A pneumatic tire having on its tread surface a tread pattern formed by grooves and having a tread rubber of a two-layer structure comprising a cap rubber layer and a base rubber layer of which the latter comprises a rubber having a lower heat generating characteristic in comparison to a rubber forming the former, the tire being characterized by having a value within the range of from 0.15 to 0.35 for (Gc/G).times.(Eb/Ec), in which G is the total thickness of the tread rubber at the bottom of grooves in the tread pattern, Gc being the thickness of the cap rubber at the bottom of the tread pattern groove, Eb being the modulus at 25% elongation of the base rubber layer, Ec being the comparable modulus of the rubber of the cap rubber layer.

BACKGROUND 
The present invention relates to a pneumatic tire and, more particularly, 
an improved pneumatic tire, the improvement relating to the rolling 
resistance and the resistance to groove crack generation of the tire. 
To minimize the rolling resistance of pneumatic tires, lately there have 
been propositions made in the tire industry to provide a two-layer 
structure to the tire tread rubber, comprising a cap rubber layer and a 
base rubber layer. The propositions are made out of consideration of the 
fact that the tread rubber tends to exhibit contradictory characteristics 
with respect to the rolling resistance and the wet skid characteristics of 
tires, and they are made in an attempt to cancel such tendency of the 
tread rubber by way of using a rubber capable of exhibiting a remarkable 
resistance to wet skid for or in the ground-contacting part of the tread 
rubber, namely the cap rubber layer, while using a rubber capable of 
providing a low rolling resistance for or in this part of the tread rubber 
which does not contact the ground or pavement, namely the base rubber 
layer. 
Based on the technical concept of the propositions referred to above, it 
falls that if, in the thickness of the tread rubber, the proportion of the 
base rubber layer is increased, by this the rolling resistance can be 
lowered. 
According to the result of experiments conducted by the present inventors, 
however, it has come to be known that when the proportion occupied by the 
base rubber layer in the thickness of the tread rubber is increased, the 
thickness of the cap rubber layer is indispensably reduced at the bottom 
of grooves forming a tread pattern on the tread surface of the tire, 
whereby the tire becomes likely to with ease undergo during its running a 
so-called groove crack trouble, a phenomenon of generation of a crack at 
the bottom of tire pattern grooves, which is more likely particularly 
where there is a large difference in the physical properties of rubber, 
particularly modulus thereof, between the cap rubber layer and the base 
rubber layer. 
SUMMARY 
Therefore, according to the present invention, it is contemplated to 
provide an improved pneumatic tire, improved in or relating to the rolling 
resistance and the resistance to the groove crack generation or growth. 
To this end, the present invention provides a pneumatic tire which has on 
its tread surface a tread pattern formed by grooves and has a tread rubber 
of a two-layer structure comprising a cap rubber layer and a base rubber 
layer of which the latter comprises a rubber having a lower heat 
generating characteristic in comparison to the rubber forming the former, 
and of which, on the supposition that the total thickness of the cap 
rubber layer at bottoms of grooves in the tread pattern is G, the 
thickness of the cap rubber layer at same bottoms of grooves in the tread 
pattern being Gc, the modulus at 25% elongation of the rubber of the base 
rubber layer being Eb and the comparable modulus of the rubber of the cap 
rubber layer being Ec, the (Gc/G).times.(Eb/Ec) value is within the range 
of from 0.15 to 0.35. 
The above and other objects and features of the present invention will 
become more apparent from considering the following description of the 
invention taken in conjunction with the accompanying drawings.

THE PREFERRED EMBODIMENT 
With reference to the accompanying drawings, the present invention will now 
be described in greater detail in connection with a preferred embodiment 
thereof. 
Initially in FIGS. 1 and 2, the reference character E denotes a pneumatic 
tire embodying the invention, which comprises a left and a right, in pair, 
of bead portions 1 and 1, a left and a right, in pair, of side walls 2 and 
2 connected to and extending from the bead portions, and a tread 3 located 
between the pair of side walls. Between the pair of bead portions 1 and 1, 
a carcass cord layer 4 is disposed, the cords of which are inclined 
substantially at an angle of 90.degree. relative to the tire 
circumferential direction, and each end of the carcass cord layer 4 is 
turned over from the inside to the outside, surrounding bead wires 5 
located at the position of the bead portion 1, to form a folded-over end 
portion 4a, which envelopes a bead filler 6 located above the bead wires 5 
and is closely contacted to the carcass cord layer 4. On this part of the 
carcass cord layer 4 which is located at the position of the tread 3, a 
plurality of cord belt layers 7 are disposed in a mutually crossing 
arrangement at cord angles of from 10.degree. to 30.degree. relative to 
the tire circumferential direction. 
