Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a pneumatic tire
having an improved tread pattern and squared shoulders, in
which uneven tread wear is reduced and at the same time
lateral skid and wandering are prevented to improve running
performance on the snowy roads.
Recently, in order to prevent dust pollution, studless
t-ires for used on snowy and-lcy-roads are going to be
brought into greater use for heavy duty vehicle e.g.
truck and bus, instead of spike tires.
In the studless tires, block tread patterns are widely
used to obtain a large traction and a large braking force.
In general, in order to improve traction and braking force,
the tread blocks are provided with axial sipes so as
to divide each block into comparatively small independent
tread elements
As the resultant small tread elements are easily moved
during running, uneven wear is liable to occur, and the
elements are liable to be torn o~f.
On the other hand, on the snowy roads, squared
shoulders can provide a great lateral road grip than sipes.
However, such squared shoulders are liable to cause
wondering of the vehicle during running on the well-paved
roads but having shallow ruts.
To reduce wondering by decreasing the
tread shoulder rigidity, the upper sidewall part is
provided with a narrow circumferential groove This
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groove can not prevent lateral skid on the snowy/icy roads.
To decrease the tread shoulder rigidity,
the number of axial sipes are increased. In this case, ~-
uneven wear and tear-off are increased, and lateral skid can
not prevented. -
Japanese Patent Publicatlon
(KOKAI) No 56-21gO5 discloses a tire having shoulder blocks
provided with two axia~~sipes - lextendlng continuously -
across the whole axial width of the block-, and one narrow
groove extending continuously~across the whole
circumferential length of the block. In this case, the
tread elements adjacent to the tread edge are liable to
be torn off, and uneven wear and lateral skid are not fully
prevented.
It is therefore, an object of the present invention to
provide a pneumatic tlre having squared shoulders, in which
by using a discontinuous sipe or narrow groove and then
speciflcally defining the position, si2e, number and the
like, lateral skid on the snowy roads and wondering on the
well-paved roads are reduced without sacrificing a
resistance to uneven wear and a resistance to tear-off.
Accordlng to one aspect of the present lnvention, a
pneumatic tire comprising,
a tread portion having a pair o-f tread edges, and
a buttress portion e~tending radially inwardly from
each of the tread edges forming a radially outer part of
each tire sidewall,
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the tread portion provided with
an axially outermost wide main circumferential
groove extending continuously in the ~ire
circumferential direction and located on each side of the
tire equator at a distance of 0.2 to 0.3 times the tread
width from the tire equator,
- two narrow circumferential grooves
extending in the tire circumferential direction and located
between the axially outermost wide main circumferential
groove and the tread edge the two narrow
circumferential grooves located within a tread
edge region defined between an axially outer position
spaced apart from the tread edge by a distance of 3 to 6
mm and an axially inner position spaced apart from the
tread edge by a distance of 0.0~ to 0.1 times the tread
width and
circumferentially spaced axial grooves
extending from the axially outermost wide main
circumferential groove to the tread edge
thereby forming circumferentially divided three rows of
blocks in each tread shoulder portion
the three rows of blocks being
an axially inner row of inner shoulder blocks
between the axially outermost wide main circumferential
groove and the axially inner narrow circumferential
groove
a middle row o~ middle shoulder blocks
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between the two narrow circumferential grooves
and
an axially outer row of outer shoulder blocks
between the axially outer narrow circumferential groove
and the tread edge ~ .;
the tread portion provided with a convex profile, the :~
convex profile extending from one tread edge to the other
- tread edge and having a radius of curvature -~of 45-O to -
800 mm,
the buttress portion provided with a concave profile
intersecting with the convex profile of the tread portion at
the tread edge to form a squared or angled tire shoulder,
each of` the inner shoulder blocks provided with
axial sipes the axial sipes including
a discontinuous axial sipe extending
discontinuously in the tire axial direction between the
~axially outermost wide main circumferential groove and
the axially inner narrow circumferential groove and
a continuous axial sipe extending
continuously from the axially outermost wide main
circumferential groove to the axially inner narrow
circumferential groove~ ,
the number of the axial sipes in each of the inner
shoulder blocks being not more than five, and
the space between the axial sipes being 8 to 16 mm.
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In the accompanying drawings:
Fig.l is a developed plan view showing the tread
portion of a truck/bus tire according to the present
invention;
: .. . . . .. .
Fig.2 is a cross sectional view taken along the line
X-X of Fig.l;
- ~ Fig;3 is a enlarged perspective view showing a tread
shoulder region thereof;
Fig.4 is a- enlarged cross sec.tional view showing the
narrow circumferential grooves;
Figs.5(A)-(C) are plan views showing other examples of
the narrow circumferential groove;
Fig.6 is a perspective view of the tread shoulder
region of another embodiment of the present invention; and
Fig.7(A) is a graph showing relationship between camber
thrust and camber angle and Fig.7~) is a diagram e~plaining
the camber thrust and camber angle;
Figs.8(A)-(C) are perspective views showing prior art.
