Note: Descriptions are shown in the official language in which they were submitted.
CA 02084741 2001-09-20
(a) TITLE OF THE INVENTION
BLOCK PATTERN TIRE WITH OPTIMIZED GROOVE DEPTH RATIOS, SIPE
DEPTH AND LENGTH RATIOS
(b) TECHNICAL FIELD TO WHICH THE INVENTION RELATES
The present invention relates to a pneumatic tire having an improved block
tread
pattern which is capable of improving a wet grip performance and uneven wear
resistance.
(c) BACKGROUND ART
Recently, in order to prevent dust pollution, studless tires for use on snowy
and icy
roads have been brought into greater use for heavy duty vehicles, e.g., trucks
and busses,
instead of spike tires.
In such studless tires, block tread patterns are widely used to obtain a large
traction
and a large braking force.
In general, tread blocks are provided with axially-extending sipes to improve
traction
and braking force. The resultant narrow tread elements are easily moved during
running,
and uneven wear, so called "heel/toe wear" occurs along the front/rear edges
thereof.
Such uneven wear is especially liable to occur in the initial stage of tread
wear life
because tread elements are tall and liable to move.
On the other hand, from the middle stage to the last stages, the rigidity of
the tread
elements becomes increased, and thereby wet grip, especially ice grip
performance,
becomes decreased.
(d) DESCRIPTION OF THE INVENTION
An object of a first aspect of the present invention is to provide a pneumatic
tire in
which the wet grip performance is improved and uneven wear is effectively
prevented.
An object of a second aspect of the present invention is to provide a block
pattern tire
in which the wet grip performance is maintained even in the last stage of
tread life.
An object of a third aspect of the present invention is to provide a block
pattern tire
in which wet grip performance is improved during both straight running and
cornering.
CA 02084741 2001-09-20
According to one aspect of the present invention, a block pattern tire is
provided,
which comprises a thread portion provided in the central part with a
circumferential row of
blocks which are divided by two wide main circumferential grooves and axial
grooves
extending therebetween. The tread portion is provided in its tread surface,
with a plurality
of first sipes which is narrower than the circumferential grooves and axial
grooves. The
axial grooves are shallower than the circumferential grooves. Each of the
axial grooves is
provided, in the bottom face thereof, with a second sipe having a width of 0.2
to 1.1 mm.
The second sipe extends along the centre line of the axial groove and has ends
terminated
within the bottom so as to have a length of not less than 0.7 times and less
than 1.0 times
the axial groove length. The depth of the axial groove is in the range of 0.2
to 0.5 times
the depth of the circumferential grooves. The depth of the second sipe is 0.2
to 0.5 times
the depth of the circumferential grooves. Finally, the total of the depth of
the axial groove
and the depth of the second sipe is not more than 0.7 times the depth of the
circumferential
grooves.
By a first variant of this broad aspect of the invention, the wide main
circumferential
grooves include two axially-outermost wide main circumferential grooves, each
being
located on each side of the tire equator, and at least one inner wide main
circumferential
groove located therebetween, and the first sipes include a circumferential
sipe and axial
sipes provided in each block located adjacent to the axially-outermost wide
main
circumferential groove, the circumferential sipe extending along the
circumferential
groove, the axial sipes extending from the circumferential sipe to the axially-
inner adjacent
wide main circumferential groove.
By a second variant of this broad aspect of the invention, and/or the first
variant
thereof, with respect to the first sipes within the ground contacting area of
the tread
surface, the quotient of the total length of the circumferential components
thereof divided
by the total length of the axial components thereof is in the range of 0.70 to
1.2, and the
quotient of the total length of the circumferential components divided by the
maximum
axial width of the ground contacting area is in the range of 4.0 to 10Ø
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CA 02084741 2001-09-20
(e) DESCRIPTION OF THE FIGURES
In the accompanying drawings,
Fig. 1 is a cross-sectional view of a tire according to an aspect of an
embodiment of
the present invention;
Fig. 2 is a developed plan view of the tread portion thereof showing a tread
pattern;
Fig. 3 is a sectional view taken along the line M-M of Fig. 2;
Fig. 4 is a sectional view taken along the line N-N of Fig. 2;
Fig. 5 is a graph showing a relationship between breaking performance on an
icy
road and the circumferential and axial component ratio;
Fig. 6 is a graph showing a relationship between sideslip performance on an
icy road
and the circumferential and axial component ratio;
Fig. 7 is a graph showing a relationship between the amount of uneven wear and
the
circumferential and axial component ratio;
Fig. 8 is a cross-sectional view of another tire according to an aspect of
another
embodiment of the present invention;
Fig. 9 is a developed plan view of the tread portion thereof;
Fig. 10 is an enlarged view of one tread block explaining the axial component
and
circumferential component of the sipe; and
Fig. 11 shows an example of the tread pattern according to an aspect of yet
another
embodiment of the present invention.
