TECHNICAL FIELD

The present invention relates to heavy duty tires.

BACKGROUND ART

Life performance is one of the important performances in heavy duty tires which are mounted on commercial vehicles such as trucks, buses, and the like. In a tire mounted on a drive shaft of a commercial vehicle, the contact pressure in a crown region including the equator tends to be high, and wear is likely to occur in the crown region.

Depending on an area where tires are used, a tire may be used under a light load condition (for example, a load that is 60% or less of a normal load) due to load regulation. In this case, the tire runs in a state where the contact pressure at each shoulder side is low. The contour line at each circumferential direction side of the ground-contact surface tends to bulge in the crown region, and wear tends to progress in the crown region.

In order to improve life performance, development of a tire that evenly wears the entirety of a tread thereof even when used under a light load condition is in progress (for example, PATENT LITERATURE 1 below).

CITATION LIST

Patent Literature

PATENT LITERATURE 1: Japanese Laid-Open Patent Publication No. 2018-127199

SUMMARY OF INVENTION

Technical Problem

In the heavy duty tire disclosed in PATENT LITERATURE 1 described above, the position of an inflection point between a crown circular arc and a shoulder circular arc is controlled in a contour of a tread surface in a state where the internal pressure of the tire is adjusted to 5% of a normal internal pressure. However, under a light load condition, a peak of contact pressure is expected to occur at the inflection point, depending on the magnitude of the radius of the crown circular arc, and there is a concern that wear may progress near the inflection point. Technology to evenly wear the entirety of a tread under a light load condition has not been established.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a heavy duty tire that has excellent uneven wear resistance under a light load condition and can contribute to improvement of life performance thereof.

Solution to Problem

A heavy duty tire according to an aspect of the present invention includes a tread having a tread surface that comes into contact with a road surface. In the tire in a normal state where the tire is mounted on a normal rim, an internal pressure of the tire is adjusted to a normal internal pressure, and no load is applied to the tire, a profile of the tread surface includes a crown circular arc as a circular arc representing a profile of a portion including an equator, and the crown circular arc has a radius of not less than 1600 mm and not greater than 2000 mm. When: a ratio of an equator ground-contact length measured along the equator, to a reference ground-contact length at a position corresponding to a width that is 70% of a maximum ground-contact width, on a ground-contact surface obtained when a load is applied to the tire in the normal state and the tread surface is brought into contact with a flat road surface, is defined as a shape index; the shape index obtained when the load is 30% of a normal load is defined as an ultra-light load shape index; and the shape index obtained when the load is 60% of the normal load is defined as a light load shape index, the ultra-light load shape index is higher than the light load shape index, the ultra-light load shape index is not less than 1.10 and not greater than 1.20, and the light load shape index is not less than 1.04 and not greater than 1.14.

Preferably, in the heavy duty tire, when the shape index obtained when the load is the normal load is defined as a heavy load shape index, the heavy load shape index is lower than the light load shape index, and the heavy load shape index is not less than 1.00 and not greater than 1.10.

Preferably, the heavy duty tire further includes a belt located inward of the tread in a radial direction. The belt includes a plurality of layers stacked in the radial direction, and each of the layers includes a large number of belt cords aligned with each other. The plurality of layers include a first reference layer having a largest width in an axial direction, and a second reference layer stacked radially outward of the first reference layer. An angle of the belt cords in each of the first reference layer and the second reference layer relative to an equator plane is not less than 14° and not greater than 17°.

Preferably, in the heavy duty tire, at least four circumferential grooves aligned in an axial direction are formed on the tread, thereby forming at least five land portions. Among the at least five land portions, land portions including ends of the tread surface are shoulder land portions, land portions located inward of the shoulder land portions in the axial direction are middle land portions, and a land portion located inward of the middle land portions in the axial direction is a crown land portion. A ratio of a width of each middle land portion to a width of the crown land portion is not less than 0.95 and not greater than 1.05, and a ratio of a width of each shoulder land portion to the width of the crown land portion is not less than 0.85 and not greater than 0.95.

Preferably, in the heavy duty tire, on each of the middle land portions and the crown land portion, a sipe is formed so as to traverse the land portion. In the sipe, an edge shape of an opening open on the tread surface has a zigzag portion that extends in a zigzag manner by repeating peaks and valleys. In a depth direction of the sipe, the edge shape of the opening is substantially retained, and first displacement portions in each of which a position of the zigzag portion is displaced to one side in a longitudinal direction of the sipe and second displacement portions in each of which the position of the zigzag portion is displaced to another side in the longitudinal direction of the sipe are repeated.

Preferably, in the heavy duty tire, a ratio of a thickness of the tire at each end of the tread surface to a thickness of the tire at the equator is not less than 1.30 and not greater than 1.35.

Advantageous Effects of Invention

In the heavy duty tire according to the present invention, the entirety of the tread is evenly worn under a light load condition. The tire has excellent uneven wear resistance under the light load condition and can contribute to improvement of life performance thereof.

