Pneumatic tire including two grooves and two rubber parts

A pneumatic tire has a tread part with two circumferential grooves continuously extending in the circumferential direction in either side of the tire's equator so as to divide the tread part into a pair of shoulder parts and a central part. The central part has a surface utilizing successive convex curves composed of a pair of inner groove walls, which extend inside in the axial direction of tire along a curve convexed outwardly in the radial direction from inner bottom edges of the circumferential grooves, and a central ground-contact surface, which is smoothly connected between the pair of the inner groove walls. The central ground-contact surface is substantially in contact with a virtual tread line connected between outer surfaces of the shoulder parts.

BACKGROUND OF THE INVENTION 
The present invention relates to a pneumatic tire that is preferably 
applicable as a low aspect radial tire for passenger vehicles, in 
particular, and capable of achieving a higher wet grip performance 
reducing tire noises and maintaining the dry grip performance. 
FIELD OF THE INVENTION 
Recently, as automobiles are operated more silently, a noise caused by a 
tire has come to contribute at a higher ratio to a total noise level of an 
automobile, and its reduction is demanded. Such noise reduction is 
specifically desired at a range about 1 kHz that is easily heard by a 
human ear, and sounds due to a columnar resonance is one of main sound 
sources of such high frequency range. 
On the other hand, in order to maintain the wet grip performance, a tire 
tread is generally provided with plural circumferential grooves 
continuously extending in the circumferential direction of tire. 
In such a tire, when it is in contact with the ground, a kind of air column 
is formed by the road surface and the circumferential groove. Then a sound 
of specific wavelength, that is, a double wave length of the air column is 
caused by an airflow within the column during running. 
Such phenomenon is referred to as a columnar resonance, and provides a main 
source of noises at 800 to 1.2 kHz. A wavelength of the columnar resonance 
sound is approximately at a constant frequency regardless of the tire's 
speed, and increases sounds inside and outside an automobile. 
In order to prevent the columnar resonance, although reduction of the 
number or volume of the circumferential grooves is known, such reduction 
leads to a lower wet grip performance. 
On the other hand, although the wet grip performance can be increased 
contrarily by increasing the number or volume of circumferential grooves, 
a simple increase causes reduction of the dry grip performance, because a 
ground-contact area is reduced, and reduction of the steering stability as 
a rigidity of tread pattern is reduced, in addition to the increase of 
tire noise. 
Conventionally, tire's performances have been adjusted by sacrificing any 
of such inconsistent performances. 
SUMMARY OF THE INVENTION 
It is hence a primary object of the invention to provide a pneumatic tire 
capable of improving the wet grip performance without affecting the dry 
grip performance and the steering stability, and reducing tire noises. 
According to one aspect of the present invention, a pneumatic tire has a 
tread part with two circumferential grooves continuously extending in the 
circumferential direction in either side of the tire's equator so as to 
divide the tread part into a pair of shoulder parts, which are located 
outside outer bottom edges of the circumferential grooves in the axial 
direction of tire, and a central part, which is located between inner 
bottom edges of the circumferential grooves in the axial direction of 
tire. The central part has a surface utilizing successive convex curves 
composed of a pair of inner groove walls and a central ground-contact 
surface. The pair of inner groove walls extend inside in the axial 
direction of tire along a curve convexed outwardly in the radial direction 
from the inner bottom edges of the circumferential grooves. The central 
ground-contact surface is smoothly connected between the pair of the inner 
groove walls. The central ground-contact surface is substantially in 
contact with a virtual tread line connected between outer surfaces of the 
shoulder parts. 
A tread rubber of the tread part may be composed of a first rubber 
composition of a loss tangent tan .delta. 1 at 0.01 to 0.35 and a second 
rubber composition of a loss tangent tan .delta. 2 at 1.2 to 10 times the 
loss tangent tan .delta.1. The first rubber composition is provided at 
least in a radially inner region of the central part so as to be adjacent 
to a belt layer. The second rubber composition is provided at least in a 
radially outer region of at least one shoulder part so as to be adjacent 
to a tread surface. 
The central ground-contact surface may be provided with a circumferential 
radiation groove continuously extending on the tire's equator and having a 
groove depth D1 of 0.4 to 0.9 times a groove depth D of the 
circumferential groove and a groove width W1 of 5 mm or less.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a sectional view of an embodiment of the invention in a 
standard state, mounted on a standard rim R and inflated with a standard 
inflation pressure specified by the JATMA standard. 
A tire 1 comprises a pair of bead parts B each having a bead core 2, 
sidewall parts S extending from the bead parts B outwardly in the radial 
direction of tire, and a tread part T for linking between their outer 
ends. And this embodiment is approximately 0.4 to 0.6 in the aspect ratio 
of the tire sectional height to the tire width, and is formed as a low 
aspect radial tire for passenger vehicles. 
Between the bead parts B, a carcass 3 with a radial structure is 
straddling, of which both ends of the main body part extending from the 
tread part T through the sidewall parts S are folded back from inside to 
outside around the bear core 2, and a belt layer 4 is provided on the 
carcass 3 and radially inward of the tread part T. 
