Note: Descriptions are shown in the official language in which they were submitted.
20 1 0462
The present invention relates to a radial tire in which uneven wear especially
between the tread central region and the tread shoulder regions is lessened.
In general, the tread part of a radial tire for heavy duty vehicles, e.g., truck,
bus and the like, is fully reinforced by rigid belt cords and carcass ply cords. The
so-called pantographic movement of such reinforcing cords which occurred in biasply tires hardly occurs.
Accordingly, when there is a difference in ~ meter between the crown region
and the shoulder regions of the tread face, the ground ~l~ssure becomes larger in the
shoulder regions than in crown region, and the amount of slip of the shoulder regions
against the road surface during running becomes larger than that of the crown region.
Further, as the tread rubber thi~ ss in the shoulder regions is usually larger
than that in the crown region, the heat generated in the shoulder regions is also
greater than that in the crown.
As a result, the tread shoulder regions are apt to wear more rapidly than the
crown region. This type of uneven wear is known as shoulder wear.
In recent years, the endeavours to prevent such uneven wear have been mainly
directed to improving the pattern or profile of the tread. The prevention of uneven
wear by such improvement in only the tread pattern or profile has recently become
insufficient.
On the other hand, an old Japanese Patent Application filed in March, 1975
and published in September, 1976 under publication No. JPA 51-100504 disclosed atread of which shoulder portions are made of a rubber having a higher wear resistance
than that of the crown portion.
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An object of a general aspect of the present invention, therefore, is to providea radial tire in which uneven wear of the tread is effectively prevented.
An object of another aspect of the present invention is to provide a tread of
which the central portion and shoulder portions are made of dirrelent rubber composi-
S tions by limiting the width of each portion in a specific range, and using a specific
rubber composition for the shoulder regions, and further providing a specific
dirr~ rei~ce in hardness between the central portion and the shoulder portions.
According to one aspect of the present invention, a radial tire is provided
comprising: a carcass extending between a pair of beads of the tire; a tread disposed
radially outside the carcass; and a belt disposed between the carcass and the tread; the
tread comprising a central portion and two shoulder portions disposed one on each
side of the central portion, the tread being provided with two circumferentially-
extending grooves; the boundary between the central portion and each shoulder
portion being located within the width of each of the grooves; the shoulder portions
being made of a rubber composition which is different from the rubber composition
of the central portion; the radially-outer surface of the central portion and the
radially-outer surfaces of the shoulder portion being exposed to define the tread face
of the tread; the axial distance of each of the boundaries between the radially-outer
surface of the central portion and the radially-outer surfaces of the shoulder portions
from an equator of the tire being not less than 0.15 times and not more than 0.3 times
the tread width; the rubber composition of the shoulder portions consisting of: natural
rubber, but~-liene rubber, of which the trans-1,4-bond content is not less than 54%
and less than 70%, and carbon black of 10 to 30 microns in particle tli~met~r, the
ratio of the natural rubber content to the butadiene rubber content in parts-by-weight
201 0462
_
unit being 85/15 to 20/80, and the carbon black content is 20 to 70 parts by weight;
the Shore-A hardness of the shoulder portions is larger than the Shore-A hardness of
the central portion, and the difference therebetween is not less than 2 degrees and not
more than 6 degrees; the Shore-A hardness of the shoulder portions being in the
range of 65 to 75 degrees; and the Shore-A hardness of the central portion being in
the range of 60 to 70 degrees.
By a variant thereof, the tire tread portion comprises: a central rubber layer;
a shoulder rubber layer disposed on each side of the central rubber layer being
directly connected thereto; and a base rubber layer disposed radially inside each of
the shoulder layers and disposed radially-outside the edge portion of the belt, the
hardness of the base rubber layers being lower than that of the central rubber layer
and that of the shoulder rubber layers; each of the base rubber layers extendingaxially inwardly from the axially outer edge of the shoulder layer to the axially inside
of the boundary between the central and shoulder layers.
In the accompanying drawings,
Fig. 1 is a sectional view showing a right-half of a radial tire of one
embodiment of the present invention;
Fig. 2 is a section view showing a radial tire of another embodiment of the
present invention;
Fig. 3 is a sch~ tic view explaining uneven wear "t" in a shoulder wear
resistance test; and
Fig. 4 is a perspective view showing a wedge-shaped tool "T" used in cut
reci~t~nee tests.
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20 1 04 6?
