TECHNICAL FIELD

The present invention relates to a tread band.

The present invention may be used to advantage on TBR (Truck and Bus Radial), i.e. heavy-duty vehicle, winter tyres.

BACKGROUND ART

Currently marketed heavy-duty vehicle tyres have a tendency to pick up road surface debris (typically stones), which clings, even for long periods of time, to the inside of the longitudinal (circumferential) tread grooves. As the tyre rolls along the road surface, the debris clinging to the longitudinal grooves in the tread is rammed cyclically to the bottom of the grooves, thus damaging the rubber (in the worst case scenario, the debris may even be elongated, with a pointed tip facing the bottom of the groove) . In addition, debris retention inside the longitudinal tread grooves forms Mams' , which prevent water from running off and draining along the grooves, thus impairing wet-pavement road-holding performance of the tyre.

Debris retention inside the longitudinal tread grooves is especially problematic in the case of winter tyres, which have at least one zigzag-shaped longitudinal groove - i.e. each sidewall of the longitudinal groove has a continuous sequence of alternating transverse (axial) projections and recesses - designed to trap snow (and so improve road-holding performance on snow-covered roads) , but which also forms a succession of longitudinal debris-retaining ^pockets' . DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a tread band designed to eliminate the above drawbacks, and which, in particular, is cheap and easy to produce.

According to the present invention, there is provided a tread band as defined in the accompanying Claims .

BRIEF DESCRIPTION OF THE DRAWINGS

A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which :

Figure 1 shows a schematic side section of a tyre featuring a tread band in accordance with the present invention;

Figure 2 shows a schematic, larger-scale plan view of part of the Figure 1 tyre tread band;

Figure 3 shows a larger-scale detail of Figure 2;

Figure 4 shows a schematic, larger-scale cross section along line IV-IV of a longitudinal groove of the Figure 1 tyre tread band;

Figure 5 shows a schematic, larger-scale cross section along line V-V of a longitudinal groove of the Figure 1 tyre tread band;

Figure 6 shows a schematic view in perspective, with parts removed for clarity, of a longitudinal groove of the Figure 1 tyre tread band.

PREFERRED EMBODIMENTS OF THE INVENTION

Number 1 in Figure 1 indicates as a whole a tyre comprising a toroidal body ply 2, which has two beads 3 and supports a tread band 4 made of cured-rubber-based material. A tread belt 5, comprising two tread plies 6, is interposed between body ply 2 and tread band 4. Each tread ply 6 comprises a rubber strip embedding a number of cords (not shown) , which are arranged side by side with a given spacing, and slope at a given angle to the equatorial plane of tyre 1. Body ply 2 also supports two sidewalls 7 between tread band 4 and beads 3.

As shown more clearly in Figure 2, tread band 4 has a rolling surface 8, which bounds tread band 4 externally (i.e. is located radially outwards) and, in use, rests on the road surface. Rolling surface 8 of tread band 4 has a raised pattern defined by a number of longitudinal or circumferential grooves 9 (i.e. extending along the circumference, and crosswise to the axis of rotation, of tyre 1) , and by a number of transverse grooves 10 (i.e. parallel to the axis of rotation of tyre 1 and perpendicular to longitudinal grooves 9) . Longitudinal grooves 9 and transverse grooves 10 form five longitudinal rows of blocks 11 projecting radially from tread band 4, and each of which is roughly parallelepiped-shaped with a rectangular or trapezoidal cross section, and is bounded laterally by grooves 9 and 10.

More specifically, tread band 4 comprises two side by side, central longitudinal grooves 9a; and two lateral longitudinal grooves 9b with a rectangular cross section and located on either side of central longitudinal grooves 9a. In one embodiment, the two lateral longitudinal grooves 9b may be shallower (even 50% shallower) than central longitudinal grooves 9a. In another embodiment, not shown, tread band 4 may comprise a further two lateral longitudinal grooves on either side (i.e. outwards) of the two lateral longitudinal grooves 9b.