In addition, a bead apex rubber 6 extending radially outward from the bead 
core 2 is provided between a main part of the carcass 3 and a folded back 
part thereof so as to maintain the shape and rigidity of the bead part B. 
The belt layer 4 comprises plural belt plies 4A of cords coated by a 
topping rubber. The cords have a high tensile rigidity, such as steel and 
aromatic polyamide, and are aligned in parallel with each other. 
In the embodiment, the belt layer 4 includes, as shown in FIG. 2(A), a band 
ply 4B placed outside a belt ply 4A for controlling lifting of the belt 
ply 4A that is associated with high-speed driving. 
In each belt ply 4A, the cords are aligned at a relatively small angle of 
15 to 30 degrees to the tire's equator so as to be crossed by each other 
between the plies. 
The band ply 4B comprises an edge band 4B1 covering the belt ply 4A in an 
outer end part thereof, and a full band 4B2 placed outside the edge band 
and covering the belt ply together with the edge band 4B1. The bands 4B1, 
4B2 are formed by spirally winding nylon band cords, for example, at an 
angle of 5 degrees or less to the tire's equator. 
A coating rubber layer 20 may be formed outside the belt plies 4A or the 
band ply 4B as shown in FIG. 2(B). The coating rubber layer 20 is a thin 
rubber layer covering an outer surface of outer belt layer 4 so as to 
increase adhesion between a tread rubber 21 and belt layer 4. As for the 
coating rubber layer 20, a rubber composition approximately same as that 
of the topping rubber is employed. It may be formed over an entire width 
of the tread, as shown in the FIG. 2(B), or in a same width as that of the 
outer belt ply. As for the carcass cords, in the case of a tire for 
passenger vehicles, such organic fiber cords as nylon, rayon and polyester 
may be generally employed. 
The tread part has two wide circumferential grooves 7, which are positioned 
in either side of the tire's equator CL and continuously extend 
substantially in the circumferential direction, so that the tread part T 
is divided into a pair of shoulder parts 8 and a central part 9. The 
shoulder part 8 is defined as an area outside an outer bottom edge 7b of 
the circumferential groove 7 in the axial direction of tire. The central 
part 9 is defined as an area between inner bottom edges 7a of the 
circumferential grooves 7 in the axial direction of tire. Preferably, the 
circumferential grooves 7 are positioned symmetrically about the tire's 
equatorial surface. More preferably, a center of a bottom 7S of the groove 
7 is located approximately in the middle of tire's equatorial surface and 
ground-contact tread end TE. The groove depth D of the groove 7 is 4 to 8% 
of a ground-contact width TW of the tread such as 7.5 to 15.0 mm, 
preferably 8.4 mm for a tire of 205/55R15 in size. 
The central part 9 has a surface with a smooth convex curve composed of a 
pair of inner groove walls 9a extending inside in the axial direction of 
tire along a curve convexed outwardly in the radial direction of tire from 
the inner bottom edges 7a of the grooves 7 and a central ground-contact 
surface 9b smoothly connected between the inner groove walls 9a. 
The central ground-contact surface 9b is defined as a tread surface area of 
the central part 9 which comes in contact with the ground when a standard 
load specified by JATMA standard is applied to a tire in the standard 
state. The ground-contact tread end TE is an outer end of ground-contact 
surface of the shoulder part 8, when the standard load is applied. The 
ground-contact surface of shoulder part 8 is crossed by an outer groove 
wall 8a extending outside in the radial direction from the outer bottom 
edge 7b of the groove 7. Thus, the circumferential groove 7 is defined by 
the groove bottom 7S and inner and outer groove walls 9a, 8a. The groove 
width GW of the circumferential groove 7 is defined by a distance in the 
axial direction of tire from an inner end Ea of ground-contact surface of 
the shoulder part 8 to the upper end of the inner groove wall 9a. The 
groove bottom edges 7a, 7b may be formed, when the groove bottom 7S is 
approximately a flat surface as in the embodiment, as bending points 
between the groove bottom 7S and groove walls 8a, 8b. When the groove 
bottom 7S is a concaved surface as shown in FIGS. 12(A) and (B), the 
groove bottom edges 7a, 7b may be formed as bending points or inflection 
points. 
The central ground-contact surface 9b is substantially in contact with a 
virtual tread line 10 connected between the ground-contact surfaces of the 
shoulder parts 8 by extending the ground-contact surfaces of the shoulder 
parts 8. 
Here, the expression "substantially in contact" means that a distance L 
between the central ground-contact surface 9b and the virtual tread line 
10 is less than 2% of the ground-contact tread width TW in the tire's 
equator CL. If it is 2% or more, because a difference between 
ground-contact pressures of the shoulder part and central part is 
increased, the grip performance is reduced, and the wear resistance is 
affected. Thus, it should be preferably 1% or less, more preferably 0.5% 
or less. 
Additionally, the virtual tread line 10 is defined as such an arcuate curve 
of a single curvature of radius that extends between the inner ends Ea of 
the ground-contact surfaces of the shoulder part 8 and is in contact with 
tangent lines to the ground-contact surfaces of the shoulder part 8 at the 
inner ends Ea thereof. When the tangent is approximately parallel, the 
virtual tread line 10 is formed as a straight line connecting between the 
inner ends Ea, Ea. 
