CHARACTERIZATION OF A BALL GAME RACKET FRAME

Abstract

The present invention relates to a method for characterizing a racket head of a ball game racket frame as well as to the representation of a racket head.

Claims

1. A method for characterizing a racket head (1) of a ball game racket frame comprising the steps of: (a) providing data of a ball game racket frame that describe the contour of the racket head of the ball game racket frame; (b) automatically determining one or more of the following parameters characterizing the racket head shape on the basis of the contour data: measure of the length-width asymmetry of the racket head, measure of the deviation of the racket head shape from a circular shape, evolute of the racket head contour; (c) outputting and/or graphically representing the determined parameter or parameters.

2. The method according to claim 1, wherein in step (b) a measure of the length-width asymmetry of the racket head is determined by: (i) identifying the longitudinal diagonal (2) through the racket head contour having the greatest length; (ii) identifying the transverse diagonal (3) through the racket head contour having the greatest width; (iii) determining the point of intersection (4) of the longitudinal diagonal (2) with the transverse diagonal (3); and (iv) determining the offset between the established point of intersection (4) and the midpoint (5) of the longitudinal diagonal (2).

3. The method according to claim 1, wherein in step (b), a measure of the length-width asymmetry of the racket head is determined by: (i) identifying the longitudinal diagonal (2) through the racket head contour having the greatest length; (ii) identifying the transverse diagonal (3) through the racket head contour having the greatest width; (iii) determining the ratio between the length of the longitudinal diagonal (2) and the length of the transverse diagonal (3).

4. The method according to claim 1, wherein in step (b) a measure of the deviation of the racket head shape from a circular shape is determined by: (i) identifying the longitudinal diagonal (2) through the racket head contour having the greatest length; (ii) identifying the transverse diagonal (3) through the racket head contour having the greatest width; (iii) establishing the point of intersection (4) of the longitudinal diagonal (2) with the transverse diagonal (3); and (iv) defining a circle (6) around the established point of intersection (4) having a diameter smaller than the length of the transverse diagonal (3).

5. The method according to claim 4, wherein the measure of the deviation of the racket head shape from a circular shape is further determined by: (v) defining a plurality of racket head contour segments (8), each extending from the circle (6) to the racket head contour and bounded by an arc of the circle, a portion of the racket head contour and two sides (7) extending in a radial direction with respect to the point of intersection (4), wherein the angle between the two sides of a racket head contour segment (8) is the same for all racket head contour segments (8); and (vi) determining the length of the sides (7) of the racket head contour segments (8) and/or the area of the racket head contour segments (8).

6. The method according to claim 4, wherein step (c) comprises the graphical representation of the racket head contour and the defined circle (6).

7. The method according to claim 5, wherein step (c) comprises the graphical representation of the racket head contour segments (8).

8. The method according to claim 5, further comprising a classification of the racket head contour segments (8) according to the determined length and/or area.

9. The method according to claim 8, wherein the classification is graphically visualized in the representation, preferably by assigning one or more of the following graphical parameters to predetermined classes of the classification: color value, tonal value, hatching.

10. The method according to claim 1, wherein in step (b) the evolute of the racket head contour is determined by: (i) identifying the normal vectors (9) to a plurality of points along the racket head contour, wherein the end point (10) of each normal vector (9) corresponds to the center of the associated circle of curvature; and (ii) optionally connecting the end points (10) to form a curve.

11. The method according to claim 10, wherein step (c) comprises the graphical representation of the racket head contour and the evolute.

12. The method according to claim 1, wherein the data of the ball game racket frame comprise CAD data of the contour, preferably the inner contour, of the racket head.

13. The method according to claim 1, wherein step (a) comprises: (i) creating an image of the racket head of a ball game racket frame with a viewing axis perpendicular to the string bed plane; and (ii) automatically determining the contour, preferably the inner contour, of the racket head from the image.

14. The method according to claim 13, wherein step (ii) comprises: (1) converting the image of the racket head into a binary pixel image, wherein the threshold value is selected such that a first value is assigned to each of the pixels of the frame and a second value is assigned to each of all other pixels; and (2) identifying the frame pixels and determining the racket head contour.

