Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to pneumatic raclial tires
for passenger cars having excellent wet skid resistance and
cornering performance and a low rolling resistance.
In general, the pneumatic radial tires for passenger
cars are required to have excellent cornering perfomance,
durability and economical value, and particularly wet skid
resistance on road surface under wet conditions is required
to be excellent in view o-f the sa-fety. Furthermore~ it is
important that the durability during high-speed running is
excellent with the advancement of highway networks.
Lately, the development of motor vehicles having
less gasoline consumption, improvement of engines, weight-
saving of motor vehicles and the like are attempted by the
social demand for saving of resource and energy.
Particularly, the research and development of low rolling
resistant tires are advanced in order to diminish power loss
resulting from the tire.
Heretofore, it has been known that the reduction
of the rolling resistance of the tire may contribute to save
the gasoline consumption o-f the motor vehicle. Moreover,
there are considered various factors exerting upon the
rolling resistance of the tire, but it is particularly well-
known from many articles that energy loss of rubber quality
in a tread portion oE the t:ire greatly contributes to the
rolling resistance oE the tire.
Now, it has been attempted to modify the quality
o-f the tread rubber so as to make its energy loss as small
as possible to thereby reduce the rolling resistance oE the
tire. However, such a modification of tread rubber conversely
tends to deteriorate the wet skid resistance. Therefore, it
becomes a common sense that the improvemen~ of the rolling
resistance conversely brings about the deterioration of the
wet skid resistance in the pneumatic radial tire for passenger
cars. In order to establish the contradictory performances
of the rolling resistance and wet skid resistance together,
there have hitherto been attempted various reforms and
contrivances of tire construction, but unsatisfactory
results are still obtained in view of practical uses, so
that the establishment of the contradictory performances is
said to be difficult till now.
The inventors have aimed at a tread of double
laminate structure composed of a cap rubber and a base
rubber (the part of the tread facing to ground is called as
the cap rubber, and the remaining part thereof is called as
the base rubber), which is usually practised in large-sized
tires for truck and bus, in order to solve the above mentioned
drawbacks of the prior art and establish the contradictory
performances of rolling resistance and wet skid resistance
in the pneumatic radial tire for passenger cars. That is,
in the large-sized tires, there is a great problem that
separation failure is liable to be caused due to heat
build-up in the tread rubber during the running because the
tread rubber gauge is -fairly thicker than that of the
pneumatic radial tire for passenger cars. In order to solve
this problem, a cap-base structure using a base rubber of a
low energy loss composition is applied to the tread portion.
On the contrary, the cap-base structure as in the large-
sized tire has not hitherto been much applied to the pneumatic
radial tire for passenger cars from the following reasons;
that is, the separation failure due to heat build-up is
little, and there are such problems in the production
technique that the tread rubber gauge is fairly thin as
compared with that of the large sized tire, the swelling
difference between the cap rubber and the base rubber is
large, and the like. And also, when the conventional rubber
composition having a low energy loss is used as the base
rubber, the modulus of elasticity lowers and as a result,
there are caused problems relating to the cornering perform-
ance, wear resistance, cut resistance and the like due to
the reduction of rigidity in the tread portion, deterioration
of high-speed durability and the reduction of rein-forcing
effect.
As mentioned above, the inventors have aimed at
the double laminate structure composed of cap rubber and
base rubber in the tread portion of the large-sized tire and
made various studies with respect to a means for establishing
the contradictory performances o-f wet skid resistance and
rolling resistance in the pneumatic radial tire for passenger
cars and as a result, it has been confirmed that the cap-base
structure is greatly effective even in the pneumatic radial
tire for passenger cars. That is, the feature that the
cap-base structure is e:Efective for establishment of the wet
skid resistance and low rol.ling resistance is due to the
:Eact that the wet skid resistance is largely dependent upon
the friction coefficient of the cap rubber contactillg with
ground surface, while the low rolling resistance is largely
dependent upon the energy loss of the total rubber content
including the cap r-ubber and base rubber, and hence the base
rubber has an energy loss composition lower than that of the
cap rubber.