In the invention, such the convex central part 9 provides a sub-tread 
having a curvature of radius comparatively small and a width sufficiently 
narrower than the tire's width in the center of tire, thus, the 
hydroplaning phenomenon is prevented, and the wet grip performance is 
increased. It is because a tire with a narrower width and smaller 
curvature of radius is generally superior in preventing the phenomenon. 
Besides, by reducing the curvature of radius of central part 9, 
specifically that of the central ground-contact surface 9a, the draining 
performance to outside in both directions is increased, and the draining 
effect on a wet road is enhanced. 
Incidentally, in the case that a curvature of radius R2 of the 
ground-contact surface of shoulder part 8 is also reduced, the grip 
performance on a dry road and steering stability in cornering are reduced 
due to a reduction of ground-contact area. Therefore, the curvature of 
radius R2 of the ground-contact surface of shoulder part 8 should be 
comparatively large, preferably 3 times or more of the ground-contact 
width TW. And it is allowable until the ground-contact surface of shoulder 
part 8 comes to be approximately a straight line parallel with the tire's 
axis. 
FIG. 1 shows an example with the surface of the central part 9 formed by an 
arc at a curvature of radius R1. The curvature of radius R1 is 
sufficiently smaller than the curvature of radius R2 of shoulder part 8, 
and the convex curve of the central part 9 is inscribed with the virtual 
tread line 10 in the example. In FIG. 1, the distance L is drawn on 
purpose to explain the meanings of "substantially in contact". 
It is also preferable that the curvature of radius R1 is set within a range 
of 0.4 to 1.5 times the ground-contact tread width TW. If it is less than 
0.4 times, a width SW of the central ground-contact surface 9b is reduced, 
and the dry grip performance tends to be significantly reduced. If it is 
more than 1.5 times, the draining effect is insufficient, and the wet grip 
performance is inferior. Additionally, both curvature of radii R1, R2 
should have the center on the tire's equatorial surface. In the 
embodiment, the shoulder part 8 is provided with an arcuate part with a 
curvature of radius smaller than the curvature of radius R2 in the 
vicinity of the ground-contact end TE. 
In order to maintain such the performances as dry grip performance, wear 
resistance and steering stability, the width SW of central ground-contact 
surface 9b is about 5 to 40%, preferably 15 to 35% of the ground-contact 
tread width TW. In addition, it is preferable that a width CW of the 
central part 9, that is a distance between the inner groove bottom edges 
7a is about 40 to 55% of the ground-contact tread width TW. 
Furthermore, in the shoulder part 8, it is desirable that the outer groove 
wall 8a of the groove 7 is formed in such relatively steep and non-arcuate 
line as a straight line at an angle .delta. of 0 to 40 degrees, preferably 
5 to 25 degrees to a radial line X of tire, so that an edge effect with a 
road surface is provided at the inner end Ea of shoulder part 8 with a 
high ground-contact pressure to help maintain the dry grip performance by 
increasing a lateral force, and thereby a cornering power. The outer 
groove wall 8a may be formed in a convex curve similar to the inner groove 
wall 9a, or extended in a zigzag, as shown in FIG. 4, to increase the 
traction ability. 
Additionally, for reduction of noises due to the air column, the groove 
width GW of circumferential grooves 7 is 15% or more of the ground-contact 
tread width TW, when a tire in contact with the ground is applied with the 
standard load, as shown in FIG. 11. 
It has been determined from a result of measuring a passage noise by 
setting the groove depth of circumferential grooves 7 at a constant value, 
and changing the groove width ratio GW/TW between the ground-contact tread 
width TW and the groove width GW of circumferential groove 7. A tire 
tested was of 205/55 R15 in size, and two each U-shaped circumferential 
grooves were employed in the tread surfaces. 
In the measurement, the tire was mounted on a domestic passenger vehicle of 
2000 cc in cubic capacity, and a passage noise at a speed of 60 km/h was 
measured according to the JASO standard (a microphone positioned at 7.5 
m). As recognized from FIG. 7, the passage noise is increased, as the 
groove width ratio is increased, to reach the maximum level at the ratio 
of 13%, and rapidly reduced thereafter. Therefore, the groove ratio is 15% 
or more, more preferably 20% or more. 
Now, FIG. 8 shows a result of frequency analysis about tires with the 
groove ratio GW/TW at 13% and 27%. It is found that noises of frequency 
about 1 kHz are reduced with the ratio of 27%. 
Regarding the circumferential groove 7, it was found that a total groove 
width ratio 2GW/TW of a total groove width 2GW of the circumferential 
grooves 7 to the ground-contact tread width TW affects the cornering power 
and wet grip performance. FIG. 9 shows a result of measuring the cornering 
power in a tire of the same size with a central part in a form of a single 
arc as shown in FIG. 1 and a conventional tire with four circumferential 
grooves G as shown in FIG. 20 by changing the total groove width ratio 
.SIGMA. GW/TW. As the total groove width ratio, a value of the ratio 
2GW/TW was employed for the embodiment, and a value of the ratio (.SIGMA. 
