Pneumatic tire

A pneumatic tire has a block pattern which is divided by plural circumferential grooves extending in the circumferential direction of a tire and a number of lateral grooves crossing the circumferential grooves. In the block pattern, the block circumferential/lateral ratio Lm/Wm of the average block circumferential length Lm to the average block width Wm is in a range from 0.38 to 0.55, and the block circumferential length ratio Lm/L of the average block circumferential length Lm to the circumferential length L of the tread part is in a range from 1/120 and to 1/85.

FIELD OF THE INVENTION 
The present invention relates to a pneumatic tire having tread patterns 
that can improve both the riding comfort and steering stability. 
BACKGROUND OF THE INVENTION 
In addition to the riding comfort and steering stability, improvements of 
the performances such as low noise characteristic and hydroplaning 
characteristic are demanded in a pneumatic tire. And, it is known that a 
tread pattern formed in the tread part is one of the factors that affect 
such performances. 
There are such tread patterns as the lug pattern mainly composed of lateral 
grooves, the rib pattern mainly composed of circumferential grooves, the 
rib and lug pattern which intermediates the former two and the block 
pattern having blocks by dividing the tread part by circumferential and 
lateral grooves. And it is known that although, specifically, a tire 
having the block pattern is generally superior in characteristics against 
the road surface such as driving performance and hydroplaning 
characteristic, it is generally inferior in cornering power and wear 
resistance due to the relatively low rigidity of the blocks. However, in 
radial tires which are widely used today, as the rigidity of the treat 
part is increased by a belt layer having superior hoop effect, and the 
wear resistance and cornering power are improved by employing harder tread 
rubbers, tires with the block pattern are being used in high-speed buses 
and passenger cars. 
As the applications of such tires with the block pattern are widened, 
however, it is required to further improve the riding comfort and steering 
stability of a tire without affecting the low noise characteristic and 
hydroplaning characteristic. 
However, as the rigidity of the tread part should be generally reduced to 
improve the envelope performance in regard to convex run-over 
characteristics for the purpose of improving riding comfort, and the 
rigidity of the tread part should be generally increased to increase the 
cornering power for the purpose of improving steering stability, the 
riding comfort and steering stability are contradictory characteristics 
that could not be compatible. There has never been any suggestion for 
improving the riding comfort and steering stability in conventional tread 
patterns that could compatibly realize the two characteristics. 
It is hence a primary object of the invention to provide a pneumatic tire 
that can improve the riding comfort and steering stability without 
affecting the hydroplaning and low noise characteristics, basically by 
mainly limiting the circumferential/lateral ratio of the blocks within a 
specific range. 
According to one aspect of the present invention, a pneumatic tire has such 
tread pattern that a tread part is divided into blocks B aligned in 
parallel by plural vertical grooves G which extend in the circumferential 
direction and a number of lateral grooves g which cross the 
circumferential grooves G, wherein A) the block circumferential/lateral 
ratio Lm/Wm of 1) the average block circumferential length Lm, which is 
the quotient of the actual length Le in the circumferential direction, 
that is, the difference between the circumferential length L of the tread 
part and the total width Lg, Lg=Lg.sub.1 + . . . +Lg.sub.n, of the lateral 
grooves g in the circumferential direction divided by the umber Ng of the 
lateral grooves g, to the average block width Wm, which is the quotient of 
the actual length We in the direction of the tire's width, that is, 
difference between the ground-contact width W of the tire, and the total 
width WG, WG=WG.sub.1 + . . . +WG.sub.n of the circumferential grooves G 
in the direction of tire's width, divided by the added number of 
circumferential grooves NG+1, that is, the number NG of the 
circumferential grooves G added by 1, is 0.38 in a range from 0.55 to and 
B) the block circumferential length ratio Lm/L, which is the ratio of the 
average block circumferential length Lm to the circumferential length L of 
the tread part, is in a range from 1/120 to 1/85. 
In equation format, the above relationships are as follows: 
EQU Lm=Le/Ng=(L-Lg)/Ng 
EQU Wm=We/(NG+1)=(W-WG)/(NG+1) 
EQU 0.38.ltoreq.Lm/Wm .ltoreq.0.55 
EQU 1/120.ltoreq.Lm/L.ltoreq.1/85 
In this invention, the block circumferential/lateral ratio Lm/WM is set to 
being a range from 0.38 from 0.55 to as mentioned hereinbefore. It has 
been obtained as the rate of the average block circumferential length Lm 
at which the envelope power EP can be reduced within a specific range so 
as to improve the riding comfort while controlling the pattern noise, to 
the average block width Wm at which the cornering power CP can be 
increased so as to improve the steering stability while controlling the 
reduction of the hydroplaning characteristic. Therefore, by specifying the 
block circumferential/lateral ratio Lm/Wm within the aforementioned range, 
the riding comfort and steering stability can be improved without 
affecting the pattern noise and hydroplaning characteristics. 
