Tire for motor-cycles

Abstract: This invention relates to a tyre particularly suitable for motor-cycles having a textile carcass(1), preferably of radial type, and a belt structure (5) on the carcass top, comprising a strip (51) of reinforcing cords in radially inner position, said cords being inclined with respect to the circumferential direction of the tyre according to an angle ranging between 22° and 35° and, in a radially outer position, a second strip (52) of reinforcing cords, made of material with a modulus lower than that of the cords of the underlying strip, crossed with the cords of the underlying strip and inclined according to an angle ranging between 18° and 30°, with a difference of inclination between the cords of the two strips, ranging between 3° and 15°.

Description: The present invention refers to tires for vehicles wheels and more particularly tires to be used on motor-cycles. These types of tires have a very particular structure capable to support conditions of use completely different from those under which the conventional tires for motor-vehicles are used: for instance, they must be able to run also with angles of camber of very high value, of about 50° against the usual value of 3° or 4° of tires for motor-vehicles. For this reason, the tires to be used for motor-cycles have a very winding tread-band, i.e.provided with a strong curvature on the top contrarily to what happens for the others tires where the tread-band is only slightly swelled and becomes as flat as possible by the decreasing of the ratio height of section/width, which is well known to the technicians as ratio H/C. Furthermore, the tires for motor-cycles must have the best degree as to steering ability, transversal stability and no sensibility to the road unevenness, due to the instability of the two-wheels vehicle in respect with the four-wheels vehicle. By way of example such tires must not develop high thrustes of drift during the running on a straightaway, as such thrustes, as not balanced by those of a twin tire mounted on the same axis, as in the case of four-wheels vehicles, would provoke an uneven run of the motor-vehicle and dangerous for the driver. Recently, by the coming of faster and lighter motor-cycles, the tires for motor-cycles and specifically those mounted on the front wheel, shows a dangerous behaviour, known to the technicians as "kick-back"; in other words the front tire, when encountering - at high speed - an obstacle even of small dimensions (joints, slight prominence of the road-bed, etc) - transmits to the fork of the vehicle an impulse force having a component along a direction which is different from that of the axis of the fork, and therefore of the front shock absorber system. For this reason it transmits to the handlebar and therefore to the arms of the driver, a sudden blow, more or less violent, which causes the side-skid or even the fall of the vehicle together with the driver, with the severe consequences that can be expected for both of them. The Applicant has come to realize that such a behaviour is connected with a strong longitudinal stiffness and a drift of the belt structure of the conventional motorcycle tires which at the above mentioned high speed makes the tire substantially rigid and unable to absorb the road unevenness and then gradually returning the elastic energy absorbed during the impact. The kick-back effect is typical of a two-wheel vehicle, and does not exist in four-wheels vehicles wherein the shock absorber system is capable of completely absorbing an impulse force caused by an obstacle on the road. FR-A-2 178 001 discloses a cross-ply tire provided with one reinforcing strip aimed to improve the behaviour of a motorcycle (or a vehicle having wheels arranged on a single rows) when cornering. US-A-4 770 222 discloses a motorcycle tire comprising a pair of beads at the side of the tire for anchoring said tire to a corresponding wheel rim; a textile carcass extending between the beads and secured thereto; a tread band radially outward of said carcass and having an arcuate cross-section whose height to width ratio has a value ranging between 0.20 and 0.35; a pair of sidewalls defined substantially between the radially outer ends of the beads and the ends of the tread band; an annular reinforcing structure positioned between said carcass and said tread band and comprising two strips of rubberized fabric reinforced with cords. The technical problem solved by US-A-4 770 222 is that of obtaining a high stiffness in the sidewall and to this aim the patent teaches a carcass structure of new type. The present invention aims at providing a belt structure which allows to eliminate the above mentioned drawbacks ensuring both to the tire and to the motorcycle a greater safety and reliability even at higher speed. Accordingly, the subject matter of the present invention is a motorcycle tire comprising a pair of beads at the side of the tire for anchoring said tire to a corresponding wheel rim; a textile carcass extending between the beads and secured thereto; a tread band radially outward of said carcass and having an arcuate cross-section whose height to width ratio has a value ranging between 0.20 and 0.35; a pair of sidewalls defined substantially between the radially outer ends of the beads and the ends of the tread band; an annular reinforcing structure positioned between said carcass and said tread band and comprising two strips of rubberized fabric reinforced with cords, characterized by the fact that the first strip of said annular reinforcing structure has