Note: Descriptions are shown in the official language in which they were submitted.
~ ~ ~7 ~ ~ 0
1 Back~round -
Substantially all tennis rackets in general use today
have a plurality of characteristics which have proven to be
positive in effect, other characteristics found to have a nega-
tive effect, and still other characteristics which are considered
to be neutral in their effect.
Size - Substantially all tennis rackets in practical
use today employ a strung surface of approximately 70 square
inches. In most cases, the head is elliptical with the strung
surface having a longitudinal axis of approximately 10-1/2
inches and a minor axis of approximately 8-1/2 inches. This ` ~
relatively small strung surface is believed to be disadvantageous ~ ;
from a playing standpoint, particularly in making no attempt to
use the size and location of the strung surface to centralize
the center of percussion of the racket and from several other -
standpoints as will be made clear hereinafter.
Center of percussion - The designation "center of
percussion", sometimes called the "sweet spot", is an important
concept in tennis racket design. For the purposes of this
disclosure, the "center of percussion" of the racket is the
location at which an impacting ball results in zero kick or jar
to the player's hand. A disadvantage of rackets proposed here- `
tofore and in common use today is that the center of percussion
is close to the throat and is spaced from the intersection of
the major and minor axes of the strung surface. For example,
see U. S. Patent 1,539,019 which suggests the use of weights
added to an otherwise light racket to change the location of
the center of percussion. A conventional racket having a center
of percussion relatively close to the throat of the racket is
120~ P/2 CA - 1 -
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1 undesirable from several viewpoints. The average player tends
to strike a ball at the geometric center of the racket and hence
at a location spaced from the center of percussion.
Coefficient of restitution - The coeficient of resti-
tution is the ratio of the relative incoming velocity of the
ball as struck by the racket as compared with its outgoing
velocity after impact. Players prefer a racket designed such
that their average strokes impact an area of maximum coefficient
of restitution in order to obtain the greatest possible return
velocity to their shots without having to swing the racket ~ ;
harder and thereby risking loss of control and accuracy. `
Strength - A disadvantage of most t~nnis rackets in
general use today, whether made of wood or metal, is their
tendency to break. The breakage most often occurs in the shaft
of the racket just below the head or in the flare of the racket
just above the throat. ~
Strings - The only two materials in common use today -
for the stringing of tennis rackets are nylon and animal gut. -:
Of the two materials, nylon has the advantage of being approxi-
mately one third the cost,~of being weatherproof, and being two
to three times more durable as compared with gut. On the other ~ ~-
hand, gut is unmistakably the preferred material for use by
players of medium-to-expert ability in the case of conventional
rackets where the relatively short strings require a material of
maximum elasticity and resilience. There has long been a great
need for a racket which utilizes strings of nylon or other
synthetic or composite materials but which produces substan-
tially the same feel as animal gut when the racket is in use.
Disclosure
:
The tennis racket of the present invention includes a
1201~ P/2 C~ - 2 -
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1 frame having a head connected to a handle grip so as to have an
overall length of 26 to 28 inches, with the optimum length being
27 inches, and a weight of 12 to 15 ounces. The head has a
strung surface and its inner periphery defines an area of
between about 85 to 130 square inches. The length of the strung
surface in a direction along the longitudinal axis of the racket
is between 12 and 15 inches while being between 45 and 58V/~ of
the total racket length. The strung surface has transverse
dimensions, that is perpendicular to the longitudinal axis, of
between 9-1/2 and 11-1/2 inches. Due to the increased length of
the strung surface in a direction from the tip toward the hand-
le, the center of percussion on the strung surface is proximate
to the geometric center thereof.
