Double strung tennis racquet

A tennis racquet has a frame which includes a frame head portion with an outwardly facing stringing groove and an inwardly facing surface which is generally convex. The stringing groove includes a pair of angled side wall surfaces. A series of stringing holes extend through each side wall surface and the opposing convex surface to define holes for a pair of generally parallel stringing surfaces. Preferably, the string holes extend at an angle of about 45.degree. relative to the central plane of the stringing area, and the angled surfaces in the stringing groove, and the opposing surface on the convex surface, are parallel to one another and generally perpendicular to the direction of the string hole.

FIELD OF INVENTION 
The present invention relates to an improvement in tennis racquets of the 
type having two, generally parallel strung surfaces. 
BACKGROUND OF THE INVENTION 
Conventionally, tennis racquets have a head portion defining an opening 
which contains a plurality of interwoven main (i.e., longitudinal) and 
cross strings which form a strung hitting surface. The strung surface lies 
generally in a plane located midway between the front and rear faces of 
the frame member. 
It has been proposed, as an alternative to a single strung surface, to 
provide two parallel strung surfaces lying generally on opposite sides of 
the frame, i.e., a double strung racquet. Double strung racquets offer 
potential advantages over a single strung racquet. However, in practice 
designing a double strung racquet that would be commercially acceptable 
poses unique problems. The frame must be strong enough to carry the load 
of the additional strings, and yet remain at an acceptable weight. 
Further, the frame must have an acceptable construction in terms of how 
the strings are secured to the frame. 
While there have been a number of previous proposals for double strung 
racquets, it would be desirable to provide a double strung racquet which, 
compared to racquets having a single stringing plane, provides two strung 
surfaces without unduly increasing the weight, which has a frame which 
provides performance comparable to a single strung racquet, and which has 
an improved construction, compared to prior proposals for double strung 
racquets, for securing the strings to the frame. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a sports racquet, preferably a 
tennis racquet, has a frame which includes a frame head portion with an 
outwardly facing stringing groove and an inwardly facing surface which is 
generally convex and which defines a stringing area. The stringing groove 
includes a pair of angled side wall surfaces disposed on opposite sides of 
the central stringing plane. The head portion includes a plurality of 
stringing holes, including a first series of string holes extending from 
one of the side wall surfaces through the convex surface and a second 
series of string holes extending from the other side wall surface through 
the convex surface. The first and second series of string holes diverge 
from the central plane at opposite angles. The racquet further includes a 
first plurality of interwoven strings extending through the first series 
of string holes and a second plurality of interwoven strings extending 
through the second series of string holes, thereby to form two parallel 
strung surfaces which extend from the convex surface on opposite sides of 
the central plane. 
Preferably, the string holes extend at an angle of about 45.degree. 
relative to the central plane of the stringing area, and the angled 
surfaces in the stringing groove, and the opposing surface on the convex 
surface, are parallel to one another and generally perpendicular to the 
direction of the string hole. 
Preferably also, the racquet frame is formed from a continuous tubular 
profile member, which defines the head portion and has opposed frame 
throat portions extending therefrom that converge to form a throat area. A 
throat bridge has opposite ends which are joined to the opposing frame 
throat portions at a pair of throat joints. The outwardly facing stringing 
groove continues down into the region of the throat joint. At the throat 
joint, the inwardly facing surface comprises, in cross section, a pair of 
shoulders on opposite sides of the central plane and a central boss 
section disposed between the shoulders and extending inwardly toward the 
strung surface. String holes extend from the angled side wall to the 
respective shoulders. The shoulders are perpendicular to the angle of the 
string holes, limit the distance through the frame necessary to form the 
string holes, and allow the strings to pass inwardly without contacting 
the central boss. 
For a better understanding of the invention, reference is made to the 
following detailed description of a preferred embodiment, taken in 
conjunction with the drawings accompanying the application.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 1 and 2 illustrate a tennis racquet which includes a frame 10 having 
a frame head portion 12 defining a stringing area 14. A throat bridge 18 
spans the throat area to enclose the lower end of the stringing area 14. A 
pair of opposed frame throat portions 16 extend from the head portion 12 
and converge to extend side-by-side and form a shaft 20, as shown in FIG. 
6. A portion of the shaft 20 is molded into the shape of a handle 22. The 
molded-in handle 22 is shown in cross section in FIG. 7. 
FIG. 9 shows the general configuration of the cross section of the head 
portion 12. As shown, the head portion 12 has a generally convex surface 
24 which faces inwardly toward the stringing area 14, and an outwardly 
facing stringing groove 26. The stringing groove 26 includes a pair of 
angled side wall surfaces 28 disposed on opposite sides of the central 
plane 30 of the stringing area. The side wall surfaces 28 are disposed at 
an angle of less than 180.degree. relative to one another, as shown. 
