The radial-ply tire,

The radial-ply tire, on the other hand, is constructed very differently from
the bias-ply tire. It was first introduced by Michelin in 1948 and has now
become dominant for passenger cars and trucks and increasingly for heavy-
duty earth-moving machinery. However, the bias-ply tire is still in use in
particular fields, such as cycles, motorcycles, agricultural machinery, and
some military equipment. The radial-ply tire has one or more layers of cords
in the carcass extending radially from bead to bead, resulting in a crown
angle of 90 degree,. A belt of several layers of cords of high modulus of elasticity
(usually steel or other high-strength materials) is fitted under the tread, . The cords in the belt are laid at a low crown angle of approximately 20 degree. The belt is essential to the proper
functioning of the radial-ply tire. Without it, a radial-ply carcass can become
unstable since the tire periphery may develop into a series of buckles due to
the irregularities in cord spacing when inflated. For passenger car tires, usu-
ally there are two radial plies in the carcass made of synthetic material, such
as rayon or polyester, and two plies of steel cords and two plies of cords
made of synthetic material, such as nylon, in the belt. For truck tires, usually
there is one radial steel ply in the carcass and four steel plies in the belt. For
the radial-ply tire, flexing of the carcass involves very little relativemovement
of the cords forming the belt. In the absence of a wipingmotion between the
tire and the road, the power dissipation of the radial-ply tire could be as low
as 60% of that of the bias-ply tire under similar conditions, and the life of
the radial-ply tire could be as long as twice that of the equivalent bias-ply
tire . For a radial-ply tire, there is a relatively uniform ground pressure
over the entire contact area. In contrast, the ground pressure for a bias-ply
tire varies greatly from point to point as tread elements passing through the
contact area undergo complex localized wiping motion.
There are also tires built with belts in the tread on bias-ply construction.
This type of tire is usually called the bias-belted tire. The cords in the belt
are of materials with a higher modulus of elasticity than those in the bias-
plies. The belt provides high rigidity to the tread against distortion, and re-
duces tread wear and rolling resistance in comparison with the conventional
bias-ply tire. Generally, the bias-belted tire has characteristics midway be-
tween those of the bias-ply and the radial-ply tire.

For instance, for a tire ‘‘P185/70 R14 87S,’’ ‘‘P’’ indicates
a passenger car tire; ‘‘185’’ is the nominal width of the cross section in
millimeters; ‘‘70’’ is the aspect ratio, which is the ratio of the height of the
sidewall to the cross-sectional width; ‘‘R’’ stands for radial-ply tire; ‘‘14’’ is
the rim diameter in inches; ‘‘87’’ is a code indicating the maximum load the
tire can carry at its maximum rated speed; ‘‘S’’ is a speed rating which in-
dicates the maximum speed that the tire can sustain without failure, S—112
mph (180 km/h), T—118 mph (190 km/h), H—130 mph (210 km/h), V—
149 mph (240 km/h), Z—149 mph (240 km/h) or more. Traction and tem-
perature capabilities are indicated on a scale fromA to C, A being the best
and C the worst. The traction rating is based on straight-line stopping ability
on a wet surface. The temperature rating is an index of the tire’s ability to
withstand the heat that high speeds, heavy loads, and hard driving generate.
Tread-wear index is an indication of expected tire life. It is rated against a
reference tire with an index of 100. For instance, a tread-wear rating of 420
means that the tire should last 4.2 times as long as the reference tire. A tread-
wear index of 180 is considered to be quite low and an index of 500, quite
high.

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