7/18/2019
Driving on the Edge of Failure

Driving on the Edge of Failure

With truck tire separations, it’s all about speed and age

By William J. Woehrle

One advantage of a national automotive accident investigations team is that the broader reach can help identify commonalities and trends among crashes across the country. Analysis of recent files reveals truck tire failures are increasing on recreational vehicles (RV), campers, trailers, and trucks due to belt separations.

Two factors—speed and age—are key contributors to tire-belt separations, and not under-inflation, overloading, or impacts. Heavy vehicle tire failures are not new, but they persist in sufficient numbers to call attention to their incidence as posted highway speeds trend higher.

The reality is that speed ratings of the current inventory of truck tires top off at around 80 mph, a mere five mph over a common 75-mph posted speed limit. Such a safety margin is uncomfortably narrow when compared to a 130-mph speed rating on a tire suitable for sport and luxury coupes and sedans. Fatigue would probably claim those tires before a speed-related failure. And even a bottom-of-the-line passenger tire could push 100 mph without failing.

Unfortunately, similar safety margins do not exist for truck tires. At present, there are three popular truck-tire speed-rating options: 65, 75, and 80 mph, which means vehicles stocked with 65-mph rated tires, for example, run on the ragged edge of failure when traveling expressways at these higher posted speeds. This is even more significant for highway systems that no longer differentiate between speed limits for trucks and passenger vehicles.

Truck Tire Essentials

The Federal Motor Vehicle Safety Standards (FMVSS) are the U.S. regulations specifying the requirements for motor vehicles, and for their components that are manufactured or sold nationally. All highway tires from passenger vehicles to light trucks, heavy trucks, and motorcycles on U.S. roadways must comply with published standards for dimensions, markings, performance, and minimum durability.

Consumers can see the imprint of FMVSS regulations molded on the sidewalls of every tire. Truck tires sometimes will show speed ratings—probably as a single letter called a speed symbol—the maximum loads, and inflation pressures in pounds per square inch (psi). In addition, a three-digit number, known as the load index, will precede the speed rating. The load index, coupled with the speed rating, form the optional service description for that tire model. That molded tire data establishes the parameters for safe usage of that specific truck tire model.

Passenger and truck tires have much in common—both are steel-belted radial ply tires—but there are significant distinctions aside from the obvious size differences. At typically four belts per tire, truck models have double the belts that passenger tires do. In addition, all the cords in a truck tire are steel, including the radial ply cords in the sidewall, while passenger tires often have polyester cords. Inflation pressures are much lower in passenger tires with a range of 35 to 51 psi. In truck tires that range is 100 to 130 psi. Lastly, the riskiest position for a tire failure in a truck is on the front, or steer, tires, whereas passenger vehicles are more vulnerable to disaster when the rear tires fail. Trucks will experience severe understeering challenges with front failures, and passenger vehicles fight oversteering with rear-wheel failures.

Speed and Load Rating Basics

Every working tire, from a bouncing wheelbarrow tire to hardworking aircraft landing gear, has a speed limit and it likely will fail should it exceed its capabilities. For a steel-belted radial ply tire—either a passenger or truck tire—that failure mode will be a belt separation. Tread separation is also common terminology, but, in reality, the belts take the tread with them when they unravel from the tire. A closer look at scattered tire remnants on the roadside, popularly known as tread alligators, will almost always include the tire’s belt cables.

Speed and load ratings result from voluntary standards and are validated through extensive laboratory testing to determine the parameters of safe usage for each tire model. “Perform as designed” is the caveat at work with tires and it does not imply that a vehicle’s tires can be pushed beyond their limits. Tires are precision products that are far superior to their predecessors, but state-of-the-art tires can be very unforgiving of the smallest of imperfections and mistakes in manufacture.

Tire speed ratings are communicated via a worldwide standard of speed symbols derived from alphabet letters. A tested tire will be assigned a letter grade from its manufacturer, signifying its maximum speed. The letters run from “A” to “Z,” with “A” denoting lower maximum speeds and increasing in speed capacity as the letters progress (with a letter or two falling out of the conventional alphabet sequence). Ultimately, the speed symbol enables the vehicle manufacturer to select a tire that is capable of going as fast as the vehicle. The symbol standard is voluntary in the U.S. and, should the imprint not show on the tire itself, one may refer to the tire manufacturer’s brochure or data sheet for that information.

The tire load index or rating shows how much weight a tire can bear. The FMVSS requires that the actual load in pounds be molded on a tire’s sidewall, so the load index code is redundant in the U.S. The total load ratings of all tires on a vehicle axle must equal or exceed the gross axle weight rating for that vehicle.

Belt Separation Physics: Speed and Age

There are three primary tire-failure modes: separation (the tire’s ply layers delaminate); sudden pressure loss (a blowout, perhaps from a rupture); or slow pressure loss from a leak that leads to a run flat (or run soft) failure.

