DESIGN DEFECTS LEADING
TO TREAD SEPARATIONS
By Hugh N. Smith
Smith & Fuller, P.A.
455 N. Indian Rocks Road, Suite A
Belleair Bluffs, FL 33770
Steel belted radial tires find their origin in a 1947 Michelin patent. The design is characterized by the employment of very flexible sidewalls and a rigid steel cord-rubber composite tread structure. Early in the commercial development of the modern passenger steel belted radial tire, it became apparent that tires of this construction had an inherent tendency toward the development of separations at both of the steel belt edges and in the lower sidewalls, which separations could lead to catastrophic failure. Ironically, these problems had their mechanical origins in stress concentrations arising from the design features of rigid tread structure and flexible sidewalls - the very features that are responsible for the excellent performance characteristics.
The primary mode of catastrophic failure of the steel belted radial tire is separation of the tread from the outer steel belt following progressive belt edge separation. Separation usually starts at the outer steel belt edge and progresses circumferentially and axially in the rubber skim stock between the inner and outer steel belt from the inner steel belt structure under the influence of the centrifugal force acting on the tread/steel belt structure due to highway speed rotation of the tire/wheel assembly.
As noted above, this tendency to separate is acknowledged to be inherent in steel belted radial tires having steel belt edges consisting of sharp, sheared, cut edges of steel cord wires. The basic causes and mechanisms of separation and the design means of mitigating or preventing these separations have been recognized since the early commercial development of steel belted radial tires.
This tendency to failure is exacerbated by design defects in the steel belted radial tires and is mitigated only by the incorporation of known mechanical and chemical design countermeasures and adherence to appropriate quality control measures during tire production.
The mechanical and chemical solutions to belt edge separations are both numerous and varied. For example, most manufactures recognize the importance of a “belt step.” Because a major cause of stress in the rubber around the belt edge is the discontinuity of the belt stiffness, belt edges are usually “stepped.” This means that the inner belt is wider than the outer belt, so that the stiffness of the belt is gradually decreased towards the edge. Oftentimes, the belt goes from two layers, to one layer before ending.
Similarly, a shoulder insert involves placing a wedge-shaped rubber insert between the ply and the belt edge which, in turn, reduces growth of the body ply around the belt edges. This insert is alternatively called a cushion because it cushions the belt edge from the ply and helps to dissipate the stresses in the area. This cushion aborbs a lot of mechanical energy, some of which is converted to heat energy. Because of this potential for heat build-up, it is important that the insert compound be formulated to retain as little heat as possible. In the industry, this is called a “cool running” compound.
This paper discusses, in outline form, selected basic countermeasures that can and should be employed by tire manufacturers to overcome the inherent tendency of steel belted radial tires to separate during normal highway application.
II. Belt Wedges
Most generally, a belt wedge is defined as an extrusion of rubber placed under the edges of the belt structure to improve durability. “Extrusion” merely refers to the process by which this wedge is manufactured. A more precise definition would be a triangular piece of rubber wedging material inserted between the edges of the inner and outer steel belts to reduce the shear stresses between them and to reduce the working temperatures at the outer steel belt edges where tread separations most commonly occur.
A. Origin of Invention
1. J. Boileau/Michelin: U.S. Patent 3,717,190, February 20, 1973.
B. Refinement of the Invention
1. A.L. Pollard, et al /Michelin: U.S,. Patent 5,711,829, January 27, 1998.
C. Important Discussions Relating to Belt Wedges
1. Ford Motor Company: Preliminary Findings–Investigation of Root Cause Of Elevated Rates of Tread Separation in Firestone ATX, prepared for joint Ford/ NHTSA meeting, Dearborn, Mi. December 11, 2001.
2. Ford Motor Company: Firestone Tire Replacement Program and Ford Explorer Safety Analysis, July 10, 2001.
3. NHTSA: Engineering Analysis Report and Initial Decision Regarding Investigation, EA00-023, October 10, 2001.
4. R. Baumgardner: Anatomy of a Radial Tire Belt Edge Breakdown, Retreaders Journal, September, 1985.
D. Practical Use of Belt Wedge Deficiencies in Tire Litigation
1. Cooper Tire & Rubber Company uses no belt wedge in any passenger tire or light truck tire, with the exception of a select few heavy load 16" tires.