In the drawings, a radial tire 1 has a pair of bead parts 3, a sidewall part 4
extending radially-outwardly from each bead part 3, and a tread part extending
between the radially-outer edges of the sidewall parts. The tire further includes a
bead core 2 which is disposed in each bead part, a carcass 6 having at least one ply
5of cords extending between the bead parts through the tread part and the sidewall
parts and turned up at the edges thereof around the bead cores. A tread 5 is disposed
radially outside the carcass, and a belt 7, which is composed of plural plies of cords,
is disposed between the carcass and the tread.
The carcass ply cords are laid at 60 to 90 with respect to the tire equator
10so as to provide a radial or semi-radial construction for the carcass 6. Steel cords,
or organic fibre cords, e.g., nylon, polyamide and the like, and glass fibre cords can
be used as the ply cords for the carcass.
The belt is composed of at least two plies 7a and 7b of high modulus cords
to provide a tight hoop effect in the tread part. For example, steel cords, organic
15fibre cords, e.g., polyamide and the like, can be used for the high modulus belt
cords.
The tread 5 comprises a central rubber layer 10 and a shoulder rubber layer
11 disposed on each side of the central rubber layer and is directly connected thereto.
The radially-outer faces of the central and shoulder layers are exposed as a
20tread face.
In this embodiment, the tread 5 further comprises a base rubber layer 15
which is disposed radially inside each of the shoulder layers and radially outside the
edge portion of the belt 7.
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20 1 04~2
The base layer 15 extends axially-inwardly from the axially-outer edge of the
shoulder layer 11 to the axially-inside of the boundary M or connection between the
central and shoulder layers.
The central rubber layer can be co~ osed of a rubber composition which
5 preferably consists of a natural rubber (NR) and a styrene-butadiene rubber (SBR).
The shoulder rubber layer is composed of a rubber composition which is
different from that of the central rubber layer and consisting of a natural rubber (NR)
and a trans-butadiene rubber (IBR), of which trans-1,4-bond content is not less than
54% and not more than 70%, whereby the wear resistance is greatly improved.
10 When the trans-l,4-bond content is less than 53%, wear resistance and processability
become low. On the other hand, when it is more than 71%, heat generation
mcreases.
The Shore-A hardness of the central rubber layer (HS) is more than 60 degrees
and less than 70 degrees. When it is not more than 60 degrees, wear resistance is
15 lowered; when it is not less than 70 degrees, wet grip performance is impaired.
Further, the Shore-A hardness of the shoulder rubber layer (HS) is larger than
the Shore-A hardness of the central rubber layer (HC), and the difference (HS-HC~
therebetween is not less than 2 degrees and not more than 6 degrees. By these means
movements of the shoulder portions during running is decreased to balance the
20 amount of wear between the central portion and the shoulder portion. When the
dirr~ ce is less than 2 degrees, the durability of the shoulder rubber layers is
lowered due to its heat generation. On the other hand, when the difference is more
than 6 degrees, a separation and a stepped dirrefe,lce in the amount of wear are liable
20 1 0462
to occur between the central rubber layer and the shoulder rubber layers. Preferably,
the Shore-A hardness of the shoulder portion is in the range of 65 to 75 degrees.
In the rubber colllposi~ion for the shoulder rubber layers, the ratio, in a unitof part-by-weight, of the NR content to the TBR content is in the range from 85/15
to 20/80. The colllposition contains carbon black whose size is 20 to 30 microns at
20 to 70 parts by weight.
When the NR/TBR ratio is more than 85/15, wear resistance becomes poor,
and shoulder wear, and uneven tread wear is caused to lower the road grip
~follllal1ce and the tire life. On the other hand, when the ratio is more than 20/80,
cut resistance and durability become poor.
Further, when the carbon black content is less than 20 parts by weight, wear
resistance becomes low. On the contrary, a carbon black content of more than 70
parts by weight is not preferable from the point of view of heat generation and
processability in a mixing process, sheeting process, a calendering process and the
like.
The above-mentioned base layer 15 is composed of a rubber composition
which is different from that of the central and shoulder rubber layers. The layer 15
has a less heat generation prop~l~y, and a hardness which is lower than that of the
central and shoulder rubber layers.
The radially-outer faces of the central and shoulder layers are exposed as the
tread face, as mentioned above, and the distance from the tire e~uator C to the
boundary P between the central and shoulder layers at the tread face is 0.15 to 0.30
times the tread width W between the tread edges N. When the distance is less than
0.15 times the tread width W, wet grip pt;lro,.llance and ride colllro.l are impaired.
