This invention relates to a dynamically balanced inflatable ovoid sports ball, and to a dynamically balanced inflatable bladder for an ovoid sports ball.

The term "ovoid" is used herein to refer to balls and bladders having principal axes x, y and z, where: x is a polar axis, and y and z are principal equatorial axes; x, y and z are mutually perpendicular and coincide at an origin o at the centre of geometry of the ball or bladder; an equatorial cross-section of the ball or bladder is generally circular; and the length of the ball along its polar axis x is greater than its equatorial diameter along either principal equatorial axis x or y. The term should be construed also to encompass prolate spheroids and ellipsoids.

Inflatable ovoid sports balls are used in the sports of rugby union, rugby league, American football, Canadian football and Australian rules football. Conventional ovoid balls are generally constructed with a bladder, inflatable via a single air inlet valve, and a stitched or moulded outer shell covering the bladder, with an aperture to permit access to the valve. In the case of American and Canadian footballs, Australian rules footballs and some Japanese rugby balls, the outer shell may also include lacing.

The presence of the single valve, which is generally located on the equatorial circumference of the ball, results in the ball having eccentric mass distribution, with the centre of mass of the ball being displaced relative to the ball's centre of geometry. This displacement of the centre of mass relative to the centre of geometry gives rise to a number of perceivable effects when the ball is in motion.

The first such effect is that the trajectory of the ball in flight, for example when kicked or thrown, cannot accurately be controlled. A second perceivable effect of the eccentric mass distribution of a conventional ovoid ball is that the ball's bounce characteristics (degree and orientation) following impact with the ground or a player, can also not be predicted with any degree of accuracy. A further perceivable effect attributable to the presence of a single valve is that the ball exhibits unstable rotation. This is perceivable as a "wobble" when the ball is in flight, caused by the valve twisting from side to side. This increases the tilt angle, causing increased drag and affects players' perception of the flight of the ball. A skilled player will attempt to compensate for these effects, or to use them to his advantage, by applying spin to the ball, with the degree and orientation of the spin being adjusted to take into account the position and orientation of the valve. Nevertheless, these and other factors combine to make the motion of an ovoid sports ball somewhat erratic.

Previous attempts to address this issue have involved adding an additional mass element at a point on the equatorial circumference, diametrically opposite the air inlet valve. By balancing the additional mass element exactly with the mass of the air inlet valve this has the effect of re-aligning the ball's centre of mass with its centre of geometry at the origin o. This modification is referred to herein as a statically balanced ball.

The static balancing of an ovoid ball serves partially to address the first and second perceivable effects described above, namely the inaccuracy of the ball's trajectory and the unpredictability of its bounce characteristics. However, the third perceivable effect, namely the instability of the ball's rotation, not only remains unaddressed by this modification, but in fact has been shown to be exacerbated by it.

The alternative approach of simply reducing the mass of the valve serves to reduce the perceivable effects - but only up to a point. The limitations of this approach are defined by the minimum mass to which a commercially viable valve can be produced. The minimum mass of a commercially available valve for a match rugby ball is presently around 12g - and at this level the three effects described above still present a technical problem to be solved.

The present invention seeks to address the above issues by providing a dynamically balanced inflatable ovoid sports ball, and a dynamically balanced inflatable bladder for an ovoid sports ball, which ball and bladder exhibit stable rotation about all principal axes, and improved accuracy of trajectory, when compared to conventional ovoid balls, ovoid balls with valves of reduced mass, and the statically balanced ovoid balls of the known prior art. The present invention further seeks to provide a dynamically balanced ball and bladder exhibiting improved bounce predictability characteristics, when compared to conventional ovoid balls, and ovoid balls with valves of reduced mass.

According to a first aspect of the present invention there is provided a dynamically balanced inflatable ovoid sports ball having a polar axis x, a first principal equatorial axis y and a second principal equatorial axis z, said axes x, y, z being mutually perpendicular and coinciding at an origin o at said ball's centre of geometry, wherein said ball comprises an air inlet valve to permit inflation thereof, and at least one additional mass element adapted to counter-balance the mass of said valve, said air inlet valve and each said additional mass element being arranged relative to one another such that:

- the centre of mass of said ball is substantially coincident with the origin o; and

- the difference between the average mass moment of inertia I _yy of said ball when rotated about the first principal equatorial axis y, and the average mass moment of inertia I ₂₂ of said ball when rotated about the second principal equatorial axis z, is less than 3.46% of the value of I _yy .

Preferably, the difference between the average mass moment of inertia I _yy about the first principal equatorial axis y, and the average mass moment of inertia I _zz about the second principal equatorial axis z, is, in increasing order of preference, less than 3%, 2%, 1 % and 0.75% of the value of I _yy . Most preferably, the average mass moment of inertia I _yy about the first principal equatorial axis y, is substantially equal to the average mass moment of inertia I ₂₂ about the second principal equatorial axis z.