2. The belt wedges in the ATX and Wilderness tires of Firestone were found by NHTSA to lack sufficient “robustness” which resulted in a determination of a defect relating to public safety.
III. Nylon Caps, Stripes or Belts
A cap ply is a layer of rubberized, parallel nylon cords wrapped circumferentially over the steel belts such that the nylon cords are aligned with the equatorial centerline of the tire. A nylon strip is an alternative name for the same type of mechanical structure. A nylon belt generally extends from belt edge to belt edge and covers the steel belt through the center of the tire. It is located directly beneath the tread or tread base. In more expensive tires, a two ply nylon belt is often employed giving rise to a construction known as the “2 X 2 X 2".
The cap ply is a mechanical device that acts as a kind of tourniquet and restricts the amount of growth due to the centrifugal load on the tire. The physical restriction of the cap ply causes the movement of the belt edge to be significantly reduced, and this has three distinct, important, and significant consequences:
1. It reduces stress, fatigue, and the loss of physical properties in the rubber surrounding the belt edges.
2. It reduces the growth of microscopic separations (sockets) that can develop into larger separations.
3. It reduces tire temperature. Heat is the enemy of tires; it weakens rubber through a process known as aging.
It has also been argued that a full cap ply prevents intrusion of moisture into the body of the tire from road nicks and cuts to preserve the integrity of the steel belts against the effects of oxidation or corrosion. A cap ply may also increase puncture resistence.
Cap plies, strips, or belts have undergone evolutionary refinements. As originally conceived, the invention used a band of fabric reinforced with parallel nylon cords arranged lengthwise along it at zero degrees to the tread equator to stabilize the steel cord tread belts. During the tire building process, the band is wound tightly around the outside of the steel belt edges.
During the tire curing process (in which the tire is heated under pressure), the so-called zero degree band, or cap ply shrinks, cinching itself in place around the steel belts like a tourniquet.
As separations between the steel cords at the steel belt edges form and grow during use of the tire, particularly at turnpike speeds and at high ambient temperatures, the zero degree band holds the steel belts, and particularly the steel belt edges, in place. A modification of this concept was described in later patents. That modification included the folding of the edges of the zero degree nylon belts to strengthen the shoulder regions and therefore prevent high speed edge separations.
Still later, the cap ply was formed with single yarns rather than twisted cords. This development was aimed at reducing the weight, thickness, and cost of the cap plies, reducing the build up of heat, and improving heat dissipation so as to further reduce belt edge separations.
As a practical note, tire manufacturers in the litigation context generally take the position that a cap ply, belt, or strip is unnecessary in a non-speed rated tire. The argument is that the cap ply is only useful to reduce the standing waves that are created through high speed operation. Empirical experience, however, tends to show the extensive use of caps in high load tires in the United States, and test data shows that cap plies are effective in substantially reducing tread separations in all tires, regardless of speed, size, or load.
A. Origins of Invention
1. H. J. Mirtain, et. al / Uniroyal: U.S. Patent 3,786,851, January 22, 1974.
2. R. H. Snyder / Uniroyal: U.S. Patnet 3,850,219, November 26, 1974.
3. H. J. Mirtain / Uniroyal: U.S. Patent 3,834,439, September 10, 1974.
4 H. J. Mirtain / Uniroyal: U.S. Patent 4,184,530, January 22, 1980.
5. H. H. Mirtain / Uniroyal: U.S. Patent 4,407,347, October 4, 1983.
6. G. J. Gasowski, et al / Goodyear: U.S. Patent 4,635,696, January 13, 1987.
7. D. J. Pogue, et al / Uniroyal: U.S. Patent 4,724,881, February 16, 1988.
8. D. J. Pogue, et al / Uniroyal: U.S. Patent 4,284, 117, August 18, 1981.
B. Important Discussions Relating to Nylon Caps
1. ASME Safety Division: An Instructional Aid for Occupational Safety and Health in Mechanical Design, ASME, 1984.
2. Uniroyal Tire Sales Promotional Literature, YHL31124 1997 MNA Inc., Uniroyal Laraedo Durashield Registered 7704185 Aug. 1997 UGTC The Laraedo Tire’s Patented 6 ply Durashield Construction.