GW)/TW for the conventional example. The cornering power was measured on 
the drum tester in the standard state. It is detected that the embodiment 
shows a higher value in comparison with the conventional tire. It is 
considered because, when the total groove width ratio defined as above is 
constant, the inner groove wall 9a of a convex curve contributes to 
increasing the tire's lateral rigidity. However, when the total groove 
width ratio exceeds 50%, the cornering power is significantly reduced. 
FIG. 10 shows a result of measuring, in a similar manner, a hydroplaning 
inducing speed. It is observed that the hydroplaning phenomenon is caused 
at a higher speed in the embodiment, compared with the conventional tire. 
It is because the circumferential groove 7 forms a widened part 13 as 
shown in FIG. 11 in front and back of a ground-contact center Q, when the 
tire comes in contact with the ground. The widened part 13 increases the 
draining performance, and prevents occurrence of the columnar resonance in 
the circumferential groove 7. 
Thus, because of the noise, dry grip performance affected by the cornering 
power and wet grip performance by the hydroplaning phenomenon, the groove 
width ratio is preferably 15% or more, more preferably 20% or more, and 
the total groove width ratio 30 to 50%, more preferably 40 to 50%. 
Incidentally, although the surface of the central part 9 is formed by a 
single arc in the embodiment shown in FIG. 1, it may be formed in a 
elliptic shape, as shown in FIG. 5, or a curve approximate to an ellipse. 
FIG. 6 shows that the groove wall 9a and the central ground-contact surface 
9b have different curvature of radii R3, R4. The curvature of radius R3 is 
less than the curvature of radius R4 of central ground-contact surface 9b 
and the curvature of radius R2 of ground-contact surface of the shoulder 
part, respectively, and the lowest limit thereof is preferably 5% or more 
of the ground-contact tread width TW. If it is less than 5%, the draining 
effect tends to be insufficient. The highest limit is at a value identical 
to the curvature of radius R4, and the central surface is formed by a 
single arc in such case. The curvature of radius R4 can be close to the 
curvature of radius R2 so long as the wet grip performance is not 
inferior. 
Additionally, in the right and left groove walls 9a, 9a, the curvature of 
radius R3 may be different between right and left such that it is larger 
in one groove wall 9a facing outside the vehicle in mounting a tire than 
the other for reducing sound radiation to the outside. 
Here, in a tire with the central part 9, the heat generated in the central 
part 9 is comparatively high. And it is preferred to control the heat 
generation so as to increase the high-speed durability. 
Accordingly, in the embodiment, as shown in FIGS. 13 to 18, the tread 
rubber 21 comprises a first rubber composition 22 with a loss tangent 
.delta. 1 at 0.01 to 0.35 and a second rubber composition with a loss 
tangent .delta. 2 at 1.2 to 10.0 times of the loss tangent .delta. 1. A 
first rubber part 25 using the first rubber composition 22 is provided at 
least in a radially inner region of the central part 9 so as to be 
adjacent to the belt layer 4, and a second rubber part 26 using the second 
rubber composition 23 is provided at least in a radially outer region of 
at least one shoulder part 8 so as to be adjacent to the tread surface. 
In this way, since the first rubber part 25 of central part 9 is formed by 
the first rubber composition 22 with the loss tangent .delta. 1 at a lower 
value, that is, lower in energy loss, and thereby the increase of internal 
temperature that comes to be excessive in the central part 9, as described 
above, is effectively controlled, and the high-speed durability is 
increased. 
On the other hand, as the second rubber part 26 of shoulder part 8 that is 
subjected to a higher ground-contact pressure is formed by the second 
rubber composition 23 with the loss tangent .delta. 2 at a higher value, 
that is, higher in energy loss, and thereby the riding comfort can be 
increased, and the steering stability in straight-forwarding and turning 
is maintained in the entire tire with enhancement of the ground tracking 
performance and grip performance. 
Now, as for a rubber structure Y of the tread rubber 21, a lateral division 
type rubber structure Y1 formed by dividing the first and second rubber 
parts 25 and 26 in the axial direction of tire as shown in FIGS. 13 to 15, 
and a vertical division type rubber structure Y2 formed by dividing the 
first and second rubber parts 25 and 26 in the radial direction of tire as 
shown in FIGS. 16 to 18 may be adopted. 
As an example of the lateral division type rubber structure Y1, as shown in 
FIG. 13, for example, two boundaries 29 extending from origins V on the 
tread surface to the belt layer 4 are provided in either side of the 
tire's equator CL, the first rubber part 25 is formed between the 
boundaries 29, 29, and the second rubber parts 26 are formed outside the 
boundaries 29. 
The origins V, V are located outside the central ground contact surface 9b, 
that is, on the inner groove wall 9a, on the groove bottom 7s or on the 
outer surface of the shoulder part 8. And the first rubber part 25 is 
formed in both of radially inner and outer regions of the central part 9. 
Therefore, the first rubber part 25 is disposed at least in radially inner 
region of the central part 9 so as to be adjacent to the belt layer 4. 
Similarly, each of the second rubber part 29 is also formed in both of 
radially inner and outer regions of the shoulder part 8, therefore, 
disposed at least in the radially outer region of at least one of the 
shoulder parts 8. 
