Radial tyre having a tread provided with sub-circumferential longitudinal grooves

Abstract: The tread pattern (3) of a radial tyre for motor-vehicles offering high performance comprises oblique longitudinal grooves (7), inclined to the circumferential direction of the tyre by an angle included between 2° and 20°, the opposite ends (7h, 7k) of said grooves being axially offset relative to one another: all grooves may be inclined (7) in the same direction or may converge (9), preferably symmetrically, towards the meridian plane of the tyre, specifically for directional patterns. The longitudinal grooves (7) are associated with a plurality of transverse grooves (8) exhibiting their central portion inclined in the opposite way relative to the longitudinal grooves, according to an angle included between 30° and 80°, with respect to the circumferential tyre direction. The combination of the longitudinal (7) and transverse (8) grooves according to the established inclinations confines plugs (6) developing, on the tyre contact-area, tangential forces generally combining with one another thereby generating a resultant force having the axial component (drift force) of substantially zero value.

Description: The present invention relates to motor-vehicle tyres and specifically to tyres having a radial carcass and suitable for medium and/or high class motor-vehicles in which tyres affording high performance are required. More particularly, the invention relates to the tread pattern in said tyres. It is known that tyres have a thick tread band disposed crown-wise to the carcass in which a relief pattern exhibiting longitudinal and/or transverse grooves is formed by molding, said pattern having the function of giving the tyre some driving behaviour features such as in particular good capability of traction and roadholding at any ground conditions, that is when the road is dry, wet or covered with snow, and irrespective of the ride conditions, that is to say on rectilinear paths or in case of drift, i.e. when there are bends or side thrusts for example due to the effect of wind. As regards roadholding on wet ground, longitudinal grooves are of the greatest importance in that, due to said orientation, they succeed in discharging the water collected in the tyre contact-area, thereby ensuring the wheel/road contact and avoiding the well-known and dangerous aquaplane phenomenon arising. For the above reason all tyres except some special types intended for machines having a very low ride speed and therefore unaffected by aquaplane phenomenon, are always provided with wide continuous longitudinal grooves which are oriented circumferentially. On the contrary, as regards the tyre capability of traction, this is connected with the presence of transverse grooves in the tread and for this reason, in the absence of said grooves, the longitudinal ones generally have a zigzag path so as to exhibit inclined edges relative to the vehicle ride direction. The efficiency of the transverse grooves as regards the capability of traction becomes increasingly greater as the inclination of said grooves approaches the axial direction: on the contrary, this axial inclination is negative as regards the possibility of discharging the water picked up from ground because this orientation makes it difficult to pump the water in the longitudinal grooves under the contact-area, and also as regards the directional stability of the tyre above all at the high speeds achieved with present motor-vehicles. Therefore the attemps to reach the best compromise between the different requirements of modern tyres have brought to the development of both symmetric and asymmetric tread patterns relative to the diametrical plane of the tyre, having rectilinear circumferential grooves and transverse grooves all inclined in the same direction or alternately in the opposite direction with respect to the axial direction of the tyre: the so-called "helix-shaped" and "fish-bone shaped" patterns are well-known and widely spread and represent an example of the foregoing. Upon increasing of the performance required from tyres as regards power and speed, the longitudinal grooves have shown a new drawback, specifically in tyres having a radial carcass. Radial carcass tyres, shortly referred to as "radial tyres", are those provided with a carcass the reinforcing cords of which substantially lie in radial planes, that is containing the rotational axis of the tyre, and an annular reinforcing structure which is circumferentially inextensible (the so-called belt) disposed crown-wise to the carcass, inserted between the carcass and the tread band and ensuring dimensional stability to the tyre, particularly in the circumferential direction, as well as the necessary drive behaviour qualities. In fact a tyre provided with the radial carcass alone, due to its structure, would not be dimensionally steady under the effect of the efforts due to inflating pressure and those connected with the centrifugal force in use, and would substantially be incapable of offering good driving features, in particular directional and transverse stability. It is known that roadholding in case of drift and the transverse stability of the radial tyre closely depend on the strength of the belt structure to axially-acting tangential efforts transmitted between the tyre and the road; on the other hand, it has been found that in tyres suitable for vehicles offering high power and speed performance said strength is also conditioned by the geometry of the tread pattern. In particular, the