Tennis racket

In a tennis racket whose frame and handle shaft are integrally made of fiber reinforced plastics and the frame defines a strung surface, the frame is so designed that its first thickness "t" in a direction of the strung surface is substantially uniform around the circumference of the frame and its second thickness "T" in a direction perpendicular to the strung surface is gradually varied. The second thickness is maximum at side sections of the frame interposing a sweet spot in the strung surface and is minimum at the top and bottom portions of the frame in such a manner that the maximum thickness is thicker in the range from 35% to 60% than the minimum thickness, which increases a weight per unit length of the frame toward the side sections.

BACKGROUND OF THE INVENTION 1. Field of the Invention 
The present invention relates to a tennis racket of a type in which frame 
and handle shaft are integrally made of fiber reinforced plastics 
(hereinafter referred to as "FRP") and the frame is strung with some 
strings such as a gut. 2. Description of the Prior Art 
In recent years, tennis rackets having enlarged frames such as so-called 
large-size racket and mid-size racket have been broadly used. These 
rackets can provide some advantages in comparison with tennis rackets 
having conventionally sized frames. In detail these new type rackets 
provide a high rebounding coefficiency and enlarge their sweet spot. On 
the contrary, such larger rackets tend to be twisted considerably when an 
user hits a ball at a point outside of the sweet spot. This tendency is 
particularly remarkable in the case of FRP-made rackets rather than wood 
or aluminum-alloy made rackets, because of the small specific gravity of 
FRP material. 
In order to reduce the twisting of rackets, it has been known effective to 
increase moment of inertia in their frames, and some proposals have been 
made to attain a relatively large moment of inertia. For example, Japanese 
Utility Model Applications as laid-open under No. 54-41364 and No. 
61-127766 show the tennis rackets in which a metal weight is fixed to a 
portion of the frame. However, such racket involves new problem. That is, 
additional component such as the metal weight is not integrally formed 
with the main component which is used for the frame per se, and thus such 
different components concentrates mechanical stress on a particular 
portion so that the frame may be broken in the vicinity of the weight. 
Also, Japanese Utility Model Application No. 58-188069 shows another 
example of improved racket whose frame is added with weight by protruding 
a part of the internal circumferential surface of the frame radially 
inwards toward the center of strung surface. However, this racket also 
causes several problems owing to its frame shape. Such protruded section 
increases an air resistance and generates turbulent flow which is not 
ignored, so that energy loss is increased during swing and user may feel 
unpleasant. 
It is therefore an object of the present invention to provide a tennis 
racket of which frame can have an increased moment of inertia for reducing 
twisting of the racket, without increasing a surface area of the frame in 
a plane of a strung surface. 
Another object of the present invention is to provide a tennis racket which 
can effectively transmit an energy to a ball and can provide a good swing 
feeling. 
Still another object of the present invention is to provide a tennis racket 
improved in its mechanical strength and durability. 
A further object of the present invention is to provide a tennis racket 
which can be manufactured in a simple work. 
SUMMARY OF THE INVENTION 
According to the present invention, a tennis racket includes a frame for 
defining a strung surface and a shaft, these frame and shaft being 
integrally formed of fiber reinforced plastic material. A first thickness 
of the frame in a direction of plane containing the strung surface is 
substantially uniform around the circumference of the frame, while a 
second thickness of the frame in a direction perpendicular to the strung 
surface is gradually varied. The second thickness is maximum at side 
sections of said frame interposing a sweet spot in the strung surface and 
is minimum at the top and bottom portions of the frame in such a manner 
that the maximum thickness is thicker in the range from 35% to 60% than 
the minimum thickness, whereby a weight per unit length of the frame 
increases toward the side sections. 
The weight is increased at the side sections of the frame without any 
protrusion to the strung surface. Therefore, twisting movement of the 
racket can be effectively reduced while maintaining an air resistance to 
the racket during a swing motion in the same level as conventional tennis 
rackets. The increase in thickness of the frame in a direction 
perpendicular to the strung surface contributes to a stabilization of the 
swing. 
In one embodiment of the invention, the side sections are located at 
portions slightly below a level of the geometric center of the strung 
surface. The frame may have a cross section of a substantially oval shape 
of which minor axis extends in a direction of the first thickness and a 
longitudinal axis thereof extends in a direction of the second thickness. 
