WO1994000203A1

WO1994000203A1 - Tennis racquet having a surface structure adapted for the reduction of air resistance

Info

Publication number: WO1994000203A1
Application number: PCT/CA1993/000257
Authority: WO
Original assignee: Velke, Willi, H.
Priority date: 1992-06-30
Filing date: 1993-06-30
Publication date: 1994-01-06
Also published as: AU4413993A

Abstract

A tennis racquet utilizes a surface modification to increase the swing speed thererof. The increase of swing speed of the racquet, as used in racquet sports such as tennis, squash, badminton or racquetball, is achieved by reducing the base drag behind the frame and strings of the racquet. The base drag is reduced by including in the surface of the frame and strings a plurality of surface discontinuities in one of a number of patterns, so as to change the fluid flow across the surface of the frame and strings to a more favourable flow, when compared to an unmodified racquet.

Description

TENNIS RACQUET HAVING A SURFACE STRUCTURE ADAPTED FOR THE REDUCTION OF AIR RESISTANCE

BACKGROUND OF THE INVENTION A racquet is a hand held device, and is used by players in games such as tennis, squash, racquetball, etc. , to move a ball from one player to another. All such racquets basically consist of a frame with string woven into it to provide a resilient surface. This resilient tightly woven surface is the part of the racquet making contact with the ball during the motion of swing, propelling the ball in its desired direction.

It is the object in most racquet games for a player to hit and manipulate the ball such, that the opponent is unable to properly return a shot, thereby losing points. The force with which a ball can be hit obviously plays an important part in the manipulation of the ball and such force depends entirely upon the speed with which a racquet can be moved through the air. A racquet is an extension of hand and arm of a player and encounters a great amount of drag during the motion of swing. Such drag limits the speed with which a racquet can be moved, in turn limiting the amount of force a ball can be hit with. It is therefore quite obvious that it would be most desirous to have available a racquet frame with reduced drag, with which a player would be able to hit a ball with more force. The object of the present invention is to provide a racquet frame which noticeably reduces the amount of drag it normally encounters during the motion of swing, thereby increasing the speed a racquet can be moved with, and hence increasing the force a ball can be hit with.

SUMMARY OF THE INVENTION

The scope of the present invention generally embraces all types of racquets, consisting of a racquet frame and tensioned string, used in a variety of racquet sports and games.

A racquet frame and its string webbing experiences drag forces inherent in flow mechanics of all bodies moving through air. Such drag forces divide into two basic forms. Firstly, drag resulting from a slow-down or non-slip condition of air moving adjacent a surface of a body, usually referred to as frictional drag or positive surface pressure, and secondly, drag resulting from a low pressure condition existing at the rear of a body moving though air, usually referred to as negative surface pressure or base drag.

The present invention deals with the effects of negative surface pressure or base drag and the means to provide significant reduction of such base drag as experienced by all racquet frames during the motion of swing.

In accordance with the present invention, the surface of the frame and string of such racquet is modified to include a surface structure which increases operating efficiency during the motion of swing, by altering fluid flow behaviour across such racquet frame and string surface, reducing or eliminating, as the case may be, negative surface pressure or base drag.

The present invention therefore comprises a racquet frame, including its tensioned string webbing, which includes in its surface a structure in the form of a pattern of distinct surface depressions, such as rows of dimples or grooves.

Such structure covers at least part of a racquet frame and all of its string surface and preferably consists of, in the case of dimpled shapes, a pattern of generally spheroidal depressions, and in the case of grooves, a pattern of generally channel-like depressions, which, in order to be fully effective, must be arranged at 90° to the mean flow direction experienced by a racquet frame surface during the motion of swing.

When employing dimples in the racquet frame and string surface, generally spherical shapes are especially preferred, although oblate and prolate shapes, as well as cylindrical, pyramidal, cubic and multi-faceted, as well as irregularly shaped depressions are also contemplated.

When employing grooves in the racquet frame surface, generally straight channels, with a radius floor, and parallel sidewalls perpendicular to the adjacent land area, are especially preferred, although flat or irregularly shaped floor channels, as well as channels with asymmetrical sidewalls, which may be arranged other than parallel to each other, or, for that matter, other than parallel in relation to adjacent channels, if a pattern consists of more than one channel, are also contemplated.

When employing grooves in the surface of a racquet string, care must be taken, that a ratio of land area to grooves is maintained of at least 50% to 50%, that is 50% land area versus 50% channel area. Further care must be taken that a sharp edge is maintained where the channel walls meet the remaining land area, so as to cause a shearing effect to adjacent fluid flows, without any deviation in flow direction away from the general mean flow direction, as occurring across the surface of the string during ah operational motion.