The average mass moment of inertia about a principal equatorial axis, is defined herein as the average value, in kgm ² , of a series of measurements taken using an oscillating torsional pendulum mass moment of inertia measurement machine, manufactured by Inertia Dynamics. For each principal equatorial axis, a mass moment of inertia measurement is taken in each of two positions, 180° apart, with each measurement repeated five times. Each such set of measurements is repeated with three like balls (or bladders) and the average value determined over the whole series of measurements.

The development of the present invention stems from the inventor's study of the properties of conventional ovoid balls and the known prior art, and the realisation that the three perceivable effects described above - and in particular the unstable rotation effect - are attributable not only to the eccentric mass distribution but also to the different mass moments of inertia which that eccentric mass distribution generates when the ball is rotated about each of its principal axes. - A -

For any ovoid ball, the mass moment of inertia I _χχ when the ball is rotated about the polar axis x will be the smallest of the mass moments of inertia about the three principal axes, with the mass moments of inertia I _yy and I ₂₂ about the principal equatorial axes y and z, respectively, being somewhat larger. In a theoretical "perfect" ovoid ball having no valve, and therefore concentric mass distribution, the mass moments of inertia I _yy and I ₂₂ will be equal to one another. The inventor has established, via experimental observation, that such a ball would exhibit stable rotation both about the polar axis x, having the smallest mass moment of inertia I _xx and about either of the principal equatorial axes y, z having the largest mass moment of inertia, I _yy , I ₂₂ , respectively (where I _yy = I ₂₂ ). However, such a theoretical model cannot be realised for an inflatable ovoid sports ball, due to the need for a mass- bearing element, the air inlet valve.

In a conventional ovoid sports ball having a single air inlet valve located at a point on the equatorial circumference, the first principal equatorial axis y is taken to intersect the air inlet valve. In such a system, the mass moment of inertia I _yy about the first principal equatorial axis y intersecting the air inlet valve is smaller than the mass moment of inertia I ₂₂ about the second principal equatorial axis z, but is larger than the mass moment of inertia I _χχ about the polar axis x. A single valve system thus has a mass moment of inertia I _yy which is intermediate in magnitude, compared to the mass moments of inertia I _χχ , I ₂₂ about the other two principal axes. By experimental observation, the inventor has determined that it is this principal axis y having the intermediate mass moment of inertia I _yy about which unstable rotation occurs, with the rotation about the principal axes x, z having the smallest and largest mass moments of inertia I _χχ , I ₂₂ , respectively, remaining stable.

For a statically balanced ball as described in the prior art, the first principal equatorial axis y is taken to intersect both the air inlet valve and the dummy valve positioned diametrically opposite the valve. The static balancing of an ovoid ball in this way results in the largest mass moment of inertia I ₂₂ being increased, whilst the intermediate mass moment of inertia I _n , remains constant - or more significantly, results in an increased difference between the mass moments of inertia I _yy , I ₂₂ about the two principal equatorial axes y, z. By experimental observation, the inventor has established that this results in the rotation of the ball about the principal axis y having the intermediate mass moment of inertia I _yy becoming more unstable, whilst rotation about the principal axes x, z having the smallest and largest mass moments of inertia Iχx, Izz, respectively, remains stable.

From the above, the inventor has concluded that in systems in which the mass moments of inertia I _yy , I ₂₁ about the two principal equatorial axes y, z are equal, an ovoid ball exhibits stable rotation about all principal axes. However, in systems in which the mass moments of inertia I _yy , I _zz about the two principal equatorial axes y, z are not equal, an ovoid ball will exhibit unstable rotation about the axis having the smaller mass moment of inertia of the two - this being the intermediate mass moment of inertia of the whole system, given that the mass moment of inertia I _xx about the polar axis will always the smallest mass moment of inertia of the three principal axes. Furthermore, the larger the difference between the mass moments of inertia I _yy , I ₂₂ about the two principal equatorial axes y, z, the greater the instability of the rotation will be. That is to say, the more frequent the change in the orientation of the valve will be, leading to a more severe "wobble" of greater frequency.

In ovoid ball sports, rotation about the polar axis x, known as rifle spin, can be seen during a spin pass or spiral kick. Rotation about the principal equatorial axes y, z, known as tumble axis spin, is most commonly seen during a place kick or drop kick, and it is in these plays that the ball can be seen to exhibit unstable rotation, unless the player is skilful - or lucky - enough to achieve rotation solely about the z axis.

The key to achieving the aim of the present invention in producing a dynamically balanced ball is therefore to reduce, or preferably eliminate, the difference between the mass moments of inertia I _yy , I _zz about the two principal equatorial axes y, z.