The origin V is preferably provided on the groove walls 8a, 9a, on the 
groove bottom 7S or on an inner end part a1 of the shoulder part 8 spaced 
outside in the axial direction of tire from the inner end Ea by a distance 
equal to groove bottom width GW1. The origin V is provided more preferably 
on the groove bottom 7S. Although the boundary 29 may be formed in the 
radial direction, that is, parallel to the tire's equatorial surface, it 
may be formed with an inclination away from or approaching to the tire's 
equator CL toward the inside in the radial direction, for example, as 
shown in FIG. 14. 
As the rubber structure Y1, as shown in FIG. 15, one boundary 29 may be 
formed in the tread part T, and the first rubber part 25 is provided 
inside the boundary 29 in the axial direction of tire. In such case, the 
second rubber part 26 is formed only in one shoulder part 8, and the tire 
is, then, mounted with the shoulder part 8 in the outer side of the 
vehicle. 
A tire of the embodiment with a tread rubber structure shown in FIG. 13 and 
a tire of a comparison example with a tread rubber structure shown in FIG. 
22, wherein the tread rubber is dimensionally divided in a same width as 
that of FIG. 13, are prepared. And a relation between the ratio tan 
.delta. 2/tan .delta. 1 and high-speed durability was measured. As 
recognized from a measurement result shown in FIG. 19, in a tire with a 
tread profile of the invention, the high-speed durability is significantly 
increased in a range of 1.2 to 2.0 of the ratio tan .delta. 2/tan .delta. 
1, and the high-speed durability can be improved to a level similar to 
that of a tire with conventional tread profile. 
In other words, the rubber structure Y is most effective within the range 
of 1.2 to 2.0 of the ratio tan .delta. 2/tan .delta. 1, and the ratio tan 
.delta. 2/tan .delta. 1 is more preferably 2.0 to 6.0. The effect of 
increasing the high-speed durability is insufficient, if the ratio is less 
than 1.2, and physical properties between the first and second rubber 
compositions 22, 23 are excessively different, if it is more than 10, 
thus, a separation is induced between the compositions 22, 23. In 
addition, properties as a rubber is lacking, if the loss tangent tan 
.delta. 1 is less than 0.01, and the high-speed durability is 
insufficient, if it exceeds 0.35. Therefore, the loss tangent tan .delta. 
1 is preferably within a range of 0.05 to 0.25. The loss tangent tan 
.delta. 2 is preferably 0.25 or more, more preferably 0.30 or more to 
obtain the steering stability required. 
Here, the loss tangent is a value measured by using a visco-elasticity 
spectrometer prepared by Iwamoto Engineering Works in conditions of a 
temperature at 70 degree C., initial strain 10%, dynamic strain 2% and 
frequency 10 Hz. 
As illustrated in FIG. 16, the vertical division type rubber structure Y2 
is, for example, constructed by a base rubber 30 and a cap rubber 31. The 
base rubber 30 provides the first rubber part 25' placed in a radially 
inner region of the tread rubber 25 over the entire tread width through 
the central part 9, groove bottom 7S and shoulder part 8. The cap rubber 
31 provides the second rubber part 26' covering the base rubber 30 by 
being placed radially outside thereof. 
In the embodiment, the base rubber 30 has an outer part 30A which extends 
below the shoulder part 8 and groove bottom surface 7S with a generally 
constant low thickness and an inner part 30B which extends below the 
central part 9 with an outer convex surface generally parallel to the 
surface of the central part 9. Thus, the base rubber 30 has the highest 
thickness on the tire's equator CL, and a thickness ta of the base rubber 
from the belt layer 4 in the tire's equator CL, which is the highest 
thickness, is higher than a total thickness tb of the tread rubber from 
the belt layer 4 in the groove bottom 7S. 
As shown in Table 2 of an example, a relation between the thickness ratio 
ta/tb and high-speed durability was measured by the embodiment tires 5 to 
12 having a tread structure shown in FIG. 16. As for measuring conditions, 
the ratio tan .delta. 2/tan .delta. 1 was constantly set at 0.30/0.15 
(=2.0), and the total thickness tb constantly at 3.0 mm. As shown in Table 
2, it is found that the high-speed durability is increased, as the ratio 
ta/tb is increased. Specifically, the high-speed durability is 
substantially increased within a range of 1.0 to 1.3 of the thickness 
ratio ta/tb. It means that the rubber structure Y2 is most effective 
within 1.0 to 1.3 of the thickness ratio ta/tb, and the thickness ratio 
ta/tb is more preferably 1.3 or more. The total thickness tb is generally 
about 3 mm in a tire, and the thickness ratio ta/tb is, therefore, 
allowable to such range of thickness ta that the base rubber 30 is not 
exposed from an outer surface of tire. 
With the rubber structure Y2, as shown in FIG. 17, the highest rubber 
thickness tc in an outer part 30A of the base rubber 30 may be increased 
to a value approximately equal to the rubber thickness ta, and the base 
rubber 30 may be formed by eliminating the outer part 30A and employing an 
inner part 30B only, as shown in FIG. 18. 
