Tread and Belt Separations

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          When a steel belted radial tire suddenly loses its tread and steel belt(s) during vehicle operation the result is frequently a loss of control, sometimes even without a blowout, and  a rollover or crash resulting in serious injuries or deaths. Generally, at least the top steel belt comes off after the tread is lost. Accompanying the separation may be a sudden loud noise and vibration. On passenger vehicles rear tire blowouts are more difficult to control, and frequently lead to vehicle rollovers, especially with vans and utility vehicles. The vehicle usually pulls to the side of the blown tire, and the driver typically attempts to correct by steering to the opposite side, as well as braking. Unfortunately, due to the loss of traction of the blown tire the vehicle’s handling is altered substantially, making it difficult to control, and it begins to yaw. At that point it is usually too late for the average driver to correct, and the vehicle leaves the road.  

    Tread and belt separation can be caused by weak rubber compounds, poor rubber to brass-plated steel cord adhesion as a result of manufacturing contamination, and rust. Any weakness will usually be manifested in the areas of greatest stress, which for steel belted radial tires is at the belt edges. Loosening at the belt edges then proceeds toward the center of the tire, and when enough of the tread becomes loose it suddenly peels off of the tire.  

      Although rubber does not bond to steel, it bonds very well to brass, so the steel cords are plated with a thin layer of brass. The bond between the rubber in the belt and the brass-plated steel cords is designed to be stronger than the tearing strength within the rubber itself. So, if a proper bond if formed when the tire is made, it will not fail at that interface, but within the rubber, and will leave the steel cords coated with rubber – not brassy appearing. Poor rubber to brass adhesion can be caused by contamination (by liquid, dust, grease or other substances), sulphur blooming of the rubber (due to old age, and other reasons), excessive swabbing with petroleum based liquids in an attempt to “freshen” skim stock that has lost its necessary tack, as well as a bad rubber compound mixture. This type of failure cannot be caused by anything the user does. It is always a problem caused at the time of manufacture.  

The appearance of bright brassy steel cords in a failed tire is a classic sign of poor skim stock rubber to steel cord at the time of manufacture.

 Although there is a surprising lack of scientific study about the effects of under inflation on tread separation, tire companies often try to place the blame on the vehicle driver or owner by this excuse. Interestingly, they seldom admit that they have long known how to make tires that don't leak significantly, or that under inflation warning devices could economically be  included in a vehicle or tire.

As explained in the section on tire manufacture, tires are built in layers on a tire building machine while the rubber layers and rubber coated fabrics and belts are in an uncured or “green” state.  The fully assembled tire (with all components in their proper places) is then subjected to high temperatures and pressures to cure or vulcanize the assembly.  It is this vulcanization process which  gives the tire its desired physical properties and bonds the multiple layers or plies of rubber together (both mechanically and chemically) so they can function as a unit.  

Although the incorporation of steel belts into the steel belted radial tire offers the advantages of additional impact and puncture resistance, it also presents a design problem.  The rubber compounds routinely used in the manufacture of bias and bias belted tires will not adhere to the steel wires used to make a steel belt.  In order to gain proper adhesion, manufacturers must coat the steel wires before incorporating them into the tire construction.  However, the steel continues to act like steel, and the rubber continues to act like rubber.  For example, there is a difference in the flexibility of steel and the flexibility of rubber compounds which continues to create a potential for tread separation in all steel belted radial tires.  This potential increases under various operating conditions - most notably, high speeds.

One on the solutions to this problem is the use of Nylon Cap Plys in the tire's construction. Even though technology exists (and has existed for more than 10 years) to prevent tread separations, including separations at high speeds, the U.S. tire manufacturers have not yet incorporated that technology into passenger and light truck tires.  The technology consists of surrounding the belt plies immediately below the tread with a nylon cap or overbelt.  This nylon overbelt not only adds another layer between the road surface and the steel belt to resist moisture intrusion, but also serves as a tourniquet which provides stability to the interior tire assembly by restricting the movement of the tire's internal components.

U.S. manufacturers have resisted this technology, largely because it would increase the cost of production by approximately one dollar per tire. Therefore, some experts in the field of tire design argue that the United States makes the worst tires in the world.  They point to the fact that the U.S. has the best roads in the world; yet the manufacturers  rely upon speed regulation to prevent the high speed operation of tires rather than incorporating a safety measure that would permit high speed performance.  Interestingly, the U.S. manufacturers have incorporated this technology into the “high performance” line of tires, a product with less price sensitivity.

Another inherent problem in the construction and performance of steel belted tires is the potential for oxidation or rust.  This condition usually occurs in tires which have been placed in operation, removed from operation, and then placed in operation again.  Water can permeate a tire through any slight cut or nick in the tire surface.  Once the water reaches the steel belt, oxidation occurs.  The rust weakens the belt and can cause the tire to disintegrate during normal road operation.  A visual inspection of a tire with a rusted belt may reveal slight tread wear, and the tire may appear to be in good condition.  Thus, the potential for catastrophic tire failure can lurk below the surface where it cannot be seen.

Again, the technology has existed for many years which would alleviate the rusted steel belt condition, but it had not been incorporated into most tires built in the U.S. (though it is widely used in Europe).  Most consumers are not aware of this cause of tire failure, and none of the literature produced by the manufacturers warn of this danger.

Perhaps more common than tread separations occasioned by rusted belts are tread separations caused by deficiencies in the manufacturing process.  These defects include the use of unsuitable raw materials (most notably old or “dry stock”), compound composition errors, contamination of the rubber stock, adhesion failures during belt construction, building errors, use of scrap material, use of outdated mixing machines, calenders, tire building equipment, or tire molds, improper cure or vulcanization, insufficient quality assurance and quality control, and any number of human errors or neglect.  Unfortunately, the development of evidence of deficiencies in the manufacturing process requires extensive (and expensive) investigation of the plant where the tire was manufactured.