Pneumatic tire with pair of grooves

A pneumatic tire having an improved tread profile capable of improving wet grip and running noise without sacrificing dry grip. The tread portion is provided with a pair of circumferential grooves to divide the tread portion into a central portion and a pair of shoulder portions. The contour of the central portion comprises a central face defined by a convex curve and a pair of side faces each defined by a concave curve. Each of the side faces extends axially outwardly and radially inwardly from each axial edges of the central face towards the bottom of the adjacent circumferential groove. The side faces intersects the central face to form an angle of less than 180 degrees. In a standard condition in which the tire is mounted on a standard rim, and inflated to 70% of the maximum air pressure, and loaded with 70% of the maximum load, the ground contacting width of the central portion is not greater than the axial width between the axial edges of the central face.

The present invention relates to a pneumatic tire having an improved tread 
profile capable of improving the wet grip performance and tire noise while 
maintaining the dry grip performance. 
In recent years, as the expressway network is developed and the vehicle 
performance is remarkably improved, a tread pattern which includes 
circumferentially extending straight or generally straight wide main 
grooves, has been widely used for its superior drainage performance. In 
the ground contacting patch, air tubes are formed between the ground and 
circumferential grooves. As a result, the air existing in the tube is 
excited by impact sound, vibrations, pulsative air flow and the like, and 
the air resonates at a certain frequency (about 800 to 1250 Hz) to 
generate so called air resonance noise. 
If the volume of circumferential groove and/or the number of the 
circumferential grooves are decreased, such a resonance noise can be 
reduced, but wet performances are greatly decreased. If the volume and 
number of the circumferential grooves are increased, wet performances such 
as wet grip are improved, but the dry grip performance and steering 
stability are liable to decrease. 
In order to solve these problems, in Japanese patent application laid open 
No. JP-A-6-127215 (appln. No. 4-802955), a pneumatic tire having a novel 
tread profile shown in FIG. 11 has been proposed, wherein the tread 
portion is divided by two cumferential grooves (e) into a central portion 
(j) and two shoulder portions (k); the central portion (j) has a contour 
(h) defined by a convex curve (g) extending continuously between the edges 
(f) of the groove bottoms, whereby the drainage is improved to prevent the 
occurence of aquaplane phenomenon and the wet grip performance is 
improved. 
In this proposal, however, as the contour (h) is defined by a single convex 
curve, if the tread wear is progressed, the groove width of the 
circumferential groove (e) is greatly decreased, in other words, the 
decrease in the groove volume is accelerated, and wet performances such as 
the resistance to aquaplaning and wet grip performance are greatly 
decreased. 
It is therefore, an object of the present invention to provide a pneumatic 
tire in which wet performances improved by the above-mentioned proposition 
are maintained, even if the tread wear is progressed, without sacrificing 
other performances such as dry grip, running noise and the like. 
According to one aspect of the present invention, a pneumatic tire 
comprises a tread portion, 
the tread portion provided with a pair of circumferential grooves extending 
substantially continuously in the circumferential direction of the tire, 
each circumferential groove disposed on each side of the tire equator to 
divide the tread portion into a central portion between the 
circumferential grooves and a pair of shoulder portions axially outwards 
of the circumferential grooves, 
the contour of the central portion comprising a central face defined by a 
continuous convex curve having axial edges and a pair of side faces each 
defined by a continuous concave curve, 
each of the side faces extending axially outwardly and radially inwardly 
from each axial edges of the central face towards the bottom of the 
adjacent circumferential groove, 
the side faces intersecting the convex central face at the axial edges to 
form an angle of less than 180 degrees, 
in a standard condition in which the tire is mounted on a standard rim, and 
inflated to 70% of the maximum air pressure, and loaded with 70% of the 
maximum load, the ground contacting width of the central portion being not 
greater than the axial width between the axial edges of the central face. 
Here, the standard rim is the rim officially approved for the tire by, for 
example JATMA (Japan), TRA (U.S.A.), ETRTO (Europe) and the like. Also the 
maximum air pressure and maximum tire load are those officially specified 
by the same association or organization. 
Therefore, the volume of the circumferential grooves is increased to 
improve the drainage, and aquaplaning phenomenon is prevented to improve 
the wet grip performance. 
Further, a sufficient ground contacting area can be obtained to maintain 
the dry grip performance. Furthermore, in a foot print, the 
circumferential groove changes its width along its longitudinal direction 
such that the width is minimum in the center and becomes wider towards 
both ends thereof as shown in FIG. 8. That is, the above-mentioned air 
tube changes its sectional area and shape. Accordingly, the resonance mode 
becomes complex. As a result, the resonance is hardly occurred and the air 
resonance noise can be reduced. 
Even if the tread wear makes progress, the convex side faces decreases the 
change in the width of the circumferential groove, which prevents the 
resistance to aquaplaning performance from being decreased.

