Function

• A tire is more than a means for your vehicle to roll down the road. It provides a cushion between the road and the wheel rims. Air, under pressure inside the tire, provides a soft barrier by pushing against the inner walls and top of the tire. Tire inner rings seal the air inside by pushing against the rim. This allows the tire'soutside structure to provide adequate road traction to help the car move, steer and stop more efficiently.
  
  

Features

• You can learn a lot about a tire by reading the markings on the outside wall. An alphanumeric code tells you the tire size, type and rim diameter. Strings of small text provide maximum load rating, weight and inflation pressure the tire can stand. Also, you can find the number of plies and ply rating used, tread wear and temperature rating and the maximum speed the tire can withstand before possible failure.
  
  

Identification

• The foundation of a tire is built through a series of piles, rubberized fabric and cords folded tightly around beads. To bond and make them leak-proof, rubber liners are spread between these layers of plies. Then, depending on tire purpose and model, a number of belts may be rolled around the piles to strengthen the body and provide a hard base for the tread or outer surface. Finally, two outside walls will help tie the tread to a pair of steel wire rings wrapped in rubber to keep the tire and air sealed against the wheel rim.
  
  

Considerations

• Variation in construction and materials has resulted in three main types of tires. The bias ply tire has plies that run at an angle without the use of belts. One of the first tire designs, a bias tire provides a good cushioning on the road, especially on rough surfaces, but reduces traction at high speeds. On the other hand, a belted bias tire provides a number of belts between the plies and tread area, which helps to stiffen its surface. This helps the tire provide a smooth ride and better traction on the road. Unlike the previous two designs, a radial ply tire has its plies running straight across from one steel ring to the other. It also provides a series of belts underneath the tread surface. This construction results in a stiffer tread area but more flexible walls, allowing more tread to contact the road with improved cornering, braking and wear.
  
  

Safety

• Photo courtesy of E. Bartholomew; Flickr.com.
  
  Beyond construction and materials, tires are further classified according to their added security features. The spare tire is usually of a small size. Mounted in its own rim and loaded in the rear of the vehicle, the spare is ready to replace a flat tire temporarily. As a complement to the spare tire, a run-flat tire is used as an extra road safety measure. Stiff sidewalls allow this tire to keep rolling even after losing air pressure, providing enough time for a driver to clear off a busy highway and reach a tire shop safely. With the same purpose in mind, a self-sealing tire is built with liners coated in sealing compound. Pushed by the air under pressure, this special substance will cover small punctures such as nails to help prevent air leakage.