15. The method according to claim 1, comprising the additional step of: (d) determining the playing characteristics of the ball game racket frame by means of the determined parameter or parameters.

16. The method according to claim 1, comprising the additional step of: (e) determining at least one alternative racket head which approximates the playing characteristics of the characterized racket head with regard to the determined parameter or parameters.

17. The method according to claim 1, comprising the additional steps of: (f) repeating the method for at least one further ball game racket frame; (g) comparing the determined parameters and optionally the playing characteristics according to step (d) of claim 15.

18. A representation of a racket head of a ball game racket frame, comprising a graphical representation of the racket head of the ball game racket frame, optionally comprising a representation of at least a portion of the string bed, and one or more of the following parameters characterizing the racket head shape: measure of the length-width asymmetry of the racket head, measure of the deviation of the racket head shape from a circular shape, evolute of the racket head contour.

19. The representation of a racket head according to claim 18, further comprising a graphical representation of the racket head contour segments, preferably with graphical visualization of a classification of the racket head contour segments according to the determined length and/or area, preferably by assigning one or more of the following graphical parameters to predetermined classes of the classification: color value, tonal value, hatching.

20. A set comprising a ball game racket frame and a representation of a racket head according to claim 18.

Description

[0028] In the following, preferred embodiments of the present invention are explained in more detail with reference to the Figures, in which:

[0029] FIG. 1 shows a top view of a tennis racket head comprising various parameters according to the invention for characterizing the racket head shape;

[0030] FIG. 2A shows a top view of the tennis racket “Babolat Pure Strike 16×19”;

[0031] FIG. 2B shows a top view according to FIG. 2A including the racket head contour segments according to the invention;

[0032] FIG. 3A shows a top view of the tennis racket “HEAD Pyramid Tour 630”;

[0033] FIG. 3B shows a top view according to FIG. 3A including the racket head contour segments according to the invention;

[0034] FIG. 4A shows a top view of the tennis racket “Head Graphene Touch Radical MP”;

[0035] FIG. 4B shows a top view according to FIG. 4A including the racket head contour segments according to the invention;

[0036] FIG. 5A shows a top view of the tennis racket “Völkl 10 PB”;

[0037] FIG. 5B shows a top view according to FIG. 5A including the racket head contour segments according to the invention;

[0038] FIG. 6A shows a top view of the tennis racket “Wilson Blade 98 V7”;

[0039] FIG. 6B shows a top view according to FIG. 6A including the racket head contour segments according to the invention;

[0040] FIG. 7A shows a top view of the tennis racket “Wilson Burn FST 99”;

[0041] FIG. 7B shows a top view according to FIG. 7A including the racket head contour segments according to the invention;

[0042] FIG. 8A shows a top view of the tennis racket “Wilson Ultra Tour 97”;

[0043] FIG. 8B shows a top view according to FIG. 8A including the racket head contour segments according to the invention;

[0044] FIG. 9A shows a top view of the tennis racket “Yonex Vcore PRO 97”;

[0045] FIG. 9B shows a top view according to FIG. 9A including the racket head contour segments according to the invention;

[0046] FIG. 10A shows a top view of the tennis racket “Yonex Vcore SV 98”;

[0047] FIG. 10B shows a top view according to FIG. 10A including the racket head contour segments according to the invention;

[0048] FIG. 11 schematically shows how the evolute according to the invention is established;

[0049] FIG. 12 shows the evolute established according to the invention for the tennis racket “HEAD Graphene Touch Radical MP 2”; and

[0050] FIG. 13 shows the evolute determined according to the invention for the tennis racket “Yonex Ezone DR100”.

