Tire Tread Separations
Tire tread separations are failures rooted in the design and manufacturing of radial tires, and result in the belt and tread layers tearing away from the carcass. Tread separations in both passenger car and light truck tires are one of the leading causes of tire-related crashes. They differ from tire “blow-outs”: A tread separation creates a yaw or pull, as the tread is yanked from the tire by centrifugal force and catches in the wheel well or wraps around an axle, locking the wheel. At highway speeds, tread separations create conditions that are often beyond a driver’s capability to control, particularly in trucks, vans and SUVs.
Today’s radial tires are highly engineered products that generally have improved in durability and performance. It is not uncommon for tire makers to market tires that are capable of remaining in service for 80,000 miles. However, along with this long tread life, the manufacturer must design and manufacture the internal materials to withstand the mechanical, environmental and chemical forces that work against component adhesion. In other words, tires must be made to wear out before these forces create loss of adhesion and tread separation.
There is no single design feature that prevents tread separations. A tire’s individual components work as a system. Critical components like nylon cap plies, belt edge wedges, anti-aging compounds, and high-quality inner liners all improve tire robustness, but are frequently missing from some models because of the increased cost.
Despite the existence of technology that can prevent tread separations entirely, this hazard continues to occur – because tire manufacturers fail to correct known design and manufacturing deficiencies. Often, cost pressures create conditions in which critical design features and materials may be diminished or simply omitted. These cost-downs can show up in the form of lax quality control and inspection during the building process, which is frequently set with high quotas for builders and incentives to produce higher volumes. The manufacturing environment plays a key role in tire quality, because adhesion depends on proper curing temperatures, pressure, and environmental controls as well as machine calibration, and tight quality process checks are critical to maintaining consistency and designs that meet specification. In some cases the cost savings emanate from the specification and cutting or eliminating design features, thinning material gauge, reducing key chemicals, or substituting materials with cheaper alternatives.
In litigation, tire makers assert that a tread separation is not a result of defects in the tire’s design or manufacture. Instead, manufacturers point the finger at the victim, claiming that improper maintenance, including under-inflation or over-deflection, caused the failure. Manufacturers also blame a prior impact – although they cannot determine the magnitude of these phantom impacts.
Lack of Nylon Cap Ply
A nylon cap ply is a layer of material placed over the steel belts in order to give increased strength to the belt structure. Tires with nylon cap plies are more durable and less likely to fail from belt separations. The addition of a nylon cap ply to a steel belted radial tire reduces the stress and fatigue in the rubber surrounding the belt edges; it reduces the growth of minute separations that can grow into larger separations; and it reduces tire temperatures that weaken the rubber through aging.
Patents dating back to 1974 describe the contribution of nylon cap plies in steel belted radial tires for reinforcement of the tread/belt package and restricting belt/edge separations. Tire manufacturers have long been aware of this technology and its benefits in decreasing the risk of tread/belt separations. Nonetheless, some tire models do not include this important feature. The design of a tire without a nylon cap ply is frequently considered a defect.
Lack of Belt Edge Wedges
A belt edge wedge is a strip of rubber placed between the two belts near the belt edges on each side of the tire to improve durability at this highest stress area. Belt edge wedges are an extra safety measure to prevent tread/belt separations. Yet it is not uncommon for tire manufacturers to omit them or use wedges of inadequate thickness – even though tires with a robust wedge and nylon cap plies don’t come back to the manufacturer as a warranty claim. The failure by the tire manufacturer to apply this countermeasure is a defect.
Tire aging is technically referred to as thermo-oxidation, or the permeation of oxygen in the tire carcass that is accelerated by heat. This internal degradation affects the material properties of compounds used to adhere the belts to the tire carcass. Over time these compounds become less elastic and more brittle, allowing cracking to begin between the belts. Under normal mechanical stress of service these cracks propagate faster, and can ultimately result in tearing between the belts and a tread-belt separation. When put into service, an aged tire is more likely to fail catastrophically from tread-belt separation.
Tires, like any other rubber product, have a limited service life regardless of tread depth and use. Tires on vehicles in hot climates are particularly susceptible to the effects of aging. Research shows that tires degrade internally more rapidly in regions with higher ambient temperatures, which is a contributing factor for tire failures, such as tread separations.
The dangers of “aged” tires are a little-known problem outside of the industry, and one that causes a significant number of tread separation problems. “Aged” tires are often unsuspectingly put into service after having served as a spare, stored in garages or warehouses, or simply used on a vehicle that is infrequently driven. In many instances these tires show no visible sign of deterioration, and absent any visible indicators, tires with adequate tread depth are put into service regardless of age.
Tire age can be determined through decoding of the required DOT number molded into the side of a tire; however, the DOT date coding is consumer-unfriendly and confusing. This alpha-numeric code identifies the manufacturing location, the tire size, and when it was built. The first two digits after the acronym DOT designate the factory where the tire was made, and the last four numbers denote the week and year the tire was manufactured. For example, 1209 would mean the tire was manufactured the 12th week of 2009.
Manufacturers address the problem by adding antidegradants and antioxidants in the rubber compounds, and through the use of less permeable inner-liners. Antidegradants and antioxidants are high-cost chemicals that may be reduced as tire companies cut costs.
Most vehicle and auto makers have added information about the hazards associated with aged tires by placing a notice in their owner’s manuals. Most recommendations state replacement is necessary after six years, regardless of tread depth, because aging can affect the safe performance of tires. The obscurity of these “warnings” and hidden date codes are key issues in tire aging cases.
Once tires are installed on a vehicle and put into service, they are considered used. Millions of used tires are sold to consumers each year – most of which are processed by large tire recycling companies. These companies then sell used tires to local dealers with little more than a cursory visual inspection. Some used tires are washed, cleaned and even painted black to give the appearance of a newer tire. Consumers, meanwhile, may only be considering tread depth. There are many threats to tire safety undetected by a visual inspection – such as age, history of use, maintenance, storage conditions, and repairs. There are no government or industry safety standards for used tires.
To download the PayneMitchell Vehicle Tire Safety Handbook click here.