structural strength of the belt is generally limited due to the fact that, at the longitudinal tread grooves, where the tread thickness is clearly reduced to the minimum value, the belt has a preferential deformation area, that is a "hinge". This hinge when the tyre in use is greatly engaged in drifting, triggers a type of stress causing the dynamic yielding of the belt structure, which gives rise to a decrease in the roadholding and general behaviour qualities of the tyre and consequently of the vehicle at high speed. In order to overcome these drawbacks tread patterns devoid of longitudinal grooves have been so far proposed, only having transverse grooves inclined to both axial and circumferential directions of the tyre, optionally emerging in the axial direction on the tread ends, such as for example the tread described in British Patent GB 2,229,974. Along with the same technology, US-A-4 953 604 discloses a directional tyre having a tread design divided into three circumferential flanked zones: the tyre is deprived of longitudinal grooves and shows, on each side zone, transverse grooves merging together, in pair, to form for each pair, a unique transverse groove in the middle circumferential zone. Other tyres show longitudinal and tansverse grooves, with longitudinal grooves inclined with respect to the circumferential direction of the tire. FR-A-2 588 514, upon which the preamble of claim 1 is based, discloses a tyre having a tread band provided with a tread pattern comprising a plurality of longitudinal grooves and transverse grooves all of which are rectilinear, parallel and continuous. Such tyre has a tread design in which the tread blocks (all having the same shape) defined by said longitudinal and transverse grooves, are arranged in rows that are inclined of a given angle to the circumferential direction. The tyre has a ground contact area including portions of at least two of said longitudinal grooves, said portions being parallel to each other and oblique to the circumferential direction of the tyre. FR-A-2 232 454 is concerned with a tread pattern aimed to generally improve the performance of the tyre, such as the traction or the resistance to skidding. The tread comprises a plurality of circumferential grooves, side by side disposed, each one following a zig-zag path inclined with respect to the longitudinal direction of the tyre, said grooves being further connected with transversal grooves following a second zig-zag path, inclined with respect to the axial direction of the tyre. However tyres provided with this type of pattern, as well as those mentioned above and exhibiting a helix-shaped pattern, suffer the drawback of developing, under rectilinear ride conditions, a drift force of high strength and greatly changing on varying the torque applied to the tyre, which hinders the driving and, without varying the steering angle, causes the vehicle to slide under acceleration and braking conditions, bringing about an uncontrolled change in the trajectory thereof. For the purpose of restraining this undesirable tyre behaviour, attempts are usually made to balance this drift force by mounting the tyres on the vehicle in a particular manner, that is with the tread patterns disposed symmetrically to the vehicle axis. In accordance with the present invention it has been found that tyres provided with a tread pattern exhibiting transverse grooves all inclined in the same way, either associated with circumferential grooves or not, so as to break the transverse ribs into a plurality of lugs, offer a qualitatively-limited performance that cannot be improved by said "symmetric" mounting of the tyres to the vehicle. Still in accordance with the invention it has been found that an overall equilibrium situation can be reached within the same tyre by relying on the tread lug to develop drift forces balancing those developed by the lugs themselves by effect of the direction taken by the transverse grooves, provided that longitudinal grooves inclined to angles of predetermined value relative to the circumferential direction of the tyre are adopted in combination with said transverse grooves. Within this scope, by adopting particular arrangements, sizes and shapes for said longitudinal and trasverse grooves, further improvements have been surprisingly reached as regards specific features of the tyre such as lack of noise and drainage capability. The present invention relates to a tyre for vehicle wheels provided with a tread pattern comprising longitudinal and transverse grooves as claimed in claim 1. Said longitudinal grooves have a portion of their extension included in the contact-area inclined to an angle ranging between 2° and 20° relative to the circumferential direction of the tyre, and preferably said inclination angle does not exceed the value of 15°. As regards the transverse grooves, these have a sinusoidal path extended astride of an axially oriented line of the tyre, and comprise five consecutive lengths, with the two side lengths being substantially oriented in the axial direction and connected by a curvilinear portion to a central rectilinear length crossing the meridian plane of the tyre and axially extending between at least two axially adjacent portions of longitudinal grooves, said rectilinear central length being inclined in an opposite sense relative to said longitudinal grooves according to an angle between 30° and 80° relative to the mid-circumferential plane of the tyre: preferably they extend axially from one end to the other of the tread pattern and open