Heavy duty pneumatic tires

A heavy duty pneumatic tire possessing excellent wet-running performances and wear resistance comprises a tread portion provided at its surface with a plurality of main grooves arranged on both sides with respect to an equatorial plane at a given interval in a circumferential direction of the tire and has a negative ratio of not more than 0.3. In this tire, the main grooves curvedly extend in a converging direction from each side end of the tread toward the equatorial plane and terminate on the equatorial plane or in the vicinity thereof within 25% of a tread width (W) centering around the equatorial plane so as to meet the terminals of opposed main grooves with each other or somewhat separate away them from each other and have particular inclination angle (.alpha.), and continuously projection lengths (L.sub.1, L.sub.2) in the widthwise and circumferential directions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to heavy duty pneumatic tires, and more particularly 
to a heavy duty pneumatic radial tire having satisfactory wear resistance 
at a relatively small negative ratio and good wet-running performance by 
enhancing an arrangement efficiency of tread grooves. 
2. Description of the Related Art 
In general, main grooves extending circumferentially of the tire and 
various grooves such as auxiliary grooves extending in the widthwise 
direction of the tire and the like are arranged in a tread portion of the 
tire, if necessary, in order to sufficiently develop wet-running 
performance such as traction and braking properties on wet road surface 
and so on. 
In order to improve wet-running performance, it is effective to increase a 
ratio of groove area occupied in a ground contact area of the tread 
portion or the so-called negative ratio. Inversely, it is favorable to 
reduce the ratio of groove area occupied in a ground contact region of the 
tread portion in order to improve wear resistance. That is, the 
wet-running performance and the wear resistance are conflicting with each 
other, so that it is difficult to simultaneously establish both the 
properties. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide a heavy duty 
pneumatic tire possessing sufficient wear resistance by setting the 
negative ratio to a relatively small value of not more than 0.3 and having 
good wet-running performance by rationalizing the arrangement and shape of 
the main grooves. 
According to the invention, there is the provision of a heavy duty 
pneumatic tire comprising a tread portion provided at its surface with a 
plurality of main grooves arranged on both sides with respect to an 
equatorial plane at a given interval in a circumferential direction of the 
tire in which the main grooves curvedly extend in a converging direction 
from each side end of the tread toward the equatorial plane and terminate 
on the equatorial plane or in the vicinity thereof so as to meet the 
terminals of opposed main grooves with each other or somewhat separate 
away them from each other and form a directional pattern by gradually 
entering into a ground contact region of the tread portion from the 
terminal toward the side end of the tread portion, the improvement wherein 
the tread pattern has a negative ratio is not more than 0.3, and the 
terminals of the opposed main grooves are located within 25% of a tread 
width (W) centering around the equatorial plane, and a part of the main 
groove located in a zone ranging from the equatorial plane to at least 25% 
of the tread width (W) satisfies that an inclination angle (.alpha.) of a 
tangential line (m) drawn to this part with respect to the equatorial 
plane gradually increases in accordance with the separation from the 
equatorial plane, and a continuously projection length (L.sub.1) of the 
main groove in the widthwise direction is not less than 30% of the tread 
width (W) and a continuously projection length (L.sub.2) of the main 
groove in the circumferential direction is not less than 70% of the tread 
width (W). 
The term "tangential line (m) drawn to the part of the main groove" used 
herein means a tangential line drawn to a phantom line passing through a 
widthwise center of the main groove. The term "continuously projection 
lengths (L.sub.1 and L.sub.2) of the main groove in the widthwise 
direction and the circumferential directions" used herein mean lengths 
produced when the phantom line passing through the widthwise center of the 
main groove are projected in the widthwise direction and the 
circumferential direction, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 is a tread portion in a typical heavy duty pneumatic tire 
according to the invention, in which numeral 1 is a tread portion, 
numerals 2a and 2b side ends of the tread portion, numeral 3 an equatorial 
plane of the tire, numerals 4a and 4b main grooves, and numerals 5a and 5b 
terminals of the main grooves, respectively. 
In the tire having a tread pattern shown in FIG. 1, a plurality of main 
grooves 4a and 4b are arranged in the tread portion 1 on both sides with 
respect to the equatorial plane 3 at a given interval in a circumferential 
direction of the tire in which these main grooves curvedly extend in a 
converging direction from each side end 2a, 2b of the tread toward the 
equatorial plane 3 and terminate in the vicinity of the equatorial plane 
3. The terminals 5a, 5b of opposed main grooves 4a, 4b are shifted from 
each other by a half pitch in the circumferential direction and separate 
away from each other at an equal distance from the equatorial plane 3. 
