Pneumatic radial tires having carcass line with plural inflection points

In a pneumatic radial tire comprising a radial carcass and a belt, when the tire is mounted onto a normal rim and inflated under an internal pressure corresponding to 5% of a normal internal pressure, a carcass line of the carcass in radial section of the tire at a self-posture under no load has at least two inflection points between positions A and C and/or between positions C and B in which A is a position corresponding to each end of the belt at its maximum width, B is a position corresponding to a width of the normal rim and C is a position corresponding to a maximum width of the carcass.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to pneumatic radial tires having improved running 
performances. 
2. Related Art Statement 
Particularly, the improvement of the running performance in this type of 
the tire is strongly demanded in pneumatic radial tires for passenger cars 
because it is required to always ensure a stable steering property in the 
passenger car in accordance with remarkable fullness of the equipment and 
performance in the passenger car as well as the enlargement and 
development of road networks 
In order to improve the running performance of the pneumatic tire, it is 
necessary to have sufficient rigidity in the tread portion and bead 
portion of the tire. For this purpose, there is considered a 
countermeasure of strengthening the reinforcement at the tread and bead 
portions, a countermeasure of raising an air pressure filled in the inside 
of the tire, and the like. 
However, the quantity of reinforcing member required and used for 
strengthening of the reinforcement is generally increased to undesirably 
raise the cost. On the other hand, as the internal pressure becomes 
higher, the ground contact area of the tread portion reduces to obstruct 
the transmission of necessary force, and consequently the steering 
performance, and traction and braking performances are degraded. 
That is, the tire should be used under such an internal pressure as to hold 
a balance capable of accepting merits through the strengthening of the 
rigidity and demerits due to the degradation of the performances. However, 
the strengthening of the rigidity without causing the increase of the cost 
can not be achieved by the above conventional technique. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide a novel pneumatic 
radial tire for automobiles, particularly a passenger car which can 
advantageously realize the reinforcement of tread and bead portions in the 
tire without excessively using the reinforcing member over the 
conventionally adopted limit and increasing the internal pressure. 
According to the invention, there is the provision of a pneumatic radial 
tire having improved running performance comprising a carcass composed of 
at least one radial cord ply toroidally extending between a pair of bead 
portions as a casing reinforcement for sidewall portions and tread portion 
connecting to the bead portions, at least one cord ply of which carcass 
being wound around a bead core of the bead portion from inside of the tire 
toward outside thereof to form a turnup structure, and a belt composed of 
plural cord layers arranged along a crown portion of the carcass as a 
tread reinforcement, characterized in that when the tire is mounted onto a 
normal rim and inflated under an internal pressure corresponding to 5% of 
a normal internal pressure, a carcass line of the carcass in radial 
section of the tire at a self-posture under no load has at least two 
inflection points between positions A and C and/or between positions C and 
B in which A is a position corresponding to each end of the belt at its 
maximum width, B is a position corresponding to a width of the normal rim 
and C is a position corresponding to a maximum width of the carcass. 
In a preferred embodiment of the invention, heights H.sub.1, H.sub.2 and M 
of the positions A, B and C measured from a rim base line of the normal 
rim are within ranges of 0.80-1.0, 0.10-0.25 and 0.35-0.70 per a carcass 
maximum height H measured from the same rim base line, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 is shown a left-half section of the pneumatic radial tire for 
passenger car according to the invention in the radial direction thereof 
at a self-posture under no load when the tire is mounted onto a rim and 
inflated under an internal pressure corresponding to 5% of a normal 
internal pressure. This tire is symmetry with respect to the equator of 
the tire. 
In FIG. 1, numeral 1 is a bead portion, numeral a sidewall portion, numeral 
3 a tread portion, numeral 4 a bead core, numeral 5 a carcass, numeral 6 a 
belt and numeral 7 a normal rim closely fitted into the bead portion 1. 
The carcass 5 is comprised of at least one radial cord ply toroidally 
extending between the bead portions 1. In the illustrated embodiment, the 
carcass is comprised of a single cord ply, but two or three plies may be 
used. In any case, at least one cord ply is wound around the bead core 4 
from inside of the tire toward outside thereof to form a turnup structure 
according to the usual manner. 
