Tread wear resistance
This is the third in a series of articles about tire design by Jacques Bajer, president of Tire Systems Engineering Inc. In the first, he analyzed the causes of tire structural integrity degradation. In the second, he explained how tire rolling resistance affects vehicle fuel consumption.
From a pure economics standpoint, tread wear resistance is the tire property consumers appreciate the most, and this we know from consumer satisfaction surveys. Lately, however, I am not so sure that vehicle producers, in the process of selecting a tire for a particular vehicle application, take this into serious consideration.
Establishing meaningful tire tread wear resistance properties is difficult. Every tire is different (there are about 1,700 sizes and types), and every tire/vehicle combination is different. Add to this the variety of road surface textures; variations of road topographies and climatic conditions; variations of vehicle architecture and maintenance; driver habits; and, last but not least, the variations that develop during tire manufacturing, and you can see why the tire/vehicle system engineer faces a monumental task to correctly validate a tire for production as original equipment.
Finally, and because of the tremendous proliferation of tire sizes, types, aspect ratios, etc., the cost to tire and wheel manufacturers of validating every tire and every tire/wheel combination for a given vehicle application has become horrendous.
Determining tire tread wear resistance
Over the years, many have attempted to determine and rank tire tread wear resistance characteristics using indoor tire testing machines. As in the case of determining tire rolling resistance -— and this regardless of the accomplishments achieved to date -— the results have not been satisfactory because of the failure to derive a reasonable correlation between real world road/tire/vehicle testing measurements and indoor tire testing machine measurements.
These machines are not, at this point, capable of duplicating the real-world vehicle operational and environmental conditions encountered. So today, vehicle road testing prevails.
When conducting vehicle road testing, it is very important that the testing conditions, as well as the degree of testing severity, be clearly established. Road surface textures, road topographies, climatic conditions, and the effects of vehicle architecture all will make a difference in wear resistance. So will operating modes such as speed, range, percent of braking, acceleration, cornering, straight-ahead driving, and city or highway modes. The major differences in the ranking of tire tread wear resistance, historically, have been established from testing under the conditions experienced by ordinary consumers.
The basic nature of tire tread wear
The tread is a vital component of the overall tire system. It directly interacts with the road surface under varying operating and environmental conditions, including climatic conditions such as winter, summer, rainfall, etc.
It is the tread that provides the miles or kilometers that determine tire life, assuming that the tire structure remains in a sound state of integrity. The energy generated by tire forces and the abrasiveness of the road surface are the main factors determining the life of a tread for a given tire architecture (radial, bias).
While the abrasiveness of the road surface is an external factor, the tire architecture is an internal factor. This internal factor modifies the effect of the external factor by varying degrees, depending upon internal tire construction details. The tread compound formulations, the tread dimensions and design, the tread void-to-solid ratio and sipe density all play important roles, and this is why tires should be removed from service no later than when the treads are worn down to 2/32nds-inch, assuming they have worn out uniformly.
To encourage uniform wear, periodic tire rotation (every 7,500 miles) is recommended. Today, due to economic conditions, many consumers delay tire replacement, sometimes to the point that the tire steel cord belts are exposed.
Tire tread wear resistance is affected by a tire’s internal architecture, tire aspect ratio, tire size and type selection for a given vehicle, and the degree to which a vehicle is radial-tuned.
Internal architecture. Fundamentally, the reason radial tires are significantly more resistant to tread wear than bias tires is related to the ratio of tire belt to radial body stiffness.
The higher the ratio, the higher the tire resistance to tread wear for a given tread design and dimension.
Years ago, I called this “decoupling,” a mechanical term that means, in this case, the tire sidewalls work independently from the tread. This allows these “tire zones” to perform the respective functions for which they are designed: one, for the sidewall to absorb road surface irregularities, and two, for the tread and its stabilizing understructure, the belt, to provide high tread wear resistance.
Back in the bias tire days in the United States, when the tire producers were confronted with complaints about poor tire tread life, they would typically answer, “We have to change the tread compound.” However, it became evident early on (1959) that, by itself, the radial tire architecture would increase tire tread life by a far greater amount than would be gained through the use of any type of tread compound.
To some extent, this low tire tread life situation still exists today, particularly with tires of ultra-low aspect ratios (which are sold on the basis of providing high grip/skid resistance and fast steering response). For quite some time now, consumers have complained about the poor tread life these ultra-low profile radial tires provide, some down to 12,000 miles.
