Background of The Invention
Field of the invention
The present invention relates to a pneumatic tire in which
both steering stability and riding comfort are enhanced by specifying viscoelasticity
characteristics of a carcass cord.
Description of the Related Art
conventionally, organic fiber cords such as nylon, rayon,
polyethylene terephthalate (PET) and the like are widely used as a carcass cord
of a pneumatic ti re for a passenger car and an automatic two-wheel vehicle. Especially,
the PET fiber cord is going mainstream of the carcass cord in views of tire rigidity,
size stability of ti re, flat spot resistance and the like. However, since modulus
of the PET fiber cord at a high temperature is largely deteriorated, steering stability
at the time of high speed running is poor. Further, as the performance of a vehicle
is enhanced and the speed is increased, performance required for a tire becomes
stricter, and in recent years, it is strongly required to further enhance the steering
stability at high speed running.
In recent years, it is proposed to use a fiber cord having
higher elasticity than that of the PET fiber cord as a carcass cord. However, if
the elasticity of the carcass cord is increased, the ti re rigidity is increased
so that the steering stability at high speed running is enhanced, but there is a
problem that the riding comfort is deteriorated.
Summary of the Invention
Hence, based on an idea that a fiber cord having high elasticity
is used as the carcass cord and the viscoelasticity characteristics at the time
of high temperature is specified, it is an object of the present invention to provide
a pneumatic tire capable of securing excellent riding comfort while enhancing the
steering stability at the time of high speed running.
To achieve the above object, the invention of claim 1 of
the application provides a pneumatic tire comprising a carcass having a carcass
ply with carcass cords extending from a tread portion to a bead core of a bead portion
through a sidewall portion, wherein the carcass cords are made of high tenacity
vinylon fiber, polyethylene naphthalate fiber or polyalylate fiber, viscoelasticity
characteristics per one cord at 120°C satisfy the following equations (1) to
(wherein "A" represents a product of complex modulus E*(N/cm2) and cross-sectional
area "S" (cm2) of cord), and "B" represents a loss tangent tan&dgr;).
As described above, according to the invention, organic
fiber cord having high elasticity and selected from high tenacity vinylon fiber,
polyethylene naphthalate fiber and polyalylate fiber, is used as the carcass cord.
Further, the viscoelasticity characteristics at the time of high temperature (120°c)
of the organic fiber cord are specified in a designated range. Therefore, balance
between extension of the cord and resilience is optimized, and it is possible to
secure excellent riding comfort while enhancing the steering stability at the time
of high speed running.
Brief Description of the Drawings
Detailed Description of the Preferred Embodiments
- Fig. 1 is a sectional view of a pneumatic tire showing an embodiment of the
present invention; and
- Fig. 2 is a graph used for explaining a relation between the viscoelasticity
characteristics, the steering stability and the riding comfort of the carcass cord.
An embodiment of the present invention will be explained
together with illustrated examples.
Fig. 1 is a sectional view when a pneumatic tire of the
invention is a radial tire for a passenger car. As shown in Fig. 1, the pneumatic
tire 1 includes a carcass 6 which extends from a tread portion 2 to a bead core
5 of a bead portion 4 through a sidewall portion 3, and a belt layer 7 disposed
inside of the tread portion 2 and radially outward of the carcass 6.
The carcass 6 comprises one or more (one, in this embodiment)
carcass ply 6A in which the carcass cords are disposed at an angle of 75° to
90° with respect to a circumferential direction of the tire. The carcass ply
6A includes a ply body 6a extending between the bead cores 5 and 5, and ply fol
ded-back portions 6b connected to both si des of the ply body 6a and folded back
from an inner side to an outer side in an axial direction of the tire around the
bead core 5. A bead apex 8 made of hard rubber having a triangular cross section
is disposed between the ply body 6a and the ply folded-back portion 6b. The bead
apex 8 extends from the bead core 5 radially outward of the tire. With this, a porti
on of the ti re from the bead porti on 4 to the sidewall portion 3 is reinforced.