In a first embodiment of the present invention, each additional mass element is a dummy valve, having a mass substantially equal to the mass of the air inlet valve. Preferably, the air inlet valve and each dummy valve each have a mass of substantially 12g. Most preferably, three or more dummy valves are provided.

The air inlet valve and each dummy valve are preferably arranged at equally spaced intervals around the equatorial circumference of said ball. In a preferred sub- embodiment, the ball comprises three dummy valves, with the air inlet valve and the three dummy valves being arranged at equally spaced intervals of 90° around the equatorial circumference of the ball. In this sub-embodiment, the first principal equatorial axis y is taken to intersect the air inlet valve and one dummy valve, and the second principal equatorial axis z is taken to intersect the other two dummy valves.

In an alternative embodiment of the present invention, the ball comprises one dummy valve, with the air inlet valve and the dummy valve being arranged at the opposed poles of said ball. In this embodiment, the polar axis x is taken to intersect the air inlet valve and the dummy valve.

In a further alternative embodiment of the present invention, the air inlet valve is arranged on the equatorial circumference of said ball, and the additional mass element is a continuous band extending around the equatorial circumference of said ball. In this embodiment, the first principal equatorial axis y is again taken to intersect the air inlet valve.

In accordance with the general principles of construction of conventional inflatable ovoid sports balls, the dynamically balanced ball of the present invention may comprise an inflatable bladder and an outer shell having an aperture therein adapted to receive the air inlet valve, said valve permitting inflation of said bladder.

Each additional mass element may be incorporated into the inflatable bladder, or alternatively, each additional mass element may be incorporated into the outer shell. For embodiments of the present invention comprising a plurality of additional mass elements, at least one said additional mass element may be incorporated into the inflatable bladder, and at least one said additional mass element may be incorporated into the outer shell.

Preferably however, the dynamic balancing of an inflatable ovoid sports ball is achieved by incorporating a dynamically balanced inflatable bladder into said ball.

Therefore, according to a second aspect of the present invention there is provided a dynamically balanced inflatable bladder for an ovoid sports ball, said bladder having a polar axis x, a first principal equatorial axis y and a second principal equatorial axis z, said axes x, y, z being mutually perpendicular and coinciding at an origin o at said bladder's centre of geometry, wherein said bladder comprises an air inlet valve to permit inflation thereof, and at least one additional mass element adapted to counter-balance the mass of said valve, said air inlet valve and each said additional mass element being arranged relative to one another such that:

- the centre of mass of said bladder is substantially coincident with the origin O] and

- the difference between the average mass moment of inertia I _yy of said bladder when rotated about the first principal equatorial axis y, and the average mass moment of inertia I ₂₂ of said bladder when rotated about the second principal equatorial axis z, is less than 3.46% of the value of I _yy .

Preferably, the difference between the average mass moment of inertia I _yy of said bladder about the first principal equatorial axis y, and the average mass moment of inertia I ₂₂ about the second principal equatorial axis z, is, in increasing order of preference, less than 3%, 2%, 1 % and 0.75% of the value of I _yy . Most preferably, the average mass moment of inertia I _yy about the first principal equatorial axis y is substantially equal to the average mass moment of inertia I ₂₂ about the second principal equatorial axis z.

It will be appreciated that the preferred features and embodiments described above with reference to the dynamically balanced inflatable ovoid sports ball according to the first aspect of the present invention, also apply mutatis mutandis to the dynamically balanced inflatable bladder according to the second aspect of the present invention.

EXAMPLES

The present invention will now be further described, with reference to experimental observations and data.

Example I

A range of prototype inflatable ovoid sports balls having different valve configurations was prepared, as shown in Table I:

Table I

(including dummy valves) Each prototype comprised one air inlet valve, with any additional valves being dummy valves, of equivalent mass. For each prototype, all valves and dummy valves were placed around the equatorial circumference of the ball. Prototype CHSI b represents a close reproduction of a standard rugby union ball such as the Gilbert ^® Synergie. Prototype CHSI a was prepared so as to study the effect of minimising valve mass. Prototype CHSI c was prepared as a reference, having equivalent total valve mass to prototypes CHS2 and CHS4. Prototype CHS2 is a statically balanced ball as hereinbefore described with reference to the prior art. Prototype CHS4 is a dynamically balanced ball according to the present invention.

Three samples of each prototype ball were prepared. The prototype balls were all manually stitched, four panel rugby balls, having similar outer panel shape, material and construction, and differing only with respect to the valve configuration, which was incorporated into the internal bladder.