As shown in Figures 4 and 5, each longitudinal groove 9a is bounded by a bottom wall 12; and two sidewalls 13 on either side of bottom wall 12 and forming respective outer edges 14 with rolling surface 8.

As shown in Figure 3, the two sidewalls 13 of each central longitudinal groove 9a have a continuous sequence of alternating transverse (axial) projections 15 and recesses 16, so that, in plan view, central longitudinal groove 9a is zigzag-shaped (i.e. with abrupt alternating right and left turns) . More specifically, each projection 15 of one sidewall 13 faces and is aligned (coincident) with a recess 16 in the other sidewall 13, so the transverse width of each central longitudinal groove 9a (i.e. the distance between the two sidewalls 13) is constant longitudinally (i.e. for a given depth or radial position).

As shown in Figures 4 and 5, each sidewall 13 of each central longitudinal groove 9a has an outer portion 17 extending between an outer edge 14 and an intermediate line 18, and which slopes at an obtuse angle a or β (i.e. of over 90°) with respect to bottom wall 12; and an inner portion 18 extending between intermediate line 18 and bottom wall 12, and which forms a right-angle (i.e. of 90°) with bottom wall 12. Outer portion 17 of each sidewall 13 slopes at an obtuse angle a to bottom wall 12 at all the projections 15, and at an obtuse angle β, different from obtuse angle a, to bottom wall 12 at all the recesses 16. Each central longitudinal groove 9a is thus asymmetrical (i.e. varies in cross section, as shown clearly in Figures 4 and 5) , by part of each sidewall 13 sloping at obtuse angle a to bottom wall 12 (at projections 15), and the rest of sidewall 12 sloping at obtuse angle β to bottom wall 12 (at recesses 16) .

Each central longitudinal groove 9a has a rectangular cross section between inner portions 19 of sidewalls 13, and a trapezoidal cross section between outer portions 17 of sidewalls 13.

In a preferred embodiment, obtuse angle β is greater than obtuse angle a. More specifically, obtuse angle β may range between 110° and 115°, and obtuse angle a between 100° and 105°. These values allow for achieving the best compromise between the need to expel debris trapped inside the two central longitudinal grooves 9a, and the need to ensure high performance (particularly road-holding performance) on snow.

In a preferred embodiment shown in Figure 6, each projection 15 is bounded laterally, inside central longitudinal groove 9a, by two shoulders 20, which are located on either side of projection 15, connect projection 15 to the two recesses 16 on either side of (i.e. adjacent to) projection 15, and are triangular in shape, with a bottom vertex at intermediate line 18.

Tread band 4 has numerous advantages.

In particular, it is cheap and easy to produce using a conventional mold.

Moreover, it provides for excellent road-holding on snow, due to the zigzag pattern of the two central longitudinal grooves 9a, which 'trap' snow inside.

Any debris trapped inside the two central longitudinal grooves 9a is expelled by virtue of the slope of sidewalls 13, which exerts outward radial thrust on the debris to expel it radially.

The narrow width of the bottom of each central longitudinal groove 9a (i.e. the part between inner portions 19 of sidewalls 13) greatly reduces the likelihood of the tip of any pointed debris, trapped inside a central longitudinal groove 9a, reaching and damaging the bottom of the groove.

Finally, tread band 4 described is ideal for year- round use. Tyre 1 is fitted, new, at the start of winter, to run on snow throughout the winter with deep longitudinal grooves 9 for good road-holding performance. By the time longitudinal grooves 9 are worn (i.e. lateral longitudinal grooves 9b are practically gone, and central longitudinal grooves 9a have practically lost their zigzag pattern and are substantially worn down to the bottom portion between inner portions 19 of sidewalls 13) , winter is over, and, though performance of tread band 4 (in particular, road- holding performance) is poor on snow (which is no longer an issue by the end of winter) , it is still good on snow-free road surfaces (which means any, by the end of winter) .