1. Field of the Invention
The present invention relates to a tyre, preferably a winter tyre, which is provided with a tread band whose blocks are advantageously provided with sipes which are so designed as to improve the performance levels of the tyre on dry and wet surfaces without impairing the grip characteristics on snow-covered or icy surfaces.
2. Description of the Related Art
In a typical structural configuration thereof, a winter tyre comprises a tread band on which there are defined a plurality of grooves which have a circumferential and/or transverse extent and which define a corresponding plurality of blocks, on each of which, in turn, a plurality of sipes are formed. Typically, the sipes extend inside the block in a radial plane of reference which is substantially perpendicular to the tread surface of the tyre and their presence is one of the most evident characteristics which also distinguish at first sight a winter tyre from conventional summer tyres.
The function of the sipes is to provide additional gripping edges on the snow-covered surface and to retain therein a given quantity of snow which, as known, has greater friction with respect to the snow present on the road surface than that provided by the tread band itself.
However, the presence of the sipes in the blocks of the tread band consistently reduces the performance levels of the tyre if the road surface is not covered with snow, and is dry or wet.
It is considered that this decrease in the performance levels may be attributed to the fact that the various portions of the block separated by the sipes, which may benefit from relative freedom of mutual movement, particularly in the radial direction, are not able to provide sufficient resistance to the tangential loads (or “shear stress”) imparted to the tread band during acceleration phases, phases in bends or braking phases, with resultant deformation of the block and reduction of the contact surface with respect to the road surface.
In that sense, it is known that, under dry road surface conditions, the tyre configuration which has the best performance levels is the one which provides a tread band having a surface which is completely smooth so as to provide a greater contact surface with the ground and a greater rigidity when subjected to tangential loads, which are typical of phases involving acceleration, braking or travel round bends.
However, the provision of grooves in the tread band is found to be indispensable in the presence of a road surface which is wet because it allows rapid discharge of the water which may be present on the road surface, thereby preventing dangerous phenomena of aquaplaning with resultant lifting of the tyre.
A tyre which is suitable for travel under all conditions of a road surface mentioned above is therefore called for in order to balance the opposing requirements involving configuration so that the provision on the tread band of grooves and, on the blocks defined thereby, of sipes, is optimized in accordance with the required demands during use.
This need is further particularly apparent in those countries where the winters are characterized by sporadic or even occasional falls of snow. In that case, the use of winter tyres is advised with respect to summer tyres because the composition of polymer materials from which the tread band is constructed is more suitable for the harsh winter temperatures but a large part of the travel time is spent on roads which are not covered with snow but are instead simply dry or wet.
It should be specified that, in the present context, the term “sipe” is intended to refer to a recess which is formed in a portion of the tread band and which has a width of from 0.1 to 1.5 millimeters and a depth of between 1 and 15 millimeters whilst the term “groove” is intended to refer to a recess which is formed in a portion of the tread band and which has a width greater than 1.5 millimeters and a depth greater than 5 millimeters.
Furthermore, considering a sipe extending from the tread surface towards the interior of the tread band in a “plane of reference” which is incident with respect to the tread surface, the term “longitudinal direction” defines a direction of that plane substantially parallel with the tread surface and the term “transverse direction” defines a direction of that plane substantially perpendicular to the longitudinal direction. If the plane of reference extends in a direction substantially perpendicular to the tread surface, the transverse direction coincides with a radial direction of the tyre.
Furthermore, within the scope of the present description and the claims which are appended, the term “sipe of complex shape” is intended to refer to a sipe which extends in a plane of reference which is incident with respect to the tread surface and which defines, on the facing surfaces of the block portions, at least one protuberance which is formed on one of said surfaces and a corresponding recess which is formed in the other of said surfaces such that the respective projections of the two block portions which are separated by the sipe, in a transverse direction of the plane of reference, are, in the region of the protuberance and the corresponding recess, at least partially superimposed.
In that manner, a relative movement of the two block portions in a direction parallel with the plane of reference is prevented by interference between the two block portions in the region of the protuberance and recess.
Conversely, the term “sipe of simple shape” is intended to refer to a sipe whose extent in a plane of reference which is incident to the tread surface does not have any zones in which there is superimposition between the respective projections of the two block portions which are separated by the sipe in a transverse direction of the plane of reference.
Furthermore, it is intended to be understood that the distance between surfaces of respective block portions facing each other in the region of a sipe remains “substantially constant” over the extent of the sipe when the distance between said surfaces is within a range of more or less 50% of a mean value of the distance calculated over the entire extent of the sipe.
In the present description and in the subsequent claims, the inclinations of the connecting surfaces between a vertex of a protuberance or a recess and the plane of reference are further defined taking into consideration the triangle formed in a plane of section perpendicular to the plane of reference, parallel with the transverse direction and extending through the vertex of the protuberance or recess, and defined by the vertex of the protuberance or the recess and the intersection points between the curves defined by the intersection of the connecting surfaces with the plane of reference and the above-mentioned plane of section. In particular, considering the side belonging to the plane of reference to be the base of the triangle, the inclination of each connecting surface is defined with the respective angle relative to the base of the triangle that is formed in the region of the intersection points set out above, respectively.
In that manner, the inclination of the connecting surface is measured in a plane of section perpendicular to the plane of reference parallel with the transverse direction and extending through the vertex and, in the case of curved connecting surfaces, a mean inclination of the connecting surface is in fact considered.
In the present description and in the subsequent claims, it is further intended to be understood that a connecting surface is “substantially planar” when it is planar over at least 50% of the surface thereof.
In the present description and in the subsequent claims, the term “profile of a recess or a protuberance in a longitudinal or transverse direction” is further intended to refer to the orthogonal projection of the recess or the protuberance in a plane perpendicular to the plane of reference and parallel with the longitudinal or transverse direction, respectively.
Finally, a protuberance is defined as being “completely internal with respect to the block” when its connecting surfaces with respect to the plane of reference are not intersected by the external surfaces of the block. This definition applies in a generally similar manner to a recess or a succession of protuberances or recesses which are “completely internal with respect to the block”.
International Patent Application No. WO 2009/077807 in the name of the same Applicant discloses a winter tyre whose blocks are affected by sipes, in which there are provided, on the surfaces of the block portions facing each other, protuberances and recesses with a curved, semi-conical profile which extends in a substantially longitudinal direction from the axial ends of the sipe towards a central region thereof. The profile of those protuberances in the radial direction is substantially semicircular with a section decreasing towards the central region of the sipe.