In FIG. 1, a tire 1 according to the present invention is a passenger 
radial tire having a low aspect ratio, and the tire is mounted on a 
standard rim R and inflated to 70% of the maximum air pressure. 
The aspect ratio, which is a ratio of the tire section height to the tire 
section width, is 0.4 to 0.6. 
The tire 1 comprises a tread portion (T), a pair of axially spaced bead 
portions (B), a pair of sidewall portions (S) extending between the tread 
edges (TE) and the bead portions (B), a pair of bead cores 2 each disposed 
in each of the bead portions (B), a carcass 3 extending between the bead 
portions (B), and a belt 4 disposed radially outside the carcass 3 and 
inside the tread portion T. 
The carcass 3 comprises at least one ply of radially arranged cords 
extending between the bead portions and turned up around the bead cores 2 
in the bead portion (B) from the axially inside to outside of the tire. 
For the carcass cords, organic fiber cords, e.g. polyester, nylon, rayon 
and the like are preferably used in case of a passenger tire. 
Between each turnup portion and the main portion thereof, a bead apex 6 
made of hard rubber extending radially outwardly from the bead core 2 is 
disposed to increase the bead rigidity. 
The belt 4 in this embodiment comprises cross plies, each made of high 
modulus cords, e.g. steel, aromatic polyamide and the like, laid at an 
angle of from 15 to 30 degrees with respect to the tire circumferential 
direction. 
The tread portion (T) is provided on each side of the tire equator CL with 
a pair of circumferential grooves 7 and 7, each extending substantially 
continuously in the circumferential direction of the tire, whereby the 
tread portion (T) is divided into a central portion 9 between the 
circumferential grooves 7 and 7 and a pair of shoulder portions 8 axially 
outwards of the circumferential grooves 7 and 7. In this embodiment, the 
circumferential grooves 7 are a straight groove. However, in order to 
provide more traction, it may be possible to use a zigzag configuration in 
the groove sidewall 8a. Preferably, each circumferential groove 7 is 
disposed in the center between the tire equator CL and the tread edge TE 
or the edge of the tread ground contacting width TW. 
In a meridian section of the tire, i.e., a tire section including the tire 
axis, the contour of the shoulder portion 8 is defined by a top face 8b 
and an axially inner side face 8a. 
The side face 8a extends radially outwardly from the axially outer edge of 
the bottom 7a of each circumferential groove 7 in a non-arc fashion, for 
example straight, inclining at an angle (alpha) of 0 to 40 degrees, 
preferably 5 to 25 degrees with respect to a radial line X, so as to form 
an edged corner (a) between the inner side face 8a and the top face. The 
top face 8b extends axially outwardly from the edge corner (a), and the 
radius of curvature R3 thereof is preferably not less than 3 times the 
tread width TW or the overall ground contacting width of the tread portion 
(T). The center of the radius of curvature R3 is disposed on the tire 
equator CL. If the radius R3 is small, the steering stability during 
cornering and dry grip performance are decreased. The larger radius is 
more preferable, and the upper limit is infinite, that is, the top face 
can be straight in parallel to the tire axis. 
As the edged corner (a) is provided in the shoulder portion 8 whose ground 
pressure is high, the cornering power is increased to maintain the dry 
grip performance. 
The contour of the central portion 9 is defined by a pair of concave side 
faces 9a and 9a and a convex central face 9b therebetween. 
Each concave side face 9a extends axially inwardly and radially outwardly 
from the axially inner edge of the bottom 7a of each circumferential 
groove 7. The convex central face 9b extends between the axially inner 
edges (F) of the side faces 9a and 9a. 
The central face 9b is defined by an arc having a radius of curvature R1. 
The radius R1 is smaller than the radius of curvature R3 of the shoulder 
portions 8, and preferably in the range of from 0.5 to 1.5 times the tread 
width TW. 
If R1 is less than 0.5 times TW, the ground contacting width CW of the 
central portion 9 decreases, and the dry grip performance is liable to 
deteriorated. If R1 is greater than 1.5 times TW, the drainage becomes 
insufficient and the wet grip is decreased. In this embodiment, the 
central face 9b has a single radius R1, and the center is disposed on the 
tire equator CL. 
The above-mentioned side face 9a has a radius of curvature R2. The radius 
of curvature R2 is preferably in the range of from 0.05 to 0.5 times the 
tread width TW. 
If R2 is less than 0.05 times TW, the rigidity of the central portion 9 is 
liable to decrease. If R2 is more than 0.5 times TW, the decrease in the 
circumferential groove width due to tread wear is liable to become 
greater. 
The side faces 9a intersects the central face 9b at the axial edges (F) to 
form an angle of less than 180 degrees, preferably 120 +-15 degrees, 
whereby an edged corner is formed. 