In a preferred embodiment of the racket of the present
invention, the stringing pattern is non-uniform. Strings normal-
ly adjacent the frame and extending in a longitudinal and trans-
verse direction have been ellminated. Further, strings adjacent `
the geometric center of the playing surface are closer together
as compared with the spacings of strings closest to the frame of
the racket. This non-uniform stringing pattern not only saves
stringing material by spacing the strings most widely where they
are least used, and concentrating them in the middle of the
strung surface where they are most used, but also produces an
unexpected and advantageous efect. ~ string close to the side
of the frame is shorter and therefore stiffer in feel. The
strings crossing near the geometric center of the racket are
longest and therefore softest in feel. By the non-uniform
string spacing in which the shorter strings are fewer in number
and the longer strings intersecting at the center of the racket
are more closely spaced with respect to an impacting ball, there
1204 P/2 C~ - 3 -
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1 results an increase in uniformity of feeling of stringing ten-
sion over the entire face of the strung surface.
A racket in accordance with the present invention
overcomes various disadvantages of the prior art while at the
same time producing a variety of desirable results. The in-
creased size of the playing surface, particularly the aspect of -
its length in a direction from the tip towards the handle end of
the racket, has the results of producing or achieving: circum- ~ .
scription of the center of percussion, a larger zone of high
coefficient of restitution, an average more accurate return of
shot by the player, an increased polar moment of inertia about
the longitudinal axis, a greater ease of applying spin to the -
ball, a longer "d~ell time" in which a returned ball is in -~
contact with the strings for greater control, and greater
strength to the racket. All of these concepts will be further
amplified in the ensuing disclosure. The non-uniform stringing
pattern discussed above adds further increments to all of these
useful advantages. -
It is an object of the present invention to provide a
tennis racket of standard length, weight, and balance structurally
interrelated in a manner so as to have a substantially increased -
length of strung surface particularly in the direction from the
tip toward the handle end of the racket.
It is another object to provide a tennis racket `~ -
wherein the center of percussion is close to the geometric
center of the strung surface and has a larger zone of high
coefficient of restitution.
Other objects will appear hereinafter.
For the purpose of illustrating the invention, there
is shown in the drawings a form which is presently preferred; it
1204 P/2 CA - 4 -
, . . . . .
7~
1 being understood, however, that this invention is not limited to
the precise arrangements and instrumentalities shown.
Figure 1 is a plan view of a tennis racket in accor-
dance with ~he present invention.
Figure 2 is a side view of the tennis racket in Figure
1.
Figure 3 is a sectional view taken along the line 3-3
in Figure 2.
Figure 4 is a sectional view taken along the line 4-4
in Figure 3.
Figures 5A and 5B are a diagrammatic comparison of
test results on the racket of this invention and a conventional
racket with respect to the coefficient of restitution.
Figures 6A and 6B are a diagrammatic comparison of
test results on the racket of this invention and a conventional
racket with respect to the angle of ball return.
Figure 7 is a diagrammatic view of the present racket
being held by a hand.
Figures 8 and 12 are diagrammatic plan views of a head
of à conventional racket.
Figure 9 is a diagrammatic view of a string at impact
as seen along the line 9-9 in Figure 8.
Figures 10 and 14 are diagrammatic plan views of a
head of a racket in accordance with this invention.
Figure 11 is a diagrammatic view of a string at impact
as seen along the line 11-11 ln Figure 10.
Figure 13 is a diagrammatic view of a string at impact
as seen along the line 13-13 in Figure 12.
Figure 15 is a diagrammatic view of a string at impact
as seen along the line 15-15 in Figure 14.
120L~ P/2 CA ~ 5 -
. - . - - . : . .
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1~47(~0
Figure 16 i8 a diagramrnatic view of a string at impact as seen along
the line 16-16 in Figure 12.
Figure 17 i9 a diagrarnmatic view of a string at impact a~ seen
along the line 17-17 in Figure 14.
Referring to the drawing3 in detail, wherein like numerals indicate
like elements, there is shown in Figure 1 a tennis racket in accordance
with the preferred embodiment of the present invention designated generally
as 10. The racket 10 includes a head 12 and a handle shaft 1~
The racket 10 includes a frame 16 preferably made from a hollow
ex-truded high strength alurninum alloy. As shown in ~IG. 4, the preferred
construction for the frame 16 i8 a substantially figure 8 configuration
in cross section. Frame 16 is bent into the desired configuration 90 as
to have a loop defining the head 12 and parallel ends which are secured
to the handle grip 18. The grip 18 is fabricated of ligh-tweight material
such a~ plastic with leather or other suitable covering and is made in a
variety of sizes for different player preferences.
