41  to form the common contact surface. In particular, the first surface indentation  40  contains first lateral surface regions  403  and  404 , each having the surface normal tilted away from the perpendicular direction to the mid-plane of the string network. Similarly, the second surface indentation  41  contains second lateral surface regions  413  and  414 , each having the surface normal tilted away from the perpendicular direction to the mid-plane of the string network. Upon mutual string contact the first and second lateral surface regions meet to form a lateral portion of the common contact surface that carries compressive stresses leading to lateral forces that oppose the movement of the upper crossing string  26  relative to the lower crossing string  27  (FIG.  4 ). 
     The transversal strings and longitudinal strings have at each string crossing a bulge  208  (FIG. 5) to provide an enlarged cross-section in the region of the string crossing. At the location of each surface indentation, the bulge produces a minimum string cross-sectional area that can carry, with a desired margin of safety, the design tension anticipated to occur in the string. The bulge also allows surface indentations with larger lateral surface regions. 
     In another embodiment on the invention, shown in FIG. 6, the first surface indentation  40  on the upper crossing string  26  has an essentially rectangular shape that mates and matches with the second surface indentation  41  of essentially rectangular shape on the lower crossing string  27 —(FIG.  4 )—upon mutual string contact. The first surface indentation  40  has first lateral surface regions  503  and  504  separated from one another by a first frontal surface region  502 , with the first lateral surface regions each having the surface normal tilted away from the perpendicular direction to the mid-plane of the string network. Similarly, the second surface indentation  41  has second lateral surface regions  513  and  514  separated from one another by a second frontal surface region  510 , with the second lateral surface regions each having the surface normal tilted away from the perpendicular direction to the mid-plane of the planar string network. 
     Upon mutual string contact the first frontal surface region  502  contacts the second frontal surface region  510  and these two surfaces are pressed against each other in the presence of string tension. Furthermore, the first lateral surface regions  503  and  504  contact the lower crossing string at  515  and at the corresponding location on the other side of the lower crossing string, respectively, while the second lateral surface regions  513  and  514  contact the upper crossing string at  505  and at the corresponding location on the other side of the upper crossing string, respectively. 
     The sharing of the common contact surface, rather than just a small region essentially limited to a point as in conventional string networks, allows the center of force acting between the upper crossing string and the lower crossing string to shift relative to the longitudinal axis of either crossing string. The shift between the center of force and the perpendicular component of the axial force carried by the upper and the lower crossing string creates a force couple that can hold the upper crossing string and the lower crossing string in their relative position and orientation during the transient motion of the string network resulting from the impact between the string network and a sports ball. 
     Since the strings maintain their relative position, the strings can have a streamlined cross-section oriented to yield lower aerodynamic drag when the string network is moved in the direction essentially perpendicular to the mid-plane of the string network, such as during the swinging motion of the sport racket. A streamlined cross-section of elliptical form with major axis to minor axis ratio of 1.6 is shown at  210  (FIG.  4 ). Although a substantially higher value for this ratio will further reduce the drag when the air-flow is in the direction of the major axis, the substantially higher ratio is undesirable when the racket is swung with an upwards or downwards component of motion, such as the swing made to impart spin on the ball. In this motion, the air-flow is at an oblique direction with respect to the major axis, and the substantially higher value of major-minor axis ratio will lead to a loss of aerodynamic efficiency. upwards or downwards component of motion, such as the swing made to impart spin on the ball. In this motion, the air-flow is at an oblique direction with respect to the major axis, and the substantially higher value of major-minor axis ratio will lead to a loss of aerodynamic efficiency. 
     The String Structure 
     A representative transversal string isolated from the string network and the frame, is shown in FIG. 7 a.  A representative longitudinal string has essentially the same string structure as the transversal string. The string contains the first surface indentation  40  at the locations along the string corresponding to crossings in which the string is the upper crossing string. Similarly, the string contains the second surface indentation  41  at the locations along the string corresponding to crossings in which the string is the lower crossing string. 
     The string has a first free-end  201  and a second free-end  202  for attachment to the first fixing means and second fixing means, respectively. The first free-end has a string enlarged section  205 . A perspective view of the string enlarged section  205  is shown in FIG.  8 . The string enlarged section  205  has a secant section removed to form a string reference plane  207  that is oriented with a predetermined angle about the longitudinal axis of the string with respect to the first and second surface indentations. The removal of the secant section makes the string enlarged section have no rotational symmetry about the longitudinal axis of the string. 
     The string portion extending from the last surface indentation (counting from the first free-end  201  to the second free-end  202 ) contains string surface corrugations  211  oriented essentially perpendicular to the longitudinal axis of the string. An example of the string surface corrugations is shown in FIG. 7 b.  The string surface corrugations improve the fixing ability of the string to the second fixing means, described below. 