Belt separations undermine two defining features of tires: their laminate structure and their function as pressure vessels. Unlike a parking-lot flat, a failing tire must be rolling at a sustained critical speed for belt separation. The sequence of events is irreversible once the process initiates and progresses.

Separations start between the belts at the shoulder of the tire where the tread meets the sidewall—the location of the belt edges. A bulge may emerge at the shoulder, distorting the tread. The fracture will grow in two directions: across the tread from one shoulder to the other and circumferentially around the tire. As the separation grows, the tire loses its round shape and smooth roll. For truck tires, a thumping noise or a vibration in the steering wheel can signal a delamination in progress. Once a critical point is reached, the tread and belt package will peel away from the body of the tire.

If the tire throws off all of its belts, the radial ply carcass will round out where the treads and belts left the tire. The tire’s center, midway between the sidewalls, expands to a larger diameter when freed from that constraint. In the final stages, the radial ply carcass cords split apart, blowing out the tire.

Physical examination of tires can reveal incipient belt separations. Bulges in the tread area can trigger uneven wear patterns on the tire tread surface. Wire cables may also unravel and pierce the tread surface, sometimes freeing stiff and sharp filaments capable of piercing the tire’s rubber cladding.

A tire must be traveling at a sustained elevated speed in order to generate sufficient centrifugal force for a tire to throw off its belts. In addition to centrifugal stress, sheer stress develops between the belts as the inflated-and-loaded tire rolls at speed. Sheer stress is the impetus for heat buildup in a tire. The rubber surrounding the belts weakens as the heat rises with the increased rate of travel, ultimately causing the tire’s steel belts to fly off in a stress failure.

A key qualifier here is sustained speed: a speed burst to pass a slower vehicle does not count. Tires need time to heat up. For passenger tires, the duration is about 10 minutes to reach equilibrium temperatures; light truck tires need around 20 minutes; and heavier truck tires need a half-hour or more.

As a passenger tire seldom reaches critical speed—a travel rate far higher than any posted speed in the U.S.—it rarely experiences the high-speed stress failure of a thrown belt. Should a belt separation on a passenger tire happen, the likely cause is tire fatigue. The required fatigue-endurance performance of any tire is straightforward: A tire must not fall apart before the tread wears out. The corollary here is that the longer the tread-life expectancy—miles to wear out—the longer the required fatigue life. This extended fatigue life is commonly characterized as aged-tire endurance, which implies a chronological age limit for tires. At present, tire manufacturers typically suggest a 10-year life expectancy for highway tires, although many vehicle manufacturers advise a more conservative six years.

There are a couple of factors—natural and human-caused—that can diminish longevity. Rubber ages by absorbing oxygen. That process causes rubber to lose its physical properties like its tear strength. An old, crumbly rubber band that snaps apart is a clear example of oxidation’s toll. That same process in a highway tire causes visible cracking on its outer surface, but when the cracking affects the tire’s belt structure, a separation failure could be in the offing. This leads to a risk of putting too few miles on a tire annually, which is a form of tire abuse. A good appearance with deep tread depth can bely an oxidized tire with compromised structural strength.

In the trucking industry, the practice of retreading tires complicates fatigue determination because each acquired layer extends the tire’s fatigue life to longer mileages. Nowadays, retreaded tire fitments customarily are limited to dual applications on a truck’s rear axle. The accepted principle seems to be that, should one tire fail, the other dual will hold things up at that end of the axle. Truckers riding dual rear tires may not even be aware of a belt separation and will drive until the failure affects the roll or is noted on a stopover.

While spewed tread alligators are occupational hazards for commercial truck drivers and potentially dangerous to vehicles following in their flight paths, that tread debris can also signal a tire that has too many miles of wear. It is common for truck drivers to push the usage of their dualed tires, especially if they are retreaded, as far as possible before retiring them from service. Ultimately, retreads blur the aging issues for truck tires and make them vulnerable to failure. Just as truck tires run with speed limits, aging presents constraints as well.

Savvy fleet managers and virtually everyone who uses trucks extensively for their livelihoods tend to be practiced in speed and load ratings when loading and deploying their vehicles. They are also aware that truck tires will put on hundreds of thousands roadway miles annually as they travel U.S. expressways. However, their working reality is that goods and services need to be transported 24/7 and drivers need to earn incomes, so those tires better perform to meet their heavy demands.

What is not so clear is the tire industry’s commitment to delivering truck tires capable of sustained travel at higher highway speeds and providing guidance on tire-aging issues. A common tactic in a commercial tire-failure lawsuit is to assign blame to the tire user for excessive loads, road-hazard mismanagement, and faulty tire pressures when a belt separation tire failure occurs. None of those factors account for delamination: Speed or aging are the culprits.



William J. Woehrle is the tire group leader at Peter R. Thom and Associates Inc. engineering@prtassoc.com

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