The head portion 12 includes a plurality of string holes therethrough, 
including a first series of string holes 32 lying on one side of the 
central plane 30, and a second series of string holes 34 lying on the 
opposite side of the central plane 30. As shown in FIG. 9, each string 
hole, 32,34 extends from its respective side wall surface 28 through the 
opposed convex surface 24. The two sets of string holes diverge from the 
central plane 30 at opposite angles, which are preferably 45.degree. 
(i.e., such that the string holes 32, 34 extend at a 90.degree. angle 
relative to one another). Moreover, the wall surfaces 28 and 24 are formed 
at an angle such that the string holes 32, 34 are generally perpendicular 
to the respective surfaces. Thus, the surfaces 24, 28, in the region where 
the string holes are formed, are generally parallel to one another. 
FIG. 4 illustrates the throat piece 18 and its hole configuration. The 
throat piece has a generally convex surface 38 which faces inwardly toward 
the stringing area 14. Surface 38 preferably has the same configuration as 
surface 24, so that the border of the stringing area is a uniform shape. 
Unlike head portion 12, however, the outwardly facing surface 40 of the 
throat piece 18, in cross section, is preferably flat. A first series of 
string holes 42, corresponding to string holes 32 in the frame portion 12, 
lies on one side of the central plane 30, and a second series of string 
holes 44, corresponding to holes 34, lies on the opposite side of the 
central plane 30. As shown in FIG. 4, each string hole, 42, 44 extends 
from the outer surface 40 through the opposed convex surface 38. The two 
sets of string holes diverge from the central plane 30 at opposite angles, 
which are preferably 35.degree.. 
As can be seen in FIGS. 3, 8, 10, and 11, the lower corners of the head 
portion 12, in the region 46 where the throat piece 18 is joined, have a 
specially shaped profile. The outwardly facing stringing groove 26 
continues down into the region of the throat joint 46 However, the 
inwardly facing surface 48 comprises, in cross section, a pair of 
shoulders 50 on opposite sides of the central plane 30 and a central boss 
section 50 disposed between the shoulders and extending inwardly toward 
the strung surface 14. As shown in FIG. 8, string holes 32a, 34a extend 
from each angled side wall to each of the shoulders 52. The string holes 
32a, 34a diverge from the central plane 30 at an angle of approximately 
35.degree.. The shoulder sections 50 are angled so as to be approximately 
normal to the direction of the string holes. In addition to providing 
surfaces which are generally perpendicular to the holes 32a, 34a, the 
shoulders reduce the length that the string holes need to travel through 
the frame profile. Also, the shoulders act to space the strings in the 
lower corners of the racquet so as to avoid contact with the central boss 
section 52. 
FIG. 3 shows an exemplary arrangement of string holes for one series of 
holes. e.g., 32, 32a, and 42, which are labelled as a.sup.1 through 
a.sup.34. The string holes for the opposite side of the racquet are a 
mirror image. In the stringing arrangement shown, holes a.sup.1 through 
a.sup.6, a.sup.8, a.sup.10, a.sup.25, a.sup.27, and a.sup.29 through 
a.sup.34 would be used for the main strings, whereas the remaining string 
holes would be used for the cross strings, as described further below. 
The racquet frame 10 may be molded utilizing known techniques. For example, 
if the frame is to be made of a fiber reinforced thermoset resin, a 
tubular profile member of, e.g., uncured fiber-reinforced resin, is placed 
in a mold having the shape of a racquet. A throat member is also 
positioned in the mold. The junction between the profile tube and throat 
piece is wrapped with additional fiber-reinforced resin material as 
needed. The mold is then closed and the profile member is internally 
pressurized and heated to conform to the mold and cure. Thereafter, the 
now formed frame is removed from the mold and string holes are drilled. 
Alternatively, the frame may be formed with a fiber-reinforced 
thermoplastic material, as disclosed in commonly owned, co-pending U.S. 
application Ser. No. 07/645,255. 
The preferred embodiment of the invention employs plastic grommet strips as 
seats for the strings in the string grooves along the sides and throat, 
and a plastic bumper strip around the outer portion of the frame to act as 
a seat for the strings and to protect the outer surface of the frame from 
damage from accidental impact with the ground or other surfaces. In the 
frame of FIG. 1, the holes extend at a 45.degree. angle relative to the 
central stringing plane yet are generally perpendicular to the angled 
surfaces 28 in the stringing groove 26. Thus, conventional bumper and 
grommet strips could not accommodate such hole angles and at the same time 
remain flat against the angled surfaces 28. 