On the tread surface indicated at Ts of the tread 3, there are provided 
grooves M forming a tread pattern, and the tread rubber shown by T of the 
tread 3 comprises a two-layer structure composed of a base rubber layer Tb 
disposed on the outside of the cord belt layers 7 and a cap rubber layer 
Tc disposed externally on the base rubber layer Tb and forming the tread 
surface Ts. The base rubber layer Tb is made of a rubber of which the heat 
generating characteristic is lower than a rubber used for the cap rubber 
layer Tc. 
Now, supposing that at the bottom of the groove M, the tread rubber T has a 
total thickness G and the cap rubber layer Tc has a thickness Gc and also 
that the modulus at 25% elongation of the rubber of the cap rubber layer 
Tc and that of the rubber of the base rubber layer Tb are Ec and Eb 
respectively, according to the present invention the value of 
(Gc/G).times.(Eb/Ec) is set to be within the range of from 0.15 to 0.35. 
Further, in FIG. 2 the reference character Gb denotes the thickness of the 
base rubber layer Tb at the bottom of the groove M. 
In the case of the embodiment of the invention illustrated in FIG. 1, the 
total thickness G is 3.0 mm, the thickness Gc is 0.8 mm, the modulus Ec is 
0.07 kg/mm.sup.2, the modulus Eb is 0.06 kg/mm.sup.2 and the 
(Gc/G).times.(Eb/Ec) value is 0.23. 
Now, a description will be entered into the circumstances in which the 
present invention has been made and also a few experiments carried out by 
the present inventors. 
EXPERIMENT 1 
The present inventors have conducted an extensive study through experiments 
on the ratio, Gc/G, of the thickness Gc of the cap rubber layer Tc at the 
bottom of the groove M to the total thickness G of the tread rubber T at 
the same bottom of the groove M, and also the relationship between the 
modulus values Ec and Eb respectively of the cap rubber layer Tc and the 
base rubber layer Tb of the tread rubber T, for one thing, and the rolling 
resistance value, for the other. 
In the present Experiment 1, use were made of radial tires of the size of 
155 SR 13, and four rating standards were set within the range of 1 to 
about 3 of the ratios, Ec/Eb, of the modulus Ec of the cap rubber layer Tc 
to the modulus Eb of the base rubber layer Tb. With the thicknesses of the 
cap rubber layer Tc and the base rubber layer Tb variously changed while 
in a green tread condition, the total thickness G of the tread rubber T 
and the thickness Gc of the cap rubber layer Tc at the bottom of the 
groove M in section in the radial direction of vulcanized tires were 
measured, the measurement being made at 6 different points on the tire 
circumference and the values found being averaged. Also, modulus values 
found of the rubber of each of the cap rubber layer Tc and the base rubber 
layer Tb are those at 25% elongation. 
Measurements of rolling resistance values were made under the conditions of 
1.9 kg/cm.sup.2 for the air pressure and 390 kg for the load, on rims of 
41/2J.times.13 and at 40 km/hr, 60 km/hr and 80 km/hr, and the values 
found were averaged for the purpose of rating. 
Results of the Experiment are shown in FIG. 3, in which the Gc/G ratios are 
taken on the abscissa, while taken on the ordinate are rolling resistance 
values (%) of tires found on a drum of 67 inches in diameter, which are 
shown in terms of indices relative to the reference case where Gc/G=1.0 
and Ec/Eb=1.16. 
From the results of the Experiment, it has been ascertained that when the 
amount of the base rubber layer Tb is increased, the rolling resistance 
can be suppressed. 
However, as the amount of the base rubber layer Tb is increased or, more 
particularly, the thickness of the base rubber layer Tb in a green tread 
condition is so increased as to be close to the total thickness G of the 
tread rubber at the groove bottom, the thickness Gc of the cap rubber 
layer Tc at the same groove bottom abruptly tends to be smaller, and also 
the effect of the base rubber layer Tb to lower the rolling resistance is 
enhanced as this layer Tb is present closer to the tread surface. From 
this, it is seen that the change of the rolling resistance does not 
uniformly or proportionally take place as the ratio Gc/G is changed, and 
as shown in FIG. 3, the effect of lowering the rolling resistance is all 
of a sudden enhanced in a region of the Gc/G values smaller than about 
0.4. 
Then, in connection with the ratio, Ec/Eb, of the modulus of the cap rubber 
layer Tc to that of the base rubber layer Tb, it has been found that as 
this ratio takes a greater value, the rolling resistance is more greatly 
suppressed. 
FIG. 4 is taken to show the data shown in FIG. 3 in simplification by 
taking (Gc/G).times.(Eb/Ec) values on the abscissa, and in this FIG. 4, 
too, the rolling resistance values are taken on the ordinate. 
By taking the (Gc/G).times.(Eb/Ec) values on the abscissa of the graph as 
in FIG. 4, all the data concerned can be arranged almost on a single 
curve, and from this FIG. 4, it is seen that the effect of lowering the 
rolling resistance can be greatly enhanced in the region of 
(Gc/G).times.(Eb/Ec) values below 0.35. 
