1. Field of the Invention
This invention relates to the design and construction of racquets, and particularly of racquets intended for use in playing the internationally popular game of tennis.
2. Description of the Prior Art
The game, once fully named "lawn tennis" but not known simply as "tennis," has been played for almost one hundred years. Within the past decade or so tennis has seen a great increase in its popularity, particularly in areas of the world which are not subjected to long cold winter periods. However, even in colder areas, indoor facilities have been constructed to permit play at any time. The game involves the stroking of a felt-covered ball by players moving on opposite sides of a net bisecting a marked court with each player employing a racquet, which, by custom and convention but not by regulation, is approximately 27 inches long with an elliptical head integrally secured on the end of a handle. The racquet head is strung with nylon or cat-gut in a crossed pattern, thereby providing a grid of approximately one-half inch square openings to minimize the air resistance to the rapid movement of the racquet head in a direction substantially normal to the plane of racquet strings, and hopefully properly opposing the trajectory of the ball.
For most of the years during which the game of tennis has been played, the racquets were almost exclusively made of wood. Initially, and for many years the racquet heads were in the shape of an inverted isosocles triangle rounded at the base angles with the apex integrated with the handle. Later the head assumed more the shape of an ellipse, the long axis of which coincided with the axis of the handle. Construction of these early racquets was accomplished by steam or otherwise bending a wooden member into such triangular or elliptical shapes with the extremities extending beyond the apex of the triangle or the ellipse and laid one upon the other to be secured together to constitute a handle.
Beginning in about the 1920's elliptical racquet heads were formed separately as laminated wood ellipses and one end of the ellipse was glued, cemented or otherwise joined to the bifurcated end of the handle. This type of construction, in various improved versions, is still employed for the standard still popular wooden racquet. Sometime in the 1920's and 1930's certain racquets were fabricated with metal elliptical heads and strung with wire instead of gut, but these never acquired any real popularity and generally disappeared entirely from the tennis scene by the early 1950's.
Although the laminated wood racquet continues to be popular, within the last few years a number of new types of metal racquets strung with gut or nylon have been made and sold and these are now in widespread use. These metal racquets have generally assumed the same shape as that of their wooden forebears, viz., an elliptical head with the long axis of the ellipse being coaxial with the axis of the handle. Such metal racquets now offered ar fabricated either from formed tubular members, stampings or from extrusions with the head frame being integral with the handle. The currently offered metal racquets are generally designed to duplicate within 5 percent both the weight and its distribution of their wooden predecessors (and still competitors). Because the head frame of a metal racquet may be thinner and thus appear to offer less air resistance than a wood frame needed to provide sufficient rigidity to resist buckling upon impact with a ball or warping under the tension imposed upon the frame by the tight stringing, some players believe that a metal head frame may be moved more rapidly through the air than a wooden frame. However, complaints are often heard that metal frame racquets lack the solid "feel" of wooden racquets to which many tennis players have been accustomed through years of play.
In addition, since the efforts of manufacturers of metal racquets have been directed toward duplicating the shape, weight and its distribution and the stringing of the long accepted wooden racquets, no consideration appears to have been given to the possibility of utilizing certain advantages which metal frame construction can offer to produce a novel racquet design with features not possibly attainable with wood construction. In this connection, it should be pointed out that the Rules of the American Lawn Tennis Association make no provision for any particular size, shape or construction of a racquet. Anything may be employed as a racquet, but the conventional form in use today is based upon the years of experience of the millions of tennis players and the historical development of racquets as essentially wooden constructed devices.
As previously mentioned, tennis racquets of both metal and wood, have traditionally been approximately 27 inches in length with an elliptical head providing a strung area of approximately 70 square inches. The long dimension of the ellipse is approximately 11 inches, the short dimension, 9 inches. The handle, thus, extends some 15 inches from one end of the ellipse.
Racquets traditionally weigh from about 12-1/2 oz. for children to as much as 16 oz. for some men. The average racquet weight, however, ranges between 13-1/2 oz. and 14 oz., normally distributed so that the center of gravity lies at between 12-1/2 and 13-1/2 inches from the gripped end of the handle. This weight distribution has not been arbitrarily selected but rather has resulted from the weight of the average type of wood of the volume which has been found necessary to provide sufficient strength in the usual racquet configuration to enable the average player to impact the ball with the greatest amount of force he normally can develop in his stroking of the ball with the racquet. When racquet manufacturers began substituting metal for wood in the racquet framing, they generally sought to simulate the weight and its distribution of the conventional wood racquet.