In addition, the block circumferntial length ratio Lm/L is set in a range 
from 1/120 to 1/85. 
The block circumferential length ratio Lm/L represents the rate of the 
average block circumferential length Lm that can reduce the envelope power 
EP so as to improve the riding comfort without affecting the pattern noise 
to the circumferential length L of the tread part. The average block 
circumferential length Lm can be practically determined by specifying the 
value and limiting it within the block circumferential/lateral ratio Lm/Wm 
.

FIG. 1 shows a ground contact surface when a tire is mounted on a standard 
rim, inflated with a specified internal pressure and loaded with a 
specified load. In FIG. 1, a tread part 2 has plural, in this embodiment, 
two circumferential grooves G and G extending in the circumferential 
direction, whereby a central rib 3 between the circumferential grooves G 
and G and ribs 4 and 4 between either the circumferential groove g and a 
side edge s of the ground contact surface are provided. And by forming a 
number of lateral grooves g which are aligned in parallel and which cross 
the circumferential grooves G, each of ribs 3 and 4 is divided into blocks 
B. The circumferential grooves G of the embodiment comprise straight 
grooves extending in the circumferential direction, and lateral grooves g 
are formed as inclined grooves having a narrow width and depth in 
comparison with the circumferential grooves G and crossing them at an 
angle. The circumferential grooves G can be formed as winding grooves such 
as zigzag and wavy forms in addition to straight grooves, and the lateral 
grooves can cross the circumferential grooves G orthogonally. 
Furthermore, the ratio of the average block circumferential length Lm, 
which is the length between the walls of the adjoining lateral grooves g 
and g, to the average block width Wm, which is the length between the 
walls of adjoining circumferential grooves G and G, or circumferential 
groove G and the side end s, that is the block circumferential/lateral 
ratio Lm/Wm, is set 0.38 in a range and 0.55 to. And the block 
circumferential length rate Lm/L, which is the rate of the average block 
circumferential length Lm to the circumferential length L of the tread 
surface, is set at 1/120 or more and 1/85 or less. 
This is based on the result of studies about the effects of tread patterns 
on the riding comfort and steering stability. 
The riding comfort relates to the convex run-over performance, that is, the 
envelope performance, and the smaller the shock (envelope power EP) 
conducted to the axle in running over a convex, the more superior is the 
riding comfort. 
The steering stability relates to the side force (cornering power CP) 
occurring in cornering. 
FIG. 5 shows the measurement result of the cornering power CP and the 
envelope power EP by using tires having tread patterns shown in FIGS. 4(a) 
to (b). A tire with a tire size of 195/60R19 mounted on a standard rim 
(15.times.51/2 JJ) and inflated with a specific internal pressure (1.9 
kgf/cm.sup.2) was employed in the measurement. In regard to the cornering 
power CP, the side force occurring at 1 deg. of slip angle was measured by 
using a dynamic tire tester. 
In regard to the envelope power EP, the shock occurring in the axle was 
measured in running over a convex having a square sectional surface of 10 
mm (H).times.10 mm (W) at a peripheral velocity equivalent to 40 km/h. The 
measured values are shown by index setting the allowance of the cornering 
power CP and envelope power EP of a tread pattern P5 at 100. 
In addition, the average block circumferential length Lm and the average 
block width Wm are 36.0 mm, 23.0 mm and 16.5 mm, and 41.3 m, 29 mm and 
21.6 mm, respectively, and the tread patterns P1 to P9 are settled as 
their combinations. In other words, the average block width Wm is 41.3 mm 
in the tread patterns P1 to P3, 29 mm in the tread patterns P4 to P6 and 
21.6 mm in the tread patterns P7 to P9, and the average block 
circumferential length Lm is 36.0 mm in the tread patterns P1, P4 and P7, 
23.0 mm in the tread patterns P2, P5 and P8, and 16.5 mm in the tread 
patterns P3, P6 and P9. 