on the pattern edges so as to promote the side discharge of the water collected in the tyre contact-area. Preferably the angle delimited by said oblique portion of the longitudinal groove and said rectilinear length of the transverse groove, of opposite inclination, is included between 35° and 70°. Conveniently each of said oblique longitudinal grooves extends from both portions of the meridian tyre plane and is of at least the same length as the maximum size of the contact-area of the tyre, measured on the inflated tyre at the exercise pressure and loaded with the maximum permissible loading value. The present invention will be better understood with the aid of the following description and the accompanying drawings, in which: - Fig. 1 is a cross -section of the general structure of a tyre in accordance with the invention; - Fig. 2 is a plan view of the crown portion of the tyre shown in Fig. 1, highlighting the tread pattern of the invention according to an asymmetric and non directional embodiment relative to the meridian circumferential plane of the tyre. While the invention is adapted to any type of tyre, the preferred tyre offering the greatest advantages is the type designed to very powerful and fast cars, from which high performance is required at any environmental and ride condition. The structure of these tyres, generally shown in Fig. 1, comprises a carcass 1 consisting of one or more plies of rubberized fabric, provided with reinforcing cords lying in radial planes, that is containing the rotational axis of the tyre, of toroidal ring-shaped configuration and exhibiting its ends axially folded back from the inside to the outside around two annular metal cores 2, usually known as bead cores, forming the bead reinforcement, that is the radially inner ends of said tyre, and having the function of enabling the tyre assembling with the corresponding mounting rim. Disposed crown-wise to said carcass is a thick band 3 of elastomeric material, that is the tread band, in which a relief pattern including longitudinal and transverse grooves is formed, which is adapted to ensure, among other things, particular qualitative features to said tyre, that is roadholding, good capability of traction, lack of noise, drainage capability and uniform wear. Finally, disposed between the carcass and tread band is an annular circumferentially-inextensible reinforcing structure 4, usually referred to as belt, comprising at least two radially-superposed layers 4a, 4b of rubberized fabric, provided with textile and/or metal reinforcing cords disposed parallel to one another in each layer and in crossed relationship with the cords of the adjacent layer symmetrically to the meridian plane of the tyre and, preferably, also a layer of textile cords 5 of the heat-shrinkable type, such as for example nylon cords oriented according to angles of value not higher than 10° relative to the circumferential tyre direction, but usually oriented circumferentially, disposed in a radially outer position at least at the ends of said underlying layers, for example as described in the US Patent No. 4.146.415 in the name of the same Applicant. As well known, this structure has the purpose of counteracting the efforts acting on the tyre in use connected with the inflating pressure and centripetal force, and ensuring the necessary drive behaviour qualities. In accordance with the present invention said structure is combined with a tread pattern comprising (Figure 2) at least a pair of oblique longitudinal grooves 7a and a plurality of transverse grooves 8 that mutually intersect, thereby delimiting a corresponding plurality of lugs 6 of general rhomboidal form, so shaped and oriented that they develop tangential forces on the tyre contact-area that generally combine together so as to generate a resultant force having the axial component of substantially zero value during the rectilinear ride at a constant speed, which value does not change even when the tyre is submitted to a positive or negative torque, that is under accelerating or braking conditions. The longitudinal grooves are defined as "oblique" in that they comprise at least an important portion of their longitudinal extension which is inclined to the circumferential direction of the tyre, represented by line m-m in the meridian plane, so that the opposite ends of said grooves are axially offset with respect to one another. For example (Figure 2), assuming that the circumferential extension of said oblique portion is coincident with the whole circumferential extension of the longitudinal groove and this extension is the same as the circumferential extension of the tyre, it is apparent that the opposite ends 7h and 7k of the groove 7a are aligned along the same axial line l , on the opposite tread edges. In the present description the demarcation between oblique longitudinal grooves and transverse grooves is represented by a value of the inclination angle of said grooves, or their most inclined portion, equal to 20° with reference to the diametrical plane of the tyre. According to the invention, the angle of inclination w of said oblique portion has a value included between 2° and 20°, preferably not higher than 15°. Conveniently its value can be maintained the same as the inclination angle of the reinforcing cords of one of the different belt layers, preferably the one located at the radially outer position. For "important portion" of the longitudinal extension of said grooves it is meant the fact that the circumferential extension of said oblique portion