A heavy-duty radial tire including a carcass and four steel cord belts bearing most of stress of the tire. The carcass and the belts cross each other. An outer side belt layer of two belt layers of different cord diameters which are defined by the belts being divided into the two belt layers, the outer side belt layer being a belt layer of the two belt layers which is disposed at an outer circumferential side of the tire in a radial direction thereof; and an inner side belt layer disposed further toward an inner circumferential side of the tire in the radial direction thereof than the outer side belt layer. A difference between belt cord angles of respective belts of the outer side belt layers is less than or equal to 5.degree.. A difference between belt cord angles of respective belts of the inner side belt layers is less than or equal to 5.degree.. Further, 1<.PHI. out/.PHI. in.ltoreq.2.5, and .alpha. in-.alpha. out.gtoreq.5.degree. and (.alpha. out+.alpha. in)/2.ltoreq.23.degree., wherein: .PHI. out=belt cord diameter of the outer side belt layer; .alpha. out=average value of absolute values of belt cord angles, defined by an equatorial plane of the tire, of the outer side belt layers; .PHI. in=belt cord diameter of the inner side belt layer; and .alpha. in=average value of absolute values of belt cord angles, defined by the equatorial plane of the tire, of the inner side belt layers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a heavy-duty radial tire, and in 
particular, to a heavy-duty radial tire which is used for a vehicle which 
often travels on bad roads. 
2. Description of the Related Art 
Conventionally, heavy-duty radial tires, and in particular, heavy-duty 
radial tires used on bad roads, are easily cut when the tires travel over 
relatively sharp protrusions such as rocks or the like. The cut may extend 
to the belt layer which is disposed at an inner side in the radial 
direction of the tire of the tread rubber layer, thereby resulting in 
so-called cut separation, in which a separation forms between the tread 
rubber layer and the belt layer, or so-called cut through, in which the 
cut tears from the outer layer which was cut by the protrusion and 
penetrates through. 
A method of preventing these cuts is known in which, as illustrated in FIG. 
6, the angles, which are defined by the equatorial plane of the tire, of 
respective cords of a first belt layer 74, a second belt layer 76, a third 
belt layer 78, and a fourth belt tire 80 of a heavy-duty radial tire 70 
are made small, e.g., the angles are changed from 25.degree. to 
21.degree.. The expansion of the outer diameter of the heavy-duty radial 
tire 70 is suppressed, rigidity in the circumferential direction is 
increased, and circumferential direction shearing distortion on the belt 
is decreased. Further, a method is known of using thick steel cords in the 
belt layer so as to increase the tearing resistance, which is 
proportionate to the cord diameter. These methods of uniformly thickening 
the cord diameters of the entire belt and uniformly decreasing the belt 
angles defined by the equatorial plane of the tire in order to avoid an 
excessive concentration of distortion are used in conventional cut 
resistant structures of belt layers. 
However, in conventional heavy-duty radial tires utilizing these types of 
methods, the methods are effective in dealing with defects caused by 
gradual tearing from the outer belt layer due to relatively sharp 
protrusions. However, on rough roads, the vehicle frequently travels over 
such relatively sharp protrusions. 
In such a case, as illustrated in FIG. 7, an entire belt layer 84 of a 
heavy-duty radial tire 82 is subject to bending deformation in the 
circumferential direction due to a relatively sharp protrusion 83. As a 
result, the circumferential direction tensile force T of an inner layer 
belt 86 at the inner side of the belt layer 84 increases, resulting in the 
inner belt layer 86 tearing before a belt 88 of the outer layers. The tear 
extends throughout the entire belt layer 84 at once, leading to a 
so-called cut burst defect. 