A preferred embodiment of a grommet strip 60 according to the invention is 
shown in FIGS. 12-14. A preferred embodiment of a bumper strip 70 is shown 
in FIGS. 15-19. 
A grommet strip 60 according to the invention includes a plurality of 
grommet pegs 62, which fit in the string holes 32, 34, and a thin, 
connecting strip 64. Each grommet peg 62 extends perpendicular to the 
connecting strip 64 and has a central longitudinal bore 66 through which a 
string can pass. 
It is not practical to mold a plastic grommet piece on a 45.degree. angle 
with all grommets pointing toward the center of curvature as required on a 
racquet frame. However, it is possible, by flattening out the curved 
portion of the cone representing surface 28, to create a two dimensional 
piece, in which the connecting strip 64 is flat, and the grommet pegs 62 
extend normal to the connecting strip 64. To do so, the shape of the 
connecting strip 64 is determined by considering each surface 28 of the 
bevelled 45.degree. string groove 26 to represent a portion of a cone 
having a vertex angle of 90.degree.. If the section of the cone 
represented by the surface 28, together with the projecting grommet pegs, 
is flattened out, the result is a part in which, as shown in FIGS. 12 and 
14, the axis of the connecting strip 64 is curved. As can be seen, the 
grommet pegs lie along the same curve, rather than in a straight line. 
However, all of the grommet pegs extend normal to the connecting strip 
surface. Such a part can be readily molded and yet, when bent to lie flat 
against the stringing groove surface 28, will resume the shape of the cone 
section and cause the grommet pegs 62 to extend inwardly at the desired 
angle. 
If the head portion 12 of the racquet were circular, the connecting strip 
64 would follow a constant curve. However, in most tennis racquets, 
including the racquet shown in FIG. 1, the head is generally elliptical in 
shape, rather than circular. As a result, the stringing groove 26 extends 
about an elliptical curve. Thus, while the 45.degree. string hole angle 
remains constant (except as discussed below), the radius of curvature of 
the stringing groove varies at different locations of the head portion of 
the racquet. In order to conform the bumper strip to the stringing groove 
at various locations on the head, the radius of curvature of the 
connecting strip is varied as a function of the radius of curvature of the 
stringing groove so that the connecting strip 64 lies flat against the 
surface 28 and the grommet pegs 62 project at the desired angle. 
Referring again to FIG. 3, in order for string hole a.sup.30 to emerge at 
the desired position on the interior side of the frame surface, due to the 
increased thickness of the frame at the throat joint 46, the string hole 
a.sup.30 extends at a different angle, in the stringing plane, than the 
adjoining holes, e.g. a.sup.29 and a.sup.28. To accommodate the angle of 
string hole a.sup.30, a joint 65 is provided between the main section of 
the connecting strip 64 and the end strip 67 holding the end grommet peg 
68. This allows the end strip 67 to be rotated and pivoted as needed about 
the centerline 69 of the grommet strip 60 to accommodate the angle of hole 
a.sup.30. 
Bumper strip 70 is formed in a similar manner to grommet strip 60, using 
the theory of cones to define the radius of curvature of the connecting 
strip 72. As shown, bumper strip includes grommet pegs 74, along with a 
bumper guard 76 that fits over the outer surface of the racquet frame. As 
shown in FIG. 16, the bumper guard 76 may be formed with rectangular 
depressions 78 (which are omitted in FIG. 17 for clarity), which can be 
used to reduce weight and provide an ornamental appearance. In addition, 
the connecting strip includes a plurality of cutouts 80 adjacent to 
selected grommet pegs 74a. The upper surface 82 of the selected grommet 
pegs 74a are raised, as described further in connection with FIG. 22. 
Throat grommet strips 110, one of which is shown in FIGS. 23-27, are 
preferably also utilized in the bridge piece 18. Referring to FIGS. 23, 
24, and 27, the grommet strip 110 includes four grommet pegs 112 and a 
connecting strip 114. The grommet pegs 112 are parallel to one another 
but, as shown in FIG. 23, the connecting strip is curved so that its upper 
surface 116 follows the curvature of the throat piece 18. Moreover, as 
shown in FIG. 27, the upper surface 116, in cross section, is sloped at an 
angle .alpha. relative to the grommet pegs 112, which angle .alpha. is 
preferably 35.degree., such that the upper surface can be flat against the 
throat piece surface and such that the grommet pegs extend at a 35.degree. 
angle up into the throat piece holes. 