In the above tire section at the self-posture under no load, an intersect 
point of a perpendicular line drawn from a widthwise end of a belt cord 
layer having a maximum width in the belt 6 toward a rim base line RL of 
the normal rim 7 with the carcass 5 is defined as a position A, and an 
intersect point of a perpendicular line drawn from the rim base line RL 
with the carcass 5 at a position corresponding to an end of the rim width 
of the normal rim 7 is defined as a position B, and a point corresponding 
to the maximum width of the carcass 5 is defined as a position C. Further, 
a straight line passing through the position C and in parallel to the rim 
base line RL is CL, and a radius of an arc having a center on the straight 
line CL and passing through the positions A and C is RA, and a radius of 
an arc having a center on the straight line CL and passing through the 
positions B and C is RB. Moreover, in the carcass 5 shown by a meandering 
curve, a mark X is an inflection point of the carcass line and a mark 
.DELTA. is a point most separated from a line segment connecting the 
position A or B to the inflection point X or a line segment connecting the 
adjoining inflection points X to each other. Among arcs passing through 
the position A or B, the inflection point or adjoining inflection points 
and the point .DELTA., a radius of an arc having a center in the inner 
direction of the tire and locating the point .DELTA. toward the belt 6 
with respect to the straight line CL as a standard is represented by 
R.sub.i.sup.U (i=1, . . . n.sup.U), and a radius of an arc having a center 
in the inner direction of the tire and locating the point .DELTA. toward 
the normal rim 7 with respect to the straight line CL as a standard is 
represented by R.sub.i.sup.L (i= 1, . . . N.sup.L), and a radius of an arc 
having a center in the outer direction of the tire and locating the point 
.DELTA. toward the belt 6 with respect to the straight line CL as a 
standard is represented by r.sub.i.sup.U (i=1, . . . m.sup.U), and a 
radius of an arc having a center in the outer direction of the tire and 
locating the point .DELTA. toward the normal rim 7 with respect to the 
straight line CL as a standard is represented by r.sub.i.sup.L (i=1, . . . 
m.sup.L). 
According to the invention, the above radii are preferable to be within 
ranges of the following equations (1)-(4) in relation with the radii RA 
and RB, respectively: 
##EQU1## 
As mentioned above, the carcass line is adopted to have at least two 
inflection points each of between the positions A and C and between the 
positions B and C, whereby the running performances of the tire can be 
improved while using only the conventionally used members without 
requiring new addition or change to the structure of the tire as well as 
the reinforcing member for the tread and bead portions, so that the 
invention has great merits in the quality and productivity of the tire. 
The aforementioned peculiar profile of the carcass line is obtained by 
controlling a distribution of rubber gauge ranging from an inner wall of a 
mold to the carcass ply in a rotational axis direction of the tire in the 
building-up and vulcanization of the tire. 
Moreover, if it is intended to strictly control the change or scattering of 
the carcass line due to a so-called "rubber flow" in the vulcanization, 
the following method is particularly effective. 
That is, the cord ply of the carcass 5 extending between the bead cores 4 
is merely arranged along the inner shape of the tire according to the 
conventional building-up and vulcanization method. On the other hand, 
according to the invention, an inner liner 8 having locally different 
thickness portions 8' may be used as shown in FIG. 2. 
In addition, the change in the curvature of the carcass line may properly 
be controlled by forming continuous protrusions on a periphery of a 
bladder for vulcanization at positions corresponding to curvature changed 
portions of the carcass line. 
By considering such a laborless means, the carcass line according to the 
invention can properly be controlled without causing the variation or 
scattering in the productivity as previously explained. 
According to the invention, the carcass line of the pneumatic radial tire 
in the radial section at the self-posture under an internal pressure 
corresponding to 5% of a normal internal pressure has at least two 
inflection points between the positions A and C and/or between the 
positions B and C. 
In FIG. 3 is shown carcass profiles having two inflection points each of 
between the positions A and C and between the positions B and C before and 
after the inflation under a normal internal pressure, in which a solid 
line is a carcass profile before the inflation and dotted lines are a 
carcass profile after the inflation. As to the deformation between the 
positions A and C, a portion of the carcass line having a center of 
curvature in an inside direction of the tire in adjacent to the position A 
deforms inward the tire without changing the curvature center, while a 
portion of the carcass line having a center of curvature in an outside 
direction of the tire deforms outward the tire with changing the curvature 
center into the inside of the tire. 
As to the deformation between the positions B and C, a portion of the 
carcass line having a center of curvature in an outside direction of the 
tire beneath the position C deforms outward the tire with changing the 
curvature center into the inside of the tire, while a portion of the 
carcass line having a center of curvature in an inside direction of the 
tire in adjacent to the position A deforms inward the tire without 
changing the curvature center. 