Further, as illustrated in FIGS. 8A and 8B, when the total gauge of the 
belt layer 84 is changed from G1 to G2 due to the use of thick cords, the 
inner layer tensile force T2 in the circumferential direction increases 
proportionately to the total gauge in comparison to T1 when the tire is 
subject to bending deformation in the circumferential direction due to the 
relatively sharp protrusion 83. Therefore, tearing of the inner belt layer 
86 begins at a point P and immediately causes a cut burst defect. Namely, 
when thick cords are used, the total gauge of the belt layer becomes 
thicker. Therefore, thick cords are disadvantageous with respect to 
defects in which the inner layer tears first. Further, reducing the belt 
angles defined by the equatorial plane of the tire is also disadvantageous 
for defects in which the circumferential direction tensile force increases 
and the inner layer tears first due to a sharp protrusion. 
SUMMARY OF THE INVENTION 
In view of the aforementioned, an object of the present invention is to 
provide a heavy-duty radial tire in which cut resistance of belt layers is 
improved. 
The present invention is a heavy-duty radial tire including a carcass 
formed of a single layer of steel cord, and at least four belts formed 
from steel cords and bearing most of stress of the tire, wherein the 
carcass and the at least four belts are disposed so as to cross each 
other. The heavy-duty radial tire also includes: an outer side belt layer 
of two belt layers of different cord diameters which are defined by the at 
least four belts being divided into the two belt layers, the outer side 
belt layer being a belt layer of the two belt layers which is disposed at 
an outer circumferential side of the tire in a radial direction of the 
tire; and an inner side belt layer disposed further toward an inner 
circumferential side of the tire in the radial direction of the tire than 
the outer side belt layer. The at least four belts are provided such that, 
when a belt cord diameter of the outer side belt layer is .PHI. out, an 
average value of absolute values of belt cord angles, which are defined by 
an equatorial plane of the tire, of the outer side belt layers is .alpha. 
out, a belt cord diameter of the inner side belt layer is .PHI. in, and an 
average value of absolute values of belt cord angles, which are defined by 
the equatorial plane of the tire, of the inner side belt layers is .alpha. 
in, a difference between belt cord angles of respective belts of the outer 
side belt layers is less than or equal to 5.degree., and a difference 
between belt cord angles of respective belts of the inner side belt layers 
is less than or equal to 5.degree., and 1&lt;.PHI. out/.PHI. in.ltoreq.2.5, 
and .alpha. in-.alpha. out.gtoreq.5.degree., and (.alpha. out+.alpha. 
in)/2.ltoreq.23.degree.. 
In accordance with the present invention having the structure described 
above, the ratio .PHI. out/.PHI. in of the belt cord diameter .PHI. out of 
the outer side belt layer to the belt cord diameter .PHI. in of the inner 
side belt layer is such that 1&lt;.PHI. out/.PHI. in.ltoreq.2.5. The outer 
side belt layer has thick cords as compared with the cords of the inner 
side belt layer. By making small the outer side belt cord angles which are 
defined by the equatorial plane of the tire, the expansion of the outer 
diameter of the tire can be restrained, and rigidity in the 
circumferential direction can be increased. Further, circumferential 
direction shearing distortion on the belt is reduced, and cut separation 
is suppressed. Cut through defects caused by relatively sharp protrusions 
can be suppressed. The inner side belt layer has thin cords so that the 
total gauge of the belt layer is restrained. Therefore, an increase in 
tensile force of the inner side belt layer due to bending deformation 
generated by relatively sharp protrusions can be suppressed. The 
phenomenon of the inner layer tearing first can be restrained, thereby 
preventing cut burst defects. Accordingly, cut resistance of the belt 
layer can be improved. 
If the belt cord diameter .PHI. out of the outer side belt layer is less 
than or equal to the belt cord diameter .PHI. in of the inner side belt 
layer, the above-described effects are not achieved. If the belt cord 
diameter .PHI. out of the outer side belt layer is greater than 2.5 times 
the belt cord diameter .PHI. in of the inner side belt layer or if the 
belt cord diameter .PHI. in of the inner belt layer becomes too thin, the 
above effects are not achieved sufficiently. As a result, the belt cord 
diameters .PHI. in and .PHI. out are set so that 1&lt;.PHI. out/.PHI. 
in.ltoreq.2.5. 
