Tire structural integrity
Historically, tires have provided consumers with outstanding service at minimum cost. In recent years, however, their traditionally low prices have escalated rapidly while, unfortunately, mileage has remained stagnant. In the case of high and ultra-high performance tires, mileage has decreased down to bias-ply tire levels (25,000 miles or less).
Degradation, an expression with a negative connotation, affects a variety of products, among them airplanes, ground vehicles, bridges, roads, tires, etc.
Degradation is the process of transition from a higher to a lower quality. With tubeless steel cord-belted, textile cord body radial-ply tires, degradation manifests itself in various ways, regardless of tire types, sizes, aspect ratios or wheel diameters. Some degradations affect vehicle operational safety, some do not.
This article is about tire structural integrity degradation. Although tire belt/tread detachment has been statistically rare since the advent of radialization, it does happen, typically due to the intrinsic radial tire durability failure characteristic. Here’s why.
Determining structural endurance
Tire producers over the years acquired a wealth of in-house tire design, development, manufacturing and vehicle application engineering expertise. It took generations, over which each of them developed their own particular “DNA.”
The most important tire performance criterion always was structural integrity retention. This aspect of tire performance became even more critical in view of the long original tread life potential the radial-belted tire architecture was capable of delivering, such as 50,000 miles minimum. This meant that this type of tire had to be able to safely absorb twice the number of deflection/deformation cycles as compared to the bias-ply tires they replaced.
However, bias-ply tires, upon providing 25,000 miles of original tread life, were retreadable, hence they were able to deliver another 25,000 miles of service, for a total of 50,000 miles, and this 80% of the time. The 20% casing reject rate was attributable to the road hazards bias tires encountered in real world vehicle operations.
Radial tires were also victims of such hazards. I, therefore, concluded early on that, from the vehicle producer point of view, the most practical, least costly, least complicated way to determine the level of structural endurance radial-ply tires were capable of was through the retreading process. The reason? Vehicle producers had no access to the tire producers’ tire structural integrity validation testing procedures which, to this day, are shrouded in secrecy.
In 1984, in a paper presented at the Tire Society Meeting in Akron, Ohio, titled “Manufacturing and Remanufacturing Radial Tires,” I stated that “the remanufacturing potential of a tire is the real measure of the original tire quality.” I still stand by this statement today, 24 years later.
Tire belt/tread detachment
Tire belt/tread detachment begins with microscopic cracks developing within the tire rubber compound components located within the tire belt edge zones. These cracks initially are not visible from the tire exterior, but propagate over time and mileage accumulations, and this at variable rates, depending upon the following:
* compositions of the rubber compounds involved;
* the precision with which they are mixed/homogenized;
* the type of tire manufacturing disciplines enforced; and
* the road/tire/vehicle system operating conditions, including climatic conditions, encountered.
Of critical importance and ideally, tire cross-sectional structures should be homogeneous, free of voids, porosity, contamination and intra-carcass pressure. Such tires should also feature inner-liners capable of preventing the pressurized air residing within the tire cavity from diffusing to the outside atmosphere. All this is easier said than done.
The intrinsic tire failure mode mentioned above is related to the basic radial-belted tire architecture, which exhibits relatively high stress/strain concentration points such as at the tire/belt edge zones. Also, it is at these points, located within the tire tread shoulders, that the tire heat build-up is the highest and pocketed (trapped). Therefore, for the tire to safely withstand its inherent operational dynamic characteristics, appropriate tire design, manufacturing and validation disciplines must be used for a given vehicle application.
Heat is the enemy of tires, regardless of design (bias, radial, broad line, high or ultra-high performance). When excessive, heat reduces the tensile strength of the textile cords used in the tire radial body plies, but not of the steel cords used in the tire belt plies, because steel is as strong at 250 degrees Fahrenheit as at 50 degrees F.
Heat, in time, ages the rubber compound constituents making up the tire cord/rubber composite cross-sectional structure, and can make them brittle. These compounds bond all the tire components together, again, ideally, in a homogeneous, porosity and air void-free fashion and, when working in harmony and within their thermo-mechanical limits, hold the tires in one piece throughout their lives.
However, when these limits are exceeded, a road shock, harmless to a cool-running tire, can structurally degrade a hot-running tire (a tire operating at a temperature much in excess of 180 degrees F).