~ 2 ~
As the low energy loss composition, there is
usually used a rubber composition consisting of at least one
rubber selected from natural rubber, polyisoprene rubber,
and cis-1,4-polybutadiene rubber, and carbon black of a
lower filling amount as compared with the cap rubber compo-
sition. Preferably, cis-1,4-polybutadiene rubber is used
owing to excellent wear resistance, flexing resistance and
the like, but there are problems in the working properties
under unvulcanized state such as roll peeling property,
swell value (shrinkage) and the like and in the cut resistance
because the reinforcing effect after vulcanization is small.
Therefore, a rubber blend of cis-1,4-polybutadiene with
other diene rubber is used, but it is still insufficient in
practical uses. In any case, when the base rubber has the
low energy loss composition as in the conventional tire,
there are caused various problems as mentioned above, so
that the cap-base structure can not e-ffectively be utilized
: in the pneumatic radial tires for passenger cars.
Now, the inventors have made further studies in
order to solve the above mentioned drawbacks due to the low
energy loss composition of the base rubber and as a result,
it was taken notice of the fact that syndiotactic (syn)-1,2-
polybutadiene graft or block polymerized to cis-1,4-poly-
butadiene is crystalli.zed and :Eormed into short :Eibers and
found out that a polymer consisting essentially of a poly-
butadiene rubber containing a graft or block polymer of syn-
1,2-polybutadiene a:nd cis~ polybutadiene improves the
reinforcing e-ffect, which has never been attained in the
conventional cis-1,4-polybutadiene rubber, makes possible to
have a resilience more higher than that o:f the conventional
- 5
base rubber rein-folced with carbon black or resin, and is
small in the swell value under unvulcanized state and also
that when -thi.s polymer is applied to the base rubber o-f the
tire having a cap-base rubber double laminate structure in
the pneumatic radial tire for passenger cars, not only the
contradictory performances of the wet ski.d resistance and
low rolling resistance are established, but also the cornering
performance, high-speed durability, cut resistance and the
like are considerably improved as compared with the conven-
tional pneumatic radial tires for passenger cars.
According to the invention, there is the provision
of a pneumatic radial tire for passe:nger cars having excellent
wet skid resistance and cornering performance and a low
rol].ing resistance, comprising a casing reinforced with a
carcass of a radial construction and a belt superimposed
about an outer periphery o-f a crown portion of the carcass,
and a tread of a cap-base rubber double l.aminate structure,
characterized in that a volume ratio of cap rubber to base
rubber is 4:1 to 3:7, said cap rubber is a styrene-butadiene
copolymer rubber having a composition and properties required
for improving the wet skid resistance on wet road surface,
and said base rubber consists essentially o:E a polybutadiene
rubber containing a polymer tormed by block or gra:Et poly-
merization of syndiotac-t:ic (syn)-1,2-polybutadiene with cis-
l,~-polybutad:iene havlng a weight average rrrolecular weight
of not :Less than 350,000, the polybutcldiene rubber having a
microstructure containing 6 to 20 wt% of syn-1,2-structure7
at least 40 w% of the syn-1,2-polybutadiene being crystal-
lized and formed into short fibers having an average diameter
ot 0.05 to 1 ~m and an average length of 0.8 to 10 ~m7 or
- 6
is a blend consisting essentially of at ].east 10 wt% of said
polybutadiene rubber and the remainder of the other diene
rubber and is compounded with 25 to 60 parts by weight of
carbon black having an iodine absorption value (IA) of not
less than 40 mg/g and a dihutyl phthalate absorption value
(DBP) of not less than 60 mQ/100 g based on 100 parts by
weight of the total rubber content of said base rubber.