However, dimples or discrete symmetrical depressions have the added advantage of dealing uniformly with changing flow directions and operate independently of the mean flow direction across the surface of a racquet frame and string webbing equipped with such a structure.

The structure, consisting of a pattern of depressions as herebefore described may be included in the surface of a racquet frame and string webbing as part of its original forming or moulding process, or may be attached to, or wrought into, or included in the frame or string surface as part of a later or separate manufacturing phase.

When viewing various racquet frame and string manufacturing methods, it becomes obvious that at times it might be more advantageous to consider a racquet frame or string surface structure which consists of distinct surface protrusions rather than surface depressions. Surface protrusions however add to the bulk and dimension of a racquet frame or string webbing, and therefore reduce aerodynamic performance to a certain degree, while only adding limited base drag improvements.

Nevertheless, a certain level of performance enhancement may be achieved using surface protrusions, and therefore, a surface structure consisting of protrusions rather than depressions may be a desirable alternative for some racquet frame or string manufacturing methods.

Such surface protrusions, in order to be at all effective, must be of a height not to protrude much beyond the unseparated boundary layer, or its first adjacent laminar flow layer, proximal the racquet frame or string surface over which they protrude, otherwise surface drag as well as base drag will be increased.

Similar to surface depressions, surface protrusions may be employed in the form of patterns of discrete protrusions or reverse dimples, or in the form of ridges or reverse channels, in all the variations and applications as previously described.

While not wishing to be necessarily bound to any particular theory of operation, it is believed, and this belief forms the basis for at least one aspect of the present invention, that reduced base drag can be realized by changing fluid flow over the surface of a racquet frame and string webbing, through including in such racquet frame and string surface a certain surface structure. Such surface structure must be operable to cause interruptions in the unseparated boundary layer flow proximal the racquet frame or string surface, along the mean direction of flow, changing surface frictional qualities with the direction of the adjacent without interfering with the direction of the adjacent laminar flow conditions, which reduce or eliminate, as the case may be, base drag normally occuring behind a racquet frame and string during the motion of swing.

Best results may be achieved if the surface structure according to the present invention is located in the surface of a racquet frame between two points along the mean direction of flow across it, in which a point upstream of the flow across the surface of a racquet frame is located more or less ahead of the line where surface pressure changes from positive to neutral, and downstream of the flow across the surface of a racquet frame where a point is located more or less beyond the line where surface pressure changes from neutral to negative.

However, in order to allow for the large number of variances in flow directions occurring over a racquet frame, due to the many different cambers and attitudes with which a racquet may be swung with, locations of patterns may deviate drastically from the described best fit location. In general one should observe the rule that a surface structure according to the present invention is only of benefit to reduce base drag, if such structure is located in surface areas where, during relative motion, fluid pressure across the surface exceeds fluid pressure against the surface.

When applying the surface structure to a racquet string surface, it is more advantageous to include a pattern in the total surface area of such string, both from a manufacturing aspect, as well as when considering placement of string into a racquet frame to form the tensioned string webbing.

A number of tests were conducted to measure the

SUBSTITUTE SHEET advantage available through the use of the surface structure on racquet frames and strings as disclosed herein, and a comparison was made between modified and unmodified racquet frames and frame tube shapes. For that purpose a back-to-back testing technique was chosen so as to ensure that identical conditions applied to both test samples and to provide reliable data. This method supplied measurements of the difference in drag between two test samples simultaneously.

The two test samples were mounted on levers of the test apparatus, side by side, and each lever was attached to a diaphragm unit, which in turn was connected to each side of a "U" tube manometer. This system supplied measurements of the difference in aerodynamic loads on the two samples under test in a scale of centimeters of manometer fluid (speci ic gravity of 1.8). The various velocities at which pressure readings were taken were then indicated by the velocity meter of the drag unit. The testing procedure was to first open an isolator valve to supply air to both load cells at rest, record the manometer reading and close the isolator valve. Readings of the pressure differential shown by the manometer where then recorded at selected air velocities to which the test samples were subjected and the recorded data from these tests were as follows.

Test 1.

Two sections of hollow tubing, arranged to represent an oval type tubing portion, in a typical configuration as used in today's tennis racquet frame construction, were mounted side by side. One tubing section was modified to include a surface structure according to the present invention, while the other tubing section remained plain.