cords parallelly arranged with respect to each other and having a density in the range of 70-80 cords/dm and inclined at an angle ranging between 22° and 35° to the equatorial tire plane, and by that the second strip of said annular reinforcing structure lies in a radially outer position with respect to said first strip and has cords parallelly arranged with respect to each other and having a density of less than the density of the cords of said first strip, the cords of the second strip having an elasticity modulus lower than the elasticity modulus of the cords of said first strip and at an angle ranging between 18° and 30° with respect to the equatorial plane, said angle being smaller than the inclination angle of the cords of said first strip, an inclination difference between the cords of said first and second strips, with respect to said equatorial tire plane, ranging between 3° and 15°. Preferably, the materials used for the cords of said first strip are selected from the group comprising steel, aramide, rayon, whilst the reinforcing cords of said second strip are selected from the group comprising rayon, polyamide, polyester. The present invention will be better understood from the following detailed description, given only by way of non-limiting example and with reference to the attached drawings in which: Fig. 1 illustrates a straight cross section of a tire according to the invention in a plane containing the rotation axis of the tire; Fig. 2 illustrates a preferred embodiment of the belt structure of the tire according to the invention; Fig. 3 illustrates an alternative embodiment of the belt structure of the tire according to the invention. With reference to Fig. 1, it must be observed that the tire of the invention consists of a carcass 1 realized with one or more plies (generally a pair), having a straight shaped section, according to a toric arrangement whose ends are turned-up: all this is, on the other hand, carried out in a conventional manner, around two annular metal cores 2, provided - in a radially outer position - with a filling 3 of elastomeric material with suitable rheometric characteristics. On the top of said carcass there is a tread band 4 and an annular reinforcing structure 5 is sandwiched between the tread band and the carcass. In particular, the annular structure according to the invention comprises a first strip 51 of rubberized fabric in radially inner position and a second strip of rubberized fabric 52, overlapped to it, in radially outer position. The tire's area comprising the annular metal core 2, usually known as bead-core, and the filling 3, represents the bead of the tire, whose aim is to ensure the anchorage of the above mentioned tire to a corresponding assembling rim, here not illustrated. The area substantially comprised between the radially outer end of the filling 3 and the end of the belt 5 takes the name of sidewall (6) of the tire and it represents the area having greater flexibility apt to ensure comfort condition during the running of the vehicle. The reinforcing cords of the carcass plies are of any conventional textile material already known; i.e. in the case hereabove mentioned they are of rayon, that is of synthetic fiber deriving from cellulose, and preferably they are arranged according to the radial direction: they are axially extending from a bead to the other lying on planes containing the rotation axis of the tire. The necessary mechanical resistance of the rubberized fabric of which the said plies are constituted, is determined by an appropriate choice of the type of the cord and by the density of the cords in the fabric, approximately about 90-120 cords/dm so that to realize a fabric having a modulus not lower to 200 Kg per linear millimeter of fabric. As can be noted from the tire shown at figure 1, under a quality point of view, the tires for motor-cycle have a tread-band strongly winding, with an accentuated curvature on the top. In particular, according to the description will be done hereinafter, such a curvature is indicated by the value of arrow "f" that is the ratio h/l, where "h" represents the radial height of the tread-band and "l" represents the maximum width of the above mentioned tread-band, as indicated at figure 1. Concerning the tire of the invention, the value "f" is ranging between 0.20 and 0.35. The exact measurement of the tread-band ends, necessary to get a reliable and real measures of l and h is easily obtained by virtue of the fact that the tread-band terminates with a live edge, which identifies the beginning of the side: this datum is proved particularly useful to the driver of the vehicle in order to appreciate when the maximum admittable angle of camber is achieved, during the curvilineal running direction or during drift. The radial height "r" of such edge, with respect to the bottom line of the bead, taken as reference line, coincides with the ends of the belt. Now, taking into consideration the belt structure of the above mentioned tire, which is the subject matter of the present invention, such a structure (fig.2) comprises, proceeding from inside to outside of the tire, a first strip 51 of reinforcing cords, parallelly arranged from each other inside the strip and inclined with respect to the m-m equatorial plane according to an angle w ranging between 22° and 35°. Adjacent to said first strip and superimposed to it, in radially outer position, is arranged a second strip 52, of rubberized fabric reinforced with cords parallelly arranged from each other inside the strip, crossed with the cords of the