Resilient tires and wheels – Tires – resilient – Anti-skid devices
Reexamination Certificate
2000-10-27
2003-02-11
Johnstone, Adrienne C. (Department: 1733)
Resilient tires and wheels
Tires, resilient
Anti-skid devices
C152S532000, C156S096000, C156S123000, C156S128100, C156S130000, C428S411100
Reexamination Certificate
active
06516847
ABSTRACT:
BACKGROUND OF THE INVENTION
In a general aspect, the present invention relates to a tire for vehicles provided with a tread having a low rolling resistance.
More particularly, the present invention relates to a tire for vehicles comprising at least one belt layer coaxially extending around at least one carcass ply, a composite tread coaxially extending around the belt layer and comprising a radially outer layer adapted to get in touch with the ground and a radially inner layer interposed between the radially outer layer and the belt layer.
PRIOR ART
In the field of tire production, the main object of an increasingly greater part of research is that of obtaining an optimum compromise between, on the one hand, the tire rolling resistance—which should be reduced to the minimum—and, on the other hand, the characteristics of handling and comfort of the tire.
This object is particularly difficult to achieve when the tire is provided with a tread of the composite type, i.e. a tread comprising a radially outer layer and a radially inner layer having mechanical and hysteretic characteristics different from each other.
This particular tread structure is, in fact, particularly affected by the conflict existing between the opposite hysteretic characteristics necessary to achieve the aforementioned object. In order to reduce the rolling resistance of the tire, in fact, it is necessary to provide a radially inner layer of the tread having a low hysteresis value (which may be related to the value of tang &dgr; at 70° C.) and, as such, suitable to dissipate a limited amount of energy during rolling: in so doing, however, handling and comfort of the tire worsen, since these characteristics may be achieved by using rubber compositions having a high hysteresis value and, as such, adapted to dissipate an amount of energy suitable to ensure a high adhesion between the tread and the ground.
In order to limit this drawback, as described for instance in U.S. Pat. No. 4,319,619, it has been proposed to use a tread of the composite type comprising a radially inner layer, obtained by vulcanizing a rubber composition including natural rubber and/or diene rubbers, having a value of tango at 40° C. not higher than 0.2 and a value of the modulus of elasticity E′ not lower than 120 kg/cm .
According to what is reported in the aforementioned patent and thanks to the use of a rubber composition having a low value of tang &dgr; and a high value of the modulus of elasticity in the radially inner layer of the tread, it is possible to reduce the rolling resistance of the tire without causing a worsening of the tire properties at high speeds or during running on a dry road.
However, the applicant has found that such a tread may give rise to a worsening of the comfort characteristics of the tire which dampens to a smaller extent the unevenness of the ground.
In order to obtain improved handling characteristics of the tires, the prior art has also proposed, as for instance described in Canadian patent CA 1 228 282, to use a composite tread comprising a radially inner layer, having a value of the modulus of elasticity E' at 25° C. comprised between 100 and 250 kg/cm
2
, and a radially outer layer, having a value of the modulus of elasticity E' at 25° C. comprised between 70 and 150 kg/cm
2
and a value of tang &dgr; not lower than 0.25, wherein the ratio between the modulus of elasticity of the radially inner layer and that of the radially outer layer (measured at 25° C.) is not lower than 1.15.
However, the Applicant has found that the tires made according to said Canadian patent, while substantially meeting the purpose, possess high hysteresis values and therefore a high rolling resistance.
SUMMARY OF THE INVENTION
The technical problem underlying the present invention is therefore that of providing a tire for vehicles provided with a tread of the composite type which would allow to obtain, with respect to the tires of the cited prior art, a better compromise between, on the one hand, the rolling resistance of the tire and, on the other hand, the characteristics Of handling and comfort thereof.
According to the invention, the aforementioned technical problem is solved by a tire for a vehicle comprising at least one carcass ply, at least one belt layer coaxially extending around the at least one carcass ply, and a composite tread coaxially extending around the at least one belt layer. The composite tread comprises a radially-outer layer adapted to contact the ground and a radially-inner layer interposed between the radially-outer layer and the at least one belt layer. A first ratio of a modulus of elasticity E′ at 70° C. of the radially-inner layer to a modulus of elasticity E′ at 70° C. of the radially-outer layer is greater than or equal to 1.1:1 and less than or equal to 3:1, and a second ratio of a loss tangent at 70° C. of the radially-inner layer to a loss tangent at 70° C. of the radially-outer layer is less than 0.8:1.
Surprisingly, the Applicant has found that in order to achieve the best possible compromise between rolling resistance, handling and comfort of the tire, it is not sufficient to optimize in a broad sense the hysteretic or modulus characteristics of the radially inner layer or those of the radially outer layer of the tread, but that it is necessary to provide that the values of the modulus of elasticity and tang &dgr; of the radially inner layer of the tread with respect to the radially outer layer— measured at a specific temperature substantially equal to the running temperature of the tire in normal use conditions (70° C.)—fall within a specific range of values.
More particularly, the applicant has found that to achieve the best possible compromise between rolling resistance, handling and comfort of the tire, it is necessary that:
i) the ratio between the value of the modulus of elasticity measured at 70° C. of the radially inner layer and the corresponding value of the radially outer layer be comprised between 1.1 and 3, and that
ii) the ratio between the value of tang &dgr; at 70° C. of the radially inner layer and the corresponding value of the radially outer layer be lower than 0.8.
By respecting these ratios between the values of modulus of elasticity and tang &dgr; at 70° C. of the two layers of the composite tread, it has been found that it is not only possible to reduce the rolling resistance of the tire, but also—and quite surprisingly—to achieve better handling and comfort performances with respect to those achieved by known tires for the same type of use.
Preferably, the ratio between the modulus of elasticity E′ at 70° C. of the radially inner layer and the modulus of elasticity E′ at 70° C. of the radially outer layer is comprised between 1.2 and 2.5 and, still more preferably, between 1.5 and 2.
Preferably, furthermore, the ratio between the value of tang &dgr; at 70° C. of the radially inner layer and the value of tang &dgr; at 70° C. of the radially outer layer of the tread is lower than 0.7 and, still more preferably, comprised between 0.2 and 0.5.
In this way it has been advantageously possible to achieve an optimum compromise between the rolling resistance of the tire and its handling and comfort performances.
Preferably, and in order to achieve the aforementioned ratios, the modulus of elasticity E′ at 70° C. of the radially inner layer of the tread is comprised between 5 and 14 MPa, while the modulus of elasticity E′ at 70° C. of the radially outer layer is comprised between 4 and 8 MPa.
More preferably, the modulus of elasticity E′ at 70° C. of the radially inner layer is comprised between 6 and 12 MPa, still more preferably between 6.5 and 9 MPa, while the modulus of elasticity E′ at 70° C. of the radially outer layer is comprised between 5 and 7 MPa.
By fulfilling the aforementioned values of the modulus of elasticity E′ at 70° C. of the radially inner layer, it has been found that it is also possible to gradually improve the handling and comfort performances of the tire, while keeping low the rollin
Amaddeo Angela
Isola Mario
Serra Antonio
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Johnstone Adrienne C.
Pirelli Pneumatici S.p.A.
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