In addition, in the embodiment, the shoulder part 8 and central part 9 are 
provided with lateral grooves 11, 12 extended substantially in the axial 
direction of tire to increase the wet grip performance. As illustrated in 
FIG. 3, for example, in the embodiment, a lateral groove 11 is employed in 
the shoulder part 8. The lateral groove 11 is extended from a position 
spaced from the circumferential groove 7 in the axial direction of tire 
toward the outside, and opens in the tread end. The shoulder part is 
prevented from reduction of rigidity by not connecting the lateral groove 
with the circumferential groove 7, and the wet grip performance is 
increased by allowing it to open in the tread end. 
A lateral groove 12 of the central part 9 opens only in one end in the 
circumferential groove 7, and an inner side thereof in the axial direction 
of tire is terminated in the vicinity of the equator CL. By terminating 
the lateral grooves in the vicinity of the equator CL, the rigidity of 
central part is maintained, and the steering stability is assured. Groove 
bottoms surface 11a, 12a of the lateral grooves 11, 12 are approximately 
parallel with the belt layer 4. And inner end surfaces 11b, 12b of the 
lateral grooves 11, 12 in the axial direction are parallel to the tire's 
equator CL, or an angle .beta. to a radial line Y are small angle of less 
than 20 degrees. 
In such manner, reduction of the wet grip performance due to reduction in 
length of the lateral grooves as a tire is worn can be controlled. Other 
factors such as a circumferential pitch and depth may be selected 
according to the particular purpose. 
As a means for controlling heat generation in the central part 9, a 
radiation groove 41 including at least a circumferential radiation groove 
40 for heat release may be formed in the central part 9, as shown in FIG. 
24 and 25. Either one or both of the radiation groove 41 and the formation 
of tread rubber by the first and second rubber compositions as mentioned 
above, may be employed. 
The radiation groove 41 comprises, in the embodiment, a circumferential 
radiation groove 40 and lateral radiation grooves 42. The circumferential 
radiation groove 40 is formed as a narrow groove continuously extending 
substantially on the tire's equator. The radiation groove 40 is capable of 
maintaining the pattern rigidity, while providing a heat radiation effect, 
by setting a groove depth D1 thereof at 0.4 to 0.9 times a groove depth D 
of the circumferential groove 7, and a groove width W1 at 5 mm or less. In 
the case that the groove width W1 is more than 5 mm, and the groove depth 
D1 is more than 0.9 times the groove depth D, the columnar resonance is 
caused. If the groove depth D1 is less than 0.4 times the groove depth D, 
the heat radiation effect is insufficient. 
The lateral radiation groove 42 extends from a position in an inner end 
spaced from the circumferential radiation groove 40 toward outside in the 
axial direction of tire at an inclination .theta. of 20 degrees or more to 
the axial direction of tire, and an outer end thereof opens in the 
circumferential groove 7. 
Thus, because the lateral radiation groove 42 is spaced from the 
circumferential radiation groove 40, the rigidity required for the central 
part 9 is maintained, and the steering stability is assured. 
A groove depth D2 of lateral radiation groove 42 is similarly 0.4 to 0.9 
times the groove depth D, and a groove width W2 is 3 mm or less at least 
in the central ground-contact surface 9b. In the case that the groove 
depth D2 is more than 0.9 times the groove depth D, the groove width W2 is 
more than 3 mm, and the inclination .theta. is less than 20 degrees, a 
pitch noise of the lateral radiation groove 42 is excessively high. If the 
groove depth D2 is less than 0.4 times the groove depth D, a sufficient 
heat radiation effect cannot be expected. 
In the circumferential and lateral radiation grooves 40 and 42, an angle 
established by a groove wall in the grooves 40, 42 and a normal on the 
tread surface, that is, an inclination gradient of the groove wall is set 
at 15 degrees or less, more preferably 5 degrees or less, respectively, 
and a dimensional change of the radiation groove 41 due to wear of tire is 
thereby controlled. 
In the embodiment, a shoulder groove 43 is additionally formed in the 
shoulder part 8. The shoulder groove 43 is a rag groove with an inner end 
thereof opening in the circumferential groove 7 and an outer end in the 
tread end. Thus, by opening it in the circumferential groove, the heat 
radiation effect is further increased, increase of temperature in the 
shoulder groove 8 is significantly reduced, the draining performance is 
enhanced, and hydroplaning performance in turning (lateral hydroplaning 
performance) is increased, for example, as shown in FIG. 26. 
An average pitch length of a lateral groove in the circumferential 
direction of tire is generally about 30 mm, and a primary frequency at a 
speed of 60 km/h, for example, comes to be 500 to 600 Hz, thus, showing a 
coincidence with a frequency of noise peak in a tire with a tread profile 
having the convex central part 9. Therefore, in the embodiment, an average 
pitch length P1 of the lateral radiation groove 42 and an average pitch 
length P2 of the shoulder groove 43 are preferably set at 40 mm or more, 
respectively, so that a primary pitch frequency of the grooves 42, 43 are 
different from the noise peak. 
FIG. 27 shows an example of the circumferential radiation groove 40 formed 
as a zigzag groove. 