The racket 10 includes a throat 20. The throat 20 is secured
within frame 16 to provide a suitable lower completion of the generally
elliptical strung surface. The throat 20 is pierced with holes matching
20 holes in the frame 16 to provide the basis for accomplishing a stringing ~`~
pattern. The throat 20 may be made from a high strength polymeric pla~tic
material. All portions of frame 16 in the region of -the strung surface
have a radius of curvature of 3 to 10 inches except for a small portion -~
thereof at the flare of the throat.
The channel formed on the outer periphery of frame 16 may be
provided with a grommet strip 22 having grommets 24 integral therewith.
The grommets 24 extend through the holes in the frame 16 whereby the
strings 26 may be threaded through adjacent grommets and partially extend
around the grommet strip - - -
1204 P/2 CA - 6 -
- ~ : : ', .
-::, ~ , . , , :
1 22. See Figures 3 and 4. The strip 22 is preferably made from
a high strength polymeric plastic material with sufficient
resilience to provide a cushioning for the strings 26 so that
they will not cut when they extend through the holes in the
frame.
The strings 26 are preferably a standard commercially
available nylon material or other synthetic or composite mater-
ial. The webbing strings 26 are preferably of synthetic mater-
ial such as nylon because of its low cost, high durability and
because of its weatherproof nature, and because it is well
adapted to the racket of this invention where a synthetic mater-
ial of longer average string length is found to provide a resil
ience similar to animal gut when gut is used in the shorter
stringing length of conventional rackets. However, animal gut
may still be used in the racket of this invention where preferred ~ ;
by individual players.
The throat 20 cooperates with the frame 16 to define a
generally elliptical area containing the strung playing surface
28. The length of the strung surface 28 is defined by the
numerals 30, 32. The width of the strung surface 28 is defined
by the numerals 3L~, 36. The length of the str~mg surface 28 is
between 12 and 15 inches while being between 45 and 58% of the
total racket length. The width of the strung surface 28 is
between 9-1/2 and 11-1/2 inches.
The racket 10 has a weight of between 12 and 15 ounces
with an overall length of between 26 and 28 inches. The preferred
embodiment of racket 10 has an overall length of 27 inches. The
center of percussion (CP) is located at or proximate to the
geometrical center of the playing surface 28. The center of
gravity (CG) of the racket 10 is located at a point between 45
1204 P/2 C~ ~ 7 ~
1 and 52%, and preferably approximately 48~/~ of the total length of
the racket 10 as measured from the butt end of the handle shaft ~ :
14 so as to be at or adjacent the throat 20.
The head 12 in the preferred embodiment has an inner
periphery defining a strung area which is approximately 112
square inches (length about 13-1/2 inches, width about 10-1/2
inches) or about 60% larger than the average corresponding area
of a conventional racket. The length of surface 28 must be at
least 10% greater than its width. The racket 10 has high
strength attained by the structural construction of the frame 16
in combination with other strength advantages inherent in the
use of a racket structurally interrelated as set forth herein
while being of standard length and weight.
In the preferred embodiment, a pair of main webbing
strings 26 are arranged in the cen-ter located symmetrical with
the longitudinal axis of the racket 10 spaced from one another
at a center-to-center distance of about .42 inches. Disposed
: . ~
outwardly from the two center strings 26, seven additional
strings 26 are arranged such that their center-to-center dist-
ance progressively increases in the following preferred sequence
.44 inches, .46 inches, .48 inches, .50 inches, .53 inches, .57
inches and .61 inches. This arrangement leaves two uns~rung ;
cordal segments along the sides of the frame 16 each of which
has a height of approximately 1-1/2 inches.