This forms a directional pattern by gradually entering into a ground 
contact region of the tread portion from the terminal 5a or 5b toward the 
side end 2a or 2b. 
In addition to the illustrated embodiment, the terminals 5a and 5b of the 
opposed main grooves 4a and 4b may take various positional relations. For 
example, these terminals 5a, 5b may be arranged in symmetry with each 
other with respect to the equatorial plane 3 without shifting them in the 
circumferential direction or join with each other on the equatorial plane 
or in the vicinity thereof (i.e. the main grooves 4a, 4b may communicate 
with each other). Alternatively, the main grooves 4a, 4b may be terminated 
in the vicinity of the equatorial plane 3 to separate away the terminals 
5a, 5b from each other in the widthwise direction and/or the 
circumferential direction. 
In the tire according to the invention, the negative ratio of the tread 
pattern is restricted to not more than 0.3, whereby good wear resistance 
can be ensured. According to the invention, the reason why the good 
drainage property is obtained even when the negative ratio is not more 
than 0.3 is based on the following fact. 
In general, when the tire is run on wet road surface under loading, water 
existing on road surface ahead the tire invades between the ground contact 
region of the tire and the ground to reduce a ground contact area of the 
tire tread directly contacting with ground (hereinafter referred to as 
actual ground contact area), whereby slipping of the tire on wet road 
surface is caused to degrade the wet-running performances. 
In tires having a conventional tread pattern such as a rib pattern, block 
pattern or the like, water invaded into the ground contact region is 
discharged through the grooves. In such a tire, however, the main groove 
is arranged in the circumferential direction of the tire therealong, so 
that water invaded into the ground contact region is pushed ahead the tire 
along the arranging direction of the main groove and hence the pushed 
water is again treaded by the tire to decrease the actual ground contact 
area, whereby the wet-running performances are degraded and particularly 
the braking performance in the running at higher speed is considerably 
degraded. 
In the heavy duty pneumatic tire according to the invention, however, the 
main groove is formed to gradually enter into the ground contact region of 
the tire from the terminal toward the tread end. The terminals 5a, 5b of 
the main grooves 4a, 4b are located within 25% of the tread width W 
centering around the equatorial plane of the tire and the inclination 
angle .alpha. of the tangential line m drawn to a part of the main groove 
in a zone ranging from the equatorial plane to at least 25% of the tread 
width W is gradually increased in accordance with the separation from the 
equatorial plane, preferably within a range of 5.degree.-25.degree.. 
Hence, water invaded into the ground contact region is pushed sideward 
from the tire along the arranging direction of the main groove and thus 
the pushed water is not again treaded by the tire, whereby the decrease of 
the actual ground contact area can be prevented to improve the wet-running 
performances. 
Furthermore, the continuous projection length L.sub.1 of the main groove in 
the widthwise direction is made to not less than 30% of the tread width W 
and the continuous projection length L.sub.2 of the main groove in the 
circumferential direction is made to not less than 70% of the tread width 
W, whereby water invaded into the ground contact region is efficiently 
discharged. When the continuous projection length L.sub.1 is less than 30% 
and the continuous projection length L.sub.2 is less than 70%, the 
drainage property is insufficient. 
Moreover, the negative ratio is preferably within a range of 0.25-0.29 from 
a viewpoint that the wear resistance and the wet-running performance are 
most balanced. 
In FIG. 1, the tread portion 1 is provided with auxiliary grooves 6a-6e 
opening to the two main grooves and two to three blind sipes 7 formed in 
an end portion of a block portion facing to the main groove. These 
auxiliary grooves and sipes may be arranged, if necessary, because the 
arrangement of the auxiliary grooves 6a-6e is to improve the traction and 
braking performance at a wet state and the arrangement of the blind sipes 
7 is to make the ground contact pressure of the block portion uniform. 
Since the arrangement and shape of the main groove in the tire according to 
the invention are rationalized as mentioned above, if the wet-running 
performance is equal to those of the conventional tire, the negative ratio 
can be made to not more than 0.3 as compared with that of the conventional 
tire, so that the satisfactory wear resistance can be obtained. When the 
negative ratio is not more than 0.3, as seen from FIG. 2, the wet-running 
performance of the tire having the conventional block pattern considerably 
lower, while the degree of lowering the wet-running performances is small 
in the tire having the tread pattern according to the invention, so that 
the invention is recognized to be superior to the conventional tire. 
The following examples are given in illustration of the invention and are 
not intended as limitations thereof. 