(f~ AT LEAST ONE MODE FOR CARRYING OUT THE INVENTION
In Figs. 1-4, pneumatic tire 1 of an embodiment of the invention is a heavy
duty tire
for a truck or a bus (tire size 10.00820, rim size 7.50V).
The tire 1 comprises a tread portion 22 having a tread face 2 having a pair of
edges,
a pair of axially-spaced bead portions 24, a pair of sidewall portions 23
extending between
the tread edges and the bead portions, a bead core 25 disposed in each bead
portion, a
toroidal carcass 26 extending between the bead portions and turned up around
the bead
cores 25, and a belt 27 disposed radially outside the carcass 26 in the tread
portion.
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CA 02084741 2001-09-20
The carcass 26 comprises at least one carcass ply of a radial or semi-radial
structure.
In this embodiment, the carcass cords are arranged radially at 70 to 90
degrees with respect
to the tire equator C.
For the carcass cords, steel cords and organic fibre cords, e.g., nylon,
polyester,
rayon, aromatic polyamide and the like can be used.
The belt 27 comprises two or more, in this example, four, cross plies, in
which the
belt cords are laid in parallel with each other but crosswise to the cords in
the next ply. For
the belt cords, steel cords can be used, but organic fibre cords, e.g., nylon,
polyester,
rayon, aromatic polyamide and the like may alternatively be used.
Fig. 2 shows the tread portion 22, which is provided with circumferential
grooves 3
extending continuously in the circumferential direction of the tire and axial
grooves 5
extending in the axial direction of the tire so as to divide the tread portion
into at least
three rows of blocks. In Fig. 2, in order to clarify the circumferential
grooves 3, they are
shaded.
The axial grooves 5 are inclined at not more than 30 degrees with respect to
the tire
axial direction.
In this embodiment, as the circumferential grooves 3, a pair of parallel wide
straight
circumferential grooves 3b and a pair of parallel narrow zigzag
circumferential grooves 3a
therebetween are provided.
Further, as the axial grooves 5, central axial grooves 5a extending from one
of the
circumferential grooves 3a to the other of the circumferential grooves 3a,
middle axial
grooves 5b extending from each of the axially inner circumferential grooves 3a
to the
adjacent outer circumferential grooves 3b, and shoulder axial grooves 5c
extending from
each of the axially outermost circumferential grooves 3b to the adjacent tread
edge are
provided.
Accordingly, a central row 15a of central blocks 13a between the narrow zigzag
circumferential grooves 3a, two middle rows 15b of middle blocks 13b each
between the
narrow zigzag circumferential groove 3a and the wide straight circumferential
groove 3b,
and two outermost rows 11 of shoulder blocks 10 each axially outward of each
circumferential groove 3b are formed.
The number of the blocks in each row is set in the range of 25 to 60.
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CA 02084741 2001-09-20
The above-mentioned central axial grooves Sa consist of narrow grooves 6a and
wide
grooves 7a which are alternately disposed in the circumferential direction of
the tire.
The axial grooves 7a have the same depth P as the adjacent circumferential
grooves
3a.
The axial grooves 6a are shallower than the axial grooves 7a or the adjacent
circumferential grooves 3a.
The above-mentioned axial grooves Sb are all shallow grooves 6b which are
shallower than the adjacent circumferential grooves 3b.
As shown in Figs. 3 and 4, each axial groove 6a, 6b is provided in the bottom
face
6S with a sipe 20.
Accordingly, in the initial stage of tread life, the tread rubber under the
bottom of the
shallower axial grooves provides a circumferential support for the adjacent
blocks, and
thereby the block movement is reduced to prevent uneven wear.
On the other hand, when the thread wear is reached to the bottom of the
shallower
axial grooves, the edges of sipes contact with the ground to improve the ice
grip
performance.
In order that the ice grip performance may be effectively improved and the
block
movement may be effectively controlled, the axial grooves and sipes are formed
as follows:
each sipe 20 extends along the centre line of the axial groove, and the
ends thereof are terminated within the bottom;
the width (Z) of the sipes 20 is 0.2 to 1.1 mm;
the length (Y) of each sipe 20 is 70 to 100 % of the length (X) of the
axial groove 6a, 6b at the bottom 6S;
the depth (A) of the axial grooves 6a, 6b is 20 to 50 % of the depth (P) of
the circumferential grooves 3, 3b;
the depth (B) of the sipes 20 is 20 to 50 % of the depth (P) of the
circumferential grooves 3a, 3b; and
the total (A+B) of the depth (A) and the depth (B) is not more than 70 %
of the depth (P)
CA 02084741 2001-09-20
If the axial groove depth (A) is more than 50 % of the circumferential groove
depth
(P), the tread pattern rigidity decreases and block movement increases to
cause uneven
wear.