     The individual transversal or longitudinal string is produced through the injection molding of a resilient plastic material, such as nylon or an equivalent polyamide, or polyester, into a die. Whiskers of glass, aramid fibers or graphite can be included in the injected material to increase the tensile strength of the composite material. 
     The spacing between surface indentations along the length of the string and the spacing between the surface indentations and the string enlarged section  205 , as shown in FIG. 7 a,  depend on the amount of axial strain the string undergoes once the string is strung to the frame. In particular, the spacing depends on the location of the string in the string network (i.e., the string length), on the string tension, and on the elastic modulus of the string. Consequently, each individual string is manufactured with its own particular length and with its own particular placement of the surface indentations, such that the string network fits a particular racket when each string is placed under its own particular, and desired, tension (i.e., the string&#39;s design-point tension). 
     In another embodiment of the invention, an adhesive, such as a cyanoacrylate based adhesive, is applied over the common contact surface to provide a strong bond at the string crossing, thus allowing the common contact surface to sustain the shear, compressive, and tensile stresses necessary to maintain the upper crossing string and the lower crossing string in their relative position and orientation during the transient motion of the string network resulting from the impact between the string network and a sports ball, even in the case when the strings are not interwoven within the substantially planar string network. 
     The Fixing Means 
     The first fixing means, shown at  30  in FIG. 8, is used to firmly hold the string enlarged section  205  of an individual string  20  to the frame  12  when the individual string  20  is placed under axial tension. 
     The first fixing means comprises a main body  300  fixedly attached to the frame and extending from the inner head surface  13  to the outer head surface  14 . When the frame is hollow, the main body has preferably a flange  301  to block the inward motion of the main body into the frame, and, thus, help maintain the main body fixedly attached to the frame. The main body  300  has a first cavity  302  extending the length of the main body to produce a first opening  305  at the inner head surface and a second opening  306  at the outer head surface. The first cavity allows the individual string  20  to be threaded through the main body by passing the second free-end of the individual string through the second opening, through the first cavity, and through the first opening. Furthermore, the first cavity is shaped to receive the string enlarged section when the string enlarged section enters the first cavity through the second opening, the first cavity being shaped to block the string enlarged section from further inward movement into the first cavity once a predetermined advancement of the string enlarged section into the first cavity occurs. In the preferred embodiment, the string enlarged section is essentially of cylindrical form, and the first cavity contains a stopping surface  304  that contacts the string enlarged section and prevents further penetration of the string enlarged section into the first cavity. The portion of the first cavity receiving the string enlarged section is absent rotational symmetry in order to allow penetration of the string enlarged section when the string enlarged section is oriented with a predetermined angle to the frame. In the preferred embodiment, the first cavity contains a guide plane  307  that receives the string reference plane. Upon placement of tension in the individual string acting towards the string network, the string enlarged section firmly presses against the main body, effectively fixing the individual string to the frame. 
     When the strings are made of a strong but stiff material, the string network may only yield a small amount upon impact with the game ball. Since several types of game balls are designed to dissipate energy on impact, the small yield of the string network causes the ball to deform too much upon impact and dissipate a significant fraction of its kinetic energy. In this case, a flexible spacer  308  can be inserted between the surface of the main body facing the first cavity and the surface of the string enlarged section to absorb part of the kinetic energy of the arriving ball, and to return this energy to the departing ball. (See FIG.  9 ). The flexible spacer is made of a compliant material such as silicon rubber, exhibiting a substantially linear stress-strain relation, in the range of the compressive strains induced in the flexible spacer upon placement of the design-point tension in the individual string. For clarity, we define the state of compressive strain of the flexible spacer after the individual string is strung but before contact of the individual string with a sports ball as the nominal compressive state. The flexible spacer has a combination of elastic modulus and cross-sectional area that allows further compressive strain within the substantially linear stress-strain relation of the flexible material in the presence of further tension in the individual string brought about by the contact of a sports ball with the individual string during a sports game. For example, the flexible spacer  308  has a an outer diameter of 5 mm and in inner diameter of 1.4 mm for the string passage, creating a surface area normal to the longitudinal string axis of the string of about 18 square millimeters. The material is an elastomer selected to have an elastic modulus of 300 Newtons/mm 2 , yielding a strain of 5 percent at a typical string loading of 28 kilograms. This strain is acceptably within the substantially linear range of elastomeric materials. 
