Runflat tire

1. A runflat tire including

a carcass which extends from a tread portion to each of bead portions through sidewall portions,

a belt layer disposed radially outward of the carcass in the tread portion,

a bead apex rubber disposed in each of the bead portions, and

a sidewall-reinforcing rubber layer which is disposed axially inward of the carcass in each said sidewall portion and which has a cross section of such a crescent-shape that it has the maximum thickness at a center portion thereof and extends radially inward and outward from the center portion with decreasing its thickness so that the radially inner end portion and radially outer end portion thereof overlap with an axially outer end portion of the belt layer and a radially outer end portion of the bead apex rubber, respectively,

wherein

the carcass consists of a single ply of carcass cords which are made of an aramid fiber and are arranged at an angle of 70 to 90 degrees with respect to the tire circumferential direction and are covered with a topping rubber,

the carcass cords each have a three-strand structure such that three bundles of first-twisted filaments are final-twisted to have a twist factor T of 0.65 to 0.7, wherein the twist factor T is represented by the following equation (1):

T=N ×√{square root over ({(0.125 ×D/ 2)/ρ})}×10 −3   (1)

wherein

N is the number of the first twists per 10 cm,

D is a total nominal dtex of a cord, and

ρ is a specific gravity of the cord material,

said single ply is composed of a main body portion extending between the bead cores and a pair of turnup portions each turned up around the bead core in each said bead portion from the axially inside to the axially outside of the tire,

said bead apex rubber is disposed radially outside the bead core and between the main body portion and the turnup portion of said single ply, and extends radially outwardly from the bead core in a tapered manner so that the radially outer end thereof has a radial height (ha) in a range of from 10 to 60% of a section height SH of the tire, each measured from a bead base line,

each said turnup portion extends radially outwardly so that the radially outer end thereof is secured between the ply main body portion and the belt layer, and

the axial width of an overlapping part of the turnup portion with the belt layer is at least 5 mm,

the radially inner end of said sidewall-reinforcing rubber layer is located radially inside the radially outer end of the bead apex rubber so that the radial width of an overlapping part of the sidewall-reinforcing rubber layer with the bead apex rubber is from 5 to 50 mm, and

the radially outer end of the sidewall-reinforcing rubber layer is located axially inside the axial outer end of the belt layer so that the axial width of an overlapping part of the sidewall-reinforcing rubber layer with the belt layer is from greater than 0 to 50 mm,

wherein

the number of the first twists and the number of the final twists of the carcass cords are 55 to 68 per 10 cm, and

the topping rubber of the carcass cords has a complex modulus E* of more than 10 MPa.

2. The runflat tire of claim 1 , wherein the complex modulus E* is not more than 13 MPa.

3. The runflat tire of claim 1 or 2 , wherein

in the meridian section of the tire attached to a standard rim and inflated at a normal internal pressure, the tire has a profile such that the radius of curvature of the tire outer surface gradually decreases from the tire equator point CP to a point P, in which the point P denotes a point on the tire outer surface which is apart from the tire equator plane C by a distance SP of 45% of the maximum section width SW of the tire, and the profile satisfies the following equations:

0.05 <Y 60 /SH≦ 0.1

0.1 <Y 75 /SH≦ 0.2

0.2 <Y 90 /SH≦ 0.4

0.4 <Y 100 /SH≦ 0.7

in which Y60, Y75, Y90 and Y100 denote radial distances between the tire equator point CP and each of points on the tire outer surface apart from the tire equator plane C by distances X60, X75, X90 and X100 of 60%, 75%, 90% and 100% of the half width (SW/2) of the tire maximum section width SW, respectively, and SH denotes a section height of the tire.

4. The runflat tire of claim 1 , wherein

the total nominal dtex of each said carcass cord (fineness of cord) is from 2400 to 3300, and

the number of the carcass cords in the carcass ply is from 35 to 53 per 5 cm.

5. The runflat tire of claim 1 , wherein the topping rubber of the carcass cords has a complex modulus E* of 10.3 to 13.0 MPa.