The height of the central portion 9 is such that the convex central face 9b 
substantially inscribes an imaginary line 10 which is drawn, smoothly 
connecting the top faces 8b of the shoulder portions 8 with each other. 
The imaginary line 10 is a straight line or a convex line having a single 
radius of curvature extending between the edges (a) of the shoulder 
portions 8 and being tangential to the top face 8b at the edge (a). Here, 
the phrase "substantially inscribe" means that the distance (L) between 
the imaginary line 10 and the central face 9b at the tire equator CL is 
within the range of not more than 2% of the tread width TW. If the 
distance (L) is not less than 2%, the ground pressure difference between 
the central portion and the shoulder portions becomes large, and the grip 
performance and wear resistance are deteriorated. 
In order to maintain the dry grip, wear resistance, steering stability and 
the like, in the above-mentioned standard condition, the axial width SW of 
the central face 9b between the edged corners (F) is set in the range of 
from 5 to 40%, preferably 15 to 35% of the tread width TW. 
In this embodiment, further, the ground contacting width CW of the central 
portion 9 is the same as the width SW of the central face 9b. In other 
words, the whole width between the edged corners (F) contacts with the 
ground. However, it is possible that the ground contacting width CW is 
smaller that the width SW. In other words, the edged corners (F) located 
axially outward of the edges of the ground contacting width CW. 
As shown in FIG. 3, if the side face 9a is defined by a convex curve as 
shown by broken line 9c, which corresponds to the FIG. 11 contour, when 
the tread wear progresses to for example 50% (such a state is shown by 
chain line M with two dots), the decrease GW0-GW2 in the width of the 
circumferential groove becomes very large, and the circumferential groove 
volume becomes very small. Therefore, the aquaplaning performance is 
greatly deteriorated. 
In the present invention, however, by the provision of the concave side 
faces 9a and 9a, the groove width decrease GW0-GW1 in the 50% wear state 
becomes very small, and the volume of the circumferential groove is 
increased to improve the drainage, and the aquaplaning phenomenon is 
prevented. 
Here, the "50% wear" means that the tread portion is worn and the 
circumferential groove 7 decrease its depth to 50% of the depth in the new 
tire state. The groove width is the minimum axially distance measured from 
the edge of the central portion 9 to the edge of the shoulder portion 8 in 
the foot print obtained under a 70% load state in which the tire is 
mounted on the standard rim and inflated to 70% of the maximum pressure 
and then loaded with 70% of the maximum tire load. 
It is preferable that the groove width GW1 in the 50% wear state is in the 
range of from 0.84 to 0.92 times the groove width GW0 in the new state. If 
GW1 is less than 0.84 times GW0, the aquaplaning performance is liable to 
be deteriorated. If GW1 is more than 0.92 times GW0, it becomes difficult 
to provide a necessary rigidity for the central portion 9. 
Further, to reduce the air resonance noise, the width GW0 of the 
circumferential groove 7 is preferably set in the range of 20 to 30% of 
the tread width TW. 
FIG. 4 shows the results of a noise test, wherein the running noise was 
measured, changing the groove width GW0, maintaining the groove depth 
constant. The test tires (size 205/55R15) were provided in the tread 
portion with a pair of circumferential grooves having a U-shaped sectional 
shape. 
According to the "Test Procedure for Tire Noise" specified in Japanese 
JASO-C606, a 2000 cc passenger car provided with test tires was coasted 
for 50 meter distance at a speed of 60 km/h in a straight test course, and 
the maximum noise sound level was measured with a microphone set at 1.2 
meter height from the road surface and 7.5 meter sideways from the running 
center line in the midpoint of the course. 
As shown in FIG. 4, the noise became maximum when the ratio GW/TW of the 
groove width GW to the tread width TW was 13%, and it is preferable that 
the ratio is greater than 15%, more preferably greater than 20%. 
FIG. 5 shows the frequency spectrum of the noise when the groove width 
ratio was 13% and 27%. When the ratio was 27%, the peak noise level at 
about 1 kHz was remarkably decreased in comparison with that of 13%. 
FIG. 6 shows the results of a cornering power test, in which the cornering 
power was measured with an indoor drum tester, changing the total width of 
the circumferential grooves 7. 
The test tires used were the tire shown in FIGS. 1 and 2 (the total groove 
width=GW0.times.2) and a conventional tire shown in FIGS. 9 and 10 having 
four circumferential groove G (the total groove width=GW.times.4), which 
had the same tire size. 
As shown in FIG. 6, from the test results, it was confirmed that the 
cornering power of the example tire is greater than that of the 
conventional tire. However, when the total groove width exceeds 50% of TW, 
the cornering power is greatly decreased. 