In a similar manner, the transverse or cross strings ~
26 are arranged with three in the center with a center-to-center ~ '
distance of .42 inches. Thereafter, both in the direction of ~ :
the tip of the racket 10 and in a direction toward the throat
20, eight additional strings are employed such that their
center-to-center distance progressively increases in the fol-
1201~ P/2 CA - 8
' ,' . ' - . .. ~ ' ~' .
; . . . . .
~7~6~
1 lowing preferred sequence. The first additional string is .44
inches from the three central strings 26, the second string is
at .46 inches, the third at .48 inches, the fourth at .50
inches, the fifth at .53 inches, the sixth at .57 inches, the
seventh at .61 inches, the eight at .66 inches. As will be seen
from Figure 1, this arrangement leaves an unstrung cordal
segmen~, at the tip of about 1-1/2 inches and a height of about
1-3/4 inches at the throat. No attempt has been made to accur-
ately illustrate this stringing pattern in Figure 1 in view of
the small dimensions involved.
The preferred stringing pattern described above
utilizes 19 strings in a transverse direction and 16 strings in
the longitudinal direction. This arrangement requires approxi-
mately 40 feet of stringing material with ample allowance for
overlaps and wastage by the stringing mechanic. The total
length of stringing material required for the racket 10 is only
25% greater than that required by a conventional racket while at
the same time the area of the strung surface of the preferred
embodiment of racket 10 is 60~/~ greater than that of a conven-
tional racket. Compare the relative sizes of the heads in
Figures 8, 10 and 12, 14. The preferred tension for strings 26
is 65-70 l~s.
As will be apparent from the above description, the
spaces between strings adjacent the geometric center of the
strung surface 28 are smaller than those closest to the frame
16. This stringing pattern produces a more uniform feeling of
resilience across the entire face of the racket for reasons
previously explained. If desired, the conventional uniform
spacing of strings 26 may be used.
The center of percussion is the point on the racket's
:~ ',`.
120~ P/2 CA - 9 -
l surface where the entire energy of the stroke goes into return
of the ball and none is wasted by the transmission of jar to the
player's hand. On a conventional racket, the center o percus-
sion is approximately equidistant from the throat and the
geometric center of the playing surface or is closer to the
throat. Attempts have been made to correct this deficiency by
the use of weights to shift the center of percussion toward the
tip of the racket head. However such designs have failed to
achieve practical use since the balance of such rackets becomes
altered producing a "head heavy" feel objectionable to tennis
players.
This inventor uses a different approach. Recognizing -
that the center of percussion must remain fixed with respect to
its geometric position along the longitudinal axis of the racket
if the racket is to maintain good balance, my solution is to
increase the length of the strung surface toward the handle in
order to surround this unalterable location of the center of
percussion and extend therebeyond for at least 80% of the dis-
tance between the center of percussion and the center of grav-
ity. The center of percussion (CP) of racket lO has been shown
by laboratory tests to lie at a point on the longitudinal axis
of the racket about l9 to 20 inches from the butt end of the ;
handle shaft 14 and therefore is adjacent the geometric center
of the strung surace 28. Rackets having a strung surface
within the above-mentioned range of 85 to 130 square inches will
have their center of percussion spaced from the geometric center
by a distance of zero to one inch. ;~
In testing for the location of the center of percus-
sion, it was assumed that the center of rotation is at a point 3
inches from the butt end of the handle as indicated by the line
120~ P/2 CA - 10 -
, ~ ' ' ,
:' ~' ' ' ' :
~7~6~
1 X-X in Figure 7 which is the point at which the racket tends to
rotate with respect to the player's hand, and that the flexi-
bility of the player's hand and wrist, no matter how strong the
player, is much greater than the flexibility of any portion of
the racket.
The "coefficient of restitution" briefl.y alluded to in
the foregoing is an important concept in the refinement of
tennis racket design. Coefficient of resti~ution as applied to
tennis rackets is the ratio of the relative velocity of the
incoming ball as it meets the racket with respect to its result-
ing return velocity. For example, if a ball moving 50 mph
strikes a motionless racket and if found to return at 50 mph,
the racke~ would be said to have a coefficient of restitution of
1Ø If the oncoming ball again has a velocity of 50 mph and if
the racket is moving in the opposite direction at 10 mph, then
the return velocity must be 60 mph in order that the coefficient
of the racket be 1Ø The above examples are for explanation
only.