In a preferred embodiment of the invention, a
rubberized layer of steel cords is used as the belt super-
imposed about the outer periphery o-f the crown portion of
the carcass
.
In the pneumatic radi.al tire for passenger cars
according to the invention, the tread has a cap-base rubber
double laminate structure usually used in the conventional
large-sized tire in order to establish the contradictory
performances of wet skid resistance and low rolling resist-
ance. In this case, the volume ratio o-f cap rubber to base
rubber is within a range of 4:1 to 3:7. When the volume
ratio is outside the above range, even if both the cap and
base rubbers satisfy the respective compositions as defined
below, the establishment of the contradictory performances
of wet skid resistance and low rolling resistance aiming at
the invention can not be achieved.
~ccordl.n~ to the invention, the cap rnbber contri-
butes to improve the wet skid resistance on wet road surface,
so that it is necessary to take any improvements extending
from a macro-cou:ntermeasure for widening a contact area of
the tire with road surface or a ground contact area as far as
possible to increase a dynamic friction coefficient between
the tire and the road surface to a micro-countermeasure for
adapting to fine unevenness of the road surface. In this
connection, the cap rubber uses a styrene-butadiene copolymer
rubber as a main ingredient. For exampleS there is used a
rubber blend consisting o:E sytrene-butadiene copolymer
rubber of at least 60 wt% based on the total rubber content
and at least one of natural rubber and other diene rubbers
such as polybutadiene rubber, polyisoprene rubber, ethylene-
propylene-diene terpolymer rubber, halogenated butyl rubber
~ and the like.
; 10 With the cap rubber o-f the above compounding
recipe, the wet skid resistance is improved, but the rolling
resistance increases owing to the wide ground contact area.
As a result, the ~ase rubber is necessary to have a compound-
ing recipe of low energy loss (i.e. large rebound resilience~
and improved durability and resistance to heat build-up in
order to suppress the increase o-E the rolling resistance and
~ establish the contradictory performances of the wet skid
; resistance and low rolling resistance. According to the
invention, therefore, the base rubber satisfying the above
mentioned requirements consists essentially of a polybutadiene
rubber containing a polymer formed by block or graft polymeri-
zation of syn-1,2-polybutadiene with cis-1,4-polybutadiene
having a weight average molecular weight of not less than
350~000, the polybutadiene rubber having a microstructure
containing 6 to 20 wt% of syn l,2-structure, at least 40 wt%
oE the syn-L,2-polybutadiene being crystallized and formed
into short fibers having an average diameter o:E 0.05 to 1 ~m
and an average length of 0.8 to 10 ~m (this polybutadiene
rubber being abbreviated as VC-BR hereinafter), or a blend
consisting essentially of at least 10 wt% of VC-BR and the
remainder of the other diene rubber and is compounded with
25 to 60 parts by weight of carbon black having an IA of not
less than 40 mg/g and a DBP of not less than 60 mQ/100 g
based on 100 parts by weight of the total rubber content.
According to the invention, it is important that
the VC-BR constituting a main ingredient for the base rubber
has a microstructure containing 6 to 20 wt% of syn-1,2-
structure. When the syn-1,2-structure is less than 6 wt%,
there is no great difference in properties between this base
rubber and the usual cis-1,4-polybutadiene rubber containing
no syn-1,2-structure, while when the syn-1,2-structure is
more than 20 wt%, the viscosity becomes too higher and
adversely exerts upon the working property.
The formation of syn-1,2-polybutadiene short fiber
in the VC-BR improves the fatigue resistance of the base
rubber according to the invention. Particularly, the
improving effect is conspicuous when the short fibers have
an average diameter of 0.05 to 1 ~m and an average length of
0.8 to 10 ~m. When the average diameter is less than
0.05 ~m~ the resistance to fracture is insufficient, while
when the average diameter is more than 1 ~m, the flexing
resistance lowers. Furthermore, when the average length is
less than 0.8 ~m, the resistance to cracking is poor, while
when the average diameter is more than 10 ~m, the working
property is deteriorated.