is at least the same as, but preferably greater than the maximum size L of the tyre contact-area A; in this way it is possible to ensure that when a groove is engaged in the tyre contact-area, it emerges from said area by at least one of its ends. This feature is of great importance because oblique grooves, due to the fact that their ends are axially offset, are generally blind, that is closed at the opposite ends, although a connection between said opposite ends could be provided for the purpose of restoring the circumferential continuity of the grooves. Size L on the tyre contact-area, generally of elliptic form the major axis of which corresponds to L, is determined on a tyre inflated to having the use pressure and loaded with the maximum permissible load. Preferably, each of said grooves extends from one end of the tread pattern to the other: the number of said grooves 7a in the contact-area varies with the tread width, a minimum of at least two grooves being provided so as to give the tread, in combination with the transverse grooves and lamellae, a value of the "recess/overall contact-area" ratio in the order of 0.26 to 0.34; referring to the oblique longitudinal grooves it is pointed out that the important element is the number of grooves in the contact-area, irrespective of the fact that said grooves are separated from one another or portions disposed in side by side relation belonging to a single spiral-shaped groove extending in the thickness of the tread band from one end of the band to the other. Conveniently the maximum width of said longitudinal grooves is included between 3 mm and 12 mm and their depth ranges between 3 mm and 10 mm; said width and depth can vary along the longitudinal groove extension. It is noted that all width and depth values of grooves cited in the present description are intended as measured on a new tyre, mounted to its rim and inflated at the exercise pressure, but not loaded. The shape of the groove cross-section can be both symmetric and asymmetric (sidewalls of different and/or varying sloping) and the same can extend not only according to a rectilinear trajectory but also a curvilinear or a zigzag trajectory provided that it is always around an average direction inclined to an angle of the above specified value. In greater detail each zigzag portion too will be preferably inclined according to an angle within the range of the above values. Still preferably, the number of said grooves keeps constant along the circumferential tyre extension: in other words the final end 7h of a groove in the axially outer position on the tread and the initial end 7k of the groove in the axially opposite position are in alignment along the same axial line l , so that where a groove ends another one begins, even if at said groove ends it is also possible to have both a thinning and a thickening (due to covering). According to a preferred embodiment of the pattern, the blind ends of said longitudinal grooves terminate in a tapering portion for the purpose of varying the value of said "recess/overall" ratio in a continuous manner along the circumferential tyre extension: said tapering portion goes on longitudinally in the form of a thin lamella, inclined as the corresponding oblique longitudinal groove, so as to create a certain number of lugs of the same size as those included between two longitudinal grooves, even in the tread portion between the tread edge and the first axially-adjacent oblique longitudinal groove. In combination with said longitudinal grooves, the pattern also comprises a plurality of transverse grooves 8 together defining said lugs 6; preferably lugs disposed in an axially inner position on the tread pattern are of rhombic configuration, their side measuring 15 to 30 mm, and they all have approximately the same sizes and area, smaller than that of the shoulder lugs located at an axially outer position. Preferably, said transverse grooves are continuous from one tyre sidewall to the other and open on the tread edges, so as to increase the drainage capability of the tyre; in fact they help in collecting water from the contact-area, pumping it into the oblique grooves and discharging it sideways of the tyre, thereby ensuring the maintaining of the tyre/road contact. Conveniently the maximum width of said transverse grooves is included between 2 mm and 10 mm and their depth ranges between 3 mm and 10 mm. The shape of the cross -section can be symmetric or asymmetric (variable-slope sides), and width and depth can vary too along the longitudinal groove extension; in particular it is convenient that the cross -section of said grooves progressively increases from the tread centre to the ends, as shown in Fig. 2. Obviously, for said grooves too, many embodiments are possible (that is rectilinear and curvilinear extensions, which may be symmetric, asymmetric and directional to the tyre centre line); however it will be advisable to choose those forms that in combination with their own size features (width and depth) and those of the oblique grooves, within the above inclination values, appear to be the most appropriate to generate lugs having moments if inertia mutually integrating so as to create an equilibrium situation preventing negative drift effects from occurring during the tyre running, both at a constant speed and under acceleration and/or braking conditions; in fact without an appropriate dimensioning of the lugs, which can be determined and at all events experimentally verified afterwards, these effects would surely be present in the