C. Practical Use of Nylon Cap Deficiencies in Tire Litigation
1. Cooper Tire & Rubber Company has an inconsistent history of the use of nylon caps. As early as 1985, in the production of the Dakota Futura, a tire directly copied from the design of the Goodyear Wrangler of the era, Cooper used a cap ply, then discontinued the use of the nylon cap in favor of a third steel belt. Cooper later deleted the third steel belt. Presently, Cooper uses a cap ply in some load range E tires, and the President of Cooper’s Tire Division has testified that the nylon cap is used in S rated tires and above. However, the cross sectioning of an S rated tire shows no cap ply. Cooper Tire owns Cooper-Avon in Great Britain. Historically, Avon has made extensive use of cap plies.
2. Bridgestone/Firestone represented to the government of Venezuela that a nylon cap ply would solve the problem with tread separations in the ATX and Wilderness tires. Cut tire analysis on tires from Venezuela which were represented on the sidewall to contain a nylon cap showed no cap ply. Bridgestone/Firestone called this omission “simple misbranding.”
3. Most European tires do contain nylon cap plies.
IV. Introduction to Chemical Aspects of Tire Litigation
The chemical or rubber compounding aspect of tire construction is among the most closely guarded secrets of the tire industry. The tire manufacturers take the position that, were a competitor to learn the specifics of its rubber recipes, those recipes, developed at tremendous expense and man hours, would lose their commercial value and place the manufacturer at a tremendous competitive disadvantage. Each manufacturer has a person or group of persons who file affidavits in ongoing litigation which are intended to raise the ordinary trade secret status of rubber recipes to “super” trade secret status. By and large, this approach has been successful, and this author is aware of only one case in which the recipes have actually been produced. Motions requesting disclosure of rubber recipes are pending in a number of other cases.
The basis for the manufacturers’ argument is the premise that a cured tire cannot be reverse engineered. In other words, when a tire is cured, the chemical composition of the rubber compounds undergo a chemical change that makes it impossible to ascertain the original formula or recipe for the compounds. While there is a scintilla of truth in this assertion, the claim is generally false.
Most major tire manufacturers subscribe to a tire analytical service where the analysis is performed by Smithers Scientific Services of Akron, Ohio. Smithers analyzes tires of many domestic and foreign tire manufacturers on an annual basis and then issues supplemental reports. The analysis consists of a thorough review of the tire structures, including carcass cord data, belt data, chafer material, adhesion, and physical properties. There is also a section of the report that is furnished to each subscribing member detailing the viscoelastic tread rubber properties.
The chemical section of the Smithers analysis looks at the tread, sidewall, liner, belt composition, subtread composition, bead filler, and rubber chafer. It then determines for each component the specific gravity, plasticizer percentage, ash percentage, carbon black percentage, total sulfur percentage, zinc oxide percentage, calcium carbonate percentage, titanium dioxide percentage, silicon dioxide percentage, rubber polymer by weight percentage, magnesium oxide percentage, and cobalt percentage.
According to the Smithers’ representative whose deposition was taken in the Ford/Firestone multi-district litigation, the Smithers’ reports will provide 95% of the rubber recipes or formula used by most major tire manufacturers. Additional testing, which can even identify the particle size of the carbon blacks used, is certainly possible. Testing in this area, however, is very expensive and requires specialized equipment. There is a need for the plaintiff’s bar to undertake testing of this nature because compounding deficiencies are often the source of tread separations.
V. Inner Liner Permeability
Given the fact that the most common durability failure in the steel belted radial tire is a belt edge separation, particular attention needs to be given to inner liner permeability. By the early 1960's, several tire companies had encountered inner liner embrittlement and premature internal separation failures, particularly in premium tubeless tires. The 4-ply carcasses and the thicker treads of premium tires produced high-heat buildup under severe running or test wheel conditions. This heat buildup caused liner cracking and premature separation failures.