If the depth (A) is less than 20 % of the depth (P), the snow grip performance
in the
initial stage of tread wear life is deteriorated.
If the sipe length (Y) is less than 70 % of the axial groove length (X), the
ice grip
performance in the middle stage is deteriorated.
If the sipe depth (B) is more than 50% of the circumferential groove depth
(P), in
particular when the total depth (A+B) is more than 70 % of the depth (P), the
tread pattern
rigidity decreases.
If the sipe depth (B) is less than 20 % of the depth (P), the breaking
performance on
the ice road is deteriorated.
Preferably, the axial groove depth (A) is set in the range of 30 to 70 % of
the total
(A+B), and the width (Z) of the sipes 20 is set in the range of 10 to 30 % of
the width (W)
of the axial grooves 6a. In this way the difference between the ice grip
performance which
is derived from the axial grooves in the initial stage to middle stage and
that from the
combination of the sipes and axial grooves in the middle stage to the last
stage is
diminished.
In this embodiment, in order further to improve the ice grip performance, the
central,
middle and shoulder blocks 13a, 13b and 10 are provided with sipes 21 (21a,
21b, 21c,
21d).
The sipes 21 are deeper than the above-mentioned sipes 20, the depth of each
sipe 21
corresponding to the above-mentioned total depth (A+B).
With respect to all the sipes 21 existing in the ground-contacting region of
the tread
surface when the tire is mounted on its regular rim and inflated to its
pressure and then
loaded with its normal or regular load, the total EXi of the lengths of the
circumferential
components thereof is 0.7 to 1.2 times, more preferably 0.85 to 1.10 times the
total EYj of
the lengths of the axial components, and the total EYj of the lengths of the
axial
components is 4.0 to 10.0 times, preferably 5.5 to 10.0 times, more preferably
6.0 to 10.0
times the maximum axial width TW of the above-mentioned ground contacting
region.
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CA 02084741 2001-09-20
Each central block 13a is provided with one axial sipe 20a extending from the
circumferential groove 3a to circumferential groove 3a in a zigzag
configuration.
Each middle block 13b is provided with a plurality of axial sipes 20b due to
its larger
circumferential length. In addition thereto, one circumferential sipe 21c is
provided. The
circumferential sipe 21c extends parallel with the axially-outwardly adjacent
circumferential groove 3b. Each of the axial sipes 21b is extended straight
and parallel with
each other from the axially-inwardly adjacent circumferential groove 3a to the
circumferential sipe 21c, and is stopped at the circumferential sipe 21c.
Each shoulder block 10 is provided with one axial sipe 20d extending straight
and
axially-outwardly from the outermost circumferential groove 3b toward the
thread edge.
In this embodiment, the axial sipes 21a, 21b and 21d are inclined at 30
degrees or
less with respect to the axial direction of the tire, and the circumferential
sipes 21c are in
parallel with the circumferential direction of the tire. Owing to the
circumferential sipes
21c, the above-mentioned total length of the circumferential components is
increased to
meet the above-mentioned limitations.
Figs. 5, 6 and 7 show the results of a braking test, a sideslip test and an
uneven wear
test.
In the braking test, as shown in Fig. 5, it was confirmed that the braking
performance was decreased if the EXi/EYj ratio was either less than 0.7 or
more than 1.2.
In the sideslip test, as shown in Fig. 6, it was confirmed that the resistance
to sideslip
or the ice grip performance during cornering was decreased if the EXi/TW ratio
was less
than 4Ø If the EXi/TW ratio is more than 10.0, block rigidity is decreased,
and both the
wet performance and dry performance are deteriorated.
Further, in the uneven wear test, the heel/toe wear was measured after 40,000
km
running, and, as shown in Fig. 7, it was confirmed that uneven wear (heel/toe
wear) was
effectively reduced by setting the EXi/ EYi ratio in the above-mentioned
range.
Figs. 8-10 show another embodiment of a pneumatic tire according to an aspect
of
this invention having five circumferential grooves to form a block pattern
comprising six
rows of blocks with sipes wherein the total circumferential component length
EXi and the
total axial component length EYj are set as explained above.
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CA 02084741 2001-09-20
In Fig. 8, the tire 1 is a heavy duty tire with square shoulders. (tire size
10.00820,
rim size 7.50V).
The tire 1 comprises a tread portion 2 provided on the tread face S with a
block
pattern, a pair of axially-spaced bead portions 4, a pair of sidewall portions
3 extending
between the tread edges and the bead portions, a pair of bead cores 5 disposed
one in each
of the bead portions 4, a carcass 6 extending between the bead portions 4 and
turned up
around the bead cores 5, and a belt 7 which is disposed radially-outside the
carcass and
inside a rubber tread.
