SUV Tire Failures

While most drivers will log many hundreds and probably thousands of miles without so much as an air leak, tire failures can and do occur regularly. While not all tire failures result in a serious accident, the sudden failure of a tire can cause a vehicle to lose control and either roll over or collide with other vehicles. Tire failures are particularly dangerous when the vehicle is traveling at highway speeds. Tire failures are also more likely to lead to rollovers when they occur on a vehicle with a high center of gravity, such as many popular sport utility vehicles.

Tire tread separation is in the news a lot these days because of the recent Firestone tire recall. Tread separation, however, its not a new problem. Most tires manufactured today are steel belted radial tires, and tread separation is the most common type of failure in steel belted radial tires. When the tread separates from the tire carcass or the inner plies, it can frequently cause a blowout. Tread separation can also cause the driver to lose control of the vehicle, resulting in collisions and rollovers. Loss control is especially likely in vehicles with a high center of gravity.

Tires are built in layers on a tire building machine. The tire is built in the green or uncured state, and the core or carcass of the tire is sometimes referred to as the "green tire." During the manufacture of a steel belted radial tire, a layer of skim stock is applied to the green tire and then the steel belts are applied around the green tire. Another layer of skim stock is then applied and then the tread is applied around the belts. The entire tire is then subjected to high temperatures and pressures to cure the tire assembly. After it has been cured, the tire is mounted on a rim and permitted to cool.

Tire tread belt separations are an inherent by-product of steel belted radial tires. While steel belted radial tires have the advantage of providing impact and puncture resistance, the use of steel belts is inherently problematic because rubber will not adhere to steel. To obtain adhesion of the rubber tread to the steel belts, tire makers coat the steel with brass. Rubber will adhere to brass, but brass has a tendency to break down quickly.

Tread separation can occur as a result of both design and manufacturing defects. Tread separations can frequently be traced to poor bonding or adhesion of the tire components during manufacturing. Tire plants frequently emphasize quantity over quality, which leads to lax quality control practices. Poor quality control and manufacturing practices can result in moisture, foreign matter and other impurities getting cured into the tire. The moisture and impurities can lead to a tread separation when the tire is put into use. Because tires are made by hand, perspiration and dirt from the workers' hands can contaminate the tire and cause tread separation. Inadequate or over aged skim stock can also lead to tread separation. Manufacturers sometimes use solvent between the belts to restore skim stock that should be thrown out. The use of certain solvents on tire components and between the belts can lead to tread separation. During the final inspection stage of tire production, defects that should otherwise be spotted are sometimes allowed to slip through the inspection. Final inspections that should take about 2 minutes per tire take as little as 15 seconds in some plants.

Design defects that contribute to tread separation include deficient skim stock and the lack of nylon overlays. Improving the skim stock can lead to better adhesion or bonding of the rubber to the steel and thus make tread separation less likely. A nylon overlay, which consists of a nylon "safety belt" between the tread and the steel belts, can also make tread separations less likely. The nylon overlay placed over the steel belts acts like a "tourniquet" by providing stability to the interior components of the tire. It also assists in resisting moisture intrusion into the tire. Nylon also contracts when heated and acts to pull the components of the tire together when the tire starts to heat up. Unfortunately, U.S. tire manufacturers have resisted the use of nylon overlays. Although it has been estimated that nylon overlays will only increase the cost of a tire by about a dollar, nylon overlays have typically been used only in "high performance" tire lines.