Example Il

The mass moments of inertia I _xx , I _yy , I ₂₂ about each of the principal axes x, y, z were measured for each of the prototype balls CHS1 a-c, CHS2 and CHS4 prepared in Example I. For each prototype ball, the first principal equatorial axis y is taken to be the axis intersecting the air inlet valve. The measurements were carried out using an oscillating torsional pendulum mass moment of inertia measurement instrument manufactured by Inertia Dynamics, with a custom fitting developed by the inventor to enable the measurement of ovoid balls. The measurement instrument was calibrated with the custom fitting empty, prior to the measurement of the balls.

For each prototype ball, the average mass moment of inertia about each principal axis was determined. For the polar axis x, a mass moment of inertia measurement was taken in each of four positions, 90° apart, with each measurement repeated five times. For each principal equatorial axis, y, z, a mass moment of inertia measurement was taken in each of two positions, 180° apart, with each measurement again repeated five times. Each such set of measurements was repeated with each of the three sample balls and the average mass moment of inertia value for each principal axis determined over the whole series of measurements.

The average mass moment of inertia values for each prototype ball are shown in Table II: Table Il

The difference between the average mass moment of inertia I _yy about the first principal equatorial axis y and the average mass moment of inertia I _7x about the second principal equatorial axis z is expressed as a percentage of the value of /„,, and is calculated as QQ. As can be seen, with a conventional ovoid sports ball (CHSI b) the difference between the average mass moments of inertia about the principal equatorial axes is around 5.77%. Minimising the valve mass (CHSI a) reduces the difference, but only to around 3.46%. As one would expect, increasing the valve mass (CHSI c) increases the difference to around 8.36%, whilst the statically balanced ball (CHS2) exhibits a further augmentation in the difference, with the value being around 9.76%. By contrast, the dynamically balanced ball of the present invention (CHS4) shows a greatly reduced difference between the average mass moments of inertia about the principal equatorial axes - down to around 0.75%. It is envisaged that by further enhancement of manufacturing techniques, it will be feasible to reduce this figure still further towards the theoretical ideal, where

In tests, the CHS4 prototype balls exhibited considerably superior results with respect stability of rotation during flight, as compared to conventional ovoid balls (CHSI b) and statically balanced balls (CHS2). The CHS4 prototype balls also exhibited considerably superior results with respect to accuracy of trajectory and bounce predictability, as compared to conventional ovoid balls (CHSI b).

In order that the present invention may be more clearly understood, a preferred embodiment thereof will now be described in detail, though only by way of example, with reference to the accompanying drawings in which:

Figure 1 shows a perspective view of a dynamically balanced bladder for an inflatable ovoid sports ball according to a preferred embodiment of the present invention; and

Figure 2 shows an end view of the bladder of Figure 1 , along the polar axis, x. Referring simultaneously to Figures 1 and 2, there is shown a dynamically balanced bladder 10 for an inflatable ovoid sports ball, according to a preferred embodiment of the present invention. Note that for ease of reference, details of ovoid ball construction such as the stitched outer panels, have been omitted. References to the bladder 10 of Figures 1 and 2 should however also be construed as being equally applicable to a fully assembled inflatable ovoid sports ball incorporating such a dynamically balanced bladder 10.

The principal axes x, y, and z, comprising a polar axis x, a first principal equatorial axis y, and a second principal axis z, and co-inciding at an origin o at the centre of geometry of the bladder 10, are superimposed on Figures 1 and 2 (note that the ball as shown in Figure 2 is viewed along the polar axis x).

The bladder 10 has a single air inlet valve 1 1 , in common with bladders for conventional inflatable ovoid balls, said air inlet valve 1 1 being located on the equatorial circumference of the bladder 10, and being intersected by the first principal equatorial axis y. Unlike conventional balls and bladders however, the dynamically balanced bladder 1 1 of the present invention further comprises three additional mass elements in the form of dummy valves 12, 13, 14.

Each dummy valve 12, 13, 14 has a mass equal to that of the air inlet valve 11. As can best be seen from Figure 2, the air inlet valve 1 1 and the three dummy valves 12, 13, 14 are arranged at spaced 90° intervals around the equatorial circumference of the bladder 10, such that the first principal equatorial axis y intersects the air inlet valve 1 1 and one dummy valve 13, and the second equatorial axis z intersects the other two dummy valves 12, 14.

The distribution of mass around the equatorial circumference re-aligns the centre of mass of the bladder 10 with its centre of geometry at the origin o. Furthermore, the bladder 10 is now dynamically balanced such that the mass moment of inertia I _yy about the first principal equatorial axis y is substantially equal to the mass moment of inertia Z ₂₂ about the second principal equatorial axis z. This has the effect that an ovoid sports ball incorporating the dynamically balanced bladder 10 exhibits greatly improved properties in respect of accuracy of trajectory, bounce predictability, and above all stability of rotation, as compared to conventional ovoid sports balls and the known prior art.