Other and further objects, features and advantages of the present invention 
will appear more fully from the following description taken in connection 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows one preferred embodiment of a tennis racket generally 
indicated by numeral 10 according to the present invention, which includes 
a frame 12, a handle or grip 14 defining the lower end of the racket, and 
a shaft 16 extending between the frame 12 and grip 14. The shaft 16 
bifurcates above the grip 14 to define a substantially inverted triangular 
hollow space 26 therebetween and between the bottom portion 12c of the 
frame 12. The area defined within the frame 12 is adapted to be strung 
with strings such as guts (not shown) in its vertical and horizontal 
directions so a to form a ball hitting face. The frame 12 and the shaft 16 
are integrally formed by covering a core 22 (see FIGS. 3 and 4) made of 
foamed resin such as foamed urethane with reinforcing fiber layers 24 
impregnated with resin material. Such reinforcing fiber layers 24 usually 
comprises plural layers and, in this embodiment, long glass fiber are 
mainly used as reinforcing fibers while external layers may be formed of 
carbon fibers or other fibers. As shown in FIG. 2, the frame 12 contains a 
groove 18 in its outer peripheral surface and a plurality of through holes 
20 formed in the groove 18. The through holes 20 are used for stringing 
the gut. 
In the illustrated embodiment, the cross section of the frame 12 is 
substantially formed in an oval shape whose minor axis extends in a plane 
of the strung surface and longitudinal axis extends in a plane 
perpendicular to the strung surface. The thickness "t" of the frame 12 
along the minor axis is substantially uniform around the circumference of 
the frame 12 except for the junctions between the shaft 16 and the frame 
12 at which the thickness "t" is somewhat increased as shown in FIG. 1. On 
the other hand, the thickness "T" of the frame 12 along the longitudinal 
axis is not uniform and is gradually varied around the circumference of 
the frame 12. Specifically, the thickness "T" becomes maximum at both side 
sections 12a-12a which interposes a sweet spot located slightly below the 
geometric center C of the frame 12, and becomes minimum at the top portion 
12b and bottom portion 12c, these maximum and minimum thicknesses being 
indicated in FIG. 2 as "T1" and "T2" respectively. The thickness "T" is 
gradually increased from the top portion 12b and bottom portion 12c to the 
side sections 12a. As can be seen from FIGS. 3 and 4 which show cross 
sections at 12b and 12a, respectively, the wall thickness of the core 22 
and the FRP layers 24 is substantially uniform around the frame 12. 
Therefore, a weight per unit length of the frame 12 is maximum at the side 
sections 12a having the thickness "T1"and gradually decreases toward the 
top and bottom portions 12b and 12c to become minimum at the "T2" 
thickness portions. 
In the illustrated embodiment, the maximum thickness "T1" is set to 30 mm 
and the minimum thickness "T2" is 21 mm. The maximum thickness "T1" is 
increased about 43% than the minimum thickness "T". This increasing ratio 
can be obtained from the following formula: 
EQU (T1-T2)/T2.times.100 .apprxeq.43% 
In the present invention, the increasing ratio can be selected from 35 to 
60%. If it is less than 35%, the side sections 12a can not satisfy the 
weight increment to increase moment of inertia. On the contrary, if the 
increasing ratio is larger than 60%, center of gravity of the racket will 
excessively be shifted downwards and thus its weight balance will be lost. 
The side sections 12a having the maximum thickness should be so located as 
to interpose the sweet spot in the strung surface, the sweet spot usually 
being near the geometric center C or slightly therebelow. 
As it could be understood from the foregoing description, the increase in 
unit weight of the frame toward the side sections thereof achieves the 
same effect as by adding weight members to the side sections, whereby 
moment of inertia of the frame is increased to reduce twisting movement of 
the racket. Further, this advantage can be obtained without providing any 
protrusion on the inner peripheral surface of the frame and therefore 
without increasing air resistance during swing motion. The frame thickness 
is increased only in the direction perpendicular to the strung surface, 
i.e. in the direction of swing motion, which can ensure a smooth and 
stable swing of the racket. This will be appreciated by, for example, 
thinking of swinging a hollow cylindrical member in its axial direction 
and in an other directions. 
Although the present invention has been described with reference to the 
preferred embodiments thereof, many modifications and alterations may be 
made within the spirit of the invention.