The strain produced along the carcass line due to the deformation of the 
tire is considered as follows. 
In the portion wherein the curvature center changes from the outside of the 
tire toward the inside thereof, the carcass line has room against the 
deformation of the tire before the inflation under normal internal 
pressure, so that the strain produced along the carcass line after the 
inflation under normal internal pressure becomes small, and consequently 
the tension is small. Similarly, when the curvature center of the carcass 
line is held inside the tire before and after the inflation, if the 
carcass line is deformed inward during the inflation under normal internal 
pressure, the tension is small due to the presence of room against the 
deformation, while if the carcass line is deformed outward during 
inflation under normal internal pressure, the tension becomes large to a 
certain extent. 
In the tension distribution between the positions A and C, the carcass line 
adjoining to the position A deforms inward with holding the curvature 
center at the inside of the tire through the inflation under normal 
internal pressure, so that the tension is small. Further, the position of 
curvature center in the carcass line located beneath the above carcass 
line portion changes from the outside of the tire toward the inside 
thereof, so that the tension is small. 
That is, since the carcass tension near the widthwise end of the belt 6 is 
small, the belt tension increases by a quantity that the carcass tension 
is reduced from a balance between the internal pressure and the sum of 
carcass tension and belt tension. 
In the tension distribution between the positions B and C, the curvature 
center in the carcass line located beneath the position C changes from the 
outside of the tire toward the inside thereof through the inflation under 
normal internal pressure, so that the tension is small. On the other hand, 
the carcass line located beneath the above carcass line portion deforms 
outward while holding the curvature center at the inside of the tire, so 
that the tension is not small. Therefore, a portion of the carcass tension 
reduced beneath the position C shifts to the lower portion of the carcass 
line from a balance between the internal pressure in the bead portion and 
the carcass tension to further enhance the tension near to the bead 
portion. 
As a result, the tension is ununiformly distributed through the carcass 
line, whereby a large tension can be given to the tread portion requiring 
a large road gripping force and to the bead portion requiring a large 
transmission force to the rim consequently the various running 
performances are considerably improved. 
Moreover, in order to give the most effective tension to the tread and bead 
portions, the adequate arrangement of radius of curvature of the carcass 
line and position of inflection point or radii R.sub.i.sup.U (i=1, . . . 
n.sup.U), R.sub.i.sup.L (i=1, . . . n.sup.L), r.sub.i.sup.U (i=1, . . . 
m.sup.U), and r.sub.i.sup.L (i=1, . . . m.sup.L), per RA and RB as shown 
in FIG. 1 are favorable to be within the following ranges, respectively: 
##EQU2## 
Further, when the thickness of the inner liner 8 is locally changed as a 
previously shown in FIG. 2, it is desirable that the portion 8' of FIG. 2 
is 1.7 to 8 times, preferably 2 to 4 times higher than the average 
thickness of the inner liner in order to sufficiently suppress the change 
of the casing line. 
The following example is given in illustration of the invention and is not 
intended as limitation thereof. 
There was provided a passenger car tire (size: 165 SR13) comprising two 
steel cord belt layers and one carcass cord ply as shown in FIGS. 1 and 2 
and having a carcass line satisfying R.sub.1.sup.U /RA=0.3, /RA=0.9, 
R.sub.1.sup.L /RB=0.3, r.sub.i.sup.L /RB=0.6, and R.sub.2.sup.L /RB=0.55. 
Then, an actual running test was made with respect to this tire together 
with the conventional tire having a carcass profile of natural equilibrium 
state. As a result, when the running stability was evaluated by the 10 
point method, it was improved by 1.5 points in the tire according to the 
invention as compared with the conventional tire. 
The profile of the carcass line in the tire according to the invention and 
the conventional tire is shown in FIG. 4, in which a solid line is a 
natural equilibrium profile of the conventional tire before the inflation 
under normal internal pressure and dotted lines are the carcass profile of 
the invention before the inflation under normal internal pressure. 
Furthermore, the distribution of belt tension after the inflation under 
normal internal pressure in the tire according to the invention and the 
conventional tire is shown in FIG. 5, while the distribution of carcass 
ply tension after the inflation under normal internal pressure in the tire 
according to the invention and the conventional tire is shown in FIG. 6. 
As mentioned above, according to the invention, the belt rigidity and the 
bead rigidity of the tire can advantageously be increased without 
enhancing the belt and bead reinforcements or increasing the internal 
pressure, which contributes to the improvement of the running performances 
in the tire.