It has been found by many of the better players of tennis that the best area of the strung head at which to impact the ball is not on the geometric center (or centroid) of the strung elliptical area, but may be as much as an inch toward the handle from that centroid. This ideal impacting area is sometimes referred to as the "sweet spot." In engineering terminology it may be called the "center of percussion" (c.o.p.), and is defined as the square of the radius of gyration about the instantaneous center of rotation, divided by the coordinate of the center-of-gravity. I have determined that such conventional off-centroid location of the sweet spot," or c.o.p., may be attributed to the fact that the center-of-gravity of the racquet is conventionally located where it is, i.e., at about 13 inches from the end of the handle instead of further away from that end toward the centroid of the strung ellipse. Because of its location, a relatively large portion of the strung area of the ellipse is not really effective in hitting, and the player's reach for ideal stroking is decreased by the distance between the c.o.p. and centroid of the ellipse.
In addition, since racquets have been designed originally for wooden construction, they have been constructed to permit bending in the direction of impact with the ball, but to minimize twisting of the strung head. This is because although a wooden racquet head can withstand bending, it cannot tolerate large twisting moments without fracturing, owing to the intrinsic grain structure of wood in which weakness is always sought out and exploited by a twisting moment. Thus, there has resulted in the genesis of racquet design based upon wood construction the conventional elliptical head oriented with its long axis coaxial with the handle.
Moreover, the wooden racquet cannot stand too great a bending moment at the throat (i.e., the portion of the racquet where the handle is joined to the head), since in order to give the appearance of reducing air resistance to the movement of the racquet, racquet manufacturers narrow the throat to the minimum width required to withstand maximum anticipated impacts of the ball upon the head. To enable the width of the throat to be so minimized, the center-of-gravity of the wooden racquet cannot be disposed much farther toward the centroid of the racquet ellipse than where it is conventionally located (i.e., about 13 inches from the gripped end of the handle). To effect any material shift of the center-of-gravity in such direction ain a wood racquet would produce excessive bending, and possibly undesired twisting, moments at the throat and shoulders, thereby resulting in their fracture when the head is impacted with an overhead smash or hard forehand shot.
Thus, racquets based upon the limitations in design imposed by the use of wood in their construction, have heretofore assumed a fairly standard shape and weight distribution even though, as pointed out above, more recently, racquets have been constructed of metal. In so constructing metal racquets, racquet designers and manufacturers have apparently ignored certain structural capabilities of the metals -- particularly of those exotic metals such as magnesium and beryllium, or the high modulus carbon fiber composites developed particularly for aerospace use, as well as certain principles of physics and rigid body mechanics which have peculiar application to the stroking of a tennis ball.
Among such principles of physics is the fact that a player's energy is most efficiently transferred to the ball as he hits it if the c.g. of his racquet is as far from his hand and as near the ball as structural mechanics permit.
SUMMARY OF THE INVENTION
The present invention has resulted from considerable study of the dynamics of the game of tennis and certain principles of rigid body mechanics. Careful consideration of these principles has led to the present redesign of a novel and improved tennis racquet. No longer limited in designing a racquet by the physical properties and characteristics of wood, I have evolved certain novel features which, if incorporated in the construction of a tennis racquet, will be found to result in a racquet of quite a radically different appearance and rigid body mechanical effect. My approach to the problems of racquet design and construction, which approach has resulted in the present invention hereinafter to be described, is as follows:
Should a racquet frame of present-day metal frame configuration be constructed of a metal having a very low ratio of weight to stiffness (and strength), such as magnesium or beryllium, it will be considerably lighter in its over-all weight, than that of present-day metal or wood racquet frames, and yet it may possess all the stiffness and strength which may be required for impacting the ball. One may then address himself to the optimum location of the c.g. of the racquet and effect such location by design and/or weight supplements so as to bring the c.g. nearer the ideal impacting spot on the strung frame and further from the player's hand. Such design and/or addition of such weight supplements may still keep the over-all weight of the racquet at or below that of present-day wood and metal racquets. The weight supplements may be in the form of lead or silver or other metals having a high specific gravity.
I have found that such weight supplements should be added at the ends of a transverse diameter of the elliptical hitting face of the racquet. In addition to more properly locating the c.g. of the racquet for ball stroking in conformity with the principles of physics hereinabove referred to, the addition of dead weight supplements at the ends of such transverse diameter will be found to contribute strongly to the racquet's moment of inertia about its longitudinal axis, thereby minimizing for the player the unfavorable consequences of his minor mis-hits about this axis.