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201 0462
When it is more than 0.3 times the tread width W, uneven wear is caused and beltseparation failure is apt to occur.
The above-mentioned tread S is provided with at least two circumferentially-
e~t~n.1ing grooves 14, as shown in Fig. 1. Each of the bo-ln~rie~ is preferably
positioned in the range of the width of the groove 14, so that separation between the
central and shoulder layers is effectively prevented, and the difference in the amount
of wear (if occurred) is well covered. Further, the grooves 14 can be formed in a
narrow width, for example, less than 1 mm, so that the groove walls come into
contact with each other when the tires contact the ground.
Fig. 2 shows another embodiment of the present invention, in which the
above-mentioned base rubber layers 15 are omitted from the tread 5, and in whichthe boundary M between the centre rubber layer 10 and the shoulder rubber layers11 is inclined toward the tire equator C from the boundary P as a start point; the
boundary may be inclined toward the tread shoulder side.
Test tires of 11/70 R 22.5 size having the construction shown in Fig. 1 were
prepared and vaAous tests were made. The spe~-ific~ions of the tires and the results
of the tests are shown in Table 1. The rubber compositions Cl-C8 and Sl-S14 usedas the central and shoulder layers are shown in Table 2 and Table 3.
1. Wet GAp Pelrol...allce Test
The running distance to stop when the wheels were locked at 80 km/h, was
measured on a wet asphalt road. In Table 1, the test results are indicated by an index
based on the assumption that reference tire 1 is 100, and where a value more than 95
is s~ f~tory.
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20 1 0462
2. Shoulder Wear Resistance Test
As shown in Fig. 3, the difference "t" in the amount of wear produced after
a running on paved roads inclusive of highways at the average speed of 60 km/h for
120,000 km was measured. The test results are indicated in Table 1 using an index
based on the assumption that reference tire 1 is 100, and where a value more than
110 is s~ti~factory.
3. Wear Life Test
As to the same tires as were subjected to the above-mentioned shoulder wear
resistance test, the depth of the rem~ining main groove GC which is formed in the
crown region of the tread as shown in Fig. 3 was measured at six different points,
and their average value was evaluated. In Table 1, the test results are indicated by
an index based on the assumption that reference tire 1 is 100, where the larger the
value, the longer the life.
4. Cut Resistance Test
The depth of the cut formed on the tread face of the tire infl~ted to the
regulated maximum internal pressure when a wedge-shaped tool "T" shown in Fig.
4 was pressed at a force was measured. The test results are indicated in Table 1using an index based on the assumption that reference tire 1 is 100, and where avalue more than 100 is s~ti~f~ctory.
5. Heat Durability Test
The total running distance to be broken by generated heat was measured, while
the test tire was running on a drum tester at a constant speed of 80 km/h but
increasing the tire load at 10% steps every 24 hours from an initial load of JIS 100%
maximum load. In Table 1, the test results are indicated by an index based on the
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20 1 0462
assumption that reference tire 1 is 100, and where a value more than 90 is
s~ti~f~tory.
6. Processability
The pfocessabilities in a rubber mixing process and a calendering process were
S ev~ t~ In Table 1, "A" and "B" marks in~ te that there was no problem in the
proces.sing, and "C" mark indicates that the processabilities were bad.
In the above-mentioned tests 1-5, the test tires were mounted on 7.5" regular
rims and infl~t~ to 8.0 kgf/cm2, and the tests 1-3 were made using 2-D.4 wheel-type
10 ton trucks loaded so that the test tires were subjected to a regulated maximum
: 5~--'
2~1~0462
-- 10 --
TABLE 1
_______________________________________________________
Tire E1 E2 E3 E4 E5 E6
_______________________________________________________
Tread radius R (mm) 900 900 900 900 900 900
Tread width W (mm~ 262 262 262 262 262 262
Distance of 40 80 55 55 55 55
boundary P (mm)
_______________________________________________________
Rubber Composition
Central part C1 C2 C1 C2 C3 C4
Shoulder parts S1 S2 S3 S4 S5 S6
_______________________________________________________
Test Results
Wet grip 100 98 98 100 100 100
Shoulder wear 110 120 115 110 115 115
Wear resistance 100 105 110 110 110 100
Cut resistance 105 110 105 120 115 115
Heat durability 100 90 100 100 95 100
Processability B A B A A A
_______________________________________________________
TABLE 1 (cont.)