lower strip 51, and inclined with respect to the equatorial plane of the tire according to an angle v ranging between 18° and 30°. Among these fields of angular values, the difference between the value of v and that of w is always ranging between a minimum of 3° and a maximum of 15°, being "v" the angle having lesser value. According to the invention, the cords of the first strip 51 are of high modulus material: appropriate materials for this use are, for instance, steel, synthetical fibers of aromatic polyamide, or aramide (a type of these synthetical fibers is better known on the market as "Kevlar", a registered trade mark of Du Pont), or the synthetical fibers of cellulose, generally known as rayon. Contrarily, the cords of the second strip 52 are conveniently of low modulus material: for this use, appropriate materials are the already mentioned rayon, the synthetical fibers of polyester, the synthetical fibers of aliphatic polyamide generally known as nylon. In particular, as regards the cited materials, the following values (Kg/mm²) of the elasticity modulus can be given: - steel 21000 - aramide 6000 - standard viscose (rayon) 1800 - polyamide (nylon) from 200 to 800 - polyester from 560 to 1000 It will be anyhow clear that previous considerations regarding high and low modulus are referred not only to the absolute value of such a modulus but, above all, to the relative value between the modulii of the two types of cords utilized in the layers: for instance, rayon in combination with nylon or polyester must be considered a high modulus fiber; on the contrary, in combination with steel or aromatic polyamide is surely a low modulus material. Among the cited materials, those preferred by the Applicant are those of aramide fibers as regards the cords of the strip 51 and of nylon as regards the cords of strip 52; the preferred values for inclination angles choosen by the Applicant are of 25°+/-1° (24°-26°) for the high modulus cords and of 22°+/-1°(21°-23°) for the low modulus cords. As to the cited angular values, it must be said that they are intended measured on the top of the vulcanized tire, at the equatorial plane (mid-circumferential line). As a matter of fact, to determine the angle value of the cords in any other area, particularly at the extremity of the belt structure on the vulcanized tire would be neither easy nor certain. With reference to the present invention, also the density of the cords in the two belt strips represents an important characteristic particularly in connection with the behaviour of the tire in use. Such a density will however be appropriately lower than that usually utilized in the known structures: for instance as far as strip 51 is concerned, the Applicant has found convenient to utilize a density of 70 cords/dm, without anyhow exceeding the value of 80 cords/dm. The cords' thickness of strip 52 is then conveniently lesser than the density of the cords of strip 51: in particular the difference between such values of density will not be lesser than 5 cords/dm. The belt structure now described can be reinforced further on (fig. 3) with two lateral bands 7 of rubberized fabric reinforced with low modulus cords arranged in radially outer position on the ends of the lower couple of strips 51 and 52. The width "b" of each band is appropriately ranging between 5% and 15% of the overall width, measured along its own peripherical length, of the belt package with the cords of said bands inclined in the same direction, in respect with the circumferential direction of the tire, of those of 10 radially outer strip 51. The inclination of said cords can vary between 15° and 25°; they are, preferably, parallel to those of the said outer strip, anyhow, also a small angular difference between the two inclination is acceptable. In this case the difference between the value of the inclination angle "y" of the band's cords and that of the angle "v" of the cords of the radially outer strip does not exceed the value of 6°, the cords's angle of the bands being preferably that of lesser value. At last, it is advisable to put into evidence that both the position of the belt strips and the carcass type of the tire are rather important in relation to the present invention. As regards the reciprocal position among the breaker belt layers, it is thought advisable to arrange the strip of the low modulus cords in radially inner position and the strip of high modulus cords in radially outer position. On the other hand, it must be taken into consideration that, when the tire meets an obstacle during its running, the breaker belt suffers localized deformations that may provoke compression stresses on the strips cords. The stresses caused by such deformations are obviously stronger with the increase of the distance from the neutral axis of flexion of the belts package and, such neutral axis, due to the strong curvature of the tread-band is always in radially inner position with respect to the strip of the belt radially innermost. It happens that, arranging the high modulus strips in radially outer position together with the belt structure, they are subjected to stronger stresses: in fact the high modulus cords and specifically those of aramide, possess a lesser resistance than those of low modulus in respect to stresses to fatigue for compression and so they must be put as near as possible to the neutral axis and so in radially inner position; the opposite arrangement produces structures having lesser resistance to fatigue and so tires having shorter life. As concerns the carcass type adopted, it