Moreover, it is necessary that at least 40 wt% of
the syn-1,2-polybutadienc are crystallized. When the
crystallized amount is less than 40 wt%, the melting point
of the short fiber becomes lower and hence the temperature
dependence of the base rubber becomes large, so that it is
not adaptable for the improvement of the durability.
Although the base rubber is originally located
beneath the cap rubber without being exposed to the road
surface, it becomes an exposed surface in the last running
stage or due to eccentric wear and the like, so that the
rubber is also required to have excellent wear resistance,
cut resistance and the like. For this purpose, it is
necessary that the cis~ -polybutadiene in the ~C-BR has a
weight average molecular weight of not less than 350,000.
When the weight average molecular weight is less than 350,000,
the wear resistance required as the tread rubber is not
ensured.
When the VC-BR is blended with the other diene
rubber, it is important that the blend contains at least
10 wt% of VC-BR. When the content of VC-BR is less than
10 wt%, the performance of syn-1,2-polybutadiene short
fibers dispersed into the VC-BR is not developed, so that
the content o VC-BR should be not less than 10 wt%. As the
diene rubber, use may be made of, -for example, natural
rubber, styrene-butadiene copolymer rubber, polybutadiene
rubber, polyisoprene rubber and the like.
According to the invention, the VC-BR or the blend
of VC-BR and the other diene rubber is compounded with
carbon black having an IA o:E not less than 40 nng/g and a DBP
o:E not less than 60 m~/100 g. When the IA is :Less than
~0 mg/g, the specific surface area of carbon black is small
and the wear resistance is insuffici.ent, while when the DBP
is less than 60 mQ/100 g, satisfactory reinforcing effect is
not obtained. Moreover, carbon black is compounded in an
amount of 25 to 60 parts by weight based on 100 parts by
- 10 -
weight of the total rubber content including the VC-BR as
a main ingredient in view of low energy loss (i.e., large
rebound resilience), wear resistance and cut resistance,
which are requirements as the base rubber. When the amount
of carbon black is less than 25 parts by weight, the re-
inforcing effect is insufficient and there is a practical
problem in the durability, when the amount of carbon black
is more than 60 parts by weight, the low energy loss composi-
tion is not achieved satisfactori]y.
According to the invention, even when using the
rubberized layer of steel cords as the belt, the tendency of
degrading the gasoline consumption due to the increase of
the weight as compared with the usual textile radial tire,
which is a defect of the conventional steel belt tire, is
considerably improved by the adoption of the cap-base
structure, whereby the gasoline consumption o-f the vehicle
can be more saved.
The invention will now be described in detail with
respect to the following examples.
Examples 1-5, Comparative Examp]es 1-5
Penumatic steel belt radial tires for passenger
cars having a tire size of 175 SR 1~ were manufactured by
using a compounding recipe shown in the following Tab]e 1 as
a tread rubber.
In order to judge the establishment Or the contra-
dictory perEormances of wet skid resistance ancl low rolling
resistance, a tire having a tread composed of styrene-
butadiene copolymer rubber composition contributing to the
improvement of wet skid resistance is used as a control,
while Comparative Examples 1-5 show tires having a tread of
cap-base structure wherein the cap rubber is the rubber
composition of Control and the base rubber is a rubber
composition used in large-sized tires~ and Examples 1-5
show tires having a tread of cap-base structure wherein the
cap rubber is the same rubber composition of Control and the
base rubber is a rubber composition using the VC-BR according
to the invention. In this case, the volume ratio of cap
rubber to base rubber is 13:7.
The thus obtained tires were inflated to an
internal pressure of 1.7 kg/cm2, respectively. Then, the
rolling resistance was measured by a drum tester and the wet
skid resistance and cornering performance were measured
according to practical model test. These measured results
are also shown in Table 1.