The surface modification in this case consisted of a pattern of shallow dimples or discrete depressions of circular configuration, with a diameter of 3.5 mm and a dept of 1.5 mm. The pattern of depressions allowed for a land area versus dimpled area of 30% land area to 70% dimpled area. The manometer high column readings (lowest pressure- representing the drag reduction) were as follows:

Air velocity in Kmph 30 £0 M 2_2_Q.

Mean readings of high column in centimetres 2 cm 5 cm 8 cm 10 cm

Test 2.

As a back-up procedure, each tubing piece, treated and untreated, was again tested individually mounted on the drag test unit, with the attached load cell connected to one side only of the "U" manometer with the other side open, and following high column readings were recorded:

Air Velocity in Kmph .30 60. .90 120

Tube section plain 5.5 cm 16.5 cm 32.5 cm 45.0 cm

Tube section modified 3.5 cm 12.0 cm 23.5 cm 34.5 cm

Drag reduction difference 2.0 cm 4.5 cm 9.0 cm 10.5 cm Test 3.

This test was performed using actual tennis racquet frames. These frames were mounted on the drag test unit and this time the manometer was replaced with an electric load cell, which recorded aerodynamic pressures in gramms.

The modified test racquet frame used here included in the surface of its frame head and throat area rows of grooves. Such grooves were of a width of 1.2 mm and of a dept of 1.2 mm and were located at a distance of 1.2 mm apart, parallel to each other and parallel to the edge of the frame. This allowed for the placement of three rows of grooves on each side of the rows of string insert holes on both the inside as well as the outside of the frame head area, and six rows of grooves on the outside of the throat area of the frame, extending to the handle portion.

These test results presented drag reduction measurements which calculated into drag reduction percentages for the modified racquet frame over the unmodified frame as follows:

Air velocity in Kmph 20 0 £0 50 60 90

Percentage of drag reduction

36% 30% 9% 7% 6% 4%

These tests certainly demonstrate that significant improvements may be obtained as a result of the use of a surface structure according to the present invention.

For a better understanding of the invention and its operating advantages, reference should be had to the drawings and descriptive matter in which there are

SUBSTITUTE SHEET illustrated and described the preferred embodiments of the invention. INTRODUCTION OF THE DRAWINGS

Figure 1 of the drawings appended hereto depicts a preferred embodiment of the present invention, comprising a portion of a typical racquet, in isometric view.

Figure 2 of the drawings depicts an isometric side view of a portion of the frame of a racquet.

Figure 3 of the drawings depicts a portion of string webbing of a racquet, in front view.

Figure 4 of the drawings depicts a section view through strings, as shown in figure 3 of the drawings.

Figure 5 of the drawings depicts a section view through a racquet frame with its modified exterior surface.

Figure 6 of the drawings depicts a typical surface area of either frame or string surface of a racquet, illustrating a variety of exterior surface structure shape.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to Figure 1 of the drawings, there is shown a portion of a typical racquet, comprising part of the frame (1) and its strings (2), including in the surface (3) of such frame a surface structure in the form of dimples (4), grooves (5) and diamond-shaped depressions (6), all illustrated in isometric view. Such surface structures cover at least a portion of the racquet frame surface and preferably all of the string surface.

Figure 2 of the drawings depicts a portion of a racquet frame in isometric view. This view illustrates surface structures in the surface (3) of such frame portion, depicting structure patterns consisting of a combination of depressions and protrusions in a cross- weave pattern (7), a pattern of triangular depressions (8), a pattern of square depressions (9), and a pattern

SUBSTITUTE SHEET of grooves or channels (10), all arranged in rows, adjacent the frame's string insert channel (11), which, if a racquet frame is equipped with such a feature, is located within the centre of the outside frame portion, which forms the annular frame opening.

A racquet frame may not be equipped with the customary string insert channel and the coresponding string grommet inserts, in which case the pattern of surface structures employed would be designed to accomodate the location of string insert holes and their connecting bridges, as may be viewed in illustrations of Figure 2 and 5, showing number (15) located at the inside frame portion.

On the inside frame portion, forming the annular frame opening, are illustrated alternative surface structures in the form of a pattern combining V-grooves (12), and discrete spherical depressions or dimples (4), and a pattern placing a wide groove with a convex radius floor (13) adjacent a narrow groove with a concave radius floor (14), all arranged in rows adjacent the row of holes (15), through which the string is located to form the string webbing of the racquet.