Soon inner liner compounds utilizing chlorinated butyl rubber blended with natural rubber at the 50-70 percent RHC level were incorporated into these tires to improve heat cracking resistence and to reduce air diffusion with resulting benefits in tire separation resistence. The reduction in air diffusion caused a corresponding reduction in intra-carcass pressures. The reduction in intra-carcass pressure buildup and oxidative degradation from diffusing inflation air was found to be a significant contributor to the elimination of propagation of belt edge separation in long-term wheel tests and road service.
Experts for the tire industry have long talked about intra-carcass pressures. These experts argue that all tires, to a greater or lesser degree, have a permeable inner liner. The natural migration of air and moisture through the liner and carcass gives rise to a buildup of intra-carcass pressure which then permits a tread separation in the event of a puncture. They assert that the escaping air from a liner puncture or an ill-repaired puncture will follow the path of least resistence along the cords and belts of the tire, giving rise first to a polished surfaces, and then to separation and ultimate failure. Though it defies logic, these tire industry experts use this intra-carcass pressure theory even when the puncture is on the opposite side of the tire from the point at which the tread detachment was initiated. While this argument lacks any foundation in the technical literature, it has convinced many a jury.
The reverse of the intra-carcass pressure argument is that properly compounded inner liners will be essentially impermeable, and there will be no buildup of intra-carcass pressures. Unquestionably, many manufactures, in attempting to make tires less expensively, will use inadequate compounds in the inner liner material. Butyl rubbers are expensive. Therefore, the use of an alternative often provides a convenient shortcut. The shortcut does not bode well for the long-term durability of the tire because the intra-carcass pressures and circulating air and moisture will degrade the adhesion of the rubbers in the belts leading to tread separations.
A. Technical literature
1. David M. Coddington, “Factors in Tubeless Radial Tire Durability.”
2. R. H. Dudley and J. V. Fusco, “Tubeless Tire Performance Related to Innerliner Properties,” presented to the Division of Rubber Chemistry, American Chemical Society, April 21, 1961.
3. S.A. Banks, F. Brzenk, J. A. Rae and C. S. Hwa, “The Effect of Intracarcass Pressure Build-Up on Tubeless Tire Performance,” “Rubber Chemistry and Technology,” Vol. 38, 1965, pages 153-169.
4. B. Costemalle, “Tire Business Loss and Intracarcass Pressure Modeling,” presented at 12th Meeting Tire Society, Akron, March 1992.
5. M. L. Janseen and J. D. Walter, “Stresses and Strains in Tires,” “Tire Science and Technology.” TSTCA, Vol.3, No. 2, May 1975, pages 67-81.
6. H. Kaga, K. Okamoto and Y. Tozawa, “Stress Analysis of a Tire Under Vertical Load by a Finite Element Method.” “Tire Science and Technology.” TSTCS, Vol.5, No. 2, May 1977, pages 102-118.
7. N. Tokita, W. D. Sigwirth, G. H. Nybakken, G. B. Ouyang, “Long-term Durability of Tires,” International Rubber Conference, Kyoto, Japan, October 1985.
B. Practical Use of Liner Permeability in Tire Litigation
1. A common defense to a tread separation claim is the reference to a penetrating object or improper puncture repair. From a mechanical standpoint, the defense makes little sense, particularly where the tread separation event occurs at a point distant from the penetrating object or the improper puncture repair. By showing that the liner was itself permeable, thereby giving rise to intra-carcass pressures, the manufacturers are on the defensive.
2. Manufacturers have historically had problems with liner permeability. Various solutions have been sought which are cheaper alternatives to the use of high-butyl- content inner liners. Two ply liners made with alternative rubber products have been tried, but this construction often gives rise to liner blisters. Cooper has battled liner blister problems for more than a decade. One solution to the blisters in finished tires has been the insertion of an awl through the tread, belts and plies to repair the liner blisters.
3. Liner permeability does give rise to intra-carcass pressures causing degradation failures at the belt edges.
4. Bridgestone/Firestone, as part of its cost reduction program (C95), increased
liner permeability which may explain in part the late-in-life failures.
VI. Antioxidants in Belt Skim Stock
Several different components are required in the construction of radial ply passenger tires. All tire components possess key physical properties which relate to tire performance. Tire compounders continue their efforts to improve tire performance by developing better fatigue resistant rubber compounds using various antidegradants and elastomers. Improved antidegradant systems have significantly extended the life of tires.