(EXAMPLE 1) 
A tire of 205/55 R15 in size was produced according to specifications shown 
in Tables 1 and 2, and measured for the steering stabilities in 
straight-forward driving and cornering and the high-speed durability. The 
result of the measurement is shown in the tables. The performances are 
indicated by index, setting a conventional example 1 at 100, and a higher 
score shows better performance. Although a tire of embodiment 5 having a 
tread profile of the invention shows a significantly high effect in the 
aspect of tire noise and wet grip performance, as described, it is higher 
in heat generation in the tread center and inferior in the high-speed 
durability in comparison with a conventional tire with an identical tread 
rubber composition. By employing such tread profile and the tread rubber 
structure, as shown in embodiments 1, 2, 3 and 4, the steering stability 
is maintained, while the high-speed durability is increased. 
In addition, as shown in embodiments 11 to 18, it is recognized that the 
durability is superior at a higher value of the thickness ratio ta/tb. 
(EXAMPLE 2) 
Tires of 205/55 R15 in size were produced according to specifications shown 
in Table 3, and measured for the noise, cornering power, a 
hydroplaning-inducing speed and high-speed durability. A result of the 
measurement is shown in the Table. Embodiments 21 to 26 have a tread 
profile of FIGS. 1 and 6, and conventional tires 21 to 23 have a tread 
profile of FIG. 20. The result is shown by index, setting a conventional 
tire 21 at 100. All tires were measured in the conditions described above, 
and a higher score shows better performance. It is observed that tire of 
the embodiment is superior in the hydroplaning characteristic, and capable 
of increasing the cornering power while reducing the noise, in comparison 
with a conventional example of tire similar in total groove width ratio. 
Moreover, because the tires of the embodiments 21 to 23 are provided with 
a radiation groove, it is recognized that increase of temperature in the 
tread is controlled, and the high-speed durability is increased to a 
similar level to that of a conventional tire. 
(EXAMPLE 3) 
Tires of 205/55 R15 in size were produced with different tread patterns 
shown in FIG. 24, 28 and 29 according to specifications of Table 4, and 
the noise performance, durability, steering stability and residual CF were 
compared. 
As shown in Table 4 and FIG. 31, a noise level (frequency not analyzed) is 
increased from that of an embodiment 31 by 0.9 dB in an embodiment 33 and 
1.4 dB in an embodiment 32, that is relatively high. It is considered to 
be caused by such reason that a peak of primary frequency at 60 km/h is 
420 Hz with a pattern of the embodiment 33, while a peak of primary 
frequency is 570 Hz with a pattern of the embodiment 32, and coincides 
with a peak (630 Hz) of the embodiment 32. 
In terms of the high-speed durability, although the embodiment 33 is 
superior to the embodiment 32, both of them are at a practical level. As 
for the steering stability, the embodiment 33 is equivalent or slightly 
superior to the embodiment 32. Regarding the residual CF, although the 
embodiment 33 is in the negative side, it is higher than the embodiment 
31. Therefore, in order to reduce the residual CF, it is preferable, for 
example, to displace a pattern limited by the tire's equator by 0.5 pitch 
in the circumferential direction, as shown in FIG. 30(A), or reduce an 
angle of the radiation groove to an axial direction, as shown in FIG. 
30(B). 
TABLE 1 
__________________________________________________________________________ 
Conven- 
Comparison 
Embodiment tional 
example 
1 2 3 4 5 6 7 example 1 
1 2 3 4 
__________________________________________________________________________ 
Ground-contact tread width 
168 168 168 168 168 168 168 168 168 168 168 168 
TW (mm) 
Circum- 
Number of grooves 
2 2 2 2 2 2 2 4 4 4 4 4 
feren- 
Groove width GW 
32 32 32 32 32 32 32 16/16 
16/16 
16/16 
16/16 
16/16 
tial 
(mm) 
grooves 
Total groove width 
64 64 64 64 64 64 64 64 64 64 64 64 
.SIGMA.GW (mm) 
Groove width ratio 
19 19 19 19 19 19 19 9.5 9.5 9.5 9.5 9.5 
GW/TW 
Total groove width 
38 38 38 38 38 38 38 38 38 38 38 38 
ratio .SIGMA.GW/TW 
Groove depth D (mm) 
10 10 10 10 10 10 10 10 10 10 10 10 
Tread 
Figure of tread 
FIG. 13 
FIG. 13 
FIG. 13 
FIG. 13 
FIG. 23 
FIG. 23 
FIG. 23 
FIG. 21 
FIG. 22 
FIG. 
FIG. 
FIG. 22 
rubber 
rubber structure 
Characteristics of tread 
rubber 
Loss tan .delta. 1 
0.30 
0.30 
0.30 
0.30 
0.30 
0.25 
0.15 
0.30 0.30 
0.30 
0.30 
0.30 
Loss tan .delta. 2 
0.25 
0.15 
0.10 
0.05 
0.30 
0.25 
0.15 
0.30 0.25 
0.15 
0.10 
0.05 
Ratio tan .delta. 2/tan .delta. 1 
1.2 2.0 3.0 6.0 1.0 1.0 1.0 1.0 1.2 2.0 3.0 6.0 
High-speed durability (index) 
83 96 100 106 80 85 98 100 101 103 105 108 
Test by 
Steering stability in 
99 97 96 94 100 94 80 -- -- -- -- -- 
actual 
straight-forwarding 
vehicle 
(index) 
Steering stability in 
100 99 99 98 100 94 80 -- -- -- -- -- 
cornering (index) 
__________________________________________________________________________ 
In a groove width A/B, A is a value of Circumferential groove width in th 
central side, and B in the shoulder side. 