[0051] FIG. 1 shows a top view of the racket head 1 of a tennis racket. The longitudinal diagonal 2 through the racket head contour with the greatest length and the transverse diagonal 3 through the racket head contour with the greatest width are also shown in the image of the racket head 1. The longitudinal diagonal 2 and the transverse diagonal 3 intersect at the point of intersection 4. Furthermore, the midpoint 5 of the longitudinal diagonal 2 is identified. The offset between the point of intersection 4 and the midpoint 5 of the longitudinal diagonal 2 (“center offset”) was determined to be 9.33 mm in the example shown.

[0052] Furthermore, in FIG. 1 a circle 6 having a diameter smaller than the length of the transverse diagonal 3 is depicted around the established point of intersection 4. As is readily apparent from FIG. 1, this graphically visualized circle 6 clearly illustrates the deviation of the racket head shape from a circular shape.

[0053] In order to better quantify this deviation, a plurality of racket head contour segments 8 can be defined according to the invention, each of which extends from the circle 6 to the racket head contour and is bounded by an arc of the circle, a portion of the racket head contour and two sides 7 extending in a radial direction with respect to the point of intersection 4, wherein the angle between the two sides 7 of a racket head contour segment 8 is the same for all racket head contour segments. In the example according to FIG. 1, this angle is 15°. If the length of the sides 7 is now established for each racket head contour segment 8, this length is a quantitative measure of the deviation of the racket head shape from a circular shape.

[0054] According to the invention, the individual racket head contour segments 8 can be classified on the basis of the determined length of the sides 7. This classification can then also be graphically visualized, for example by coloring the sides 7 of the racket head contour segments 8. This coloring was carried out in the example of FIG. 1, but can only be seen to some extent due to the black-and-white representation. It should be clear, however, that coloring using an appropriately selected color spectrum can clearly visualize for the viewer the measure of the deviation of the racket head shape from a circular shape as discussed here.

[0055] Instead of the length of the sides 7 of the racket head contour segments 8, their area can also be determined. The racket head contour segments 8 can then be classified and colored according to the determined area, for example.

[0056] In FIGS. 2B to 10B, corresponding classifications for the racket head shapes according to FIGS. 2A to 10A are visualized by means of different shades of gray, each indicating the area of the racket head contour segments. The scaling can be seen on the right side of each of FIGS. 2B to 10B, according to which the shades of gray from white to black classify the following area ranges (each measured in cm.sup.2): <3.0-3.0; 3.0-4.5; 4.5-5.9; 5.9-7.4; 7.4-8.9; 8.9-10.3; 10.3-11.8; 11.8-13.3; 13.3-14.7; 14.7-16.2; 16.2-17.7; 17.7-19.1; 19.1-20.6; 20.6-22.1; 22.1-23.5; 23.5-25.0; 25.0->25.0.

[0057] As revealed by a comparison of FIGS. 2A to 10A, the individual racket head shapes are in some cases visually hard to distinguish from one another. In fact, however, the differences with respect to the parameters according to the invention are considerable, as is apparent from FIGS. 2B to 10B. Since these differences also have a corresponding influence on the playing characteristics of the corresponding ball game racket, the objective and quantifiable characterization made possible according to the invention on the basis of the parameters according to the invention is of high value for the player.

[0058] From the point of view of maximum error forgiveness, each player will try to hit the ball wherever possible at the point of maximum racket width. However, as Brody points out (cf. H. Brody, Medicine and Science in Tennis, vol. 8, no. 1, April 2003), the point of “maximum ball acceleration” varies depending on the kinematics of the swing or stroke as well as depending on the relationship between the velocities of the incoming ball and the struck ball. For example, a volley, in which there is a strong translational movement of the racket and virtually no rotational component and in which the speed of the ball is significantly higher than the speed of the racket, should ideally be hit rather in the lower area of the hitting surface in order to achieve both good acceleration and low rotation around the center of gravity due to the contact with the ball. In contrast, a forehand topspin stroke performed in a modern manner, in which strong acceleration is achieved primarily through the use of the wrist as the last link in the kinematic chain and the stroke movement therefore contains a strong rotational component, should be hit wherever possible in the upper area to achieve maximum ball acceleration.