As a smaller envelope power EP indicates superiority in riding comfort, and 
the stronger cornering power CP means superior or steering stability, it 
was found that as the cornering power CP of the tires with the tread 
patterns P1, P2 and P3 where the average block width Wm is 41.3 mm is 
stronger, they are superior in steering stability, and as the envelope 
power EP of the tires with the tread patterns P3, P6 and P9 where the 
average block circumferential length Lm is 16.5 mm is smaller, they are 
superior in riding comfort. Thus, as the riding comfort and the steering 
stability depend on the average block circumferential length Lm and the 
average block width Wm, respectively, the riding comfort and the steering 
stability depend no different dimensions of the tread pattern, and it was 
found, therefore, that they can be compatibly realized from the 
measurement result. 
FIG. 6, additionally, shows a result of measuring the relation between the 
average block circumferential length Lm and the noise characteristic. The 
noise characteristic was measured at a distance of 1 m in the direction of 
tire's axis by loading the tire with a specific load and rotating it at a 
speed equivalent to 60 km/h. It was found, as recognized in FIG 6, that 
the envelope power EP is reversely related to the pattern noise in regard 
to the average block circumferential length Lm, and a preferably range of 
the average block circumferential length Lm to control the pattern noise 
at 75 dB(A) or less and the index of envelope power EP at 100 or less is 
in a range from 16 mm 23 mm in the average block circumferential length. 
Curve A illustrates the relationship between EP and Lm. Curve D 
illustrates the relationship between hydroplaning onset speed and Wm. 
When obtaining the average block circumferential length Lm in the block 
circumferential length ratio Lm/L that is the ratio t the circumferential 
length L of the tread surface, the ratio Lm/L comes to be within a range 
between 1/120 and 1/85. 
FIG. 7, furthermore, shows a result of measuring the relation between the 
hydroplaning characteristic and the average block width Wm. In regard to 
the hydroplaning characteristic, a speed at which hydroplaning occurs was 
measured by a dynamic tester. 
In the average block width Wm, the hydroplaning characteristic is reversely 
related to the cornering power CP. And although the cornering power CP is 
reduced by the reduction of the average block width Wm, the hydroplaning 
characteristic is improved, on the contrary. Curve C illustrates the 
relationship between CP and Wm. Curve D illustrates the relationship 
between hydroplaning onset speed and Wm. 
Therefore, it is recognized that an optimum range of the average block 
width Wm to obtain the hydroplaning characteristic of 75 km/h or higher 
speed and the index of cornering power CP of 100 or larger number is in a 
range from 28 to to 56 mm. 
When the average block width Wm is obtained as the block width ratio Wm/W, 
that is, the ratio to the ground-contact width W of the tread in the 
direction of tire's width, the range of the ratio Wm/W comes to be within 
a range from 1/5 to 2/5. 
From the measurement results shown in FIGS. 4 to 7, the range to improve 
both the riding comfort and the steering stability without affecting the 
noise characteristic and hydroplaning characteristic is as follows: 
EQU 16 mm.ltoreq.Lm.ltoreq.23 mm 1 
EQU 28 mm.ltoreq.Wm.ltoreq.56 mm 2 
EQU 1/120.ltoreq.Lm/L.ltoreq.1/85 3 
EQU 1/5.ltoreq.Wm /W.ltoreq.2/5 4 
Here, the block circumferential/lateral ratio Lm/Wm, that is the ratio of 
the average block circumferential length Lm to the average block width Wm, 
can be obtained to be in a range from 0.38 to 0.55 by formulas 1 and 2. 
That is, by treating the deviation of the maximum and minimum values from 
the center value (Lm/Wm=19.5/42=0.46) in the block circumferential/lateral 
ratio Lm/Wm as well as the deviation of the maximum and minimum values 
from the center value (Lm32 19.5) in the average block circumferential 
length Lm. The maximum and minimum values of the ratio Lm/Wm are obtained 
by follows: 
EQU Lm/Wm ; (max)=23/19.5.times.Lm/Wm=0.55 
EQU Lm/Wm ; (min)=16/19.5.times.Lm/Wm=0.38 
It is obviously recognized that the form ratio of a block can compatibly 
realize the riding comfort and the steering stability without negatively 
affecting the other performances by regulating the block 
circumferential/lateral ratio Lm/Wm within the range. 
Considering that, in the average block circumferential length Lm, so-called 
pitch variation is to be employed where the block length is changed in 
plural types to avoid a periodic noise, the average block circumferential 
length Lm can be obtained as an average quotient of the actual length Le 
of the tread surface in the circumferential direction, that is, the 
difference between the circumferential length L of the tread surface and 
the total width Lg of the lateral grooves g in the circumferential 
direction divided by the number Ng of lateral groves g. 