All such rows of grooves, channels, discrete depressions and dimples must always be arranged at a more or less 90° angle to the direction of flow to which they are subjected, or, in other words, be located more or less parallel with the leading edge or trailing edge of the racquet frame.

A variety of surface structures are included herewith for the purpose of demonstrating the principle and effectiveness of the present invention, but, it will nevertheless be understood, that no limitation of the scope of the invention is thereby intended, such further modifications and alterations in the illustrated surface structure, and such further applications of the principle of the invention as illustrated therein being contemplated as would

SUBSTITUTE SHEET normally occur to one skilled in the art to which the invention relates.

Figure 3 of the drawings depicts a portion of a typical string webbing of a racquet, comprising strings (2a) and (2b), arranged into a webbing, thereby creating the surface which makes contact with the ball.

The string type preferred to incorporate the present invention is herein shown as a multi-filament string, usually consisting of an inner core string, which is surrounded by layers of finer string, or whiskers (16), arranged in a spiral fashion around such inner core string, to give a certain amount of elasticity as shown in string illustration (2a), or it may be surrounded by layers of finer string or whiskers, arranged in the outer most layer as a netting (17), again to give a certain amount of elasticity, as shown in string illustration (2b).

Such core string and its surrounding whiskers are all imbedded in a resin-type material which is then covered with an outer level of resin coating (18). This resin coating is the final surface which incorporates the desired surface structure, and, in the case of string (2a), will receive its grooves or channels (5) by way of an extrusion process or by heat impression, while the surface structure in string (2b) will be heat impressed into the string surface coating, placing the pattern of dimples (4) precisely within the design pattern of the netting (19).

This type of string construction is much preferred over the mono-filament construction, as a multi-filament string incorporates the much sought- after string elasticity and elasticity memory, which provides a modern string with the effect and feel closest to the original cat-gut string, still used sometimes in top of the line racquets.

Figure 4 of the drawings depicts a view through string (2a and 2b) . Such section view shows the inner

SHEET core string (21) surrounded by whiskers (16) and/or (17), all imbedded in a resin compound (22), which is covered by the outer layer of resin skin (18), into which the surface structures according to the present invention are impressed or extruded.

The dimple shape (4) adjacent its land area (23) shows the preferred ratio of 80% for dimples and 20% for land area, while the groove or channel shape (5) adjacent its land area (24) shows the preferred ratio of 50% for the groove and 50% for its land area. The location (25), at which a groove sidewall meets the land area should consist of a sharp and precise edge, especially for lower velocity applications.

Locations (26) and (27) illustrate a surface structure consisting of protrusions rather than depressions, which must not exceed a height of 10% of the diameter of the string over which they protrude (28) .

Figure 5 of the drawings depicts a view through a racquet frame. Such section view through a typical moulded racquet frame tube (1), with its string insert channel (11) and string holes (15), shows a variety of surface structures and illustrating their methods of application. Both discrete depressions (4) and grooves (5) are moulded into the surface area as part of the racquet frame's cast moulding process, and the shape of the depressions in the exterior frame surface are therefore communicated to the interior surface of the racquet frame tube, as viewed at (4a) and (5a). Depressions (4b - 5b), which may represent dimples or grooves, have been wrought into the surface at a later manufacturing phase and the thickness of the frame tube wall (20) has been increased above its average thickness during the original moulding process to allow for the reduction of material when carving out such depressions.

Frame wall area (29) is of average thickness, and has protrusions (30) added to its exterior surface, to

SUBSTITUTE SHEET. form a surface structure according to the present invention, but as a separate phase, and included as part of a later manufacturing stage independently from the moulding process. Such protrusions could be attached to the racquet frame exterior surface in the form of individual pieces or as part of a patterned, embossed surface covering.

As previously described, the surface structure according to the present invention should never reach into the high pressure area of a racquet frame surface manifested during the motion of swing, as this would be completely counter-productive, and as a racquet is normally used in a swing motion in either direction, the high pressure area during one swing motion direction would therefore relocate to the opposite side of the frame during a reverse swing motion, or when a racquet is actually turned 180° in the player's hand.

