Pneumatic bias tire with carcass plies having different cord angle

A pneumatic bias tire including a two-ply carcass layer comprising an inner carcass ply and an outer carcass ply, which respectively comprise cords of an aromatic polyamide fiber and cords of nylon, the cords of the inner carcass ply crossing the cords of the outer carcass ply, the outer carcass ply having a cord angle .beta. to the circumferential direction of the tire which is smaller than a cord angle .alpha. of the inner carcass ply to the tire circumferential direction.

BACKGROUND OF THE INVENTION 
The present invention relates to a pneumatic bias tire incorporating a 
carcass layer utilizing an aromatic polyamide fiber cord, which can 
exhibit remarkable steering stability and high-speed durability. 
Pneumatic bias tires utilizing an aromatic fiber cord for carcass ply cords 
are known per se. For instance, Japanese patent publication No. 57-12586 
discloses an example of such tires. Aromatic polyamide fiber cords have a 
high tenacity and a high modulus of elasticity, and the bias tire 
incorporating such cords, disclosed in the above patent publication, can 
exhibit a remarkable tire performance including a remarkable steering 
stability in particular and, because of this, has already been 
industrially produced particularly in the product form of pneumatic tires 
for racing cars. 
Although aromatic polyamide fiber cords can exhibit highly desirable cord 
characteristics as above, their bondability with or adhesion to rubber is 
relatively limited, and due to this, their characteristic of having a high 
modulus of elasticity tends to take a negative or disadvantageous effect 
in tires and cause them to undergo rupture through buckling. Therefore, 
particularly in the cases of racing-car tires which are run under severe 
service conditions of for example running at a super high speed, problems 
are likely such as separation of carcass ply cords and bursting, and the 
known tires incorporating an aromatic fiber cord tend to involve a 
difficulty to do with the high-speed durability. 
SUMMARY OF THE INVENTION 
Therefore, it is a primary object of the present invention to provide a 
pneumatic bias tire which has an improved high-speed durability and yet 
can fully exhibit the effect of improvement of its steering stability 
brought about by the utilization of an aromatic polyamide fiber cord. 
It is also an object of the invention to provide a pneumatic bias tire for 
racing cars which is highly desirable with respect to both of the steering 
stability and the high-speed durability. 
The pneumatic bias tire to attain the above objects according to the 
present invention includes a two ply carcass layer comprising an inner 
carcass ply and an outer carcass ply, in which the inner carcass ply 
comprises aromatic polyamide fiber cords, while the outer carcass ply 
comprises nylon cords, the cords in the inner carcass ply and those in the 
outer carcass ply cross one another, and the cords in the outer carcass 
ply has a cord angle .beta. to the circumferential direction of the tire, 
which is smaller than a cord angle .alpha. which the cords in the inner 
carcass layer have relative to the tire circumferential direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As illustrated in the drawings, the tire according to the present invention 
comprises a sidewall portion 3 and a bead portion 4 on each side of a 
tread portion 1. The carcass layer shown at 2 has a two ply structure 
comprising an inner carcass ply 2a disposed on the inner side of the tire 
and an outer carcass ply 2b disposed on the side of the inner carcass ply 
2a closer to the tread portion 1. Each of the inner and the outer carcass 
plies 2a and 2b extends over a whole of the meridian section of the tire 
through the sidewall portion 3 and the bead portion 4 in each meridian 
half portion of the tire, and has its respective ends turned up around a 
bead core 5 of the bead portion 3 from the inner side towards the outer 
side of the tire. The outer carcass ply 2b has respective turnup ends 
located in the vicinity of a maximum width region of the tire, while the 
inner carcass ply 2a has respective turnup end portions covering the 
turnup end of the outer carcass ply 2b and ending at a location above the 
turnup end of the outer carcass ply 2b. 
In the tire according to the invention, the inner carcass ply comprising 
cords of an aromatic polyamide fiber takes the effect of improving the 
mechanical strength and the dimensional stability of the carcass layer 
constituting the skelton of a tire as a high pressure vessel and thereby 
improving the tire performance characteristics such as the steering 
stability, particularly that at times of a high-speed running. On the 
other hand, nylon cords having a remarkable adhesion to or bondability 
with rubber, the outer carcass ply comprising nylon cords can integrally 
adhere to the inner carcass ply and also can strongly adhere to a rubber 
layer of the outer located tread portion and a rubber layer of the 
sidewall portion. Using an outer carcass ply comprising nylon cords as 
above, an inner carcass ply comprising aromatic polyamide fiber cords can 
be integrally joined to the tread portion and the sidewall portion, so 
that the generation of a separation trouble of the aromatic polyamide 
fiber cords and that of a rupture due to buckling can be effectively 
suppressed. 
Also in the tire according to the invention, as earlier stated the cord 
angle .beta. of the outer carcass ply relative to the tire circumferential 
direction is set smaller than the cord angle .alpha. of the inner carcass 
ply relative to the tire circumferential direction. Nylon cords have a 
higher ratio of heat shrinkage than aromatic polyamide fiber cords, so 
that if the above cord angles .alpha. and .beta. (cord angles of nylon 
cords and aromatic polyamide fiber cords, respectively) are set identical 
with each other, then the inner carcass ply comprising aromatic polyamide 
fiber cords are prone to undergo an influence of shrinking behavier of the 
outer carcass ply comprising nylon cords during vulcanization of the tire 
and the desirable characteristic of the aromatic polyamide fiber cord 
tends to be lost or reduced. Thus, the cord angle .beta. of the nylon cord 
should be set smaller than the cord angle .alpha. of the aromatic 
polyamide fiber cord. If it is met, then it is possible to suppress the 
difference in the dimensional change between the inner and the outer 
carcass plies likely due to a difference in the heat shrinkage between the 
inner and the outer cords at the time of vulcanization of the tire and 
attain an improvement in or relating to the tire building efficiency. It 
is also possible to permit the desirable characteristic of an aromatic 
polyamide fiber cord to be fully reflected on the tire performance, and as 
a result of this, it is possible to improve the steering stability of 
pneumatic bias tires, and in accordance with this, shorten lap times which 
vehicles, particularly racing cars, may take. 