Further, when the difference between the average .alpha. out of the 
absolute values of the belt cord angles, defined by the equatorial plane 
of the tire, of the outer side belt layer and the average .alpha. in of 
the absolute values of the belt cord angles, defined by the equatorial 
plane of the tire, of the inner side belt layer is less than 5.degree., 
either .alpha. in is too small, or .alpha. out is too large. In this case, 
it is difficult to suppress the tensile force of the inner belt layer and 
the expansion of the outer diameter of the tire. As a result, the belt 
cord angles are set such that .alpha. in-.alpha. out.gtoreq.5.degree.. 
Further, in order to obtain the optimal growth of the outer diameter of the 
tire, the average belt cord angle of all of the belt layers of the inner 
side belt layer and the outer side belt layer must be set such that 
(.alpha. out+.alpha. in)/2.ltoreq.23.degree..

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment of the present invention will be described hereinafter 
with reference to FIGS. 1 through 3. 
As illustrated in FIG. 2, a carcass 12 of a heavy-duty radial tire 10 is 
formed of a layer of steel cord which extends in a toroidal shape between 
bead cores 14 which are disposed in a ring shape on an inner 
circumferential portion in the radial direction of the tire. Both end 
portions of the carcass 12 in the transverse direction of the tire are 
turned around the respective bead cores 14 from the inner side in the 
transverse direction of the tire toward the outer side in the transverse 
direction of the tire. 
A belt layer 15 is disposed on the outer side of the carcass 12 in the 
radial direction of the tire. A tread rubber layer 16 formed of thick 
rubber is disposed on the outer side of the belt layer 15 in the radial 
direction of the tire. 
As illustrated in FIG. 1, the belt layer 15 is formed from four belt 
layers. A first belt layer 18 is disposed at the innermost area of the 
belt layer 15 in the radial direction of the tire. A second belt layer 20 
is disposed at an outer side of the first belt layer 18 in the radial 
direction of the tire. Both end portions of the second belt layer 20 in 
the transverse direction of the tire are provided further outwardly in the 
transverse direction of the tire (i.e., wider) than both end portions of 
the first belt layer 18 in the transverse direction of the tire. Further, 
in the present embodiment, the first belt layer 18 and the second belt 
layer 20 are defined as an inner side belt layer 21. 
A third belt layer 22 is disposed on an outer side of the second belt layer 
20 in the radial direction of the tire. Both end portions of the third 
belt layer 22 in the transverse direction of the tire are provided further 
inwardly in the transverse direction of the tire (i.e., narrower) than 
both end portions of the second belt layer 20. A fourth belt layer 24 is 
disposed at the outer side of the third belt layer 22 in the radial 
direction of the tire. Both end portions of the fourth belt 24 in the 
transverse direction of the tire are provided further inwardly (i.e., 
narrower) than both end portions of the third belt layer 22 in the 
transverse direction of the tire. In the present embodiment, the third 
belt layer 22 and the fourth belt layer 24 are defined as an outer side 
belt layer 25. 
The width of each belt layer is not limited in the present embodiment. 
As illustrated in FIG. 3, cords 18A of the first belt layer 18 are inclined 
toward the right at an angle .alpha.1 with respect to the equatorial plane 
of the tire. Cords 20A of the second belt layer 20 are inclined to the 
left at an angle .alpha.2 with respect to the equatorial plane of the 
tire. Further, cords 22A of the third belt layer 22 are inclined to the 
right at an angle .alpha.3 with respect to the equatorial plane of the 
tire. Cords 24A of the fourth belt layer 24 are inclined to the left at an 
angle of .alpha.4 with respect to the equatorial plane of the tire. 