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Note) *l : styrene-butadiene copolymer rubber
*2 : cis-1,4-polybutadiene rubber
*3 : VC-BR is composed essentially of a polymer formed
by graft polymerization of syn-1,2-polybutadiene
with cis-1,4-polybutadiene, the polymer having
a microstructure containing 12.6 wt% of syn-1,2-
structure and the syn-1,2-polybutadiene being
formed into shor~ fibers having an average diameter
of 0.25 ~m and an average length of 3.1 ~m.
*4 : N-phenyl-N'-isopropyl-p-phenylenediamine
*5 : N,N'-diphenylguanidine
*6 : N-cyclohexyl-2-benzothiazolyl sulfenamide
*7 : N,N-diisopropyl-2-benzothiazolyl sulfenamide
*8 : The hardness of valcanized rubber with a thickness
of 8 mm was measured by a spring-type JIS hardness
meter.
*9 : The rebound resilience oE vulcanized rubber was
measured at 30C by means of a Dunlop tripsometer
defined in BS-903-223.
*10 : The test tire inflated to an internal pressure of
; 1.7 kg/cm2 was trained on a steel drum having
a diameter oE 1707.6 mm and a width oE 350 mm~
which was rotated/by the driving oE a motor, at
a speecl of 80 km/hr under a load o-f 385 kg for
30 minutes and thereafter the rotating speed of
the drum was raised to 200 km/hr. l`hen, the
driving of the motor was stopped to run the drum
by inertia, during which the rolling resistance of
the tire to the drum at a speed of 50 km/hr was
measured on a basis of deceleration speed of
- 15 -
the drum and time change. Next, a net rolling
resistance of the tire was determined by subtract-
ing the previously ca].culated drum resistance
from the measured value~ The rolling resistance
is evaluated by an index based on the tire of
Control being 100. The higher -the index, the
lower the rolling resistance.
*11 : A vehicle provided with the tes-t tire was run on
a wet asphalt road at a speed o-f 40 km/hr, 70 km/hr
or 100 km/hr and thereafter subjected to rapid
braking at the above speed. Then, a running
distance -for completely stopping the vehicle was
measured at each speed of 40, 70 and 100 km/hr.
Thereafter, an average value was calculated from
the measured values and evaluated by an index
based on the tire of Control being 100.
The smaller the value, the poorer the wet skid
resistance.
*12 : A vehicle provided with the test tire was run on
an asphalt road provided with pyrons set at an
interval of 25 m at a speed of 70-80 km/hr o-ver
a given section. The cornering performance is
estimated by the feeling and time required for
passing through the above section ancl evaluated
by an index based on the tlre of Control being 100.
The higher the index, the better the cornering
performance.
*13 : A time till the occurrence of cracks was measured
by using a de Mattia machine. The flexing
resistance is evaluated by an index based Oll the
- 1~ -
tire of Control being 100. The higher the index,
the better the flexing resistance.
As apparent from the results of Table 1, if it is
intended to improve the rolling resistance o-f the tire of
Control by adopting the cap-base structure to the tire tread
and using the same rubber composition for base rubber as in
the conventional large-sized tire, the rolling resistance is
certainly improved, but the wet skid resistance somewhat
reduces and also the cornering performance lowers. Therefore,
it is obvious from Comparative Examples 1-5 that when the
cap-base structure used in the large-sized tire is merely
diverted to the pneumatic radial tire for passenger cars,
the contradictory performances of the wet skid resistance
and low rolling resistance can not be established.
On the other hand, it is apparent from Examples
1-5 that in the tires having a tread of cap-base structure
wherein the VC-BR according to the invention is used in the
base rubber, the wet skid resistance is maintained at a
level equal to that of Control, the rolling resistance is
considerably improved and hence the contradictory performances
of the wet skid resistance and low rolling resista-nce is
completely establishecl. Pa-rticularly, as shown in P~xamples
2-~, the hardness (modulus of elasticity) can be increased,
while retaining the rebound resilience (cnergy loss) at
substantially level as that of Control, by using the VC-BR
according to the invention together with the other diene
rubber as the base rubber. As a result, the invention makes
it possible to simultaneously improve three performances of
the wet skid resistance, rolling resistance and cornering
performance, which has hardly been achieved in the prior art.