If the rubber hardness of the bead apex 8 i s low, bead
ri gi di ty and bead endurance are insufficient, and if the rubber hardness is excessively
high, riding comfort and vibration characteristics are deteriorated. From such a
view-point, it is preferable that the lower limit value of the rubber hardness is
70° or higher, and more preferably 80° or higher, and the upper limit
value is 95° or lower. The "rubber hardness" is hardness by a durometer type
A as measured under atmosphere of temperature of 25°C. From the same poi nt
of vi ew, a radial height Ha of an outer end of the bead apex 8 from a bead base
line BL is preferably 25% or higher of a ti re cross secti on height H0, and more
preferably 30% or higher, and its upper limit value is preferably 45% or lower.
The belt layer 7 comprises a plurality of (two, in this
embodiment) belt plies 7A and 7B in which belt cords having high strength such as
a steel cord are disposed at an angle of 15° to 40° with respect to circumferential
direction of the tire. In each of the belt plies 7A and 7B, the belt cords intersect
with each other between the plies, thereby enhancing the belt rigidity, and substantially
the entire width of the tread portion 2 is strongly reinforced with a hoop effect.
In this embodiment, a band layer 9 is formed radially outside
of the belt layer 7 to enhance the high-speed endurance. The band layer 9 comprises
a band ply 9A in which a band cord is spirally wound in the circumferential direction
of the tire at an angle of 5° or less. As the band ply 9A, it is possible to
use a pair of left and right edge band pl i es whi ch cove only an outer end of
the belt layer 7, or a full band play which covers substantially entire width of
the belt layer 7. The band layer 6 is formed of solely the edge band ply or the
full band ply, or a combination thereof.
Next, an organic fiber cord having high elasticity made
of high tenacity vinylon fiber, polyethylene naphthalate fiber (polyethylene 2,
6 naphthalate fiber) or polyalylate fiber is employed as the carcass cord. Among
the organic fiber cords having high elasticity, especially the polyethylene naphthalate
fiber (polyethylene 2, 6 naphthalate fiber) is preferably used because it has appropriate
rigidity and the tire steering stability and the riding comfort can easily be balanced.
The organic fiber cord having high elasticity has high
tensile elastic modulus as is well known. Therefore, it has an effect to enhance
the ti re rigidity and steering stability. On the other hand, since the rigidity
of the sidewall portion 3 is increased at the same time, there is a problem that
the riding comfort is deteriorated.
Here, a tractive force that is periodically repeated at
the ti me of runni ng is applied to the carcass cord. The present inventor researched
based on a view-point that the running performance of a tire was largely influenced
by dynamic viscoelasticity characteristics rather than static characteristics of
the carcass cord. More specifically, carcass cords were prototyped using various
organic fiber materials, and using the carcass cords, tires for a passenger car
were formed. A relation between the running performance and the viscoelasticity
characteristics of the carcass cord was checked.
As a result, it was found that the steeri ng stability
and ri di ng comfort at the time of high speed running has a strong correlation
with respect to the viscoelasticity characteristics at the time of high temperature,
particularly the complex modulus E*(N/cm2) at 120°C and loss tangent
tan&dgr;. It was found that excellent riding comfort could be secured while enhancing
the steering stability at the time of high speed running by using a carcass cord
which satisfied the following equations (1) to (3) when a product (E* × S)
of complex modulus E*(N/cm2) and cross-sectional area S (unit cm2)
of the carcass cord i s defi ned as a variable A, and a loss tangent tan&dgr;
of the carcass cord is defined as a variable B:
Fig. 2 shows a portion of a result of an experiment in
the carcass cord in which steering stability and riding comfort at the time of high
speed running of the prototyped tires were evaluated while a value of the variable
A is shown in a horizontal axis and a value of the variable B is shown in a vertical
axis. Here, ○ shows tires in which both steering stability and riding comfort
were excellent, and □ shows tire in which at least one of the steering stability
and riding comfort is inferior. It can be found from Fig. 2 that at least one of
the steering stability and riding comfort is inferior in a region of f1(A) >
0 exceeding a straight line f1(A) of B = 0.0000437 × A + 0.042857, and a region
exceeding of f2(A) > 0 a straight line f2(A) of B = -0.00003636 × A + 0.163636.