The ranges of coefficient of restitution obtainable on
conventional tennis rackets is generally in the order of 0.3 to
0.5. As will be seen below, the racket 10 of this invention
develops higher coefficients of restitution than conventional
rackets. It is clear that the largest possible zone of high
coefficient of restitution will be of great advantage to a
tennis player. His return shots will then have more velocity
with the same power of swing, or alternatively he can slow down
his swing for greater control and still obtain satisfactory
velocity on his return shot. The zone of optimum coefficient of
restitution will lie in the region between the center of gravity
and the center of percussion.
1201~ P/2 CA - 11 -
7QS~ : ~
1 High speed motion pictures were taken in an effort to
find the coefficient of restitution of points approximately one
inch apart on the preferred embodiment of the racket 10 of this
invention and a conventional racket constructed of the same
materials. A calibrated air-pressure ball shooting machine was
used to propel the balls a distance of 5 feet toward the racket
faces. In all tests, parallel data was established under
identical conditions for the preferred embodiment of the racket
of this invention and compared with the conventional racket.
Zone lines for both types of rackets were established for
coefficients of restitution greater than .30, greater than .40,
greater than .50. The tests, conducted with a camera at a frame
speed of approximately 400 frames per second, charted the
incoming velocity of the ball as compared with its return vel~
- ocity. Various incoming ball velocities were employed varying
from approximately 60 miles per hour to approximately 30 miles
per hour. Both the incoming and return velocities of the ball
were readily observable from this high-speed pho~ography tech-
nique and it was further possible to measure precisely the
location of each impacting ball. ;~
In one test, the rackets were mounted vertically
disposed in a vise simulating a player's hand. This test setup
was used most extensively because of its inherent characteristic
of assurance of uniformity and reproducibility. However,
additional tests at the same camera speed were also conducted on
hand held rackets and on rackets supported on blocks simulating
a "free spacel' condition where there would be no de~ined center
of rotation. In all tests, it was established that the efect~
ive playing surface of a racket must be considered to terminate `~
at a zone designated 41, 41' located 1-1/4 inch inside the frame
120L~ P/2 C~ - 12 -
.
', ' ' , :
1 of the racket (whether the racket of this invention or a con-
ventional racket). In zone 41 or 41', the ball strikes the
~rame or throat of the racket.
Film frames were analyzed one by one to determine the
return velocity. The coefficient of restitution of points
approximately one inch apart were plotted, using powdered chalk
on the ball to prove the point of impact. A composite of tests
dealing with coefficient of restitution is shown in Figure 5.
In Figure 5A, the zones of coefficient of restitution as estab-
lished by the laboratory tests are shown for the preferred
embodiment of the racket of this invention. In Figure 5B, the
same zones of coefficient of restitution are shown as determined
for a conventional racket under the same test conditions. The
zones on the respective rackets were for all practical purposes
symmetrical about the longitudinal axes of the rackets. '~
As shown in Figure 5A, zone 33 represents a coefficient ~ ~'
of restitution greater than .30 for the racket of this invention
and is substantially larger (approximately four times as great '
in area) than zone 33' representing the extent of a coefficient
of restitution greater than .30 for the conventional racket. In
a similar manner, zone 35 and 35' map coefficients of restitu-
tion greater than .40. It will be seen that zone 35 for the ''',
racket of this invention is substantially larger (approximately '~ '
four times as great with an area of about 20 square inches) than
zone 35' for the conventional racket which was found to have an
area of only about 5 square inches. Similarly, zones 37 and 37'
which map coefficients of restitution of greater than .50 show a
similar advantage of the racket of this inventi,on over the
conventional racket.