The tread portion 2 is provided with five circumferential grooves G and axial
grooves
M to form a block pattern of which the sea/land ratio is 35/65.
The five circumferential grooves G include one central zigzag circumferential
groove
GO on the tire equator C, two middle straight circumferential grooves G1, and
two axially-
outer straight circumferential grooves G2.
The zigzag circumferential groove GO consists of alternate groove segments
(g1)
which are inclined at 15 to 30 degrees and groove segments (g2) which are
inclined at 75
to 60 degrees with respect to the tire circumferential direction.
The axial grooves M include axial grooves MO extending from the central
circumferential groove GO to the middle circumferential groove G1, axial
grooves M1
extending from the middle circumferential groove G1 to the outer
circumferential groove
G2 and axial grooves M2 extending from the outer circumferential groove G2 to
the tread
edge.
Each of the axial grooves MO and Ml consists of two segments (ml and m2) which
are inclined at 15 to 30 degrees with respect to the tire direction of the
time to have a V-
shaped configuration.
Each of the axial grooves M2 consists of a segment (m3) which is inclined at
15 to 30
degrees with respect to the axial direction of the tire, and a segment (m4)
extending at
substantially 0 degrees with respect to the axial direction of the tire.
The axial grooves M0, M 1 and M2 are arranged such that they are regarded as
extending in a zigzag configuration from one tread edge to the other tread
edge, either with
a left side up inclination (Fig. 2), or with a right side up inclination. In
Fig. 2, the axial
groove ends on both sides of each circumferential groove G2 are slightly
staggered. The
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CA 02084741 2001-09-20
axial grooves are arranged at regular intervals or pitches, but they can be
arranged at
irregular intervals or variable pitches.
Each block BO in the central block rows Ri0 is provided with four parallel
axial sipes
Ka0 extending from groove GO to groove G 1 in parallel with the axial grooves
MO thereby
having a V-shaped configuration.
Each block B1 in the middle block rows Ril is provided with one
circumferential sipe
Kbl extending in parallel with the axially-outwardly adjacent circumferential
groove G2,
and four parallel axial sipes Kal extending from the axially-inwardly adjacent
circumferential groove G 1 to the circumferential sipe Kb 1 in parallel with
the axial grooves
M 1, thereby having a V-shaped configuration.
Each block B2 in the outer block rows Rs is provided with one axially-inner
circumferential sipe Kb2 extending in parallel with the axially-inwardly
adjacent
circumferential groove G2, two parallel axially-outer circumferential sipes
Kb3 extending
in parallel with the tread edge, and three parallel axial sipes Ka2 extending
between the
axially-inner and outer circumferential sipes Kb2 and Kb3 in parallel with the
axial grooves
M2.
Those sipes K (KaO, Kal, Ka2, Kbl, Kb2 and Kb3) are 0.3 to 1.5 mm in width,
preferably 0.5 to 1.0 mm.
As explained above, in the ground contacting area Q under a condition such
that the
tire is mounted on its regular rim and inflated to its regular pressure and
then loaded by its
regular or normal load, the ratio ~Xi/EYj of the total EXi of the lengths Xi
of the
circumferential components of the sipes K to the total EYj of the lengths Yj
of the axial
components of the sipes k is set within the range of 0.70 to 1.2, and the
ratio EXi/TW of
the above-mentioned total circumferential component length EXi to the ground
contacting
width TW is set within the range of 4.0 to 10Ø In case of tire size
10.00820, the regular
rim size is 7.50V, the regular inner pressure is 7.25 kgf/sq.mm, and the
regular tire load is
2700 kg.
Fig. 10 provides a pictorial explanation of the lengths Xi of the
circumferential
components and the length Yj of the axial component of one sipe K (Ka0).
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CA 02084741 2001-09-20
The ground contacting width TW is the maximum axial width of the ground
contacting area Q. In this embodiment, the ground contacting width TW is
substantially
equal to the tread width between the tread edges.
Fig. 11 shows another embodiment of the block tread pattern of an aspect of
the
present invention. In this embodiment, the block pattern comprises a central
straight
circumferential groove G0, two middle zigzag circumferential grooves G1, two
axially-
outermost straight circumferential grooves G2, and axial grooves to form two
central block
rows RiO, two middle block rows Ril, and two outer block rows Rs. Each block
is
provided with a plurality of axial sipes K. Each block in the middle block
rows Ril is
provided with two circumferential sipes Kbl in the same manner as above, and
the axial
sipes Kal thereof are terminated at, or on, the axially-inside of the axially-
innermost
circumferential sipe Kb 1.