Accordingly, the belt layers of the belt layer 15 which substantially bear 
the tensile force, i.e., the first belt layer 18, the second belt layer 
20, the third belt layer 22 and the fourth belt layer 24 in the present 
embodiment, are disposed such that the respective cords of adjacent belt 
layers are inclined in different directions with respect to the equatorial 
plane of the tire. Namely, the cords are alternately inclined to the left 
and the right. 
Further, when the belt cord diameter of the third belt layer 22 and the 
fourth belt layer 24 which are the outer side belt layer 25 is .PHI. out, 
the average of the absolute values of the belt cord angles, which are 
defined by the equatorial plane, of the outer side belt layer 25 is 
.alpha. out, the belt cord diameter of the first belt layer 18 and the 
second belt layer 20 which are the inner side belt layer 21 is .PHI. in, 
and the average of the absolute values of the belt cord angles, which are 
defined by the equatorial plane, of the inner side belt layer 21 is 
.alpha. in, the difference between the respective belt cord angles is 
within a maximum of 5.degree., and 1&lt;.PHI. out/.PHI. in.ltoreq.2.5, and 
.alpha. in-.alpha. out.gtoreq.5.degree., and (.alpha. out+.alpha. 
in)/2.ltoreq.23.degree.. 
The cord angle and the cord diameter of each belt layer may be, for 
example, as shown in Table 1. "L" in the angle column of the table denotes 
that the cords are inclined toward the left side with respect to the 
equatorial plane, and "R" denotes that the cords are inclined to the right 
side with respect to the equatorial plane. "L" and "R" will be used in a 
similar manner throughout the other tables as well. 
TABLE 1 
______________________________________ 
type of A B C D 
tire angle, angle, angle, angle, 
cord diameter diameter diameter 
diameter 
______________________________________ 
4th belt layer 
L18, .phi. 4.5 
L20, .phi. 5.0 
L17, .phi. 4.5 
L19, .phi. 4.5 
3rd belt layer 
R18, .phi. 4.5 
R20, .phi. 5.0 
R20, .phi. 4.5 
R19, .phi. 4.5 
2nd belt layer 
L26, .phi. 3.0 
L25, .phi. 2.0 
L25, .phi. 4.0 
L25, .phi. 3.5 
1st belt layer 
R26, .phi. 3.0 
R25, .phi. 2.0 
R27, .phi. 4.0 
R27, .phi. 3.5 
______________________________________ 
Next, operation of the present embodiment will be described. 
In the heavy-duty radial tire 10 of the present embodiment, the outer side 
belt layer 25 has thicker cords than those of the inner side belt layer 21 
and the belt cord angle of the outer side belt layer 25 which is defined 
by the equatorial plane of the tire is set smaller than that of the 
conventional art so that the ratio .PHI. out/.PHI. in of the belt cord 
diameter .PHI. out of the outer side belt layer 25 to the belt cord 
diameter .PHI. in of the inner side belt layer 21 is 1&lt;.PHI. out/.PHI. 
in.ltoreq.2.5. Expansion of the outer diameter of the tire can be 
suppressed, circumferential direction rigidity can be increased, and 
circumferential direction shearing distortion on the belt can be 
decreased. Further, the occurrence of cut separation can be restrained so 
that cut through defects caused by relatively sharp protrusions can be 
suppressed. Moreover, because the cords of the inner belt layer 21 are 
thin and an increase in the total gauge of the belt layer 15 is 
controlled, the increase in tensile force of the inner side belt layer 21 
due to bending deformation generated by relatively sharp protrusions can 
be controlled. The inner layers are prevented from being torn first, 
thereby preventing cut burst defects. As a result, the cut resistance of 
the belt layer 15 can be improved. 
