Tyre for a two-wheeled vehicle

Abstract: A tyre for a two-wheeled vehicle comprising a ground contacting tread (10) having at least one tread groove (11) formed therein and extending axially from the tread centre to the tread edge, characterised in that a first region (1) of the tyre tread (10), which is the central tread region and is the region of contact between the tyre and the ground when the motorcycle is in the condition of moving in a straight line, the alignment of the tread groove is substantially circumferential, and in a second region (2) of the tyre tread (10), which is at the axial edge of the tread and is the region of contact when the motorcycle is in the condition of turning under maximum cornering, the alignment of the tread grooves is substantially transverse, and in a third region (3), which is the tread region between the first central region (1) and the second axial region (2) and which is the ground contacting region when the motorcycle is turning at less than the maximum cornering condition, the groove alignment changes progressively axially outwardly from substantially circumferential to substantially transverse so that in the first, second and third axial regions of the tyre tread (10) the tread grooves (11) are substantially aligned with the resultant forces acting on the tyre tread surface in the contact region between the tread and with the ground.

Description: The invention relates to a tyre for a two-wheeled vehicle such as a motorcycle or motorscooter, of the type disclosed by JP-A-63-121 505. Such tyres utilise treads which in transverse cross-section are sharply curved to provide good contact with the road surface when the motorcycle is steeply banked during cornering. In common with most other types of tyres, in order for these tyres to perform well under all conditions it is usual for the ground contacting tread region of the tyre to be provided with relatively deep grooves in the surface. In wet conditions such grooves assist in clearing water from the contact area between the tyre tread and the road surface and thus help to retain steering control of the vehicle by preventing aquaplaning. However the provision of such grooves effectively weakens the tyre tread and renders it more prone to wear. This occurs because such grooves divide the tread into discrete elements such as ribs and blocks which individually are more susceptible to deformation in the tyre/ground contact region due to the tractive and cornering forces transmitted therein and therefore wear by abrasion against the road surface. In making the present invention, the inventors studied the alignment of the grooves of the tyre tread and determined that it was possible to arrange these so as to minimise wear of the tread rubber and thus extend the useful life of the tyre. Accordingly it is an object of the present invention to provide a tyre for a two-wheeled vehicle and a method of designing said tyre which is improved in tread wear. According to the present invention a tyre for a two-wheeled vehicle comprises a ground contacting tread having tread grooves formed therein, characterised in that in a first region of the tyre tread, which is the central tread region and is the region of contact between the tyre and the ground when the motorcycle is in the condition of moving in a straight line, the alignment of the tread grooves is substantially circumferential, and in a second region of the tyre tread, which is at the axial edge of the tread and is the region of contact when the motorcycle is in the condition of turning under maximum cornering, the alignment of the tread grooves is substantially transverse, and in a third region, which is the tread region between the first central region and the second axial region and which provide the ground contacting regions when the motorcycle is turning at less than the maximum cornering condition, the groove alignment changes progressively axially outwardly from substantially circumferential to substantially transverse so that in the ground contacting region when it is in the first, second or third region the tread grooves are substantially aligned with the resultant forces acting on the tyre tread surface. Preferably in the third region of the tread the alignment of the groove at any axial point is the same as the direction of the force resultant from the addition of the longitudinal and lateral forces acting on the tread at that point when the motorcycle on which the tyre is mounted is cornered at constant longitudinal acceleration such that said axial point is at the centre of the contact region. The tread grooves are preferably continuous from the tread centre to the tread edge. According to another aspect the invention provides a method of determining the required tread groove angle to form the tread pattern of a tyre for a two-wheeled vehicle to give optimum tread wear comprising at successive axial points across the tread from the centre to the tread edge aligning the tread groove in the direction of the typical slippage of the tread at that axial point when the point is in the tyre/road contact region. A preferred method of the invention provides a method of determining the required tread groove angle to form the tread pattern of a tyre for a two-wheeled motorcycle to give optimised tread wear in use on a motorcycle comprising deciding on the typical longitudinal acceleration of the motorcycle during cornering in the particular manner of riding and determining the corresponding longitudinal force on the tyre, and subsequently for successive axial points across the tread surface from the tread centre to the tread edge corresponding to successive values of camber of the tyre determining the lateral acceleration of the tyre and the corresponding lateral force on the tyre, determining the direction of the force resultant from the addition of the longitudinal and lateral forces and aligning the tread groove portion at that particular axial point with the direction of the determined resultant force. Further aspects of the invention will become apparent from the following description by way of example only of embodiments of the invention in conjunction with the following drawings in which: Figure 1 shows a plan view of a part of a motorcycle tyre tread in accordance with the invention; Figure 2 shows the axial profile of the tyre of Figure 1; Figure 3 is a schematic diagram being a developed view of a tread showing the alignments of three further grooves A,B,C in accordance with the invention; Figure 4 shows a further example of tread pattern groove alignments in accordance with the invention; and Figure 5 shows another example of tread pattern groove alignments according to the invention. Shown in