A high pressure area at a racquet frame exterior surface is therefore defined as the area, where, during the motion of the frame in the direction as indicated by arrow (31), fluid flow attack would be anywhere between 90° (32) or 45 degrees (33) to the surface of the racquet frame. The pressure point (33), where fluid attack occurs at 45 degrees, is a variable pressure line location, which will shift somewhat in either direction, that is to say, it will locate at a point higher or lower than 45°, depending on racquet swing velocity and racquet attitude during swing motion. Figure 6 of the drawings depicts a view of the surface of either frame or string of a racquet, illustrating surface structures in a variety of shapes of depressions in section view, identified as a discrete depression or groove with asymmetrical side walls (34), a discrete shape or groove consisting of a combination of depression and protrusion (35), a

SUBSTITUTE SHEET. variation of a discrete shape or groove consisting of a combination of depression and protrustion (36) and protrusions (37) and (38), which could represent either a discrete protrusion or reverse channels.

Claims

I Claim:

1) An improved tennis racquet, adapted for increased swing speed, comprising: a frame having a handle portion connecting to a head portion, said head portion being generally ring-shaped, providing an opening wherein a tensioned webbing, formed from a plurality of strings, is mounted; whereby a surface structure is provided on at least a portion of the exterior frame surface of said racquet, consisting of a pattern of presized and preselected discontinuities in such a pattern, so as to alter the flow behaviour of a fluid moving adjacent the surface of said frame during the motion of swing and so as to reduce the base drag therebehind.

2) A surface structure according to Claim 1, wherein said discontinuities comprise a plurality of depressions extending below the surface of said frame.

3) A surface structure according to Claim 1, wherein said discontinuities comprise a plurality of protrusions extending above the surface of said frame.

4) A surf ce structure according to Claim 1, wherein said discontinuities comprise a combination of depressions and protrusions.

5) A surface structure according to Claim 1, wherein said discontinuities are provided on the exterior surface of the string webbing mounted in the racquet frame.

6) Depressions according to Claim 2, which consist of discrete indentations of symmetrical and asymmetrical shapes.

7) Protrusions according to Claim 3, which consist of discrete rises of symmetrical and asymmetrical shape.

8) A string webbing according to Claim 5, wherein the string is of multi-filament construction. 9) A string webbing according to Claim 8, wherein the string includes in its surface a pattern of discrete surface depressions.

10) A string webbing according to Claim 9, wherein the string is of mono-filament construction.

11) Depressions according to Claim 2, wherein the plurality of depressions are evenly spaced within the frame surface area.

12) Depressions according to Claim 2, wherein the plurality of depressions are unevenly spaced within the frame surface area.

13) Discontinuities according to Claim 3, wherein the plurality of protrusions extend only past the boundary layer flow region and into the laminar layer flow region of the frame surface, to a height sufficient to effect adjacent fluid flow behaviour but without disrupting laminar flow conditions.

14) Protrusions according to Claim 13, wherein the plurality of protrusions are arranged in an evenly spaced pattern.

15) Protrusions according to Claim 13, wherein the plurality of protrusions are arranged in an unevenly spaced pattern.

16) Depressions according to Claim 9, which consist of a pattern of elongated depressions or grooves in a parallel pattern.

17) Elongated depressions according to Claim 16, which occupy less than 51% of the total surface area to which they are applied.

18) Elongated depressions according to Claim 17, which have a depth of not more than 25% of their width.

19) Depressions according to Claim 2, which are arranged in a pattern of rows located more or less parallel with the frame's leading or trailing edge.

20) Protrusions according to Claim 3, which consist of elongated ridges in a parallel pattern.

21) . Depressions according to Claim 2, wherein the depressions consist of elongated grooves. 22) Elongated depressions according to Claim 21, which are arranged in a parallel pattern.

23) Elongated depressions according to Claim 22, which are arranged in a pattern of rows located more or less parallel with the frame's leading or trailing edge.

24) Depressions according to Claim 6, which comprise concave dimples in spheroidal, prolate and oblate shapes as well as triangular, square and diamond shape.

25) Protrusions according to Claim 7, which comprise convex rises in spheroidal, prolate and oblate shapes as well as triangular, square and diamond shapes.

26) A surface structure according to Claim 4, in which the combination of rises and depressions form a woven pattern or a lace pattern.

27) A surface structure according to Claim 4, in which the pattern of rises and depressions comprise symmetrical and asymmetrical shapes.

28) Protrusions according to Claim 13, which extend above the surface area by not more than 0.5 mm.

29) Protrusions according to Claim 13, which are applied to the frame surface by way of a pre-fabricated film.

30) Depressions according to Claim 2, wherein the floor of a depression is concave.

31) Depressions according to Claim 2, wherein the floor of a depression is flat.

32) Depressions according to Claim 2, wherein the floor of a depression is convex.

SUBSTITUTE SHEET