In a paper I presented at the 2004 International Tire Exhibition & Conference, I stated that “it remains to be seen, if these trends persist, if long-life tires will remain a selling point, and if consumers will accept radial-ply tires providing bias-ply tire mileage at radial-ply tire prices.” Since that time, the economic situation has deteriorated, tire prices have increased dramatically, and nothing really has changed.
Maximum “decoupling” is only possible with radial tires consisting of a thin, highly flexible, 90-degree angle monopoly steel cord radial body, and a rigid, triangulated (three-ply) steel cord belt lattice incorporated into a tire cross-sectional structure of, by today’s standards, high aspect ratio, such as .75. Such basic architecture is still widely used today for medium heavy-duty radial truck tires (example: 285/75R24.5) in order to provide maximum original tread life, retreadability and low rolling resistance, all economically required by the trucking industry.
As for passenger car applications, the radial internal tire architecture consists of a two textile cord-ply radial body, at times at a few degrees of angle between plies (therefore not purely radial), in conjunction with two 22-degree angle steel cord belt plies, over which nylon cords, so-called “cap plies,” are circumferentially wound to contain possible belt detachment, hence contributing to a definite amelioration of tire structural integrity retention.
Radial tires that feature maximum decoupling stabilize the tread to such an extent that they significantly reduce the tread elements’ relative motion to the road surface, to the point of doubling or tripling the tire tread wear resistance. To reach such a level of resistance to tread wear, the radial tire must use a tread of minimum thickness, consistent with providing other tire desirable properties, such as traction, grip and resistance to aquaplaning (the closer the tread is to the belt, the higher the tread resistance to wear).
Also, the thinner the tread is, the lower the tire rolling resistance. As you may conclude by now, tire traction, rolling resistance and tire resistance to tread wear are highly interwoven, and must be balanced. (For more information, see Bajer’s “Tire rolling resistance” story in the August 2008 issue of MTD or at www.moderntiredealer.com.)
Tire aspect ratio. Today, many radial passenger car tires are of very low or ultra-low aspect ratio, down to .50, or even .25. These tires are mounted on large diameter (17-inch and above) wider wheels, and therefore have much reduced “decoupling.” Consequently, they are less resistant to tread wear, again for a given tread design, volume and tread compound composition — down to 25,000 miles or less. These tires also have a higher spring rate (i.e., they are much stiffer), which results in poor vehicle ride and, at times, depending upon the vehicle involved, unacceptable impact harshness and road noise, and variable vehicle steering response characteristics. They can also rapidly run out of stroke, due to their insufficient section height, resulting in tire and wheel fractures.
Therefore, vehicle manufacturers should offer consumers the choice of a higher aspect ratio tire mounted on a smaller diameter wheel. This would not only provide the consumer with a more economical, more durable, smoother running tire, but also eliminate the necessity for the consumer to purchase wheel insurance.
Tire size and type selection for a given vehicle. The car manufacturer’s selection of the type and size of OE tire to be fitted to the vehicle is of prime economic significance to consumers. The selected tire should feature enough load reserve, enough section height and enough deflection for a given tire load and inflation pressure.
Except for a small degree of compromise determined among tire manufacturers, tire sizes, types, loads and inflation pressures are established through their trade associations. This includes rim dimensions and profiles, as well as tire valves.
However, from a pure tire engineering and development standpoint, tire types, sizes, loads and inflation pressures are determined from the level of strains, stresses and temperatures they can withstand under real-world vehicle operating conditions.
The operating deflection of a tire, including its stiffness or softness (spring rate), is key to the level of vehicle operating smoothness achievable. Theoretically, an ultra-low section height tire (a tire significantly lower than .60 in aspect ratio) should provide the same deflection and spring rate as a high aspect ratio tire (.65 to .75).
Practically, this is not possible, because it would mean that the ultra-low section height tire deflection, as a percentage of the tire section height, would be too high, resulting in tire operating stresses, strains and temperatures beyond the limits of currently used tire materials. Therefore, such a tire must operate at a lower deflection and a higher inflation pressure than a high section height/high aspect ratio tire.
Again, keep in mind that a tire, to provide a good balance of performance, must have a sufficient section height or stroke, in order to absorb road surface irregularities without structural damage to the tire and wheel, just as the vehicle suspension/shock absorber (damper) system must have sufficient travel to perform its functions efficiently.
The tire, being the only vehicle component in contact with the ground, is, therefore, the first component of the overall system to deflect, the suspension being the second. Both, for practical purposes, deflect simultaneously. Also, and of prime significance, the deflected tire footprint must have the proper shape and dimensions.
“Radial tuned” vehicles and tread wear. One major drawback with which tire and tire/vehicle system engineers, including myself, have struggled over the years is the radial tire’s inherent longitudinal stiffness and limited enveloping power.