tread of the invention, due to the oblique (sub-circumferential) and unidirectional inclination of the grooves. In the preferred embodiment of the tyre shown in Fig. 2, referring to a prototype tyre (size 196/60 R 15) manufactured by the same applicant, the tread is comprised of four oblique longitudinal grooves 7a, 7b, 7c, 7d, all parallel to one another, two of which exhibit, still with reference to a specific contact-area A, their oblique portions 7a, 7d always lying in the same half -tread zone with reference to the meridian plane m-m of the tyre, at a position axially external to any other oblique portion 7b, 7c, contiguous in the axial direction. In combination with said longitudinal grooves, the pattern also comprises said plurality of transverse grooves 8 axially extending between said two axially external oblique portions; each of them comprises at least a substantially rectilinear central length 8c axially extending between two oblique axially-adjacent longitudinal grooves 7b, 7c, inclined in the opposite sense to said couple of longitudinal grooves relative to the same circumferential direction of the tyre, according to an angle v of absolute value included between 30° and 80°. It will be noted that the established inclination values for the oblique portions of said longitudinal grooves and said central rectilinear length of the transverse grooves make it possible to provide a width for the angle w+v defined between said oblique portion and rectilinear length, included between 32° and 100°, said width being however preferably maintained in the range of 35° to 70°. In accordance with the foregoing, in the embodiment shown in Fig. 2 the corresponding inclinations are of 7° for the oblique portions of the longitudinal grooves and 50° for the central lengths of the transverse grooves respectively, the value of the included angle being equal to 57°. Still in accordance with the inventive pattern shown in Fig. 2, the transverse grooves have a sinusoidal shape substantially extending astride of a line l which is axially oriented: more particularly, said grooves substantially consist of five consecutive lengths 8a, 8b, 8c, 8b, 8a the two side lengths 8a being oriented in the axial direction, substantially tangent to said line l , and connected by a curvilinear portion 8b to said central length 8c. It will be noted that the tread shown in Fig. 2 could be reproduced in a mirror image relative to the centre line m-m, thereby achieving a directional pattern applying to each half -tread zone the concept of having a resultant of the tangential force developed by the lugs having an axial component of substantially zero value. The inventive tyre attains the intended purposes and achieves many and important advantages. First of all said drift effect causing driving unreliability, directional unsteadiness and undesirable trajectory variations is substantially eliminated. In fact, extension, shape and sizes of the longitudinal and transverse grooves are determined so as to generate such an equilibrium condition (mutual compensation) of the momenta of inertia of the different lugs that the occurrence of drift effects is prevented both at a constant speed and during the application of torque and/or braking torque. In this connection the applicant has been able to verify and quantify the improvement achieved with the tyre of the invention by a series of tests made on prototypes of tyres in accordance with the invention as compared with the best traditional tyres, measuring the drift force developed by the tyres set in rotation with a zero drift angle against a metal drum (wheel road) for different values of the applied positive and negative torque, as well as for a torque of zero value. The results have proved that in tyres of the invention the development of a drift force is of very reduced quantity and above all substantially constant on varying the torque value applied to the tyre axis. The efficiency of the tyre under aquaplane conditions does not appear to be reduced as compared to the best level already achieved by known tyres provided with rectilinear circumferential grooves, because said oblique grooves by virtue of their length, greater than the maximum size of the tread contact-area, always have at least one end emerging at the outside of said area thereby ensuring the discharge of the water collected from the road. The water drainage is also maximized by the presence of oblique longitudinal grooves in combination with the particular sinusoidal shape of the continuous transverse grooves extending in the axial direction and emerging on the tyre shoulders, which create a "milling" effect of the liquid front, particularly in the presence of deep water layers and when the tyre is under torque. The noise of the tread pattern is still more reduced and can be more easily governed since the positioning of the lugs in the transverse direction which does not take place as repeatedly as before maximizes the spreading and reduces the level of the "acoustic trouble" during the tyre rolling. In fact, as compared with the typical pitch sequence of a general tread pattern, in which the only available variable for intervening on the tyre noise is constituted by the pitch length (lug and circumferentially adjacent recess) the pitch sequence of the pattern of the invention comprises another variable consisting in the position of the lugs in the transverse direction. In other words, two axially offset consecutive lugs generate less acoustic trouble than when they are circumferentially aligned.