Because the holes in the bridge piece, a.sup.31 -a.sup.34, are drilled 
parallel to the racquet axis, the distance through the string holes 42, 
44, i.e., the distance from the outer surface 40 to the inner surface 38 
(see FIG. 4), varies, i.e., is slightly greater for string hole a.sup.31 
than for string hole a.sup.34. Because the string holes 42, 44 emerge at 
the same location on concave inner surface 38, the angle of the string 
holes a.sup.31 -a.sup.34 relative to the flat, outwardly facing surface 
40, while approximately 35.degree., varies slightly from string hole 
a.sup.31 to string hole a.sup.34. 
As a result the angle .alpha. of the grommet pegs 112 of the throat grommet 
strip 110, when placed on the surface 40, will need to vary accordingly. 
It would be desirable to mold the grommet strip 110 so that the grommet 
pegs 112 are initially parallel to one another, and yet when inserted 
through the string holes vary the angle .alpha. to conform to the varying 
string hole angle. In order to do so, the connecting strip 114 is curved, 
as shown in FIGS. 25 and 26, such that the longitudinal axis of the 
grommet pegs 112 also are disposed along a curve rather than a straight 
line. In this manner, when the strip 114 is bent straight to be mounted on 
the throat piece 18, the angle of the grommet pegs will bend to 
accommodate the variable angle of the string holes a.sup.31 -a.sup.34. 
Racquet assembly and stringing will be described with reference to FIGS. 
20-22. Referring to FIGS. 20 and 21, in order to string the racquet frame 
10, a pair of grommet strips 60 and 60a are positioned in the stringing 
groove 26 along each side of the frame 10, starting above the throat. 
Grommet strip 60a is a mirror image of grommet strip 60, so that it is 
molded with opposite curvature. As discussed above, when the grommet 
strips 60, 60a are bent to follow the curvature of the head portion 12 of 
the frame, the connecting strips 64, 64a will lie flat against the 
surfaces 28, and the grommet pegs 62 will extend through the 45.degree. 
stringing holes at the proper angle. 
A pair of bumper strips 70 (see FIG. 22) are then positioned in the 
remaining portion of the stringing groove 26, so as to extend between the 
grommet strips 60, 60a. Separate bumper strips do not need to be molded 
for the opposite sides of the stringing groove, in that the two bumper 
strips 70 are merely oriented in opposite directions so as to fit the 
respective surfaces 28. 
Finally, four throat grommet strips are positioned so that grommet peg 
project through the holes in the throat piece 18. Two of the grommet 
strips are grommet strips 110, whereas the other two grommet strips are 
mirror images of strip 110, with an opposite curvature. 
Once the grommet strips and bumper strips are in place, each strung surface 
90 and 90a is strung with a plurality of interwoven main strings 92, 92a 
and cross strings 94, 94a (one cross string for each surface is shown in 
FIG. 20). 
Although the specification refers to a plurality of main and cross strings, 
normally a racquet is strung with only one or two long strings, which are 
directed from hole-to-hole and extend along the grommet or bumper strip 
between holes. As used herein, the terminology main strings and cross 
strings includes the use of an elongated string where each cross string or 
main string is only a segment of a longer string. 
Stringing may be done with any desirable stringing pattern. As shown in 
FIG. 21 and 22, however, preferably the ends of the strings are tied off 
in the stringing groove, rather than on the inside of the strung surface 
14, as is customary. In this connection, the grommet strips and bumper 
strips are provided with a tie-off system located in the stringing groove. 
More particularly, the tie off system includes a raised portion on the 
bumper strip or grommet strip that allows a string to pass over the raised 
portion and leaves an interstitial area below the string which can be used 
to tie off another string. 
An exemplary tie-off system is shown in FIGS. 21 and 22. In the case of the 
grommet strip 60, a raised portion 120 is formed on the connecting strip 
64, in this case on the end strip portion 67. In addition, the point 65 is 
disposed to the side of the strip 64, 67 so as to leave a recessed area 
113 directly adjacent the raised portion 110. As shown, string portion 
124, which passes between holes a.sup.29 and a.sup.30, extends over the 
recessed area 113 and is raised by raised portion 120. As a result, loose 
string end 124 exiting from the string hole a.sup.28 can readily be tied 
around the string portion 124, utilizing the recessed area 113. Moreover, 
the raised portion 110 is provided with a groove 130. After tying the 
string end, 126, it can be trimmed, and the remaining end inserted into 
the groove 130. 
The bumper strip 70 similarly includes raised areas 82, over which a string 
portion 132 extends, which cooperate with adjacent recesses 80 formed in 
the connecting strip 72, to provide a tie-off area for a loose string end 
134. 
The foregoing represents a preferred embodiment of the invention. 
Variations and modifications will be apparent to persons skilled in the 
art, without departing from the inventive concepts disclosed herein. All 
such modifications and variations are intended to be within the skill of 
the art, as defined in the following claims.