It is a feature of the present invention to provide means on the frame to receive such supplementary dead weights and to secure them along the sides or even end of the frame. They may be added by the player himself after he has purchased the strung frame or by the vendor of the racquet at the time it is purchased. Conceivably, a retailing professional may even engage in some type of "try-out" session with the purchaser to arrive at the optimum disposition of the weight supplements to the racquet head in order to give the purchaser the best "feel" for ball stroking.
By this lightweight metal racquet construction and the addition of such supplementary dead weights, there may be attained one object of the invention, namely, to shift the center of gravity of the racquet toward the head and away from the conventional location about 13 inches from the handle end. This center of gravity shift will be found to result in moving the c.o.p., or "sweet spot," of the racquet from its off-center location in the string area nearer to the centroid of the strung ellipse, thereby increasing the strung area for the most effective response to ball impact.
However, this and other objectives may also be accomplished by perhaps the most iconoclastic feature of the present invention wherein the disposition of the elliptical head may be such that the long axis of the strung ellipse is transverse to the axis of the handle. This can readily be accomplished with metal framing and has a number of unexpected and interesting results. Particularly noticeable among end results are the following:
First, it inherently facilitates shifting the center of gravity away from the handle.
Secondly, stability in roll about the longitudinal axis is naturally increased to improve particularly volleys and overheads, i.e., shots which, being hit in the air, are especially vulnerable to small mis-hits about the long axis.
Third, since most players'errors occur through mishitting in the cross dimension of the head, the greater string area in that cross dimension tends naturally to attenuate the effects of such mis-hitting.
Lastly, a chopped, undercut or "topped" ball, accomplished during "ground stroking" or "serving," will receive a longer "dwell" on the strings since each of those strokes is made by a movement which subjects the ball to string action in the cross dimension.
A racquet frame of such head disposition is preferably also made of lightweight metal such as magnesium or beryllium or the so-called high modulus carbon fiber composites, and may be constructed to receive dead-weight supplements.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a plan view of a metal racquet of conventional shaping;
FIG. 2 is a plan view of a metal racquet constituted in accordance with the present invention;
FIG. 3 is a section taken on the line 3--3 of FIG. 2.
FIG. 4 is a section similar to FIG. 3, but showing the addition of a weight supplement on the cross-axis.
FIG. 5 is a partial view similar to FIG. 2, but showing the addition of further outwardly disposed weight supplements.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a metal racquet of generally conventional shape and dimensions. It comprises an integral frame 10 which may be constructed by bending a metal member 12 back upon itself in an elliptical loop configuration to form a handle 13 and head 20. The loop 14 may be completed by inserting and securing as by welding an arcuate segment 15 inside the member 12. The long axis 22 of the elliptical head coincides with the axis 24 of the handle 13 and the short head axis 23 is perpendicular to handle axis 24.
To be compared with the conventional metal racquet illustrated in FIG. 1 is the racquet constructed in accordance with the present invention which is shown in FIG. 2. This racquet differs substantially from the conventional tennis racquet configuration in that the elliptical head 20a is turned 90 degrees so that the long axis 22a of the ellipse is disposed perpendicularly to the handle axis 24a and the short head axis 23a is coaxial with the handle. It should be pointed out, however, that in order for the racquet of this novel configuration to be accepted by tennis players who have learned and long played the game with the racquets of the conventional configuration, the centroid 26a of the ellipse of the racquet of FIG. 2, should be located the same distance from the end 28a of the handle 13a as is the centroid 26 of the elliptical head 20 of the conventional racquet of FIG. 1, i.e., about 21-1/4 inches from the end 28 of its handle 13. In order to effect such location of the centroid of the ellipse in a racquet where the elliptical head is disposed with its long axis 22a transverse to the axis 24a of the handle 13a, the overall length of the racquet is necessarily shortened from the standard 27 inches to about 26 inches.
Although a racquet in the configuration of FIG. 2 may be constructed in any of the modes by which metal racquets now on the market are constructed, I would prefer to construct it in the form shown in FIGS. 2 and 3. Thus, the handle 13a is formed about a pair of members 30, 32, secured together at their handle-forming extremities 34, 36 and at several other points by struts 38. These members 30,32, are curved arcuately away from each other and then back to embrace a separately formed elliptical head 20a. Members 30, 32 are provided with channels 40 (FIG. 3) along their embracing arcuate segments 42, 46 respectively. The head frame 20a is made of a thickness and configuration to fit into said channels 40. The arcuate segments 42, 46, may be welded or otherwise secured to the inserted frame 20a. The latter preferably includes an inwardly extending rib 48 with a plurality of orifices 50 to receive the gut or nylong stringing 52.
