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Radial tires differ from traditional diagonal bias-ply tires in their construction, which minimizes tread wear and improves flexibility of the sidewall for better handling. They contain belts of steel fibers that go around the circumference, which helps stabilize the tire.
While tires may look simple, they are actually made up of a series of layers of different materials. If someone peeled back the tread in bias-ply tire, he would see a nylon weave (these are the plies) beneath, running in a diagonal pattern and meeting in the centerline of the face of the tire. The weave runs at an angle down the sidewalls to the bead, the part of the tire that fits on the wheel's rim. Nylon ply is stronger than polyester but compresses and sets when under load, especially when left for periods of time, resulting in "flat areas" on the tire. Due to the direction the ply runs on these tires, the sidewalls don't bulge even if the tire is low. This design allows the tread to wrap down over the sidewall because there is no transition point from the circumferential face to the sidewall.
A radial tire is constructed differently. Rather than diagonal nylon plies that meet in the centerline, it's made with polyester cords that run perpendicular from bead to bead, up over the face of the tire and down each sidewall. In other words, it's "wrapped" at a right-angle to the direction of the tread.
On the face of the tire over the polyester wrap is a belt that runs below the tread. The belt is nearly the width of the tire and runs around the circumference, giving the tire a "squared" look. Though belts used to be made of rubber-coated fibers, nearly all belts today are made from steel fibers, leading to name "steel-belted" radial. This belt helps stabilize the tread, reducing wear. Because of the construction of the tire, the sidewall will always have a bulge at the point of contact, and as a result, some people may think that they are under inflated.
The average steel-belted radial gets about 100,000 miles (161,000 km) of wear, while the bias-ply tires are generally rated at about 30,000 miles (48,280 km). By reading the model number on the sidewall of any tire, a car owner can quickly see what type of tire he has, though most radial tires include the word "radial" in the model name. There are several symbols used to decode a common tire, such as the P205/65R15:
- P — stands for passenger (LT would mean light truck)
- 205 — indicates width in millimeters
- 65 — refers to the aspect ratio, or how tall the tire is compared to how wide it is; in this example, the tire height is 65% of its width
- R — radial ("D" here would indicate diagonal bias ply)
- 15 — the diameter of the tire in inches
The tire might also have other designations like XL or RF, which indicates Extra Load or Reinforced respectively. It might also have tread type or other codes included. Most of the tires on the road today are steel-belted radial tires.