The average size of the corresponding zones for the
120~ P/2 CA - 13 -
~(~476~6~
1 racket of this invention was found ~o be 3.7~ times as great as
those of a conventional racket. This 3.78 times increase is
remarkable in view of the fact that the strung surface 28 is
only 60% larger than the strung surface on a conventional
racket. This increase in size of the zones was noted to be a
combination of increased length and width.
Finally, zone 39 which represents a coefficient of
greater than .60 has a substantial area (more than ~ square
inches for unrestrained racket and ball speed of 38.5 mph and
more than 8 square inches when ball speed increases to 60 mph)
for the racket 10 of this invention while being found not to
exist at all on the conventional racket 40. nn the racket 10,
the favorable coefficient of restitution greater than .4 was
found to extend from the region of the center of percussion
toward the region of the center of gravity. In all cases, it
will be seen that the contour lines mathematically developed
from the test data terminate abruptly at zone 41, 41' located 1
1/4 inches inside the frame of the rackets. From this fact, it
will be clear that embodiments of the racket of this invention
larger than the preferred embodiment will show still greater
advantage over a conventional racket with respect to larger
zones of favorable coefficient of restitution.
A well known concept in tennis instruction and develop~
,. : ,
ment of proficiency of play, deals with the concept of maxi-
mizing the "dwell" period of the ball on the strings of the
racket as the ball is stroked. With a conventional racket, the
"dwell" can only be increased by encouraging the player to
follow through while stroking the ball in order to maintain
contact between the ball and the strings of the racket for the
maximum possible time to thereby increase the stability and
:,
1204 P/2 CA - 14 -
7~
1 accuracy of the return shot.
This inventor believes that an important advantage of
the racket 10 of the present invention is that the "dwell" is
automatically increased by the inherent geometrical construction
of the racket 10. In Figure 8, let it be assumed that the ball
strikes the strings of a conventional racket 40 at the point 43
while traveling in the direction of arrow 56. As shown in
Figure 9, the web strings 54 is deflected at a point 43 by a
distance identified by the numeral 58.
Referring to Figure 10, let it be assumed that the
same ball strikes the strings Z6 on the racket at point 46 while ~-
the ball is traveling in a direction of arrow 56. Further, it
is assumed that the points 43 and 46 lie at the geometrical
center of the strung surfaces of the respective rackets. As
shown in Figure 11, the deflection of point 46 is indicated by
the distance 60. The distance 60 is greater than the distance ;~
58 which thereby results in the "dwell" period being greater on
the racket 10 of the present invention by approximately 20~/o.
In connection with Figures 8-11, it is assumed that
both rackets are strung with the same material at the same
tension and that the oncoming ball approaches at the same
velocity and is struck with the same power. Further, it is
assumed that the strings 26 are 20~/~ longer than the string 54
which assumption ls clear from the above disclosure. While the
points 43 and 46 are in the exact center of the strung surface
on the rackets 10 and 40, it will be clear that the same princi-
ples apply when a ball is struck off center. No definitive
tests have been performed substantiating increased dwell time
due to the difficulty of making accurate measurements. However,
subjective reaction in actual play with racket 10 as compared to
1204 P/2 CA - 15 -
~0 ~ 7 ~6~
l conven~ional rackets 40 repeatedly bear out a "feeling" of
increased "dwell" of the ball on the strings and resulting
greater control.
The inheren~ geometrical construction of the racket 10
provides an advantage whereby a player who is less skilled in
the art of "follow through" will nevertheless have a better
chance of returning a true shot. At the same time, an expert
tennis player using the racket 10 of the present invention with
fully developed s~roking skill will benefit by any increase in
"dwell period" achieved in racket 10 to stabilize and increase
the accuracy of his return stroke.
The racket 10 of the present invention minimizes the
inherent error of angle of return due to deflection of the ball
when the ball strikes the strung surface 28 off its geometric
center. A disadvantage of conventional racket 40 is that a ball
struck off center tends to be returned at an angle materially
differing from its incoming trajectory. See Figures 13 and 16.