One important advantage of constructing a racquet in the manner illustrated in FIG. 2 is believed to be that because more of the head frame mass is disposed further away from the handle, the center of gravity is inherently disposed further away from the handle and toward the centroid of the ellipse, thereby enabling the c.o.p. to be located more nearly at such centroid. Other advantages are:
a. Since the overall length of the racquet is slightly decreased, the bending moment generated by a smash toward the racquet tip will be somewhat less than that sustained by the conventional racquet;
b. It has been found that most hitting errors of players occur as lateral deviations in the head from the axis passing through the handle. Where the short axis of the ellipse is transverse to such handle axis, such deviations more naturally involve the ball striking the frame or just inside the frame than they would where the long axis is transverse to the handle axis. However, where the long axis of the ellipse is transverse to the handle axis, there is a greater string area in the head where such lateral deviations are wont to occur. The effects of lateral mis-hits may, therefore, be substantially decreased. In other words the stringing in the radially greater lunes of the ellipse, which stringing is less utilized for mis-hits when the ellipse is oriented with its longer axis coaxial with the handle, is better utilized when the racquet head is turned so that those lunes are disposed on an axis transverse to the axis of the handle;
c. The disposition of the head ellipse with its long axis transverse to the axis of the handle provides an increased inertial stability against roll. The moment of inertia of the racquet about the longitudinal axis is believed to be typically increased by more than 200 percent without the addition of supplementary weights at the ends of the long axis, and by more than 300 percent with the addition of such weights;
d. Lastly, such disposition of the head permits greatly increased "dwell" of the ball on the strings for undercut, chopped and topped stroking of the ball, which may occur in the course of ground strokes, serves and volleys.
With respect to item (c) above, it is known from elementary mechanics that the torque required to produce a specified rotational acceleration of a body about an axis is proportional to the moment of inertia of the body about that axis. Hence any increase in the moment of inertia of a tennis racquet about its longitudinal axis increases the inertial stability of the racquet against rotation about that axis. That is to say, a given applied torque, such as results when a ball strikes the strings at a point laterally offset from the longitudinal axis, causes a smaller rotational disturbance of the racquet.
The moment of inertia of a body with respect to a specified axis may be defined as the sum or integral, taken over all mass elements of the body, of the product of each mass element by the square of its distance from the axis. Since the distance term is squared, the handle of a tennis racquet, being concentrated close to the longitudinal axis, contributes relatively little to its moment of inertia about that axis, and the value of that moment of inertia is determined predominantly by the distribution of mass in the head. A comparison of FIGS. 1 and 2 shows that the mass of the head is distributed on the average appreciably farther from axis 24a in FIG. 2 than from axis 24 in FIG. 1. Taking account of the squaring of the distance, that redistribution of mass yields a significant increase in stability against roll.
FIG. 4 illustrates a possible manner of attaching supplementary weights 54 to each side of the elliptical frame 20a in the vicinity of the cross axis 22a in order to provide such further increase in inertial stability against roll. The weight 54 may comprise a relatively thin arcuately-shaped strip of lead with an inner contour 56 mating with the outer contour 58 of the arcuate segment 42 of the member 30. Such lead strip 54 may be held by a pair of straps 60, the ends of which are held by a fastener 62 which passes through a hole provided in the inwardly projecting rib 48 of the frame 20a.
The present invention also contemplates that supplementary weights 64 could be applied to the racquet frame through strut-like extensions 66 shown in FIG. 5, to provide lateral mass disposition even further outwardly along the cross axis 22a' from the centroid 26a' of the ellipse. It should be appreciated, however, that although FIG. 5 shows such strut-like extensions 66 supporting supplementary weights 64 on the sides of a frame of the configuration of FIG. 2, they could be similarly applied to a conventional racquet of the type shown in FIG. 1, and a noticeable improvement in the form of an increase of inertial stability against roll of the FIG. 1 racquet would be obtained. The quantum of such improvement would, of course, be a function of the square of the distance of the two oppositely disposed masses from the centroid of the ellipse. Obviously, with such a functional relationship, a much greater roll stability effect is inherently obtained with a racquet head of the configuration shown in FIG. 2, since the sides of the racquet head to which the supplementary weights are attached, are disposed at a substantially greater distance from the ellipse centroid than are the sides of the FIG. 1 racquet from its centroid along the cross axis 22.
It will further be appreciated that many factors will affect the particular design of the racquet which is considered optimum for each player, but the present invention offers new parameters and design factors which have not heretofore been considered or appear to have been taken into account by manufacturers in design even for metal racquets.