     The additional compressive strain in the flexible element causes the length of the string between fixing means to lengthen, hence absorb part of the ball&#39;s kinetic energy. The flexible spacer returns to the nominal compressive state as the ball leaves the string network, thereby returning to the sports ball part of the ball&#39;s initial kinetic energy. 
     The second fixing means  31  (FIG. 10) is used to firmly hold the second free-end  202  of an individual string  20  to the frame  12 . The second fixing means comprises an enclosure body  310 , a string clamping member  320  and a pressing means  330 . These three parts are made from a resilient and light-weight material, such as plastic. 
     The string clamping member  320  has a wedge shaped outer surface  321 , preferably of conical form, ending with an edge  322 . The string clamping member has an inner passageway to allow the passage of the individual string  20  through the string clamping member when there are no compressive forces acting on the wedge shaped outer surface. The inner passageway contains transversal corrugations  325  (see FIG. 11 b ) to match the string surface corrugations  211  on the individual string. The string clamping member is made of a compliant material, such as nylon or similar polyamide, that allows the passageway to radially contract when a compressive force is brought to bear on the wedge shaped outer surface. In the preferred embodiment, the radial contraction is aided by a cut  323  extending from the inner passageway to the wedge shaped outer surface and running the entire length of the string clamping member. 
     The pressing means  330  has a cylindrical body  331 , a small flange  332  connected to the cylindrical body, and a wedge shaped bore  335  (FIG. 11 a ) preferably of conical form to match the preferably conical form of the wedge shaped outer surface  321  (FIG. 10) of the string clamping member  320 . The wedge shaped bore extends the entire length of the cylindrical body and the small flange so as to create a passage for the individual string through the pressing means. The wedge shaped bore opens in the direction away from the small flange, and is sized to completely receive the string clamping member  320 . Upon full insertion of the string clamping member  320  into the pressing means  330 , the surface of the wedge shaped bore  335  pushes in a wedge fashion against the wedge shaped outer surface  321  (FIG.  10 ), thereby providing a compressive force to the wedge shaped outer surface and causing the inner passageway of the string clamping member to radially contract. The pressing means furthermore comprises a locking means, preferentially in the form of an engaging lip  336  located at the larger opening of the wedge shaped bore for engagement with the edge  322  of the string clamping member, the locking means locking the string clamping member inside the wedge shaped bore when the string clamping member is fully inserted into the wedge shaped bore. 
     The enclosure body  310  extends from the inner head surface  13  to the outer head surface  14  and is fixedly attached to the frame. When the frame is hollow, the enclosure body has preferably an enclosure flange  312  to block the inward motion of the enclosure body into the frame, and, thus, help maintain the enclosure body fixedly attached to the frame. 
     The enclosure body  310  contains a second cavity extending through the enclosure flange to produce a main opening  314 , and extending partially into the enclosure body to produce a base surface  315  (see FIG. 11 a ). The base surface is connected to the inner head surface by a simple bore  313  to allow the passage of the individual string through the enclosure body. Furthermore, the second cavity is sized to receive through the main opening the cylindrical body  331  of the pressing means, but not the small flange  332  of the pressing means. 
     To fasten the individual string to the frame, the individual string is threaded through the enclosure body  310  in the direction from inner head surface to outer head surface, and further threaded through the string clamping member in the direction of decreasing thickness of the wedge shaped outer surface, and through the pressing means in the direction of decreasing cross-sectional area of the wedge shaped bore. The individual string then proceeds to a conventional string tensioner to receive the desired tension. Once the desired tension is reached, the string clamping member is slid along the individual string into the enclosure body until coming to rest against the base surface of the enclosure body. The enclosure body in the preferred embodiment has a protrusion  316  at the base surface  315 . The protrusion separates the string clamping member from the enclosure body to make the edge  322  reachable by the engaging lip  336 . Afterwards, the pressing means is inserted into the enclosure body and over the string clamping member, causing the wedge shaped bore to slide over the wedge shaped outer surface of the string clamping member and to cause a compressive force on the wedge shaped outer surface. Upon full insertion, the engaging lip  336  engages with the edge  322  to lock the pressing means and the string clamping member together. Since the protrusion prevents further motion of the string clamping member into the second cavity, the wedging action between the string clamping member and the pressing means causes the string clamping member to radially contract, whereby the surface of the passageway in the string clamping member contracts and firmly presses against the individual string to fasten the individual string to the frame. 
     To remove the pressing means and the string clamping member from the enclosure body after the pressing means and the string clamping member are interlocked with each other, the individual string is cut. With each new, individual string replacement, a new pressing means and a new clamping member are used. 
     There are locations on the head of the frame where the throat can interfere with the step of pushing the pressing means  330  into the enclosure body  310 . 
