Tire Design Defects
In an effort to save money, tire companies knowingly manufacture tires with design defects. The following are examples of design defects that cause tread separations:
The Lack of a Wedge With Adequate ThicknessThe wedge is a rubber component that extends circumferentially around the tire between the edges of the two steel belts (see Figure 6 below). The wedge prevents separations by suppressing the formation of cracks at the belt edge. The Firestone ATX and Wilderness recall tires failed, in part, because they were only manufactured with a .020 inch wedge. Firestone subsequently increased the wedge gauge to .040. One major tire manufacturer does not use any wedge at all. Any wedge that is less than .040 inches in diameter is defective in design.
Figure 6
Tire companies are constantly trying to save money by reducing the amount of rubber in a tire, including rubber coating the steel belts. The thickness of the rubber coating between the two steel belts – the inner belt gauge – affects the ability of the tire to resist belt edge cracking leading to separations. If a tire is manufactured without sufficient inner belt gauge, this is an additional design defect.
Nylon Overlays/Cap StripsNylon overlays, also known as cap plies, are layers of nylon that extend circumferentially around the two steel belts as shown in the illustration below (see Figure 7).
Figure 7
A cap strip is an abbreviated form of nylon overlay consisting of two strips that run circumferentially around the tire over the belt edges (see Figure 8 below).
Figure 8
There are extensive tire company documents and testimony establishing the effectiveness of nylon overlays and cap plies in preventing tread/belt separation accidents. There is also extensive patent literature discussing nylon overlays, including the 1991 Goodyear Patent that states that nylon overlays “encircle belts for re-enforcement and to prevent separation”.
Defective Belt Skim StockThe adhesion between the rubber components of a tire is created through a chemical reaction in which sulphur bonds with polymers to form chains. These polymer chains create the adhesion between adjacent rubber components, including the steel belts.
During the life of a tire, air from within the tire passes through the inner liner into the body of the tire. Air that migrates into the body of the tire contains oxygen. Oxygen breaks down the sulphur polymer bonds, a process known as oxidative degradation or chain scission, that reduces the adhesion between adjacent rubber components. This process of oxidative degradation is accelerated by heat within the tire, including heat resulting from the operation of a tire in a region with a hot climate, such as the Southwestern United States.
Antioxidants are chemicals that combat oxidative degradation and it is critical that tire manufacturers include a sufficient amount of antioxidants in the formula of the rubber components of a tire, including the belt skim stock. A tire manufactured without sufficient antioxidants may very well fail due to a tread/belt separation before the tire tread wears out and the tire is discarded.
Historically, tire companies have been successful in preventing plaintiffs from obtaining the formulas for the rubber components of a tire, including the belt skim stock. However, several years ago in Buxbaum v. Continental Tire, the Montana Supreme Court affirmed an order of the trial court compelling Continental Tire to produce its formulas. In Lavelock v. Cooper Tire, the Missouri Court of Appeals affirmed a trial court order compelling Cooper to produce the formula for its inner liner.
In order to determine whether a tire lacks adequate antioxidants, the plaintiffs should attempt to discover the type and level of antioxidants used in the accident tire that failed. Plaintiffs should also attempt to discover documents and other information concerning any pre-production and post-production changes made by the tire manufacturer in the type and level of antioxidants.
Inner Liner DefectsThe inner liner is the layer of rubber on the inside of the tire that serves the purpose of a tube in a tube type tire (see Figure 9). The inner liner should be manufactured of 100% halogenated butyl (halobutyl) rubber, an expensive rubber that is far more resistant to air permeation than the cheaper rubbers used in other components of the tire.
Figure 9
Historically, tire companies have used inner liners with far less than 100% halobutyl rubber. These cheaper inner liners have allowed much more air to permeate into the body of the tire leading to much higher rates of oxidative degradation and separation related tire failures.
The NHTSA publication entitled “What NHTSA Applied Research Has Learned From Industry About Tire Aging” dated July 31, 2003, contains the several statements about the composition and thickness of inner liners.
The publication claims that according to tire manufacturers:
- Tires with more expensive, 100% halogenated-butyl inner liners lose air at a rate of 2.0 - 2.5 percent per month.
- Tires with cheaper, blended butyl inner liners lose air at a rate 4.0 - 5.0 percent per month.
- For the same inner liner compound, a thicker inner liner will lower the air loss rate.
- A reduction in air loss rate, by a factor of 2, may be achievable for some tires.
The importance of having an inner liner with high levels of halobutyl rubber is documented in a paper authored by the Exxon Chemical Company entitled “Tire Inner Liners: Advantage Processing Improvement, New Products” presented at the ITE Conference in September of 1994. The first paragraph of the paper states:
It has long been recognized that to achieve the maximum life of a tire, the diffusion of air through the liner into the carcass and belt areas must be minimized. This is most often achieved by the use of a liner compound containing a halobutyl polymer. This paper will demonstrate the advantage of a high content halobutyl liner and present guidelines for the factory processing of these compounds.
The authors identify causes of belt edge separations as Cyclic Flexural Strain, Heat Build-up, Intracarcass Pressure Build-up and Oxidative Attack of the Structural Components. Intracarcass pressure builds up in the tire when excessive amounts of air permeate through the inner liner into the body of the tire.
On its website, Goodyear also stresses the importance of using 100% halobutyl rubber in a tire’s inner liner to reduce air permeation and oxidative degradation leading to separations. Specifically, Goodyear states:
That makes the specialty formulated inner liners engineered into some tires doubly important for helping tires to hold air. Unfortunately, not all tiremakers use 100 percent concentrations of halogenated butyl rubber – the best choice for air pressure impermeability – to help protect the integrity of a tire’s Internal components, Egan said.
Halogenated butyl rubber, when used, is the most expensive material in a blackwall tire. “To omit it or to use it in less than 100 percent concentrations saves money.”Egan said. “However, as a way to retain air in your tires, it’s a case of being a penny wise, and a pound foolish”.
As noted in the findings of the National Highway Traffic Safety Administration, the percentage of halobutyl rubber in the inner liner is critical in determining the tire’s ability to withstand oxidative degradation and aging without developing separations that lead to catastrophic tire failure.
Manufacturing defects resulting in cracks in the inner liner and open inner liner splices also lead to excessive oxidative degradation and tread separations (see Figure 10).
Figure 10