Further, changing the total width of the circumferential grooves, the 
critical speed for aquaplaning phenomenon was measured. The results are 
shown in FIG. 7. 
From the test results, it was confirmed that when the total groove width is 
greater than about 25% of TW, the critical speed of the example tire is 
higher than that of the conventional tire. 
Therefore, it is preferable that the width GW0 of each circumferential 
groove 7 is not less than 15% of the tread width TW, and the total of the 
widths GW0 of all the circumferential grooves 7 is in the range of from 30 
to 50% of the tread width TW. 
Further, the depth of each of the circumferential grooves 7 is preferably 
in the range of from 4 to 8% of the tread width TW. 
In the embodiment shown in FIG. 1, the side face 9a and the central faces 
9b are defined by a single radius arc. However, a multi radius arc, an 
ellipse, a curve closely resembling an ellipse may be used. 
In the present invention, the shoulder portions 8 and central portion 9 can 
be provided with axially extending grooves to improve the wet grip 
performance and road grip. 
FIG. 2 shows an example of the tread pattern including such axial grooves, 
wherein each shoulder portion 8 is provided with axial grooves 15 and 16. 
Each of the axial grooves 16 extends axially outwardly from one of the 
circumferential grooves 7 over the tread edge TE, with both the axial ends 
opened. Each of the axial grooves 15 extends axially outwardly from a 
position axially outward of the circumferential groove 7 to the tread edge 
TE, with both the axial ends closed. 
The axial grooves 15 and the axial grooves 16 are arranged alternately in 
the tire circumferential direction, whereby the wet grip performance can 
be improved without decreasing the rigidity of the shoulder portion 8. 
The central portion 9 is provided with axial grooves 12. Each of the axial 
grooves 12 has an axial inner end terminates near the equator CL and an 
axial outer end opened to one of the circumferential grooves 7. 
No axial groove is provided near the tire equator CL, whereby the rigidity 
of the central portion is maintained, and a steering stability can be 
provided. As shown in FIG. 1, the axial groove 11(15 and 16) and 12 has a 
groove bottom 11a and 12a being substantially parallel with the belt 4. 
The axially inner closed end 11b,12b of the axial groove 11,12 is parallel 
to the tire equator CL or inclined at a small angle (beta) of less than 15 
degrees with respect to a radial line (Y). Therefore, when the tread wear 
is progressed, the length of the axial groove is hardly decreased, and the 
wet grip performance can be maintained. 
Test tires of size 205/55R15 having specifications shown in Table 1 were 
tested for the aquaplaning performance and running noise. 
Aquaplaning Performance Test 
A test car provided on all the four wheels with test tires was run on a wet 
asphalt road with a water depth of 5 mm along a 100 meter radius circle at 
a speed of 70 kilometer/hour, and the maximum lateral-G was measured. The 
test results are shown in Table 1, wherein the results are indicated by an 
index based on that the conventional tire is 100. The larger the index, 
the higher the resistance to aquaplane. 
From the test results, it was confirmed that the example tires according to 
the invention are superior in the aquaplaning performance to the reference 
tires in both the new tire state and 50% wear state. The running noise was 
also improved without sacrificing the aquaplaning performance. 
As explained above, in the pneumatic tire according to the present 
invention, even if the tread wear is progressed, the wet grip performance 
can be maintained at an improved level, without deteriorating the dry grip 
performance and air resonance noise. 
TABLE 1 
__________________________________________________________________________ 
Tire Ex. 1 
Ex. 2 
Ex. 3 
Ref. 1 
Ref. 2 
__________________________________________________________________________ 
Tread width TW 
(mm) 
FIG. 1 
FIG. 1 
FIG. 1 
FIG. 9 
FIG. 11 
168 168 168 168 168 
Central portion 
Radius 
R1 (mm) 
85 85 85 -- 85 
R2 (mm) 
25 15/160*.sup.1 
40 -- -- 
Ground contacting 
(mm) 
38 38 38 -- 38 
width 
Shoulder portion 
Radius R3 (mm) 
900 900 900 900 900 
Circumferential groove 
Number 2 2 2 4 2 
Depth (mm) 
8.4 8.4 8.4 8.4 8.4 
Width 
New GW0 (mm) 
38 38 38 9/9.5*.sup.2 
38 
50% wear 
GW1 (mm) 
32 34 29 -- -- 
50% wear 
GW2 (mm) 
-- -- -- -- 27 
Width change from 
(%) 16 11 24 -- 29 
new to 50% wear 
Test Result 
Aquaplane 
New 142 142 141 100 141 
50% wear 99 103 94 77 90 
50% wear/new 
(%) 70 73 67 77 64 
Running noise 
New dB(A) 
-2.5 -2.4 -2.5 
0 -2.5 
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*.sup.1) double radius, axially inside/axially outside 
*.sup.2) axially inner groove/axially outer groove