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July 15, 2009 > TechKnow Talk: Where the rubber meets the road

TechKnow Talk: Where the rubber meets the road

Most of us think of our tires only when we find ourselves on snow or ice, when making an emergency maneuver, or when a tire suddenly deflates. Most of the time, these underappreciated workhorses roll along, the critical interface between the driver and the road, providing the traction necessary to keep us in control of our vehicle and cushioning the ride. Tire failures on passenger cars have become rare due to improved materials, designs, and manufacturing methods.

Pneumatic tires, those pressurized with air, have been around even longer than the automobile. They have been used on bicycles since the 1880s. The explosive growth of the automobile industry, beginning in the early 20th century, triggered an accompanying expansion of the market for pneumatic tires. About 300 million tires were produced in the U.S. in 2008, in excess of one billion worldwide. More than half of these were for passenger cars.

The top four U.S. tire-producing companies hold more than 75% of the market. They manufacture many different types of passenger tires, intended for different driving conditions or driver preferences. These include tires designed for high performance, snow or mud, all season, all terrain, racing, and undersized spares.

Tire design is a compromise between traction and wear. High performance and racing tires are designed for maximum traction, achieved by tread designs that place as much tire in contact with the road as possible and rubber formulations yielding a soft material that best adheres to the road surface. This optimizes traction, but the softer rubber wears quickly, reducing tire life, and the tread offers fewer voids to channel away water, compromising wet weather performance.

Harder tires with wider channels offer superior wear characteristics, providing a long life, but sacrifice traction. Most passenger cars are sold with "all season" tires as original equipment. These do not offer the best traction, the best water or snow performance, or the longest wear. They strive instead to provide a balance of all of these characteristics.

Tire manufacturing is a surprisingly complex process involving a broad array of raw materials and operations. The tire itself is composed of several components, assembled in many different steps. Due to the complexity of the product and process, and the number of different types of tires produced in the same factory, there is less automation than in many other industries.

Working from the outside inward, tires are composed of the tread that contacts the road, one or more layers or "plies" of rubber and fabric materials (such as polyester, nylon, or Kevlar), two or more layers or "belts" of rubber alternating with steel wires, and an inner liner constructed of a tough rubber sheet to contain the air pressure within. At the inner edge of the tire on each side is a "bead" of steel wire coated with rubber that seals the tire tightly against the wheel. Between the tread and the bead is the sidewall, prominently featuring the name of the manufacturer.

Each of these major rubber components requires a different formulation, depending on the material properties needed. For example, the plies provide strength and puncture resistance, the belts contribute stiffness and toughness, and the sidewall must offer elasticity to cushion the ride as well as resistance to weathering and scuffing.

The first step in manufacturing is to mix together the many ingredients used for each rubber compound. These recipes are unique to each tire component, each type of tire, and each manufacturer. Both natural and synthetic rubber may be used, as are carbon black, sulfur, silica, resins, pigments, and other materials.

Natural rubber comes from the sap of rubber trees in Southeast Asia. Synthetic rubber and carbon black are petroleum products. Carbon black is a sooty material that is something of a miracle ingredient: in various formulations it adds pigmentation, reinforcement, wear resistance, and traction, as well as reducing heat buildup in the tire.

After the ingredients for a particular component have been thoroughly mixed, the compound is pushed through a die to produce the desired profile or squeezed into sheets and cut into strips. The rubber is then vulcanized by placing it in a furnace. The heat forms strong chemical bonds among the rubber molecules, using the sulfur as a catalyst. Vulcanization makes the rubber harder and more durable and also stabilizes its properties under hot and cold conditions.

Once the rubber strips are cut to size, the plies, belts, and beads are constructed by layering the fabric or steel materials with the appropriate rubber compounds and bonding them together. All these various component strips are then laid up and spliced together to form the circular tire.

Finally, the tire is "cured." It is placed in a mold and subjected to heat and pressure. This bonds the various components together and molds the tread pattern as well as the lettering on the sidewall. Typical cure conditions are about 350 degrees Fahrenheit for about 15 minutes at a pressure of several hundred pounds per square inch (psi). After curing, the tire is ready to be trimmed, inspected, tested, and prepared for shipping.

In addition to tires for passenger cars, manufacturers produce tires for bicycles, motorcycles, airplanes, farm equipment, buses, large trucks, and even earthmovers. The huge tires for large construction vehicles may be 15 feet in diameter and cost more than $40,000 each.

These large tires, including those for commercial trucking, are likely to eventually be subjected to retreading, a labor-intensive process of stripping off worn tread and bonding a new tread component onto the tire. A large tire retread is significantly less expensive than a new tire, but is more prone to failure. For passenger cars, new tires are sufficiently inexpensive that retreads are usually not economical.

Despite our tendency to ignore tires until we are forced to think about them, drivers should inspect their tires regularly for punctures, foreign objects, inflation pressure, and wear. Many tires have wear bars--raised areas of rubber at the base of the grooves. When the tread surface is even with these bars, it is time to replace the tire. In the absence of wear bars, insert a penny into the groove, Lincoln's head first. If the top of his head is visible, replace the tire. Driving with too little tread is not only dangerous, it is illegal.

The most common consumer negligence is inattention to inflation pressure. Drivers should check tire pressures at least once a month, and maintain the pressures recommended by the car manufacturer. These may be found in the owner's manual and are also usually posted on a sticker on the frame of the driver's door. Underinflated tires reduce fuel economy, promote heat buildup, wear more rapidly, and most importantly compromise the driver's ability to steer the vehicle.

Because rubber degrades over time, most manufacturers suggest replacing tires more than six to eight years old, regardless of wear. The date of manufacture is molded into the tire sidewall, though it may be on the side facing in toward the car. The date code is four digits, with the first two indicating the week, and the last two the year, of manufacture. If a date code has only three digits, the tire was manufactured prior to 2000 and should be replaced.

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