Before describing various embodiments of the invention, a
description of the prior art shown in Figs 8(A)-(c) will be given.
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In general, in order to improve traction and braking-force,
the tread blocks (a) are provided with axial sipes (s) so as
to divide each block into comparatively small independent
tread elements, for example as shown in Figs.8(A)-(C).
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.
As the resultant small tread elements are easily moved
during running, uneven wear is liable to occur? and the
elements are liable to be torn off.
On the other hand, on the snowy roads, squared
shoulders can provide a great lateral road grip than sipes.
However, such squared shoulders are liable t:o cause
wondering of- khe vehicle during running on the well-paved
roads but having shallow ruts. - ~ -
In Fig.8(A), to reduce wondering by decreasing the~read shoulder rigidity, the upper sidewall part (k) is
provided with a narrow circumferential groove (g). This
groove can not prevent lateral skid on the snowy/icy roads.
In Fig.8(B), to decrease the tread shoulder rigidity,
the number of axial sipes (s) are increased. In this case,
uneven wear and tear-off are increased, and lateral skid can
not prevented.
As shown in Fig.8(C), Japanese Patent Publication
(KOKAI) No 56-21905 discloses a tire having shoulder blocks
(a) provided with two axial sipes ~s) extending continuously
across khe whole axial width of the block, and one narrow
groove (g) extending continuously across the whole
circumferential length of the block. In this case, the
tread elements adjacent to the tread edge (e) are liable to
be torn off, and uneven wear and lateral skid are not fully
prevented.
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In Figs.1-4, pneumatic tire 1 has a tread portion 12, a
pair of axially spaced bead por~ions 15, and a pair of
sidewall portions 13 extending between the tread edges E and
the bead portions 15.
The tire 1 comprises a pair of bead cores 16 each disposed
in each of the bead portions 15, a toroidal carcass 17
extending between the bead portions, and a belt 19 disposed
- -radially outside the carcass 17 and inside a rubber tread. . -
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The carcass 17 comprises at least one ply of cords
arranged radially at 70 to 90 degrees with respect to the
tire equator C to provide a radial or semiradial carcass
structure.
For the carcass cords, steel cords and organic fiber
cords, e.g. nylon, polyester, rayon, aromatic polyamide and
the like can be used.
- - The belt 19-in th-is embodiment comprises three plies of
cords laid bias with respect to the tire equator C so that
- each ply cross the other plies.
For the belt cords, steel cords and organic fiber cords,
e.g. nylon, polyester, rayon, aromatic polyamide and the
like can be used.
The above-mentioned tread portion 12 is provided
between the tread edges E with a convex profile (F) having a
single radius curvature having the center on the tire
equatorial plane. The radius TR of the curvature is in the
range of 450 to 800 mm.
In-order to form a so called square shoulder to provide
resistance to lateral skid on the snowy/icy roads, the tread
profile (F) intersects with a concave profile (W) of the
buttress portion or sidewaIl upper portion at the tread edge
E.
The intersecting angle is determined such that the angle of
the tread e~ge E is 90 to 140 degrees.
The tread portion 12 is provided with a plurality o~
wide main circumferential grooves G1 and GM extending
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continuously around the tire, which grooves include a pair
of axially outermost grooves G1 each disposed on each side
of the tire equator C and a central groove GM disposed on
the tire equator C.
Further, between each axially outermost wide main
circumferential groove Gl and each tread edge E, only two
narrow circumferential grooves G3 and G4 extending around
the tire are provided.
Therefore, by the three wide circumferential grooves G1, GM
and four narrow circumferential grooves G3, G4, the tread
portion 12 is divided into eight axial parts.
The tread portion 12 is further provided with axial
grooves. The axial grooves includes
crown axial grooves GP extending from the central main
circumferential groove GM to the adjacent outer main
circumferential groove Gl and
shoulder axial grooves G2 extending from the axially
outermost main circumferential groove G1 to the adjacent
tread edge E to cross the narrow circumferential grooves G3
and G4.
Therefore, the above-mentioned eight axial parts are
c1rcumferentially d1vided by the axial grooves into eight
rows of blocks:
two rows LC of central blocks BC, each between the central
circumferential groove GM and each of the outermost
circumferential grooves Gl;
two rows LI of inner shoulder blocks BI, each between each
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21~7~712
of the axially outermost wide main circumferential grooves
G1 and the adjacent axially inner narrow circumferential
groove G3;
two rows LM of middle shoulder blocks BM, each between each
of the axially inner narrow circumferential grooves G3 and
the adjacent axially outer narrow circumferential groove G4;
and
two rows LO of outer shoulder blocks BO, each between each
of the axially outer narrow circumferential grooves C4 and
the adjacent tread edge E.