To permit the above described advantageous effect or results according to 
the present invention to be further effectively demonstrated, preferably 
the cord angle .alpha. of the inner carcass ply should be set within a 
range of 26.degree. to 32.degree., while the cord angle .beta. of the 
outer carcass ply is set within a range of 24.degree. to 30.degree., and 
the difference between the two cord angles, .alpha.-.beta., is to be 
limited to be 1.degree. to 4.degree.. Also, preferably the total denier 
number of nylon cords should be greater than the total denier number of 
aromatic polyamide fiber cords, and respective cord plies should have a 
same end count. 
The above described preferred arrangements of cords in the inner carcass 
ply and the outer carcass ply can take a particularly remarkable effect in 
the cases of racing-car bias tires having an aspect ratio of 30 to 60%. 
Although racing-car bias tires having such a high aspect ratio is 
possessed of a flat profile having a very large tread radius of 800 to 
1500 mm and a relatively broad contact area, if the above described ply 
arrangement is made in such tires, then it is possible to ideally provide 
such flat tire profile. 
The aromatic polyamide fiber cord for use for or in the present invention 
is for example poly-p-phenyleneterephthalamide fiber cord, while useful 
nylon cords may be any of aliphatic polyamide fiber cords to be 
represented by cords of nylon 66 and nylon 6, preferably nylon 66 fiber 
cord. 
As described above, according to the present invention an outer carcass ply 
comprising nylon cords is utilized in a pneumatic bias tire, and thereby 
the desirable cord properties of an aromatic polyamide fiber cord can be 
reflected in a desirably balanced state on the tire performance to improve 
the steering stability of the tire, so that it is possible to shorten lap 
times which vehicles mounting penumatic bias tires thereon take when they 
are run. In addition, an outer carcass ply comprising nylon cords having a 
remarkable adhesion to rubber is arranged on the outer side of the inner 
carcass ply to improve the adhesion of the inner carcass ply, so that the 
separation and rupture of the aromatic polyamide fiber cord due to 
buckling can be suppressed to bring about an improvement in or relating 
the high-speed durability. Further, as before described, it is sufficient 
according to the present invention if the carcass consists of two plies 
only, so that such as an undesirable increase in the weight of tires, a 
complication of tire building steps and an impairing of the tire 
production efficiency can all be effectively avoided. 
EXAMPLES 
There were produced three different tires as shown in the below table, 
namely a tire of the present invention and comparative tires I and II, 
which commonly had a section structure as shown in the appended FIG. 1 but 
of which the cords of the inner carcass ply and the outer carcass ply and 
cord angles .alpha. and .beta. were varied as also shown in the below 
table. 
In the above produced tires, for the inner carcass ply, use was made of a 
ply comprising 1500 D/2 poly-p-phenyleneterephthalamide fiber cords and 
having an end count of 49 cords/5 cm width, and for the outer carcass ply, 
use was made of a ply comprising 1890 D/2 nylon 66 fiber cords and having 
an end count of 44 cords/5 cm width. Further, each of the tires was made 
in respective tire sizes of 180/515-13 (aspect ratio: 50%) and 210/570-13 
(aspect ratio: 55%). 
In connection with each of the three different tires, the following 
described sterring stability test on an actual car was carried out by a 
racing driver, and lap time measurements and steering stability 
evaluations (by feeling) were conducted. 
Steering Stability Test 
Testing Car: an RJ 1600 Class racing car 
Front Tires: 
tire size: 180/515-13 
front rim size: 13.times.8JJ 
Rear Tires: 
tire size: 210/570-13 
rear rim size: 13.times.10 JJ 
Further, test results are shown in the below table in terms of indices, 
taking the test result found of the comparative tire II as 100 (reference 
value). A larger index value represents a more desirable performance. 
Also, in connection with each of the three different tires having the tire 
size of 210/570-13 (rim size: 13.times.10 JJ), a high-speed durability 
test was conducted on an indoor drum under the conditions of an inflation 
pressure of 1.0 kg/cm.sup.2 and a load of 450 kg. 
Test results are shown in the below table in terms of indices, taking the 
test result found of the comparative tire II taken as 100 (reference 
value). A larger index value represents a better high-speed durability. 
______________________________________ 
Comparative 
Comparative Tire of 
Tire I Tire II Invention 
______________________________________ 
Inner Carcass Ply 
nylon 66 aromatic aromatic 
polyamide polyamide 
fiber cords fiber cords 
Outer Carcass Ply 
nylon 66 aromatic nylon 66 
polyamide 
fiber cords 
Cord Angle .alpha. 
29.degree. 
29.degree. 29.degree. 
Cord Angle .beta. 
27.degree. 
27.degree. 27.degree. 
.beta.. 2.degree. 2.degree. 2.degree. 
Lap Time 97 100 100 
Steering 95 100 101 
Stability 
High-Speed 110 100 112 
Durability 
______________________________________ 
From the above table, it is seen that with respect to the lap time and the 
steering stability, the comparative tire II is superior to comparative 
tire I but is inferior to the comparative tire I with respect to the 
high-speed durability. It is also seen that the tire of the invention is 
not only of a same level as the comparative tire II with respect to the 
lap time and the steering stability but is it also superior to the 
comparative tire II with respect to the high-speed durability.