Another drawback is related to vehicle steering response and pull. From the vehicle NVH (noise, vibration, harshness) standpoint, the entire burden of “tuning” had to be put on the vehicle.
This became quite evident with the early radial tires -— those originally developed to maximize resistance to tread wear. Today, more of the tuning burden is placed on the tire producers.
As for the tire steering response, the tire architecture had to be modified. A compromise, therefore, was reached, and consequently the radial tire had to be “de-tuned” from its original promise of maximizing resistance to tread wear, yet still providing 40,000 to 45,000 miles of service.
In 1968, I conducted an analysis of the effect of tire size on resistance to tread wear, and concluded that the selection of the proper tire type and size was, indeed, crucial to maximizing tire resistance to tread wear, and that oversized tires were better than borderline/undersized tires.
Vehicle producers should remember this, particularly in view of consumer expectations of the long life that radial tires are supposed to provide. To this date, unfortunately, perfectly radial-tuned vehicles are few and far between even though it can be done, even on lighter unitized-body vehicles.
Current trend of radial tires affecting tire resistance to tread wear
Again, the trend has been toward the use of low and ultra-low section height tires mounted on large diameter (17 inches and above), wider and heavier wheels, so-called “high performance.”
These tires are stiff in all directions (vertically, torsionally, longitudinally and laterally), with all the consequences this entails, such as impact harshness and road noise.
They also are highly sensitive to inflation pressure variations, and can be prone to fracture (as are the wheels) when the vehicle is operating on rough road surfaces. Last, but not least, they do not deliver the long tread life for which radial tires are (or were) designed.
Radial tires, when properly sized and developed to maximize resistance to tread wear, perform best when fitted to low-drag, efficiently powered, relatively light (3,000 pounds or less) two-wheel, rear drive vehicles, rather than when fitted to large, heavy (4,300 pounds or more) four-wheel drive, high-drag vehicles. This is due to the significant difference in tire force/energy.
A good correlation between measured value of tire force severity and tire tread wear severity was established in the 1960’s when I was involved with developing vehicles for radial tire application at Ford Motor Co. This, in a way, is a situation similar to the one I described in the rolling resistance article published in the August 2008 issue of MTD.
The type, size, weight, drag, powertrain and chassis systems of the vehicle involved are key factors in maximizing tire resistance to tread wear.
A significant portion of U.S. consumers are struggling economically. If current economic conditions persist, I foresee a return to more “normal” tire sizing and aspect ratios, and more normal wheel sizes.
The development of radial tires, with their long life, low rolling resistance, low operating temperature and retreadability (see sidebar on page 52) has provided tire engineers with the opportunity to significantly reduce the degree of tire design and performance compromise that had to be accepted in bias-ply tire days. To fully reach the long tire life goal, the vehicle must be “radial tuned,” and the correct tire type and size must be selected.
When these objectives are achieved, consumers can readily see tangible proof of the performance and economic advantages such tires can provide. However, this economic advantage has been significantly reduced with the increased adoption of ultra-low section height (so-called “high performance”) tires.
With the current state of the U.S. economy, the high operating cost of vehicles (monthly payments, cost of gasoline, low mileage per gallon of fuel and tire pressure monitoring system maintenance), and now radial tire life down to the bias-ply tire level, consumers are hurting.
This is the time to provide them with smoother running, more structurally robust tires, mounted on reasonably sized wheels, and capable of delivering 80,000 miles of service.
The next chapter will be on tire grip/traction/skid.
For more than 50 years, Jacques Bajer has championed the cause of tire structural integrity in one way or another. His work with Ford Motor Co., beginning in 1955, led to the creation of a tire uniformity grading machine. In 1964, he almost single-handedly developed low-profile tires for the 1964 Lincoln and Thunderbird.
They performed so well that Ford made them standard equipment on all its vehicles the following year.
The French-born engineer also owns a number of patents, including one for a “casing preparation method” used in producing quality retreaded tires.
He is perhaps best known, however, for radializing the domestic automotive industry. Bajer didn’t invent the radial tire, but when he convinced Ford to offer them as options on its vehicles, the radial tire era began.
Bajer opened his own consulting company, Tire Systems Engineering, Inc., in Grosse Point, Mich., in 1970. The firm specializes in the design of advanced manufacturing systems for the economical mass production of tires, power transmission belts, lathe-cut seals and air springs.
More than 30 years later, he still stresses the need to analyze the relationship between the vehicle, tire and road. He can be reached at (313) 886-6860.