In general, it is known that the cornering perform-
ance is dependent upon the rigidity of the tread rubber.
Now, when the tread is made into a cap-base structure and
the base rubber having a low energy loss (high rebound
resilience) as compared with the cap rubber is used, the
hardness (modulus of elasticity) conversely tends to reduce.
As apparent from Examples 1-5, however, when the base rubber
is formed by using the VC-BR according to the invention, the
hardness increases and the cornering performance is improved.
In the steel belt radial tire for passenger cars,
it has been confirmed from Table 1 that the cornering perform-
ance is improved as the hardness (modulus of elasticity) of
the base rubber increases in the tread of cap-base structure.
Now, the wear resistance was tested by changing the kind and
amount of carbon black to be compounded in the base rubber
to obtain a result as shown in the following Table 2.
In this test, there was used a steel belt radial tire for
passenger cars having a tire size of 175 SR 1~ and comprising
a tread of cap-base structure (volume ratio of cap rubber to
base rubber of 13:7) wherein the cap rubber is the same
rubber composition as described in the tire of Control and
the base rubber is made :Erom a compounding recipe shown in
Table 2.
- 18 -
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-
- 19 -
2~
From the results of Table 2, it can be seen that
the practical use is sufficiently accomplished when using
carbon black having an IA o~ not less than 40 mg/g and a DBP
of not less ~han 60 mQ/100 g. As apparent Erom Comparative
Example 6 using carbon black N660 having an IA of less than
40 mg/g and a DBP of less than 60 mQ/100 g~ there is
practically a problem in the wear resistance level.
In the above mentioned tread of cap-base structure,
it is considered that the base rubber is not necessary to
have a wear resistance because it is not originally exposed
at the tread surface. However, the base rubber is actually
exposed from the tread surface in the middle and last stages
of the tread wear or due to eccentric wear and the like.
Now, the feature that the exposed base rubber is rapidly
worn becomes unacceptable as a practical tire. Particularly,
the tread gauge of the pneumatic radial tire for passenger
cars is thinner than that of the large-sized tire, so that
it is required to give a sufficient wear resistance to the
base rubber. In order to satisfy this requirement, carbon
black having an IA of not less than 40 mg/g and a DBP of not
less than 60 mQ/100 g is compounded into the base rubber
using the VC-BR according to the invention as apparent from
the results of Table 2.
As a~parent from the comparison of Comparative
P.xamples 7, 8 and 9 with Examples 6, 10 ancL 11 in Table 2,
when the amount oE carbon black is less than 25 parts by
weight, the rolling resistance is improved (which is coinci-
dent with the increase oE the rebound resilience), but the
wear resistance is considerably poor and there is caused a
problem in practical uses, while when the amount of carbon
- 20 -
~ 2 ~
black is more than 60 parts by weight, the wear resistance
is improved, but the rebound resilience conversely lowers
and the improvement of the rolling resistance is not achieved.
Therefore, it can be seen from the results of Table 2 that
the amount of carbon black used is within a range of 25 to
60 parts by weight per 100 parts by weight of the total
rubber content of the base rubber in order to achieve a
practical balance between the rolling resistance and the
wear resistance.
Examples 12-16, Com_arative Examples 10-12
Pneumatic steel belt radial tires for passenger
cars having a tire size of 175 SR 1~ were manufactured by
changing the volume ratio of cap rubber to base rubber as
shown in the following Table 3 in the tread of cap-base
structure wherein the cap rubber is the same rubber composi-
tion as described in the tire of Control and the base rubber
is the same rubber composition as described in Example 1.