_________________________________________________________________
Tire R1 R2 R3 R4 R5 R6 R7 R8
_________________________________________________________________
Tread radius R (mm) 900 900 900 900 900 900 900 900
Tread width W (mm) 262 262 262 262 262 262 262 262
Distance of - 55 37 81 80 40 80 80
boundary P (mm)
_________________________________________________________________
Rubber Composition
Central part C1 C2 C1 C2 C5 C6 C7 C8
Shoulder parts S7 S8 S9 S10 S11 S12 S13 S14
_________________________________________________________________
Test Results
Wet grip 100 98 100 95 100 98 95 100
Shoulder wear 100 110 95 100 95 100 80 100
Wear resistance 100 110 100 95 100 110 110 110
Cut resistance 100 100 110 110 110 95 110 90
Heat durability 100 85 100 100 80 85 100 90
Processability C A C A A A A A
_________________________________________________________________
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-- 11 --
TABLE 2
___________________________________________________________
Composition C1 C2 C3 C4 C5 C6 C7 C8
___________________________________________________________
Content (parts by weight)
NR 60 60 60 60 60 60 60 60
SBR1500 40 40 40 40 40 40 40 40
Carbon black 45 58 60 40 45 50 62 40
Stearic acid 2 2 2 2 2 2 2 2
Zinc oxide 4 5 5 4 4 5 5 4
Sulfur 1.5 1.6 1.8 1.5 1.5 1.6 1.8 1.5
Accelerator 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75
Antioxidant 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
Others 8.5 5.5 5.5 12.5 8.5 3.5 2.5 8.5
___________________________________________________________
Shore A hardness 63 69 70 60 64 68 71 59
___________________________________________________________
20~0a~6;;~
- 12 -
TABLE 3
_______________________________________________________
Composition Sl S2 S3 S4 S5 S6 S7
_______________________________________________________
Content (parts by weight)
NR 85 20 20 85 50 50 70
Trans-BR *1 15 80 80 15 50 50 30
(54) (70) (54) (65) (70) (68) (50)
Carbon black ISAF 30 70 30 70 50 48 50
Stearic acid 2 2 2 2 2 2 2
Zinc oxide 5 5 5 5 5 5 5
Sulrur 2.0 1.8 1.6 1.8 1.7 1.6 1.4
Accelerator 0.75 1.2 0.75 1.2 1.0 0.75 0.75
Antioxidant 2.5 2.0 2.5 2.5 2.5 2.5 2.0
Others 28.5 3.5 28.5 5.0 18.5 16.5 12.5
_______________________________________________________
Shore A hardness 65 75 65 75 72 65 63
_______________________________________________________
*1: The numerals in parentheses express
the trans-1,4-bound content in weight %.
TABLE 3 (cont.)
_______________________________________________________
Composition S8 S9 S10 Sll S12 S13 S14
_______________________________________________________
Content (parts by weight)
NR 20 86 19 85 20 85 20
Trans-BR *1 80 14 81 15 80 15 80
(72) (53) (71) (75) (71) (50) (65)
Carbon black(ISAF) 71 50 50 50 50 29 50
Stearic acid 2 2 2 2 2 2 2
Zinc oxide 5 4 5 4 5 4 5
Sulfur 1.7 1.5 1.7 1.5 1.6 1.6 1.7
Accelerator 1.2 0.75 1.0 1.0 1.25 0.75 1.0
Antioxidant 2.5 2.0 2.5 2.5 2.0 2.5 2.0
Others 5.2 16.5 16.5 11.5 16.5 28.5 16.5
_______________________________________________________
Shore A hardness 75 65 75 65 75 63 71
_______________________________________________________
*1: The numerals in parentheses express
the trans-1,4-bound content in we ight %.
20 1 0462
-- 13 --
As mentioned above, in the present invention, the width of
each of the central portion and shoulder portions are limited in
a specific range, and a specific rubber composition is used for
the shoulder regions, and further a specific difference in
hardness is provided between the central portion and the shoulder
portions. Therefore, the tread is effectively prevented from
shoulder wear, and the tire life is elongated, and further
durability of the tire can be improved without deteriorating the
ride feeling.
The present invention is preferably applied to a radial tire
for heavy duty vehicles having a curved tread profile.