     The second fixing means is modified for usage at these locations. A modified fixing means  32  (FIG. 12) comprises: the string clamping member, the second cavity within the enclosure body, and the pressing means are axis-symmetric about an axis aligned with the longitudinal axis of the individual string, allowing the pressing means to rotate inside the enclosure body and around the pressing means; the string clamping member and the protrusion are fixedly attached to the enclosure body; and the locking means comprises a first set of threads  384  on the enclosure body, and a second set of threads  393  on the outer surface of the pressing means, the second set of threads made to match and engagement with the first set of threads to pull and lock the pressing means within the enclosure body. Furthermore, the flange of the pressing means is modified into an angular shape  391 , such as hexagonal, to facilitate the screwing of the pressing means into the enclosure body. 
     It is to be noted here, that the purpose of the main body in the first fixing means and of the enclosure body in the second fixing means is to provide material into which a cavity can be made. The disclosed embodiments of the first fixing means and of the second fixing means are designed for a hollow frame. In case that the frame is full and composed of a resilient material, another embodiment of the invention has the main body and the enclosure body composed of the same material as the frame, so that the main body and the enclosure body are united with the frame material without a seam to become a monolithic part of the frame. 
     Further Descriptions of the Preferred Embodiments 
     The rules of the game of tennis require a tennis ball to dissipate roughly half its kinetic energy upon impact with a solid surface. When the ball impacts a network of conventional strings, the amount of dissipation is reduced in proportion to the flexibility of the strings. Players judge the most flexible strings, such as those made from delicate natural gut, as the most playable, and strings made from stiff and durable Kevlar as the most unresponsive. Thus, a dilemma exists between string playability and string durability. 
     As stated above, the flexible spacer absorbs, stores, and returns part of the ball&#39;s kinetic energy during impact with the string. This process beneficially alters the feel of the strings. As an example, accurate computer simulations show that a single 16-gauge Kevlar string of 12 inch length conventionally mounted and tensed to 57 lbs. of tension experiences a tension rise to over 70 lbs. during impact with a 0.45 kg pendulum approaching at 2.6 m/s. The pendulum simulates the forces on a string produced during a 130 mph serve. The addition of a single flexible spacer with elastic modulus of 15 N/mm 2 , diameter of 7 millimeters, and height of 14 mm (without loading), shown at  308 ′ in FIG. 13, lowers the tension rise to 40 lbs. Furthermore, the computer simulation shows that the flexible spacer stores about half the total kinetic energy of the pendulum. For comparison, a conventionally mounted multifilament Nylon string of the same length and pre-impact tension produces a tension rise of about 34 lbs. at impact. Thus, the presence of the flexible spacer makes Kevlar feel like Nylon in terms of playability, while maintaining the durability of Kevlar. 
     The energy incorporated into the flexible spacer is the integral sum of the rise at impact of the tension force acting on the spacer times the change in height of the spacer under tension. Since the tension rise in strings is typically in the range from 20 to 70 lbs., the change in height of the spacer should lie in the millimeter range in order for the spacer to absorb a significant part of the ball&#39;s kinetic energy. In the example above, the spacer compressed 2.5 millimeters under impact. 
     The stress-strain relationship in the compression of the flexible spacer  308 ′ during ball impact need not be linear, but should preferably be a one-to-one function in order to avoid losses due to hysteresis effects. 
     In another embodiment of the second fixing means  31 ′ (see FIG.  14 ), the hole  16 ′ on the outer head surface  14 ′ and the hole  15 ′ on the inner head surface  13 ′ form a passageway within the frame sufficiently large to allow the passage of the enclosure flange  312 ′ of the enclosure body  310 ′ into the racket frame. A flexible ring  350 ′, preferably made of an elastomeric material, is located between the enclosure flange  312 ′ and the frame, so that at the end of the string mounting procedure, when the string is under tension and fully locked into the second fixing means, the flexible string presses against the enclosure flange  312 ′ and the frame surface  18 ′ thereby supporting the second fixing means relative to the frame. Under a further increase in string tension during ball impact, the flexible ring  350 ′ compresses further, thereby storing part of the ball&#39;s kinetic energy during impact. The energy storage occurs as the ball is decelerating relative to the string and the string tension is rising. The stored energy is returned to the ball during the re-acceleration of the ball away from the string. 
     To further increase the traction force between the surface of the inner passageway of string clamping member  320  and the surface of the string  20  contacting the string clamping member  320 , an adhesive is placed in the area where the string  20  and the clamping member  320  make contact. 
     Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Thus, the scope of the invention should be determined by the appended claims in the formal application and their legal equivalents, rather than by the examples given.