Furthermore, considering that the circumferential grooves G may not be 
formed at a regular interval, the average block width Wm can be defined as 
an average quotient of the actual length We in the direction of tire's 
width, that is, the difference between the ground-contact width W of the 
tread and the total width WG of the circumferential grooves G in the 
direction of tire's width divided by the added number NG+1of the 
circumferential grooves where 1 is added to the number NG of 
circumferential grooves G. 
Moreover, the circumferential length L of the tread surface is defined as 
the total circumferential length of the tread part passing the tire's 
equator in a tire inflated with a specific internal pressure. And the 
ground-contact width W is defined as the maximum ground-contact width in 
the direction of tire's axis where the tire contacts the ground in such 
state that it s inflated with a specific internal pressure and loaded with 
a specific load. Even when a circumferential groove G is thin, it should 
be counted in the number NG. 
Although the measurement was performed in a tire with a tire size of 
195/60R15, it was confirmed that the same results were obtained in those 
with different tire sizes such as 025/60R15. 
Thus, the block circumferential/lateral ratio Lm/Wm is the ratio f the 
value of the average block circumferential length Lm which reduces the 
envelope power EP within a specific range and improves the riding comfort 
while controlling the pattern noise to the average block width Wm that 
increases the cornering power and improves the steering stability while 
controlling the reduction of hydroplaning performance. Therefore, by 
setting the block circumferential/lateral ratio Lm/Wm within the range, a 
form ratio circumferential to lateral of the block B that can improve the 
riding comfort and steering stability without affecting the pattern noise 
and hydroplaning performance can be obtained. 
The block circumferential length ratio Lm/L is a ratio of the block length 
in the circumferential length L of the tread part, that can reduce the 
envelope power EP and improve the riding comfort without affecting the 
pattern noise, and the average block circumferential length Lm in a 
specified tire can be determined by this range. 
The block width ratio Wm/W is a ratio of the block width in the 
ground-contact width W, which maintains the hydroplaning performance and 
the cornering power CP at specified levels or higher levels, respectively. 
And a preferable width of the block B can be determined by this ratio. 
EXAMPLES 
Prototypes of tires with a tire size of 195/60R15 were produced according 
to the specifications shown in Table 1. After mounting the tires on a 
standard rim, inflating with a specific internal pressure and loading a 
specific load, the envelope power EP, cornering power CP, pattern noise 
and hydroplaning characteristic were measured in the same manner as 
mentioned hereinbefore, with the results being shown in Table 1. In 
addition, the tires were employed on all wheels of a passenger car (2000 
cc), and the evaluation result of riding comfort and steering stability by 
the driver's feeling was shown as well. Moreover, as comparison examples, 
conventional tires with tire patterns shown in FIGS. 2 and 3 were 
produced, and the measurement result in the same manner is also presented 
in Table 1. 
It is known that a tire of the invention, as shown in Table 1, improves the 
riding comfort and the steering stability by reducing the envelope power 
EP and increasing the cornering power CP without negatively affecting the 
noise pattern and hydroplaning characteristics. 
TABLE 1 
__________________________________________________________________________ 
Embodiment 
Comparison example 1 
Comparison example 2 
FIG. 1 2 3 
__________________________________________________________________________ 
Circumferential length of 
1940 1940 1940 
the tread part L (mm) 
Ground-contact width (mm) 
140 140 140 
Number of circumferential grooves NG 
2 7 4 
Width of the circumferential 
10 5 8 
groove (mm) 
Number of lateral 90 60 60 
grooves Ng 
Width of the lateral 
2 3 3 
groove (mm) 
Average block circumferen- 
19.56 29.33 29.33 
tial length Lm (mm) 
Average block width Wm (mm) 
40 13.13 21.6 
Block circumferential/lateral 
0.49 2.23 1.36 
ratio Lm/Wm 
Block circumferential 
1/99.2 1/66.1 1/66.1 
ratio Lm/L 
Block width ratio 0.29 0.09 0.15 
Envelope power EP 218 kgf (98) 
227 kgf (100) 
232 kgf (102) 
Conering power CP 126 kgf/deg (115) 
110 kgf/deg (115) 
118 kgf/deg (107) 
Pattern noise 73.6 dB(A) 
73.9 dB(A) 74.5 dB(A) 
Hydroplaning occurrence speed 
79 km/h (100) 
79 km/h (100) 
84 km/h (106) 
Steering feeling evaluation 
3.5 3.0 3.5 
Riding comfort feeling 
3.5 3.0 2.5 
evaluation 
__________________________________________________________________________ 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.