TABLE 2 
__________________________________________________________________________ 
Embodiment 
11 12 13 14 15 16 17 18 
__________________________________________________________________________ 
Ground-contact tread width TW (mm) 
168 168 168 168 168 168 168 168 
Circum- 
Number of grooves 
2 2 2 2 2 2 2 2 
feren- 
Groove width GW (mm) 
32 32 32 32 32 32 32 32 
tial 
Total groove width .SIGMA.GW (mm) 
64 64 64 64 64 64 64 64 
grooves 
Groove width ratio GW/TW 
19 19 19 19 19 19 19 19 
Total groove width ratio .SIGMA.GW/TW 
38 38 38 38 38 38 38 38 
Groove depth D (mm) 
10 10 10 10 10 10 10 10 
Tread 
Figure of tread rubber structure 
FIG. 16 
FIG. 16 
FIG. 16 
FIG. 16 
FIG. 16 
FIG. 16 
FIG. 16 
FIG. 17 
rubber 
Characteristics of tread rubber 
Loss tan .delta. 1 
0.30 
0.30 
0.30 
0.30 
0.30 
0.30 
0.30 
0.30 
Loss tan .delta. 2 
0.15 
0.15 
0.15 
0.15 
0.15 
0.15 
0.15 
0.15 
Ratio tan .delta. 2/tan .delta. 1 
2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 
Gauge ta/tb (mm) 
2.0/3.0 
2.5/3.0 
3.0/3.0 
3.5/3.0 
4.0/3.0 
4.5/3.0 
5.0/3.0 
5.0/3.0 
High-speed durability (index) 
80 82 85 93 102 108 113 120 
__________________________________________________________________________ 
TABLE 3 
__________________________________________________________________________ 
Embodiment Conventional example 
21 22 23 24 25 26 21 22 23 
__________________________________________________________________________ 
Ground-contact tread width TW (mm) 
168 168 168 168 168 168 168 168 168 
Circumferential grooves 
Number of grooves 
2 2 2 2 2 2 4 4 4 
Groove width GW (mm) 
25 32 38 25 32 38 9/9.5 
12/13 
16/16 
Total groove width .SIGMA.GW (mm) 
50 64 76 50 64 76 37 50 64 
Groove width ratio GW/TW 
15 19 22.6 
15 19 22.6 
5.5 7.5 9.5 
Total groove width ratio .SIGMA.GW/TW 
30 38 45 30 38 45 22 30 38 
Groove depth D (mm) 
10 10 10 10 10 10 10 10 10 
Circumferential radiation groove 
Groove width W1 (mm) 
2.5 2.5 2.5 -- -- -- -- -- -- 
Groove depth ratio D1/D 
0.75 
0.75 
0.75 
-- -- -- -- -- -- 
Lateral radiation groove 
Groove width W2 (mm) 
1.8 1.8 1.8 -- -- -- -- -- -- 
Groove depth ratio D2/D 
0.65 
0.65 
0.65 
-- -- -- -- -- -- 
Shoulder groove 
Groove width W3 (mm) 
3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7 
Groove depth ratio D3/D 
0.84 
0.84 
0.84 
0.84 
0.84 
0.84 
0.84 
0.84 
0.84 
Curvature of radius R1 mm 
-- 95 85 -- 95 85 -- -- -- 
Curvature of radius R2 mm 
520 520 520 520 520 520 520 520 520 
Curvature of radius R3 mm 
40 -- -- 40 -- -- -- -- -- 
Curvature of radius R4 mm 
140 -- -- 140 -- -- -- -- -- 
Noise level dB (A) 
73.8 
72.5 
71.5 
73.8 
72.5 
71.5 
73.8 
74.5 
75.0 
High-speed durability (ECE30) 
&lt;Kph&gt; 270 280 290 240 250 250 290 290 290 
&lt;min&gt; 15 10 5 5 1 15 10 5 5 
Cornering power 97 94 89 97 94 89 100 92 86 
Hydroplaning-inducing speed 
118 137 145 118 137 145 100 114 130 
__________________________________________________________________________ 
In a groove width A/B of conventional tires 1 to 3, A is a value of 
Circumferential groove width in the central side, and B in the shoulder 
side 
TABLE 4 
__________________________________________________________________________ 
Embodiment 31 
Embodiment 32 
Embodiment 33 
Embodiment 34 
Embodiment 
__________________________________________________________________________ 
35 
Tread Pattern 
Plane FIG. 25 FIG. 28 FIG. 29 FIG. 29 
Band ply -- -- -- -- exist 
Noise level 
70.7.sup.dB 
72.1.sup.dB 
71.6.sup.dB 
70.5.sup.dB 
70.9.sup.dB 
High-speed durability 
250 km/h . . . 7 min 
270 km/h . . . 15 min. 
280 km/h . . . 20 min. 
270 km/h . . . 16 
280 km/h . . . 7 
min. 
Durability 30,000 km no fail 
30,000 km no fail 
30,000 km no fail 
30,000 km no fail 
30.000 km no fail 
Residual CF 
-55.5 +8.6 -3.2 +1.1 +20.30 
__________________________________________________________________________