Invention tire 
A heavy duty pneumatic tire used in this example has a tread pattern shown 
in FIG. 1 and a tire size of 11R22.5, in which a plurality of main grooves 
4a and 4b are arranged in a tread portion 1 on both sides with respect to 
an equatorial plane 3 at a given interval in a circumferential direction 
of the tire to curvedly extend in a converging direction from each side 
end 2a, 2b of the tread toward the equatorial plane 3 and terminate in the 
vicinity of the equatorial plane 3 to shift the terminals 5a, 5b of 
opposed main grooves 4a, 4b from each other by a half pitch (50 mm) in the 
circumferential direction and separate away them from each other at an 
equal distance (8 mm) from the equatorial plane 3 and form a directional 
pattern by gradually entering into a ground contact region of the tread 
portion from the terminal 5a or 5b toward the side end 2a or 2b. In this 
case, the negative ratio is 0.27. 
When a portion of the main groove 4a, 4b is located in a zone ranging from 
the equatorial plane 3 to 25% of the tread width W, an inclination angle 
.alpha. of a tangential line m drawn to this part with respect to the 
equatorial plane 3 is gradually increased in accordance with the 
separation from the equatorial plane 3 so as to be 5.degree. at the 
positions of the terminals 5a, 5b and 25.degree. at the position 
corresponding to 25% of the tread width. 
Further, the continuously projection length L.sub.1 of the main groove 4a, 
4b in the widthwise direction is 48% of the tread width W, and the 
continuous projection length L.sub.2 of the main groove 4a, 4b in the 
circumferential direction is 105% of the tread width W. 
The main groove has a groove depth of 16.5 mm and a groove width of 2.5 mm 
at a position of the terminal 5a or 5b and 7 mm at a position of the tread 
end 2a or 2b. 
Furthermore, the tread portion 1 is provided with auxiliary grooves 6a-6e 
opening to the two main grooves and two to three blind sipes 7 formed in 
an end portion of a block portion facing to the main groove. 
Moreover, a carcass is comprised of a single rubberized radial cord ply 
containing steel cords therein, and a belt is comprised of four rubberized 
cord layers in which a cord angle with respect to the circumferential 
direction of the tire is 50.degree. upward to the right in a first layer, 
18.degree. upward to the right in a second layer, 18.degree. upward to the 
left in a third layer and 18.degree. upward to the left in a fourth layer 
and cords of the second and third layers are crossed with each other. The 
other tire structures are substantially the same as in the conventional 
tire. 
Comparative tire 
A tire used in this comparative example is substantially the same as in the 
above example except that a portion of the main groove 4a, 4b located in a 
zone ranging from the equatorial plane 3 to 25% of the tread width W is 
straightway extended from the equatorial plane 3 at such a certain 
inclination angle that the inclination angle .alpha. of the tangential 
line m drawn to this part with respect to the equatorial plane 3 is 
15.degree. at the positions of the terminals 5a, 5b and 15.degree. at the 
position corresponding to 25% of the tread width. 
Conventional tire 
A tire used in this conventional example is a tire having the conventional 
block pattern as shown in FIG. 3, in which the negative ratio is 0.32%. 
Test Method and Results 
The wet-running performance and wear resistance are evaluated with respect 
to the above tires as follows. 
Each of the tires to be tested is run on wet road surface and braked at a 
speed of 40 km/h or 60 km/h, during which a running distance till the stop 
of a vehicle is measured. The wet-running performance is evaluated by a 
reciprocal of the running distance as an index of braking performance at 
wet state on the basis that the comparative tire is 100, in which the 
larger the index value, the better the wet-running performance. The 
measured results are shown in FIG. 4. As seen from FIG. 4, the invention 
tire is excellent in the wet-running performance at either brake-starting 
speeds as compared with the comparative tire. 
Then, each of the tires to be tested is actually run on road over a 
distance of 80,000 km and thereafter an amount of the tread rubber worn 
away is measured, from which a running distance (km) per 1 mm of worn 
amount is calculated. The wear resistance is evaluated by an index of such 
a running distance on the basis that the conventional tire is 100, in 
which the larger the index value, the better the wear resistance. As a 
result, the index value of the comparative tire is 120 and that of the 
invention tire is 133, from which it is apparent that the invention tire 
is most excellent in the wear resistance. 
As mentioned above, according to the invention, water invaded into the 
ground contact region is pushed sideward from the tire along the arranging 
direction of the main groove and hence the pushed water is not again 
treaded by the tire different from the conventional tire, whereby the 
decrease of the actual ground contact area can be prevented to improve 
wet-running performance. Furthermore, even when the negative ratio is made 
sufficiently small as compared with the conventional tire, sufficient 
wet-running performance is obtained together with the excellent wear 
resistance. 
Thus, the invention can provide heavy duty pneumatic tires simultaneously 
satisfying the wet-running performance and wear resistance, which have 
never been attained in the tires having the conventional tread pattern.