In Figure 12, let it be assumed that a ball strikes the strung
surface on the racket 40 at a point 62 which lies along the
longitudinal axis of the racket but is spaced from the geometric
center of the strung surface. As shown in Figure 13, a ball ` ;
striking point 6~ with an incorning trajectory indicated by arrow
66 leaves the point 62 with a trajectory indicated by the arrow
68. The angle defined by arrows S6, 68 is slightly exaggerated
for the purposes of illustration.
On the racket 10 in Figure 14, point 62' is on the
longitudinal axis thereof and spaced from the geometric center
of the strung surface by the same distance as point 62. As
shown in Figure 15, an incoming ball having the trajectory of
arrow 66' will strike point 62' and depart with a trajectory
120L~ P/2 CA - 16
~7~0
1 indicated by the arrow 68'. The included angle between the
arrows 66' and 6~' is less than the included angle between
arrows 66 and 68. As a result thereof, the return show with the
racket 10 of the present invention is more accurate than that
with racket 40.
In Figure 12, racket 40 is provided with a point 64
which does not lie along the longitudinal a~is. In Figure 14, a
similar point 64' is provided on the racket 10. The distance
between point 64 and 64' and the center of the strung surface of
the respective rackets is the same. As shown in Figure 16, the
included angle between the arrows 70 for the incoming trajectory
and the arrow 72 for the outgoing trajectory is greater than the
corresponding angle indicated by the arrow 70' indicating an
incoming trajectory and arrow 72 indicating the outgoing trajec-
tory in Figure 17. Thus, the racket 10 of the present invention
increases the accuracy of the return of a ball which is struck
at a point spaced from the geometric center of the strung
surface as compared to a conventional racket 40 for the reasons
set forth above.
Comparative laboratory tests were conducted in regard
to "angle of return". The majority of the tests were conducted
with the racket held horizontally in a vise simulating the hand
and with the face of the racket perpendicular to the ground.
Emphasis was placed on these vise-held tests due to their
inherent reliability and reproducibility. However, for control
purposes, considerable additional tests of the same nature were
conducted on hand-held rackets. All tests compared the prefer-
red embodiment of the racket of this invention against a conven-
tional racket. Test conditions for both types of rackets were
identical. ~acket 40 was the same racket used for the tests
120L~ P/2 C~ - 17 -
109~7~60
l reflected in Figures 5A and 5B.
The tests concerning error of angle of return were
conducted at various ball velocities ranging from 38.5 miles per
hour to 62.2 miles per hour. Sufficient points were plotted to
map the zones on the respective rackets where the error in angle
of return was greater than 10 degrees, and greater than 20
degrees. The average of these zones compiled from the various
tests is shown in Figure 6A for the racket 10 of this invention
with comparable zones shown in Figure 6B for a conventional
racket 4Q. Numerals 76 and 76' show respectively the zones
where the error in angle of return was less than 20 degrees.
Numeral 74 and 74' indicate respectively for the two rackets the ~
zones in which the error in angle of return was less than 10 ~;
degrees. From an examination of Figures 6A and 6B, it will be
seen that the area of the respective zones of equal error of ~-
angle of return are in all cases larger for the racket of this
invention as compared to a conventional racket. The average
zone area is 2.7 times as grèat for the racket 10 of this in-
vention where the ball is returned with the same accuracy as
compared to the comparable zones for conventional racket ~0. As
in the case of the test data on "coefficient of restitution", it
will be seen that the operative zones may terminate at the zones
41, 41' located 1-1/4 inch inside the frame of the racket.
Still further improvement in average accuracy of return can be
expected in the larger headed embodiments within the scope of
this invention.
In general, there are two conditions in which a
skilled player consciously arranges his racket so that the
impacting ball strikes the webbed surface from a non-perpendic-
ular direction. One such case is an "undercut'~ or "chip" causing
1204 P/2 C~ - 18 -
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~g;l 4~
1 the ball to drop dead with back spin on the opponent's court.