EXPERIMENT 2 
Of the same tires as used in the above described Experiment 1, 
determinations and rating were then made of their ratios of growth of 
groove cracks. 
In the groove bottom of each tire, a cut of 5 mm in length and 1.5 mm in 
depth was preparatively formed as an initial crack, and each tire was run 
for 2000 km on a drum of 67 inches in diameter under the conditions of 1.9 
kg/cm.sup.2 for the air pressure, 540 kg for the load and at 81 km/hr. The 
groove-crack growth ratio means the ratio of the length of the crack found 
of the tire after the above running test to the length of the initial 
crack (5 mm). 
Test results are shown in FIG. 5, in which the Gc/G values are taken on the 
abscissa, while the groove-crack growth ratios (%) are taken on the 
ordinate, of the graph. 
From considering the test results, it is seen that when the volume 
proportion is increased of the base rubber Tb so as to suppress the 
rolling resistance, the thickness of the cap rubber layer Tc at the bottom 
of the groove M is reduced and the growth of cracks in grooves is 
promoted. It at the same time is seen that with tires having a larger 
value for the ratio Ec/Eb of the modulus of the cap rubber layer Tc to 
that of the base rubber layer Tb, their values of resistance to groove 
crack generation are abruptly lowered with tires having a greater Gc/G 
value. 
In a same manner as in the above considered FIG. 4 pertinent to Experiment 
1, in FIG. 6 values of (Gc/G).times.(Eb/Ec) are taken on the abscissa of 
the graph, while groove-crack growth ratios (%) are taken on the ordinate, 
to show the data entered in FIG. 5 in simplification. 
By taking the (Gc/G).times.(Eb/Ec) values on the abscissa as in FIG. 6, all 
the data can be arranged almost on a single curve, and from FIG. 6 it is 
seen that the resistance to groove crack abruptly lowers as soon as the 
(Gc/G).times.(Eb/Ec) values are smaller than 0.15. The 
(Gc/G).times.(Eb/Ec) values should preferably be 0.20 or above. 
EXPERIMENT 3 
With use of same tires as in Experiment 1 and by setting G at 3.0 mm, Gc at 
0.8 mm, Ec at 0.07 kg/mm.sup.2, Eb at 0.06 kg/mm.sup.2 and 
(Gc/G).times.(Eb/Ec) at 0.23, same tests as in the above Experiments were 
carried out to obtain desirable results with respect to both of the 
resistance to groove crack generation and the effect of lowering the 
rolling resistance. 
Also, by setting G at 3.0 mm, Gc at 0.5 mm, Ec at 0.07 kg/mm.sup.2, Eb at 
0.06 kg/mm.sup.2 and (Gc/G).times.(Eb/Ec) at 0.14, same tests as above 
were performed to find that although a sufficient effect was obtained of 
lowering the rolling resistance, the resistance to groove crack generation 
was inferior to the result of the above recited case where Gc was set at 
0.8 mm. 
Moreover, by setting G at 3.0 mm, Gc at 1.5 mm, Ec at 0.07 kg/mm.sup.2, Eb 
at 0.06 kg/mm.sup.2 and (Gc/G).times.(Eb/Ec) at 0.43, same tests as above 
were conducted to find that although a desirable result was obtained of 
the resistance to groove crack generation, the effect of lowering the 
rolling resistance was inferior to the result of the above recited case 
where Gc was set at 0.8 mm. 
As stated above, when in order to suppress the rolling resistance of a 
tire, the tread rubber T of the tire is made comprising a two-layer 
structure composed of a cap rubber layer Tc and a base rubber layer Tb, 
the latter comprising a rubber serving to render the tire exhibiting a 
remarkably low rolling resistance, as the proportion of the base rubber 
layer Tb in the thickness of the tread rubber T is increased, the 
thickness of the cap rubber layer Tc at the bottom of grooves M forming a 
tread pattern of the tire is inevitably reduced already at the tire 
building steps, and the so-called groove crack trouble, a phenomenon of 
generating a crack at the bottom of the tread pattern grooves, tends to 
easily take place. Therefore, in order to provide a tire which is 
satisfactory with respect to both of the rolling resistance and the 
resistance to groove crack generation, it is seen from the above recited 
test results from Experiments 1 and 2 to be advisable to set the 
(Gc/G).times.(Eb/Ec) value within the range of from 0.15 to 0.35 or, more 
preferably, from 0.20 to 0.25. 
According to the present invention, the value of (Gc/G).times.(Eb/Ec) is 
set within the range of from 0.15 to 0.35, so that not only a remarkable 
effect can be obtained of lowering the rolling resistance but also the 
resistance to groove crack generation can be enhanced.