Figure 2 is a schematic diagram of the axial profile of a motorcycle tyre according to the present invention. The curved tread surface 10 is nominally divided into a first region 1 at the tread centre, second regions 2 at each tread edge and third regions 3 between the first region and each of the second regions. Each of these three nominal regions corresponds to that part of the tread surface which is in contact with the road surface under various conditions encountered in riding a motorcycle or motorscooter. Thus the first or central region 1 corresponds to that part of the tread which is in contact with the road surface when the vehicle is running in a straight line. The second or edge regions 2 correspond to that part of the tread which is brought into contact in extreme cornering when the vehicle is fully banked over, whilst the third or middle regions 3 are those which contact the road surface when the vehicle is cornering moderately. According to the present invention the alignment of tread grooves in these three nominal regions of the tread is such that at every axial position the groove direction is substantially the same as the direction of typical slippage due to the resultant forces acting on the tread surface at that position when the position concerned is in the contact patch. Thus as can be seen in Figure 1 in the first or central region 1 the grooves 11 in the tread are orientated substantially parallel to the circumferential direction C/L of the tyre so that they are aligned with the predominantly longitudinally directed acceleration or braking forces acting on the tread when the vehicle is travelling in a straight line and therefore in an upright position. Conversely in the second region 2 at the tread edge the tread grooves are orientated substantially axially, i.e. normal to the circumferential direction C/L, so that they are aligned with the predominantly laterally directed force acting on the tyre when the vehicle is fully banked under extreme cornering. In the third or middle region 2 the orientation of the tread groove 11 changes smoothly and progressively from circumferential to axial moving in the axially outwards direction. Within the third region the alignment of the groove at any axial position is the same as the direction of the force which is the resultant force from the addition of both the longitudinal and lateral forces acting in the tread contact region centred on that axial position. The tread grooves may be continuous from the tread centre to the tread edge as in Figure 1 or they may be discontinuous as in Figures 4 and 5 wherein each groove portion (40a-40g,50a-50g) is aligned according to the invention. The tyre tread may also have circumferential grooves as common in the art. In an alternative embodiment the tread between adjacent grooves is substantially continuous. Thus there are no other major tread grooves intersecting the grooves aligned according to the invention and therefore the tread is not divided into blocks which are susceptible to wear on their leading and trailing edges. For the purposes of providing a generally optimised groove alignment such as depicted in Figure 1 the inventors have found that a cosecant curve gives an acceptable shape which improves tread pattern wear. This is implemented as y = cosecant (x) wherein y is the circumferential tyre direction and x is the axial direction. The invention also provides a method of determining the required tread groove angle or alignment to form the tread pattern having optimum, i.e. minimum, tread wear. This method comprises at successive axial points across the tread from the centre to the tread edge aligning the tread groove in the same direction as typical slippage due to the resultant force acting on the tread when the axial point concerned is at the centre of the tread/road contact region. This method may be implemented either by practical means or by calculation as will be explained below. For the method by calculation it has been found expedient to determine by direct calculation the direction of the resultant force acting on the tread at an axial point in the contact region and aligning the groove at that point in this same direction. In the practical implementation of the method the tyre, preferably an unused tyre, is provided with an array of small holes, preferably circular in shape at the tyre surface, to provide a hole at each axial position across the tread surface. The tyre is then run on the vehicle under all riding conditions to initiate wearing of the tread over its entire width. The tread wear will become apparent with the appearance of a pattern of wear emanating in a particular direction from each small hole according to the axial position of the hole. Subsequently the tread groove pattern can be formed or cut into the tread of another tyre by aligning the tread groove at any particular axial position in the same direction as the wear pattern emanating from the hole at that position on the worn tyre. Furthermore in accordance with the calculation method of the invention the alignment of tread grooves may be further optimised according to the anticipated manner of riding or anticipated use of the vehicle. Thus the invention provides a method of optimising the groove alignment for example of a tyre for a high performance racing motorcycle which could be expected to be ridden with high acceleration and braking and under extreme cornering conditions of high acceleration with full camber, and contrastingly a tyre for a lower performance motorcycle or motorscooter such as a small-engined moped which can reasonably be expected to be ridden more sedately. Thus according to the method of the invention the tyre design engineer firstly decides on the typical longitudinal acceleration which the tyre will experience in cornering according to the anticipated manner of riding or anticipated use of the vehicle, and the corresponding longitudinal force acting on the tread in the tyre/road contact region. For example the tyre design engineer may decide that for a tyre intended for a high performance high powered motorcycle a longitudinal acceleration value in excess of 0.5g may be appropriate whereas for a low powered moped a value of less than 0,1g is appropriate. Subsequently for successive axial points across the tread from the centre to the edge, which points when considered to be at the centre of the contact region correspond to particular camber values, the lateral acceleration and thus the lateral force acting on the tyre is determined. Now knowing the longitudinal and lateral forces acting at successive axial point of the tread when such points are centred in the tyre/road contact region, the direction of the resultant force from the