Internal components, such as carcass, apex, liner, breaker, cushion, base tread, and bead compounds, are protected by antioxidants. External tire components such as sidewall, ring flange/chafer, toe strip, and tread, are exposed to oxygen, sunlight, and ozone, all of which cause rubber products to degrade over time. These external tire components are protected by chemical antiozonants and wax. Chemical antiozonants are required to protect all of the external tire compounds which are based on traditional rubbers containing unsaturated backbones, such as NR, SBR, and BR.
Antioxidants do not completely eliminate oxidative degradation, but they markedly retard the rates of autoxidation by interfering with radical propagation. Depending on the types and combinations used, antioxidants can provide suitable polymer protection during the phases of a tire’s life cycle.
In theory, tire compounders continue to develop better performing steel belt skim compounds which require high unaged and aged adhesion to brass coated wire, stiffness for less movement and improved handling of tires, improved heat aging, and dynamic flexing. This compound is of utmost importance to tire durability.
According to defense experts and the technical literature, it is far preferable to use proper antioxidants in the compounding of belt skim stocks than to rely upon various antioxidative coatings. Some manufacturers, however, mistakenly rely on the migratory nature of antioxidants used in the tread skim stocks to provide adequate protection against the effects of aging and heat in the belts.
A. Technical Literature
1. F. Ignatz-Hoover: Antidegradants, Rubber Technology, Hanser Publishers, 2001.
2. E. Tuval, et. al.: Deterioration of Rubber Articles in Various Exposures and their Protection, Rubber World, August 2000.
3. E. Tuval, et. al.: The Aging of Tires and Similar Objects in Open Storage, Journal of Polymer Engineering, Vol. 19, No. 3, 1999.
4. F. Ignatz-Hoover: Stabilization of Tire Compounds with QDI, Rubber World, August 2000.
5. J. R. Beatty, “Fatigue of Rubber,” Rubber and Chemistry and Technology.
6. Engineering with Rubber, 2nd Edition (2001).
7. W. W. Barbin and M.B. Rogers, “The Science of Rubber Compounding”,
Science and Technology of Rubber, 2nd Edition (1994).
8. D. Barnard and P. M. Lewis, “Oxidative Ageing,” Natural Rubber Science and Technology.
9. J. I. Cunneen and R. M. Russell, “Occurrence and Prevention of Changes in the Chemical Structure of Natural Rubber Tire Tread Vulcanizates During Service, “ Rubber Chemistry and Technology (1969).
10. K. Grosch, “Rolling Resistance and Fatigue Life of Tires,” Rubber Chemistry and Technology (1987).
11. A. J. M. Suner, S. A. Kelbach and U. G. Eisele, “Crack Growth Performance of Tire Compounds,” Rubber World (1995).
12. S. W. Hong, et. al./Uniroyal: Antidegradants Can Help Improve Tire Performance if Used Right, ITEC, 1996.
13. S. W. Hong, et. al./Uniroyal: Improved Flex Fatigue and Dynamic Ozone Crack Resistance through Antidegradants, Rubber World, August 2000.
B. Practical Use of Inadequate Belt Skim Stock in Tire Litigation.
Because of the expanded scope of discovery surrounding the Bridgestone/Firestone/Ford recall of the ATX and Wilderness tires, a tremendous amount of information now exists showing that Bridgestone/Firestone engaged in an aggressive program of cost reduction in the manufacture of its tires beginning in 1994. There were significant changes in virtually every aspect of the design and manufacture of the recalled tires, including the reduction or elimination of antioxidants in the skim stock formula for the belts. Not unexpectedly, the incidence of tread separation soared after1994. Dr. Govindjee noted in his root cause analysis of the failed BFS tires that tread separations in the recalled tires generally occurred three or four years from the date of manufacture. While he made no cause and effect conclusion, it is logical to assume that the failures were, in part, aging failures as described in the literature. The report of the Office of Defect Investigations of NHTSA is also instructive on this issue.
This paper has lightly touched on select issues relating to design of steel belted radial tires. Each of the subjects mentioned is complex, and no one of the design issues provides a winning formula for successful litigation. A combination of design issues, bolstered by other similar incident evidence and internal memoranda critical of design, are often required to prove a design case.