If the belt cord diameter .PHI. out of the outer side belt layer 25 is less 
than or equal to the belt cord diameter .PHI. in of the inner side belt 
layer 21, the above-described effects are not achieved. If the belt cord 
diameter .PHI. out of the outer side belt layer 25 is greater than 2.5 
times the belt cord diameter .PHI. in of the inner side belt layer 21 or 
if the belt cord diameter .PHI. in of the inner belt layer 21 becomes too 
thin, the above effects are not achieved sufficiently. As a result, the 
belt cord diameters .PHI. in and .PHI. out are set so that 1&lt;.PHI. 
out/.PHI. in.ltoreq.2.5. 
Further, when the difference between the average .alpha. out of the 
absolute values of the belt cord angles, defined by the equatorial plane 
of the tire, of the outer side belt layer 25 and the average .alpha. in of 
the absolute values of the belt cord angles, defined by the equatorial 
plane of the tire, of the inner side belt layer 21 is less than 5.degree., 
either .alpha. in is too small, or .alpha. out is too large. In this case, 
it is difficult to suppress the tensile force of the inner belt layer 21 
and the expansion of the outer diameter of the tire. As a result, the belt 
cord angles are set such that .alpha. in-.alpha.out.gtoreq.5.degree.. 
Further, in order to obtain the optimal growth of the outer diameter of the 
tire, the average belt cord angle of all of the belt layers of the inner 
side belt layer 21 and the outer side belt layer 25 must be set such that 
(.alpha.out+.alpha.in)/2.ltoreq.23.degree.. 
A second embodiment of the present invention will be described hereinafter 
in accordance with FIG. 4. 
Parts which are the same as those used in the first embodiment are denoted 
with the same reference numerals, and description thereof is omitted. 
As illustrated in FIG. 4, in the heavy-duty radial tire of the present 
embodiment, a first cut protective layer 27 is disposed at an outer side 
in the radial direction of the tire of the fourth belt layer 24 of the 
belt layer 15. Both end portions of the first cut protective layer 27 in 
the transverse direction of the tire are provided further outwardly in the 
transverse direction of the tire (i.e., wider) than both end portions of 
the second belt layer 20 in the transverse direction of the tire. A second 
cut protective layer 29 is provided at an outer side of the first cut 
protective layer 27 in the radial direction of the tire. Both end portions 
of the second cut protective layer 29 in the transverse direction of the 
tire are provided further inwardly in the transverse direction of the tire 
(i.e., narrower) than both end portions of the first cut protective layer 
27. The effects of the present embodiment are the same as those of the 
first embodiment. 
The width of each belt layer is not limited in the present embodiment. The 
cord angles and cord diameters of the respective belt layers and cut 
protective layers may be, for example, as shown in Table 2. 
TABLE 2 
______________________________________ 
type of A B C D 
tire angle, angle, angle, angle, 
cord diameter diameter diameter 
diameter 
______________________________________ 
2nd R23, .phi. 1.9 
R23, .phi. 2.5 
L21, .phi. 2.0 
L23, .phi. 1.5 
protective 
layer 
1st L23, .phi. 1.9 
L27, .phi. 2.5 
R23, .phi. 2.0 
R23, .phi. 1.5 
protective 
layer 
4th belt 
L18, .phi. 4.5 
L20, .phi. 5.0 
L17, .phi. 4.5 
L19, .phi. 4.5 
layer 
3rd belt 
R18, .phi. 4.5 
R20, .phi. 5.0 
R20, .phi. 4.5 
R19, .phi. 4.5 
layer 
2nd belt 
L26, .phi. 3.0 
L25, .phi. 2.0 
L25, .phi. 4.0 
L25, .phi. 3.5 
layer 
1st belt 
R26, .phi. 3.0 
R25, .phi. 2.0 
R27, .phi. 4.0 
R27, .phi. 3.5 
layer 
______________________________________ 
In the first embodiment, four layers of the belt layer 15 mainly bear the 
stress of the tire. However, the number of belt layers is not limited to 
those in the above-described embodiments. As illustrated in FIG. 5A, a 
structure may be provided in which five belt layers mainly bear the stress 
of the tire. Three layers form the outer side belt layer 25, and two 
layers form the inner side belt layer 21. Alternatively, as illustrated in 
FIG. 5B, the outer side belt layer 25 may be two layers, and the inner 
side belt layer 21 may be three layers. There is one cut protective layer 
31 in each of these examples. Further, six belt layers may mainly bear the 
stress of the tire. In this case, the outer side belt layer 25 may be 4, 
3, or 2 layers, and the inner side belt layer 21 may be 2, 3, or 4 layers, 
respectively. 