Accordingly, a block type tread pattern is formed.
Incidentally, a -tread part between the two axially outermost
wide main circumferential grooves is called tread crown, and
a tread part between the axially outermost wide main
circumferential groove and the tread edge is called tread
shoulder.
In this embodiment, the axial grooves GZ and GP have
the same groove depth as the maximum depth DO of the wide
main circumferential grooves Gl and GM.
However, the depths of the axial grooves can be differed
from that of the wide main circumferential grooves.
Further, the depth of the axial grooves GP can be differed
from that of the axial grooves G2. For example, the axially
inner axial grooves are formed to be shallower than the
axially outer axial grooves so that the blocks becomes a
continuous rib in the middle of the tread wear life.
Furthermore, the depth of the axial grooves GP may be zero,
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that is, omitted.
The axial distance of the axially outermost wide main
circumferential groove &1 from the tire equator C is 0.2 to
0.3 times the tread width TW which is defined as the axial
distance between the tread edges E and E.
The narrow circumferential grooves G3 and G4 are formed
in a straight configuration in this example, but the grooves
can be formed in a zigzag configuration, a wavy
configuration and the like.
The narrow circumf'erential grooves G3 and G4 are formed
within a range M between points P and Q.
Here, the point P is located axially inward of the tread
edge E by an axial distance W3 of 3 to 6 mm, and
the point Q is located axially inward of the point P and at
an axial distance W2 being 0.04 to 0.1 times the tread width
TW from the tread edge E, which mean that
the distance from the tread edge E to the axially outer
narrow circumferential groove G4 is 3 to 6 mm, and
the distance from the tread edge E to the inner shoulder
block BI is 0.04 to 0.1 times the tread width TW.
Preferably, each narrow circumferential groove G3, G4
has
a width WG in the range of 2.0 to 4.5 mm, and
a depth D1 in the range of 0.3 to 1.2 times the maximum
depth DO of the axially outermost circumferential groove Gl.
Therefore, the narrow circumferential grooves increase the
road grip in the axial direction of the tire by the
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increased edges and thereby improving resistance to lateral
skid.
By forming only two narrow circumferential grooves in the
specific range M, the rigidity of the blocks BO and B~ is
decreased to prevent wanderin~ of the vehicle, but an
excessive decrease is avoided, and loss of steering
stability during straight running and cornering can be
avoided.
If the number of the narrow circumferential grooves in
more than two, steering stability is disturbed.
If the groove depth Dl is less than 0.3 times DO, the
rigidity of the tread shoulder blocks BM and BO increases
and the resistance to lateral skid can not be improved. If
Dl is more than 1.2 times DO, lateral skid and uneven wear
are liable to occur, and the steering stability is
deteriorated.
If the groove width WG is less than 2.0 mm, the
resistance to lateral skid can not effectively improved.
If ~he width is more than 4.5 mm, the shoulder blocks BM and
BO become narrow to decrease the rigidity and lateral skid
and uneven wear are caused.
Further, as shown in Fig.4, the center line of each
narrow circumferential groove G3, G4 is inclined at an angle
within +5 degrees with respect to the normal direction N to
the tread profile F.
Preferably, the angle is zero, that is, in parallel with the
normal direction so as to maximize the resistance to lateral
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207~7~
skid.
Each of the inner shoulder blocks BI is provided with
at most five circumferentially spaced sipes S, in this
embodiment three sipes SM and SE, extending between the
axially outermost circumferential groove G1 and the axially
inner narrow circumferential groove G3.
The width WS of the sipes S (SE and SM) is in the range
of 0.3 to 1.5 mm.
Each of the sipes SM and SE has open ends opened at the
circumferential grooves G1 and G3.
The sipe SM located in the circumferential center of
the bloc~ ~I extends straight in the axial direction of the
tire and is discontinuous in the middle thereof.
Preferably, the length of the discontinued portion 22
is 1.0 to Z.O times the width WS of the sipe.
The sipe SE located on each side of the central sipe SM
extends continuously and zigzag as shown in Fig.1 so as to
form three axial parts:
an axially inner part extending in parallel with the axial
direction;
an axially outer part extending in parallel with the axial
direction and shifeed towards the central sipe SM than the
inner part; and
a middle part extending obliquely between the axially inner
end of the axially outer part the axially outer end of the
axially inner part.
The depth DZ of the sipes S (SE and SM~ is 0.5 to 1.0
7~
times the depth D1 of the axially inner narrow
circumferential groove G2.
In this embodiment, the depth of the sipe SE is varied
such that the sipe SE provided at the a~ially outer and
inner ends with shallower portions 23.
