Many of the news stories about the tire recalls of 2000 and 2001 referred to tires losing their treads and vehicles-mostly sport-utility vehicles-flipping over allegedly as a result of this happening.
But are tread separations really as common as these news stories seemed to indicate? Are they always dangerous?
The answer to both questions is ``no,'' according to tire industry experts interviewed by Tire Business.
``Tread separations are increasingly rare,'' said Guy Edington, managing director of Kumho Tire Co.'s technical center in Akron.
The reason for that is that the design of the radial tire has matured. ``Thirty years ago, we were all learning how to make radial tires, except for Michelin, which had been involved in it since the '40s,'' he said. ``The materials weren't what they are today, both in terms of the bonding systems and the wire themselves. And we've learned a lot. We've learned how to make a more robust product.''
It's a case of ``the bad gets the news,'' said Raymond Labuda, vice president of technology at Hankook Tire Co. Ltd.'s Akron Technical Center, referring to the stories about vehicles losing a tread.
Certainly tread separations do happen for a variety of reasons, but they're not an every day occurrence, he said. ``Most tires don't fail. Most tires do not have separations.''
So what is a tread separation and what causes it to happen?
There are actually two occurrences that are commonly referred to as tread separations, the experts said. One is when the two belts in a passenger or light truck tire begin to separate starting usually in and around the belt edge. Over time, this anomaly grows larger with the two belts eventually separating from each other. Technically, this is a belt separation, Mr. Labuda said.
Sometimes, the separation moves up over the belts and the tread delaminates off the belt package. This more accurately describes a tread separation.
But regardless of how the tire comes apart, ``it's really a belt separation that initiates the whole event,'' Mr. Labuda said.
To understand the dynamics of a tread separation requires an understanding of how the tire carries the load, Mr. Edington explained. Two major things determine this: the internal volume of the tire and the air pressure pushing out against the inside of the tire. If, for example, the pressure goes to zero, the tire won't carry any load.
Tires are designed to deflect, and a passenger tire will deflect an average of 15 percent, Mr. Edington said. ``But the more it deflects, the more the belts actually shear.''
Shearing, he explained, is when the tire's two steel belts start acting against each other. When they do that, beyond the normal deflection rate, they start building up heat similar to what happens when a person rubs both hands together.
This friction takes place at the thickest part of the tire-the tread and the belts underneath it. It's at the belt edges where the vast majority of separations get their start, Mr. Labuda said.
As the separation grows, due to continued use of the tire, either the belts separate from each other or the tread delaminates from the belt package.
So, for example, when a tire's air pressure drops to 18 or 12 psi from 35 psi, the amount of deflection taking place in the tire increases and more heat is generated.
Heat is the killer of tires. ``You'll get to the point where the tire is deflecting so much that the energy going into the tire starts building up heat, and it will fail,'' Mr. Edington said, noting that rubber is a ``notoriously poor'' conductor of heat. ``Most of the time this happens on a test wheel.''
While overloading a vehicle will increase tire deflection, Messrs. Edington and Labuda said they don't see that as the primary culprit causing tread separations, since a certain amount of extra load capability is built into tires. Instead, it's the lack of proper inflation pressure that's the primary villain because underinflation in effect is overloading the tire, Mr. Edington said.
``The main cause-and people are tired of hearing of this-the main cause of separations is lack of maintenance of the air pressure,'' Mr. Edington said. ``That's all you've got to carry the load. This is either due to lack of maintenance, a leaking valve, something that penetrates the inner liner, like a nail, poor repairs or not putting enough air in to start with.''
Goodyear's analysis and forensics group looks at tires on a regular basis to see what's happening in the marketplace, said Linda M. ``Lyn'' Lovell, director of product performance at the Akron-based tire maker. Usually this group has found evidence of abuse, significant underinflation and overloading in tires that have experienced a tread separation, she said.
``Our conclusion is that the primary cause is a tire that has been neglected and underinflated. That's why there's so much enthusiasm to get the word out to the consumer to take notice of their tires.''
But even in those rare instances when a tire's tread and/or belts do separate, it's not necessarily a catastrophic event, and drivers usually can pull off to the side of the road. In fact, in many instances, the tire doesn't even lose air.
``Most commonly, I think if you have a separation in your tires, the thing that's going to alert you first is that you're going to feel a vibration,'' Mr. Labuda said.
Other things to look for include bulges, which might indicate a separation, as well as unusual wear in the tire's shoulder area.
``If you see something on the shoulder of the tire that's starting to wear funny, it could be that that part of the tire is starting to separate from the belt and starting to lift up, and it's starting to wear off, creating an anomalous wear pattern,'' Mr. Edington said.
While a tire with a tread separation likely will lose air over time, it usually won't happen in a rapid fashion, Mr. Labuda said.
Instead, a driver's more likely to go out to the parking lot one day and find the tire flat.
While most tread separations wind up being nothing more than an inconvenience, they can be dangerous, particularly if they cause a driver to lose control of the vehicle. ``This is what happened in the Ford Explorer situation,'' Mr. Edington said, referring to the rash of Ford Explorer rollovers in 2000 linked to tread separations.
``If the tread comes off the tire, usually the tire maintains air pressure,'' he said. ``It is only when the tread comes off and wraps around the axle and causes the vehicle to lose control (that a separation becomes dangerous). Loss of control is serious.''
Such loss of stability is particularly troubling for vehicles with a high center of mass, such as SUVs, which roll over more easily than a sedan, according to Mr. Edington.
``That type of vehicle, with that situation, is more predisposed to roll over,'' Mr. Edington said.
Nylon cap plies
To prevent the shifting of belts in high performance tires, usually those with H speed ratings (sustained speed tested up to 130 mph) and higher, manufacturers employ what's commonly called a nylon cap ply. This additional nylon ply, which lies over the top of the belts, not only keeps the belts in place when the tire is running at extremely high speeds, it has the added benefit of containing them should a separation occur.
Asked whether tread separations can be prevented or greatly reduced through the use of nylon overlays, Mr. Edington's response was ``Absolutely.''
So why aren't they used more widely?
``The bottom line is cost,'' he said. ``Nylon overlays increase mass. Any component you put into the tire increases mass and will increase the cost and therefore the price of the vehicle (on which the tires are fitted).''
They also aren't needed on tires with S- and T-speed ratings, which make up the vast majority of passenger and light truck tires in the U.S. today, he said. These lower speed-rated tires don't reach temperatures high enough, as a result of the tire deflecting, to require a nylon cap to hold the belts in place, although some companies do put this feature in their S- and T-speed rated tires.
In the U.S., which doesn't have the autobahns of Europe with unlimited speed limits, most vehicles have engine management systems and are not capable of ultra-high speeds.
``A guy with a Buick LeSabre, he does not need a V-rated, 149 mph tire,'' Mr. Edington said. ``The car probably won't go that fast. It probably has a chip in the engine that will cut the thing off at 108 mph. So you put an S-speed rated tire on it that's rated to 112 mph. That means the speed rating of the tire matches the speed rating of the vehicle.''
This is starting to change, with more vehicles in the U.S. being fitted with higher speed-rated tires.
But for now, taking into consideration the types of vehicles in the U.S. and ``our road system, an S-rated or T-rated tire is fine,'' he said.