CA2084741C - Block pattern tire with optimized groove depth ratios, sipe depth and length ratios - Google Patents

Abstract

A block pattern tire is provided herein. It comprises a tread 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 thread portion is provided, in its tread surface, with a plurality of first sipes which are 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 extending along the centre line of the axial groove and having 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. 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.

Description

(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.

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

(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.

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.

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) 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.

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.

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 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).

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.

Claims (3)

1. A block pattern tire comprising:
a tread 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;
said tread portion being provided in its tread surface with a plurality of first sipes which are narrower than the circumferential grooves and axial grooves, said axial grooves being shallower than said circumferential grooves;
each said axial groove being provided, in the bottom face thereof, with a second sipe having a width of 0.2 to 1.1 mm, said second sipe extending along the centre line of said axial groove and having ends terminated within said bottom face of said axial groove so as to have a length of not less than 0.7 times and less than 1.0 times the length of said axial groove, the depth of said axial groove being in the range of 0.2 to 0.5 times the depth of said circumferential grooves, and the depth of said second sipe being 0.2 to 0.5 times the depth of said circumferential grooves; and the total of said depth of said axial groove and said depth of said second sipe being not more than 0.7 times the depth of said circumferential grooves.

2. The tire according to claim 1, wherein:
said wide main circumferential grooves include two axially-outermost wide main circumferential grooves, each said groove being located on a respective side of the tire equator, and at least one inner wide main circumferential groove located therebetween; and said first sipes include a circumferential sipe and axial sipes provided in each block located adjacent to the axially-outermost wide main circumferential groove, said circumferential sipe extending along said circumferential groove, said axial sipes extending from said circumferential sipe to the axially-inner adjacent wide main circumferential groove.

3. The tire according to claim 1 or claim 2, wherein, with respect to said first sipes within the ground contacting area of the tread surface:

the quotient of the total length of the circumferential components thereof divided the total length of the axial components thereof is in the range of 0.70 to 1.2; and the quotient of said total length of the circumferential components divided by the maximum axial width of said ground contacting area is in the range of 4.0 to 10Ø