The heavy-duty radial tire of the present invention illustrated in FIG. 1, 
a heavy-duty radial tire having a conventional structure, and heavy-duty 
radial tires of Comparative Examples 1 through 3 were manufactured in 
accordance with the specifications shown in Table 3. (The size of each 
tire was ORR1800R25). Each tire was subject to a cut separation drum test 
(C/S resistance evaluation) as described below. In Table 3, the index 100 
is used for the heavy-duty radial tire of the conventional structure. 
Larger indices are more preferable. 
In the cut separation drum test, each tire was cut in four places on the 
circumference thereof such that the cuts reached the belt. The tire was 
driven on a drum at a speed of 20 km/h and at 120% of standard load. The 
running time which elapsed until a defect occurred was measured, and the 
value of the measurement was evaluated. 
Next, each tire was subject to a cutter test (C/T resistance evaluation) as 
described hereinafter. In Table 3, the index 100 is used for the 
heavy-duty radial tire of the conventional structure. Larger indices are 
more preferable. 
In the cutter test, a relatively sharp cutter blade (height 200 mm, 
included angle 60.degree.) was used. For each tire, the cutter blade was 
placed at a position directly beneath the load direction of the tire. The 
tire was made to run over the cutter blade so that the cutter blade 
penetrated the belt layer. The maximum load at the time of penetration was 
measured, and the value of the measurement was evaluated. 
Next, each tire was subject to a plunger test (C/B resistance evaluation) 
as described below. In Table 3, the index 100 is used for the heavy-duty 
radial tire of the conventional structure. Larger indices are more 
preferable. 
A protrusion having a rounded tip was used in the plunger test. (The entire 
height of the protrusion was 200 mm, and the hemispherical tip end 
thereof had a radius of 25 mm). For each tire, the protrusion was placed 
at a position directly beneath the load direction of the tire. The tire 
was made to run over the protrusion so that the protrusion penetrated the 
belt layer. The maximum load at the time of penetration was measured, and 
the value of the measurement was evaluated. 
TABLE 3 
__________________________________________________________________________ 
type conventional 
comparative 
comparative 
comparative 
of example 
example 1 
example 2 
example 3 
embodiment 
tire angle, angle, angle, angle, angle, 
cord diameter 
diameter 
diameter 
diameter 
diameter 
__________________________________________________________________________ 
4th belt 
L23, .phi. 2.5 
L21, .phi. 4.0 
L23, .phi. 4.0 
L20, .phi. 2.5 
L20, .phi. 4.0 
layer 
3rd belt 
R23, .phi. 2.5 
R21, .phi. 4.0 
R23, .phi. 4.0 
R20, .phi. 2.5 
R20, .phi. 4.0 
layer 
2nd belt 
L23, .phi. 2.5 
L21, .phi. 4.0 
L23, .phi. 2.0 
L25, .phi. 2.5 
L25, .phi. 2.5 
layer 
1st belt 
R23, .phi. 2.5 
R21, .phi. 4.0 
R23, .phi. 2.0 
R25, .phi. 2.5 
R25, .phi. 2.5 
layer 
C/S 100 130 100 130 130 
resistance 
C/T 100 140 140 100 140 
resistance 
C/B 100 60 80 140 110 
resistance 
__________________________________________________________________________ 
From the results shown in Table 3, it is clear that the heavy-duty radial 
tire of the present invention is superior.