The other condition is where a skilled player strikes the
impacting ball while the racket face is being moved upwards to
impart top spin to the ball so that it lands in the opponent's
court with a forward bounding motion which is difficult to
return. The upward motion of the racket face results, in
effect, in a non-perpendicular impact between the ball and the
strung surface. In both cases, it will be clear that the longer
"dwell" time during which the ball is in contact with the
strings, provides more time for the returned ball to acquired
angular acceleration~ increasing the amount of backspin, or top
spin, as the case may be. This effect is further augmented by
the simple fact of the increased width of the strung surface in
that there is more transverse room, as well as time, for the
ball to roll across the racket strings 26, thus picking up
~ ....
greater spin. Experienced players consistently note increased
spin on the return baLl when struck with the racket 10 of this
invention as opposed to that achieved from the smaller strung
surface of a conventional racket. The same increased spin is
also predictable, and observed in practice while an experienced
player elects to employ a "slice serve" or "overspin serve" as
opposed to a "flat serve".
A surprising advantage of the racket 10 of this in-
vention is that it is stronger and more durable in actual use
than conventional racket 40. Laboratory tests of racket 10
using a tennis ball hitting machine resulted in one racket 10
breaking after 90,000 strokes (this is twice the accepted level
for a conventional racket) while another racket 10 was still in
good condition when the test was stopped after 100,000 strokes.
This highly desirable, and unexpected, increase in strength
120~ P/2 CA - 19 -
1 experienced in the racket 10 of this invention with respect to
conventional racket 40, in spite of the increased size of head,
appears to be the result of a far greater proportion of balls
striking in the zone of high coefEicient of restitution circum
scribing the center of percussion with the result that less
vibration and fatigue is introduced into the racket frame 16
over a prolonged period of play. Regardless of the cause, the
increase in strength is a fact proved by test and experience in
actual play, and is a great advantage to the tennis playing
public which has been inconvenienced and caused expense by the
breaking of conventional rackets.
A final advantage of the racket of this invention 10
over conventional racket ~0 has been found to be a marked allevi~
ation of the ailment common to tennis players of all classes
generally referred to as 'Itennis elbow" which is an inflammation
in the elbow joint caused by twisting of the racket when a ball
is struck off center and jar to the player's arm when a ball is ~-
struck remote from the center of percussion. A marked all
eviation of this ailment has been noticed by a wide variety of
players who have tested prototypes of the racket 10 of this
invention. This unexpected advantage is believed to follow from
the fact that more balls are struck in the region of the center
o percussion, eliminating jar from the player's hand, as well
as from the fact that the high polar moment of inertia of this
wider racket decreases the tendency for the racket to twist when
a ball is struck off center. Hence, I believe this racket will
be a boon to players who have or are prone to tennis elbow.
In sum~ary, the racket 10 of this invention is of
standard length, weight, and balance, and at the same time: (a)
the strung surface us 20V/o to 90% larger than that of a conven-
1204 P/2 CA - 20 -
0
l tional racket, (b) the strung surface is located to centralize
the center of percussion. These desirable features are attained
without resort to weights, springs, or other complications but
instead are attained by increasing the size of the head primar-
ily in the direction of the mid-point of the racket. Thus, the
inventor enlarges the strung surface to move the geometrical
center of the strung surface toward the center of percussion of
the racket instead of attempting to move the center of percus- ;
sion toward the geometric center of a conventional racket.
While the frame 16 of the racket of this invention is
preferably made from extruded aluminum, other types of frames
made from wood, fiber reinforced plastic, and other materials
having the necessary strength, lightweight and resilience, may
be utilized. Other modifications will be apparent to those
skilled in the art such as the elimination of the grommets 24,
and may be made without departing from the scope of the inven-
tion. While the grip 18 is preferably a polymeric plastic ;
material molded onto the ends of the frame 16, other convention-
al techniques may be utilized for joining the ends of frame 16 ~ ~ -
to the grip 1~. No special equipment is needed for stringing
racket 10 whereby it may be sold without strings as i9 conven-
tional with presently available rackets.
The present invention may be embodied in other specific
forms without departing from the spirit or essential attributes
thereof and, accordingly, reference should be made to the ap-
pended claims, rather than to the foregoing specification as
indicating the scope of the invention.
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