The circumferential distance or spacing (L) between the
circumferentially adjacent sipes is set in the range from 8
mm to 16 mm.
As explained above, a plurality of sipes are disposed
in each of the inner shoulder blocks BI, but no sipe is
disposed in the middle shoulder blocks BM and the outer
shoulder blocks BO to maintain the rigidity thereof.
If the number of the sipes in one block BI is more than
five, and if the sipe spacings ~L) are less than 8 mm, the
inner shoulder block BI decreases in rigidity and uneven
wear is liable to occur, and further cracks are liable to
start from the s1pe bottoms.
On the other hand, if no sipe is formed, and if the sipe
spacings are more than 16 mm, the rigidity of the inner
shoulder blocks B1 increases and lateral skid is liable to
occur.
The circumferential grooves G3 and G4 are provided with
shallow portions or discontinued portions so that the
adJacent shoulder blocks support each other.
In the circumferential grooves G3 and G4 in this
embodiment, each circumferential groove part between the
adjacent axial grooves G2 extends continuously straight and
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2079712
has ends opened at the groove sidewall of the axial grooves
G2.
Further, each circumferential groove part of the outer
groove G4 is provided with a shallow portions 20 in the
middle of the length so that the adjacent narrow shoulder
blocks BM and B0 support each other.
In the case that the depth of the narrow
circumferential groove is varied, the depth is preferably
restricted within the above men~ioned limit therefor.
However, if the Iength of the shallow portion 20 is less
than 30% of the whole, the depth may be decreased to 50 % of
the lower limit.
Fig.5(A) shows another example of the narrow
circumferential groove G3, G4, which has a widened p~rtion
(WL) between the adjacent axial grooves G2 and G2. This
portion is not more than 20% in length.
Fig.5(B) shows stlll another example which has a wavy
configuration.
Fig.5(C) shows still more another example which has a
zigzag configuration, wherein the zigzag at the groove top
is steeper than the groove bottom so that the zigzag angle
becomes gentle and zigzag amplitude becomes small with the
tread wear progresses.
Fig.6 shows another embodiment of the present invention,
in which the narrow circumferential grooves G3 and G4 and
the sipes SM and SE are modified.
In this embodiment, the axially inner narrow circumferential
2~7~7~2
groove G3 has discontinued portions 22 one between the
central straight sipe SM and each zig~ag sipe S~.
The a~ially outer narrow circumferential groove G4 has
shallow portions 20 at the opened ends on both side of each
axial groove G2.
The central straight sipe SM extends continuously from the
wide main circumferential groove Gl to the axially inner
narrow circumferential groove G3, and accordingly, both the
ends are opened at the grooves Gl and G3.
The zigzag sipe SE extends from the axially inner narrow
circumferential groove G3, but the axially inner end is
termin~ted within the block ~I near the main circumferential
groove G1. Therefore, the sipe SE is discontinuous.
WORKING EXAMPLE
Test tires of size 1l.00R20 14PR having the tread
pattern shown in Fig.1
and similar patterns and the tire structure shown in Fig.Z
were prepared and the following tests were made. The tire
specifications and test results are given in Table 1. The
test conditions are as follows.
A) Camber Thrust Test
Using a drum, the camber thrust was measured under a
camber angle of 6 degrees, a pressure of 7.25 Kgf/sq.cm and
a tire load of 2,~40 kg. The larger plus value is better.
B) Self Alignment Effort Test
Using a Z-D wheel type truck, the self alignment effort
was evaluated by a skilled test driver. The results are
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indicated by an index based on that Example tire 3 is 100.
The larger the value, the better the result.
C) Tear-off Test
By rubbing the tire sidewall against edging stone, the
resis~ance to tear-off was evaluated. The results are
indicated by an index based on that Example tire 3 is 100.
The larger the value, the better the resistance.
D) Shoulder Wear Test
Running a test vehicle on a 100% paved road at an
average speed of 80 km/Hr for 60,000 km, the amount of tread
wear was measured.
In Table 1, the reciprocal number of the wear amount is
indicated as the wear resistance by using an index based on
that Example tire 3 is 100. The larger the value, the
better the resistance to shoulder wear.
E) Snow Performance Test
A braking test and a hill climb test were made on
icebound test courses. The results are indicated by an
index based on that Example tire 3 is 100. The larger the
value, the better the snow performance.
It was confirmed frorn the test results that in
comparison with the reference tires the working example
tires had a large camber trust and a good self alignment
effort and were improved in wandering, lateral skid,
.
tear-off resistance shoulder wear resistance and steering
stability on the snowy roads.
It was also confirmed from the graph shown in Fig.7(A) that
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the working example tire 3 was superior in lateral skid
performance to the re~erence tires 1 and 2.
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