[0059] Thus, for a player who acts a lot from the baseline with strokes that have a high proportion of rotational movement, a racket exhibiting a large center offset, such as the racket in FIG. 3B (HEAD Pyramid Tour 630) or the racket in FIG. 7B (Wilson Burn FST 99), would be particularly suitable. In contrast, for a player who plays a lot of volleys, a racket exhibiting a small or even negative center offset, such as the racket in FIG. 5B (Völkl 10 PB) or the racket in FIG. 9B (Yonex VCore PRO 97), would be particularly suitable.

[0060] What is noticeable here is that optically the two rackets in FIG. 3B (HEAD Pyramid Tour 630) and FIG. 9B (Yonex VCore PRO 97) do show certain similarities. Both would be referred to as teardrop head shape in current usage. However, when determining the respective center offset, these values are extremely different, namely 44.71 mm and 5.32 mm. Visually, by looking at the racket head, this value of the center offset can thus be determined only with great difficulty or insufficiently.

[0061] By using the racket head contour segments, the differences can also be seen very clearly when comparing the two head shapes.

[0062] Even the basic strokes can be performed in various different ways. For example, some players play rather “smoothly”, i.e., without generating a lot of ball spin. This is expressed in a stroke movement in which the racket is swung relatively horizontally through the ball. Since in this case the racket movement is relatively similar to the direction of motion of the incoming ball (only directly opposite), the risk of hits outside the longitudinal axis of the racket is rather low in this case. Therefore, these players typically prefer a racket with a low ratio of width to length. This style of play and the associated rackets were particularly strongly represented in the 1990s, and the racket in FIG. 5B (Völkl 10 PB) is still a representative of this category.

[0063] Meanwhile, most players play their basic strokes with a lot of spin generation by accelerating the racket in contact with the ball not only horizontally but also strongly vertically (mainly by forearm rotation or the wrist as the last link in the kinematic chain). The players practically hit or wipe past the ball at the point of impact and thus achieve a high spin generation. Naturally, when the movement is performed in such a way, i.e. when the direction of movement of the racket in contact with the ball is clearly different from the direction of movement of the incoming ball, there is a greater risk of hitting the ball not on the longitudinal axis of the racket but more to the left or right of the longitudinal axis of the racket (off-center). In the case of extreme forms of these strokes, even professional players can be seen time and again hitting the ball with the racket frame, i.e. the ball is hit on the racket so far to the side that the frame is “in the way”. For these types of players, it is thus clearly more important to have a larger width-to-length ratio of the racket head, as is shown, e.g., by the racket in FIG. 9B (Yonex VCore PRO 97).

[0064] Hence, depending on the movement kinematics with which the player hits, with which tactics he/she plays (a lot of volleys or many basic strokes) and with how much spin he/she plays, very different ideal racket head shapes can result for a plurality of players having the same playing strength, but so far the differences between these racket head shapes can only be insufficiently described with the known methods and data.

[0065] Additionally or alternatively, the evolute of the racket head shape can be established according to the invention, as already mentioned. This is schematically illustrated in FIG. 11. Accordingly, the normal vectors 9 to a plurality of points along the racket head contour of the racket head 1 are identified, wherein the end point 10 of each normal vector 9 corresponds to the center of the associated circle of curvature. The plurality of end points of the normal vectors then form the evolute according to the invention, wherein this plurality of end points can optionally also be connected to form a curve.

[0066] In FIGS. 12 and 13, such evolutes are shown together with the racket head contours for the tennis rackets “HEAD Graphene Touch Radical MP 2” and “Yonex Ezone DR100”, respectively. Alternatively or additionally to the actual evolute, the length of each normal vector (i.e., the radius of the respective associated circle of curvature) as a function of the corresponding angular position on the racket head contour can also be represented as a curve, as is the case at the bottom of each of FIGS. 12 and 13.

[0067] Even though the two racket head contours in FIGS. 12 and 13 reveal to the naked eye that the two rackets have different head shapes, it should be clear that the graphical representations of the evolutes allow far more precise and quantitative conclusions to be drawn.