The influence of the volume ratio of cap rubber to base
rubber on the running performances of the thus obtained tire
was observed to obtain a result as shown in Table 3.
- 21 -
32~
~ N ~ i''
.,1 ,_, O ,~D a> o u~
t~ ~D L/~L~ 00 0 0 N r~ O Ll~ t
f~ ~ ~ ~ ~ O 1-- h 5~ h ~ a~
~ L~ ~ ~ ~ ~ ~d X ~ ~ u~
O X N ¢ 0 V~ O X
~ LLl __
~O
o.s a
O ~ ~ O
~-1 ~ t~ oo O O N N IJ'~ ' p~
~ t~ o ~1 0
X , ¢rQ~v)
Lt~ O
~D ~ CO O O N (`I N
N Cr~ O ~1 0 0 0
O ~ .~
~¢ ~t
_~ O
_~d' ~ CO O O N N ~1
X O N a'i 0 ~1 0 0 0
~1 __
U~
~1 N ~ O O O N N N
$ L~ ~1 0 0 r-l O O O
~1 ~ u~ ~
N _ .
E- ~ o
r ~ O O ~0~ o r O
~ O ~
a~
~r~ L~l
~d ~,~ Lr~ o o O O O O
h r1O O O O O O O
00 r~
~) ~ _
~ O
.r~ r~
~d ~ O O O O O O O O
~d ~ roo~ o o o o o o o
~1
_ .
h *
D
':~ a~ d ~rl a) ~ !~ Q
h ~ ~ ~3 ,I N ~ 1~
~ ~ ~ ~ g ~ * ~ ~d o o g
~d ~ 1 h ~ ~ o o o
O ,9 Ulrl h ::~ O o ~
~rl ~ ~
h ~ ~ N ~) d'
h D ~ ~d,c~ O o O
O ~ ~ ~ O o
~ ~ ~chv ~ o` ~'`
r l ~ r~/ ~ d h N
O e~ ,~ ~ h bO ~d h
p ~ O
_
- 22 -
2~
Note) *l : Durability according to a drum test of FMVSS No. 109
The tire was trained on a drum at a speed of
81 km/hr for 120 minutes and thereafter the running
speed was raised at a constant rate (8 km/hr)
every 30 minutes. In this way, the limiting speed
till the fracture of the tire was measured to
evaluate the level of high-speed durability~
*2 : The tire o:E Comparative Example 10 and test tire
were mounted on left and right front and rear
wheels of the passenger car, respectively.
This car was run while interchanging the left and
right tires at the front and rear wheels every
the running distance of 5000 km. Every the given
running distance~ the depth of the remaining
groove was measured to evaluate the wear resistance.
In Table 3, each performance of the tire of
Comparative Example 10 is indicated as an index of 100, and
the performances of the other tires are calculated therefrom.
The higher the value, the more the improvement of the
performance.
From the results of Table 3, it can be seen that
the volume ratio of cap rubber to base rubber is within a
range oE 4:1 to 3:7 in view of practical tire per-formances.
Particularly, the volume ratlo of cap rubber to base rubber
is preferable within a range of 7:3 to 2:3 considering the
balance of the tire performances.
In the volume ratio of cap rubber to base rubber
o-f 85/15 shown in Comparative Example 11, the contribution
of the base rubber to the rolling resistance is small, while
- 23 -
~ 3~ ~
when the volume ratio of cap rubber to base rubber is 25/75
as shown in Comparative Example 12, a part of the base
rubber is exposed from the tread surface in the running
distance of 20,000-30,000 km and completely exposed in the
running distance o-f 30,000-40,000 km, so that the wear
resistance is considerably poor and there is caused a problem
in the practical use.
Moreover, it is apparent from Table 3 that the
high-speed durability is improved by the low energy loss
composition of the base rubber ~heat build-up is small) in
the tread of cap-base structure.
- 2~ -