must be pointed out that the problem faced and solved by the present invention is that of belt structure with lesser longitudinal stiffness and of drift with respect to the known structure: it is obvious that a carcass having crossed plies holds a longitudinal stiffness on the top which partially cancels the advantages achieved by the structure of the invention, so the preferred matching of the belt structure according to the invention is that of a radial type carcass. As already hinted, the belt according to the invention has successfully solved the problem faced, in particular that of "kick-back": the use of two strips of cords having differentiated modulii and densities and inclination angles of the cords greater than those usual, has originated a belt that - although maintaining a strong belting effect - has a low longitudinal stiffness and consequently a strong capacity of absorbing impacts and localized deformations without transmitting dangerous impulses to the crank of the vehicle. Furthermore, the geometrical characteristics of the structure, in particular with a critical delimitation of the angular values of inclination of the reinforcing cords, allows to realize a tire which offers a dynamic behaviour, absolutely uniform, unable to generate in use serious anomalous thrustes in particular of drift (the low longitudinal stiffness implies also a lesser stiffness of drift); in such a situation the tire has a strong directional capacity and high transversal stability. Lastly, it is advisable to point out that the tire of the invention is specifically designed to be mounted on the front-wheel of the motor-cycle: as a matter of fact, as far as the rear wheel is concerned, although from a side the phenomenon of kick-back does not exist thanks to the the mechanical properties of the vehicle (by virtue of this all the impulses transmitted from the road to the tire are absorbed by the shock-absorber system of the motor-cycle), while - on the other hand - are preferred tires with a very strong belting capacity and also with high longitudinal stiffness, in order to develop very high running speed (up to 280 Km/hour) previously indicated.

Claims: A motorcycle tire comprising a pair of beads (2) at the side of the tire for anchoring said tire to a corresponding wheel rim; a textile carcass (1) extending between the beads and secured thereto; a tread band (4) radially outward of said carcass and having an arcuate cross-section whose height to width ratio (h/ℓ) has a value ranging between 0.20 and 0.35; a pair of sidewalls defined substantially between the radially outer ends of the beads (2) and the ends of the tread band (4); an annular reinforcing structure (5) positioned between said carcass (1) and said tread band (4) and comprising two strips of rubberized fabric reinforced with cords, characterized by the fact that the first strip (51) of said annular reinforcing structure (5) has cords parallelly arranged with respect to each other and having a density in the range of 70-80 cords/dm and inclined at an angle ranging between 22° and 35° to the equatorial tire plane, and that the second strip (52) of said annular reinforcing structure (5) lies in a radially outer position with respect to said first strip (51) and has cords parallelly arranged with respect to each other and having a density of less than the density of the cords of said first strip, the cords of the second strip having an elasticity modulus lower than the elasticity modulus of the cords of said first strip (51) and laying at an angle ranging between 18° and 30° with respect to the equatorial plane, said angle being smaller than the inclination angle of the cords of said first strip, the inclination difference between the cords of said first and second strips, with respect to said equatorial tire plane, ranging between 3° and 15°. A motorcycle tire according to claim 1, characterized in that the inclination angle of the cords of said first strip (51) is comprised between 24° and 26°, and the inclination angle of the cords of said second strip (52) is comprised between 21° and 23°. A motorcycle tire according to claim 2, characterized in that the reinforcing cords of said first strip (51) are of material selected from the group comprising steel, aramide, rayon. A motorcycle tire according to claim 2, characterized in that the reinforcing cords of said second strip (52) are of material selected from the group comprising rayon, polyamide, polyester. A motorcycle tire according to claims 1 to 4, characterized in that the difference of density between said first and second strips (51, 52) is not lesser than 5 cords/dm. A motorcycle tire according to claims 1 to 5, characterized in that said annular reinforcing structure (5) further comprises a pair of bands (7) of rubberized fabric reinforced with cords laid, in a radially outer position, upon said first and second strips (51, 52), the cords of said bands (7) having a low elasticity modulus and being inclined in the same direction as the cords of said second strip (52), according to an angle between 15° and 25° to the equatorial tire plane. A motorcycle tire according to claim 6, characterized in that the difference between the inclination value of said second strip (52) is not greater than 6°. A motorcycle tire according to any of claims 1 to 7, characterized in that said carcass is of radial type.

Inventors: GIANCOLA GUIDO, GIANCOLA, GUIDO
Applicants: PIRELLI, PIRELLI COORDINAMENTO PNEUMATICI SOCIETA PER AZIONI
Application year: 1989
Main classification: B60C9
IPC codes: B60C9, B60C3