vector addition of this two known forces can be determined, and the groove portion at that point should be then aligned with the direction of this resultant force. By way of example of the results of the application of the above method Figure 3 shows the alignment of three tread grooves denoted schematically by the single lines A,B,C which represent the optimum groove arrangements for three tyres with widely different anticipated manners of use. Figure 3 shows the developed shapes of these alignments. Thus the groove pattern A represents an optimised shape for a tyre for a low performance usage as for example on a small-engined motorcycle or a moped. For this tyre a typical longitudinal acceleration value in cornering of only 0.1g was used. In contrast groove pattern C represents the optimised shape for a high performance usage, such as on a racing motorcycle. For this tyre a longitudinal acceleration value in cornering of 0.6g was used. The groove shape B represents the optimised alignment for a tyre for intermediate performance usage. For this tyre a longitudinal acceleration value in cornering of 0.3g was used. In the above references to longitudinal acceleration it is understood to include longitudinal deceleration, i.e. braking, which is regarded, as is normal in the art, as an acceleration negatively signed. In view of load transfer considerations the optimisation of the groove alignment of the tyres should be carried out on the basis that the vehicle front tyre is optimised for braking whilst the rear tyre is optimised for acceleration. Such a basis gives rise to the directional fitment of the tread pattern indicated in Figure 1 in which when the vehicle is moving forward the front tyre rotates in the direction of arrow F whilst the rear tyre rotates in the direction of arrow R. Whilst the above embodiments describe the groove angles or alignments at the tread centre and tread edges as 0° and 90° respectively to the tyre circumferential direction other angles such as 10° and 80° respectively may be used which will retain to a large degree the advantage of reduced tread wear in accordance with the invention.

Claims: A tyre for a two-wheeled vehicle comprising a ground contacting tread (10) having in both axial halves of the tyre a plurality of tread grooves (11) formed therein and extending axially from the tread centre to the tread edge, wherein in a first region (1) of the tyre tread (10), which is the central tread region and is the region of contact between the tyre and the ground when the motorcycle is in the condition of moving in a straight line, the alignment of the tread groove is substantially circumferential, and in a second region (2) of the tyre tread (10), which is at the axial edge of the tread and is the region of contact when the motorcycle is in the condition of turning under maximum cornering, the alignment of the tread grooves is substantially transverse, and in a third region (3), which is the tread region between the first central region (1) and the second axial region (2) and which is the ground contacting region when the motorcycle is turning at less than the maximum cornering condition, the groove alignment changes progressively axially outwardly from substantially circumferential to substantially transverse, characterized in that in the first, second and third axial regions of the tyre tread (10) the tread grooves (11) are substantially aligned with the resultant forces acting on the tyre tread surface in the contact region between the tread and with the ground, and in that the tread grooves (11) in both halves of the tyre are arranged such that each of the tread grooves overlaps in the circumferential direction with the subsequent tread groove in the same half of the tyre. A tyre according to claim 1, characterised in that in the third region (3) of the tread (10) the alignment of the groove (11) at any axial point is the same as the direction of the force resultant from the addition of the longitudinal and lateral forces acting on the tread at that point when the motorcycle on which the tyre is mounted is cornered at constant longitudinal acceleration such that said axial point is at the centre of the contact region. A tyre according to either of claims 1 or 2, characterised in that the tread grooves (11) are continuous from the tread centre to the tread edge. A tyre according to any of claims 1 to 3, characterised in that the alignment of the groove (11) follows the shape of a cosecant curve. A tyre according to any of claims 1 to 4 characterised in that the tread between circumferentially adjacent grooves (11) is substantially continuous. A method of determining the required tread groove angle to form the tread pattern of a tyre for a two-wheeled vehicle to give optimum tread wear comprising at successive axial points across the tread from the centre to the tread edge aligning the tread groove substantially in the direction of the resultant force acting on the tread at that axial point when the point is in the tyre/road contact region. A method in accordance with claim 6 of determining the required tread groove angle to form the tread pattern of a tyre for a two-wheeled vehicle to give optimum tread wear in use of the motorcycle comprising deciding on the typical longitudinal acceleration of the motorcycle during cornering in the particular manner of riding and determining the corresponding longitudinal force on the tyre, and subsequently for successive axial points across the tread surface from the tread centre to the tread edge corresponding to successive values of camber of the tyre determining the lateral acceleration of the tyre and the corresponding lateral force on the tyre, determining the direction of the force resultant from the addition of the longitudinal and lateral forces and aligning the tread groove at that particular axial point substantially in the direction of the determined resultant force. A method in accordance with claim 6 comprising forming small holes in the tread surface at successive axial positions across the tread from the centre to the edge, running the tyre on the vehicle to initiate a pattern of wear emanating from each small hole, and forming tread grooves in the surface of a subsequent tyre such that at any particular axial point the tread groove is aligned in the same direction as the pattern of wear emanating from the small hole formed at that point on the worn tyre. A method according to claim 8, characterised in that the small holes are circular in shape at the tyre surface.

Inventors: JACKSON MICHAEL, WATKINS DAVID ROBERT, JACKSON, MICHAEL, WATKINS, DAVID ROBERT
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1998
Main classification: B60C11
IPC codes: B60C11