In the experiment, PET fiber, PEN fiber (polyethylene naphthalate
fiber), nylon, high tenacity vinylon fiber, polyalylate fiber, aramid fiber, rayon
were used as the organic fiber material , specifications such as thickness (fineness)
of the cord, the number of twist was changed, and carcass cords in which the variables
A and B were different were prototyped. Using the carcass cords, tires for a passenger
car of tire size of (195/65R15) is formed, and the steering stability and riding
comfort were evaluated by a driver's feeling when the vehicle actually runs on a
tire test course.
Even when the carcass cords are the same, if the stri ki
ng number is increased, the steering stability is enhanced (riding comfort is deteriorated).
Therefore, in the actual case, in the prototyped carcass cord, prototyped tires
in which the striking number M was adjusted and the steering stability was enhanced
to a satisfactory reference level, the riding comfort was evaluated, and it was
evaluated whether the prototyped ti re was excellent in both steering stability
and riding comfort. It can be confirmed from Fig. 2 that a carcass cord having viscoelasticity
characteristics which satisfy the equations (1) to (3) can exhibit excellent riding
comfort while enhancing the steering stability at the time of high speed running.
It becomes apparent from the inventor's research that the same result can be obtained
also in tires for a passenger car having different tire sizes and tires having different
categories (e.g., tires for an automatic two-wheel vehicle).
It is estimated that the steering stability and riding
comfort are also influenced by elongation of the carcass cord and restoration from
elongation. That is, it is conceived that a carcass cord using an organic fiber
having large complex modulus and high elasticity is not relatively extended easily
and has excellent steering stability. When the loss tangent tan&dgr; is high,
however, hysteresis loss when a load is released from a state where the cord is
once extended and the carcass cord is restored to its original state is large. Therefore,
the restoration is relatively poor, respond is deteriorated, and steering stability
is deteriorated. On the other hand, even in the case of a cord in which complex
modulus is low and the cord is easily extended relatively, if the loss tangent tan&dgr;
is low, the restoration is excellent and response is quickened and thus, the steering
stability can be enhanced. If the extending properties and the restoration are optimally
balanced, a margin for enhancing the riding comfort is generated, and both the riding
comfort and steering stability can be enhanced.
To enhance both the riding comfort and steering stability,
it is preferable to satisfy the following equation (4):
The loss tangent tan&dgr; which is the variable B is
roughly determined by a material of the cord fiber. The product (E* × s) of
complex modulus E*(N/cm2) which is the variable A and cross-sectional
area S (unit cm2) of the carcass cord can be adjusted by changing specifications
such as the thickness (fineness) of the cord and the number of twist.
Here, in the case of the aramid fiber, since the loss tangent
tan&dgr; is large and the tensile elastic modulus is excessively high, even when
the cord thickness is reduced and the number of twist is increased, it is difficult
to obtain a cord which satisfies the equations (2) and (3).
