1. Field of the Invention
This invention relates to tyres for vehicle wheels, in particular tyres intended for use on the steering axle of trucks or articulated lorries for medium/heavy transport on long journeys, generally on motorways, at relatively high speed.
2. Description of the Related Art
Medium/heavy transport vehicles generally have different types of tyre fitted on the driving axles and steering axles depending on the different behavioural and functional prerogatives required of the tyres under the different working conditions in which they are used.
In general, tyres intended for use on the steering axle and for long motorway journeys need: good evenness of wear, combined with a good mileage; good directional stability and steering precision on dry surfaces; good grip on wet surfaces; good resistance with respect to tearing of the tread and to lateral shocks; low rolling resistance; and the ability to expel small stones trapped in the grooves. Factors contributing to producing these features are the tread design, the profile of the inflated tyre and the structure of the tyre.
In connection with the tread design, it is preferably of the type comprising a plurality of continuous circumferential ribs, two shoulder ribs and one or more centrally positioned ribs, delimited by a corresponding plurality of circumferential grooves.
The problem of uneven wear in tyres of this type is well-known in the art. For most of its life, the tyre is used for long journeys on straight stretches, with minimal stresses due to maneuvering and/or bends. In long journeys, the failure to optimize the distribution of the tyre/ground contact pressures on the tyre's footprint may lead to continuous microsliding of parts of the tread on the asphalt, consequently inducing premature wear of these parts of the tread and, in any case, uneven wear.
One of the parts of the tread particularly sensitive to this unevenness of wear is the shoulder zone comprising both the axially external edge of the shoulder rib and also the zone straddling the shoulder groove, i.e. the groove delimiting the aforementioned shoulder rib in the position axially internal to the latter. Various solutions have been proposed to solve the problem of uneven wear in the shoulder zone.
This invention derives from the perception of the technical problem of uneven wear in the shoulder zone of the tyre, dependent on the radially external profile of the surface of the tread in the said zone.
U.S. Pat. No. 4,480,671 describes a tyre for wheels of vehicles for heavy transport characterized in that the tread comprises in each of the two lateral zones a narrow circumferential groove which defines a lateral rib having a meridian profile substantially parallel and lowered with respect to the meridian profile of the geometrical envelope of the median zone of the tread. The width of the said lateral rib is at most equal to the maximal depth of the groove; the difference in level between the surface of the lateral ribs and that of the surface of the median zone of the tread is between 40% and 200% of the deflection of the tread under load and is such that, under normal working conditions, the said lateral ribs are in contact with the ground.
The patent application WO 92/2380 describes a tyre for wheels of vehicles for heavy transport comprising a radial carcass, a belt structure comprising at least two strips of different axial width and a tread band having in the lateral zones a circumferential groove of width preferably between 0.6 and 8 mm, delimiting a lateral rib the meridian profile of which is substantially parallel and lowered with respect to the meridian profile of the geometrical envelope of the median zone. The difference between the width of the tread band and the axial distance between the median axes of the lateral grooves is between 34 and 80 mm. This solution improves the separation resistance between the extremities of the belt strips and between the extremities of the entire belt structure and the carcass structure.
The patent application WO 98/26945 describes a tyre for wheels of vehicles for heavy transport comprising a tread band having a plurality of main ribs defined by circumferential grooves and a sacrifical rib in both the lateral zones of the tyre, separated from the main ribs by a narrow shoulder groove having a maximum width of 1.5 mm. The sacrifical rib has a width of between 2.5% and 12% of the width of the tread band and a constant radial lowering with respect to the transversal profile of the main ribs of between 0.5 and 2 mm, when the tyre is new.
The abstract of patent JP 05-77608 describes a radial tyre for wheels of heavy transport vehicles in which a pair of thin circumferential grooves is defined on the tread band. A first arc of a circle defines the tread profile between the equatorial plane and the thin groove and a second arc of a circle defines the profile of the tread between the thin groove and the shoulder edge, in order to radially lower the tread profile in the region around the thin circumferential groove, both towards the centre of the tread and towards the shoulder edge.
U.S. Pat. No. 5,660,652 describes a tyre for use on the steering axle comprising a tread having a plurality of continuous circumferential grooves, a pair of thin grooves in the shoulder zone, a pair of radially lowered ribs local to the shoulder edges and a plurality of main ribs. The tread has a radially external surface having a curvature initially defined by a radius of curvature internal to the tyre, followed by a curvature defined by a radius of curvature external to the tyre. The main shoulder rib has a progressively increasing thickness starting from the axially internal, adjacent circumferential groove and reaching a maximum value close to the thin groove. Straddling the said circumferential groove adjacent to the main shoulder rib, the tyre/ground contact pressures equalize, whereas a greater contact pressure is obtained on the shoulder edge. This distribution of tyre/ground contact pressures is obtained thanks to the progressive increase in thickness of the tread from the point of contraflexure corresponding to the inversion of the curvature to the thin shoulder groove. The added rubber effectively provides material in the tread shoulder region, where the propensity to wear is high, and increases the contact pressure of the region adjacent to the thin groove.