So, how much heat can a tire take? Ambient temperatures experienced in the southwest of the United States, such as in Death Valley, can easily reach 120 degrees F during the summer months. In the Middle East, ambient temperatures can reach 130 degrees F or more. So I was not surprised when, in the late 1990s, a particular well-known brand of tire fitted to an equally well-known SUV started to fail when operating under such elevated ambient temperature conditions.
The subject of tires operating under such severe/hostile environments raises the importance of precisely determining at the initial tire design stage, and throughout tire development, the type of materials and manufacturing processes required to derive a consistently high quality tire production, with the primary goals being:
1. maximizing tire structural integrity (which equates to long tire life, including retreadability when applicable),
2. maximizing tire manufacturing precision, and
3. minimizing the number of tire manufacturing process anomalies or variations.
The latter is not easily fulfilled, considering the 20-plus component parts required in a typical radial passenger car tire, the complexity of the tire manufacturing process used, and the very tight quality controls this type of tire demands.
All in all, if tires are properly designed, developed, manufactured and matched to a given vehicle, they should be able to safely withstand the variable rigors of vehicle operating conditions encountered worldwide. This includes tolerating some levels of underinflation due to ambient temperature variations, the inaccuracies of tire pressure gages, typical consumer neglect, and the diffusion of the pressurized air from the tire cavity to the outside atmosphere.
Consequences of belt/tread detachment
When a tire belt/tread detachment occurs, particularly at high speed, the vehicle involved is destabilized, drifting and spinning out of the lane beyond the control of most drivers. Generally, it is when the vehicle leaves the road that it can roll over, depending on the type and condition of the surface/terrain encountered, and this regardless if the vehicle is equipped with a stability/traction control system or not.
Such systems, in general and to the best of my knowledge, are validated with the vehicle fitted with structurally sound tires. Under real world operating conditions, however, when the vehicle experiences a tire belt/tread detachment, one tire runs on its steel cord belt or what is left of it, and the other one, on the same axle, runs on its rubber tread, hence creating a highly asymmetrical vehicle handling/tractive operating condition.
Of interest during the tire belt/tread detachment event, the tire involved does not necessarily lose its inflation pressure.
A few years ago, the U.S. Congress, in a response to a statistically highly abnormal increase in tire belt/tread detachments experienced with one particular tire brand, determined that to prevent such situations from recurring, all vehicles of up to 10,000 pounds gross vehicle weight should have a tire pressure monitoring system, and that all tires should be subjected to higher endurance validation standards.
To date, have these measures really been effective in reducing the number of vehicle crashes, fatalities and serious injuries related to tire belt/tread detachments? Or should tire producers, in collaboration with vehicle producers, have aimed at significantly increasing the robustness of tire structures?
Tires of the future
Maintaining correct tire inflation pressure is, indeed, desirable for tires to operate, by design, at stresses, strains and temperatures that are within the capabilities of the tire architecture used (bias or radial) and the physical characteristics of the materials that, also by design, must be utilized for tire manufacture. Maintaining correct tire inflation pressure is equally important to meet the tire/vehicle performance criteria of vehicle producers, such as ride, handling, directional stability, traction, NVH (noise, vibration and harshness), tire tread life and tire rolling resistance.
Today, U.S. and Canadian customers are forced to purchase their vehicles with tire pressure monitoring systems when, in my opinion, what is required, but has yet to be developed, is a system capable of reliably detecting tire structural integrity degradation at its incipient stage, and before it propagates to the point of creating a vehicle operational safety hazard.
Again, although statistically structurally highly reliable, tires are sensitive to high operational stresses, strains and temperatures, some more than others. Some tires run cooler than others, yet fail structurally, while others operate at relatively high temperatures, but withstand them without structural failure. It is all a matter of tire design, development, manufacturing and vehicle application engineering disciplines.
In the meantime, to protect current tire structures from degradation over time and mileage accumulation, tire producers have reduced their tire mileage to the level of bias-ply tires of yesteryear (25,000 miles). One can only hope that such radial tires, marketed as high and ultra-high performance, are retreadable 80% of the time, as bias-ply tires were, thereby conserving limited sources of energies and raw materials.
Let us also hope that tire producers have learned from the problems encountered these past few years.
As for the near future, in view of the ever-increasing costs of energies and raw materials required for tire production, I do not expect quantum leaps of technological progress. What I do expect is judicious use of old and new materials and technologies, with the overall objective of producing structurally more robust, smoother running, longer life radial tires.
Coming up: The next chapter in this series on "Tires by design" will be on “Tire rolling resistance.” Bajer can be reached at (313) 886-6860.