Generally, a carcass cord having a two-ply yarn structure
is used. At that time, if the number of twist n per 10 cm length of the cord (the
number of upper twist and the number of lower twist are the same) is excessively
large, a resistance between filaments at the time of expansion and contraction becomes
excessively large, and the restoration of the cord is deteriorated. If the resistance
is excessively small, the resistance against fatigue is increased and the endurance
is deteriorated. Therefore, it is preferable that the upper limit of the number
of twist n is 70 (/10 cm) or lower, more preferably 60 (/10 cm) or lower, and the
lower limit of the number of twist is more preferably 30 (/10 cm) or higher and
more preferably 35 (/10 cm) or higher. For the same reason, it is preferable that
the twist coefficient T of the cord defined in the following equation (5) is in
a range of 0.45 to 0.75. In the equation, n represents number of twist (/10 cm),
D represents total fineness based on corrected mass (dtex) of cord, and p represents
specific gravity of fiber material:
If the vari abl e A whi ch i s the product (E* × S)
of the complex modulus E*(N/cm2) and the cross-sectional area S (unit
cm2) of the carcass cord is excessively large, the cord becomes hard
and this is disadvantageous for enhancing the riding comfort. Therefore, the upper
limit of the variable A is preferably 2000N or less, preferably 1900N or less and
more preferably 1800N or less. The lower limit value of the variable A is preferably
340N or higher in view of the steering stability. It is preferable that the lower
limit value of the variable B which is the loss tangent tan&dgr; is 0.02 or higher,
and more preferably 0.03 or higher. If the variable B is lower than this value,
the impact moderating effect is lowered, and the response to the external force
is excessively excellent, and there is a tendency that the riding comfort is deteriorated.
In the carcass cord, the carcass drag K which is the produce
(A × M) of the variable A and the cord striking number M is set to preferably
2.0 × 104 to 5.0 × 104, and more preferably 2.5
× 104 to 3.8 × 104 in view of enhancing both the
steering stability and riding comfort. The cord striking number M means the number
of cords per 5 cm width of the carcass ply.
As a topping rubber of the carcass ply covering the carcass
cord, a preferably employed rubber is one having complex modulus at 70°C of
4.5 MPa or higher and loss tangent of 0.14 or lower. This is because that since
the loss tangent of the topping rubber is small, if this is combined with a cord
having relatively small loss tangent carried out in the invention, the energy loss
of the tire is reduced and the steering stability is more enhanced. The fact that
the complex modulus is slightly high can also enhance the steering stability. In
the conventional topping rubber, complex modulus is 4.0 MPa and loss tangent is
Although the especially preferable embodiment of the present
invention has been described in detail, the invention is not limited to the illustrated
embodiment, and the invention can variously be modified and carried out.
carcass cords were prototyped based on the specifications
described in Table 1, and tires for a passenger car having tire size of 195/65R15
were formed using the carcass cords. The steering stability, stability feeling and
riding comfort of the prototyped tires at the time of high speed running were tested.
The specifications other than the carcass cord are the same. The complex modulus
and loss tangent of the topping rubber of the carcass are values measured under
the condition that the temperature is 70°C, the frequency is 10Hz and initial
distortion is 10%, and dynamic distortion is ±1%.
<Steering stability, stability feeling and riding comfort>
The prototyped tires were mounted on all of wheels of a
passenger car (2500 cc, FR vehicle) under the condition of rim (15 inches) and internal
pressure (200 KPa), and the car runs on a tire test course of dry asphalt at high
speed, and the steering stability, stability feeling and riding comfort were absolutely
evaluated by the driver's feeling on a scale from 1 to 5.
Comparative Example 1
Comparative Example 2
Comparative Example 3
High tenacity vinylon fiber
High tenacity vinylon fiber
Fi neness based on corrected mass (dtex)
specific gravity of cord &rgr;
Cross-sectional area of cord s (cm2)
Complex modulus E* at 120°C (N/cm2)
Coefficient A at 120°C (E* × S) (N)
Coefficient B at 120°C (tan&dgr;)
Number of twist (upper/lower) (/10 cm)
Twist coefficient *
Striking number of cords (/5 cm)
Satisfaction of equation (2)
Satisfaction of equation (3)
Complex modulus (Mpa) of topping rubber
Loss tangent of topping rubber
• Steering stability
• Stability feeling
• Riding comfort
It can be confirmed that the tires of the example can exhibit
excellent riding comfort while enhancing the steering stability at the time of high