Claims: A tyre for vehicle wheels comprising an annular carcass (1) of toroidal form terminating in two beads, each provided with at least one annular circumferentially inextensible reinforcing core (2) for assembling said tyre to a corresponding mounting rim, at least one carcass ply provided with reinforcing cords disposed in the radial direction, having its ends axially folded back from the inside to the outside around said bead reinforcing annular cores (2), an annular circumferential inextensible belt reinforcing structure (4), circumferentially extending around and radially outward of said carcass and comprising at least two radially superposed layers (4a, 4b) of rubberized fabric provided with reinforcing cords disposed parallel to one another in each layer and in crossed relationship with those of the adjacent layer, inclined to the circumferential direction of the tyre, and a tread band (3) radially superposed to said reinforcing structure (4) and provided with a relief tread pattern comprising a plurality of longitudinal parallel grooves (7a, 7b, 7c, 7d) and transverse grooves (8) continuously extended from one side to the other of the tread pattern (3), said tread having a ground contact area with at least two of said longitudinal grooves having portions thereof included in said area all of which are rectilinear and continuous, said portions being parallel to each other and oblique to the circumferential direction of the tyre according to an angle ranging between 2° and 20°, characterized in that said transverse grooves (8) have a sinusoidal configuration extending astride of an axially oriented line of the tyre and comprising five consecutive lengths, of which the two side lengths (8a) are substantially oriented in the axial direction and connected by a curvilinear portion (8b) to a central rectilinear length (8c) crossing the meridian plane of the tyre and axially extending between at least the two axially adjacent oblique portions of the longitudinal grooves, said rectilinear central length (8c) being inclined in an opposite sense relative to said pair of oblique portions according to an angle between 30° and 80° relative to the mid-circumferential plane of the tyre; whereby the intersection between said oblique portions and said transverse grooves defines lugs which develop tangential forces in the contact area of the tyre that combine together so as to generate a resultant force having an axial component of substantially zero value. A tyre according to claim 2, characterized in that the angle of inclination of said oblique portions does not exceed 15°. A tyre according to claim 1, characterized in that said oblique portions of the longitudinal grooves (7a, 7b, 7c, 7d) are oriented in parallel to the cords of the belt layer (4b) disposed in the radially external position. A tyre according to claim 1, characterized in that said oblique portions of the longitudinal grooves (7a, 7b, 7c, 7d) exhibit tapering ends (7h, 7k). A tyre according to claim 1, characterized in that the rectilinear central length (8c) of said transverse grooves (8) is inclined according to an angle between 30° and 80° relative to the mid-circumferential plane of the tyre. A tyre according to claim 1, characterized in that the angle delimited by said oblique portion of the longitudinal grooves and said rectilinear central length of the transverse grooves (8), of opposed inclination, is included between 35° and 70°. A tyre according to claim 1, characterized in that said oblique portion of the longitudinal grooves is at least of the same length as the maximum size of the tyre contact area and extends from both sides of the meridian tyre plane towards the tread ends. A tyre according to claim 1, characterized in that there are two of said oblique portions disposed within the contact area, always lying in the same half-tread portion with reference to the meridian plane of the tyre, in a position axially external to any other axially contiguous oblique portion.

Inventors: BOIOCCHI MAURIZIO, CARETTA RENATO, COLOMBO GIANFRANCO, BOIOCCHI, MAURIZIO, CARETTA, RENATO, COLOMBO, GIANFRANCO
Applicants: PIRELLI, PIRELLI COORDINAMENTO PNEUMATICI S.P.A.
Application year: 1992
Main classification: B60C11
IPC codes: B60C11, B60C9