What's Out There in the World of Tires

By Les Jackson

AIADA Contributing Editor

Without doubt, the most important – and by far the most ignored - safety component of any vehicle is its tires. Four contact patches, the surface areas where the tires are in contact with the pavement, control how well the car handles, stops and accelerates because they provide the sole traction for the vehicle. A great deal is expected from our tires and the vehicle’s well being – not to mention our own – depends upon how well they do their job.

Tire technology advances over the years have resulted in an immense range of characteristics that can be tailored to any vehicle or driver’s preferences. Tread design, rubber compounds, inner cord structure, tire profile (aspect ratio), speed, tread life and load rating all factor into how a tire can be designed for traction and braking in specific or varying weather conditions.

How the vehicle performs, rides, corners and brakes depends upon the choice of tire with which it is equipped. Designers and engineers work extensively with tire manufacturers to give each vehicle model the "feel" that is intended while maintaining safety and tread life.

Radial tires are the most prevalent on the road today, for a number of important reasons. In decades past most tires were Bias-belted. That is, the inner cords ran circumferentially around the tire and strengthening cord belts were laid at 45-degrees to each other, sharing both the surface under the tread and sidewalls. This resulted in sidewall flexing being transmitted to the tread and high rolling resistance, both of which made the tires less stable and more prone to "lane wander" and hydroplaning in wet conditions. Radial tires, by contrast, are constructed with the cord belts running 90 degrees to the circumference of the tire. More compliant sidewall flex allows the tire’s tread to stay in contact with the road surface, increases tread life and improves fuel consumption and handling. Thanks to the advent of the radial tire, a wide range of tire types available to the consumer offer an impressive number of qualities that should satisfy almost anyone.

There are a few rules of thumb that anyone contemplating a tire purchase should remember:

It’s always safe to replace your vehicle’s tires with the same brand/model/size that was fitted when new. A great deal of effort was made in the selection process and identical replacements will operate exactly as the existing ones did.
The greater the performance capability of a tire (cornering, high speed, etc.) the more likely it will be to ride harshly and produce noise on all but smooth pavement surfaces.
The greater the performance capability, the higher the price.
The lower the tire’s profile the harder the ride will be. Conversely, the taller the tire the smoother the ride.
The more aggressive the tread pattern, the shorter the tread life.

Passenger Car Tires

All season tires are designed to provide excellent traction and steering response in cold and light snow conditions. They also provide the best combination of wear, smooth ride and predictable handling in warm weather. These are the best tire choice for most vehicles and are generally used as OEM equipment on passenger cars and minivans. They are cost-effective and safe. Examples include: Dunlop SP; Michelin Energy; Firestone Affinity; Goodyear Assurance.

Grand Touring all-season tires are similar to standard all-season tires but are designed for higher speed capability. Examples include: Continental ContiTouring; Goodyear Eagle ResponsEdge; Pirelli Super Touring

Summer tires are designed for maximum handling for sports cars and sports sedans. Using special rubber compounds, unidirectional tread design and special internal reinforcements, these tires offer very quick turning response and high speed capability. They also handle well in wet weather conditions, but are not intended to be driven in near-freezing temperatures or through snow and ice. Examples include: Bridgestone Potenza; Michelin Pilot Sport; Yokohama ADVAN Sport.

There are two other levels of summer tires available. Ultra High Performance tires are low profile models intended for use in wet and dry conditions. Extreme Performance versions offer the best possible handling, but only in dry conditions. Their special tread designs and compounds can’t be used in snow or ice or at higher speeds on water-soaked roads.

Winter tires have special compounds and tread designs that are resistant to the effects of extreme cold and snow/ice conditions. They are available in many different sizes and profiles and offer very good ride and braking characteristics. Traction in snow and ice is superior to that of other tires. Because the tread compounds are quite specialized, such tires tend to wear relatively quickly in warm, dry conditions and manufacturers recommend removing them in the other seasons. For light truck and SUVs, studdable winter tires are offered for use in states/countries that allow studded tires. Examples include: Bridgestone Blizzak; Dunlop Winter Sport.

High Efficiency tires that minimize rolling resistance are now being introduced to the market. They utilize a special tread design and lighter/stronger materials, along with a limited-flex sidewall and higher pressure, to achieve the goal of higher fuel mileage. Manufacturers are developing special suspension and brake systems to help reduce rolling resistance as well.

Run-Flat tires are designed to be nearly impervious to punctures or other loss of air. Once deflated these tires can typically be driven for 50 miles or more at highway speeds. The chief advantage of run-flat technology is that vehicles so equipped no longer have the need for a spare tire, resulting in cost, weight and space savings.

Two general types of run-flat technology are in use today. The first utilizes stiff sidewalls that can support the weight of the vehicle for a reasonable period of time. There are limitations on sidewall height in such tires, which means that the technology is limited to low-profile designs. Consequently, these tires are almost exclusively used on sports cars and performance sedans. Run-flats of this type have relatively poor ride quality and aren’t repairable.

The second type of run-flat is a four-component tire/wheel package currently used by Michelin, called PAX. It consists of an outer tire, inner support ring, inflation monitor and special wheel incorporating a special locking bead that prevents the tire from peeling off. These tires have the advantages of being repairable and of being produced in a wide range of sidewall height. They can be used on passenger cars, minivans and SUVs, and the ride quality is nearly identical to that of conventional tires.

Competition tires are designed with purposeful tread patterns, compounds and internal construction to provide state-of-the-art acceleration, cornering and braking for use on racetracks and in autocross events. Some of these types of tire are legal for street use but their ride and wear characteristics make them highly undesirable for daily driving.

Light Truck Tires

Highway All Season tires are designed for overall driving conditions. They provide very good on-road performance and traction on gravel roads and in sand and light snow. These are typical of OEM tires on SUVs. Examples include: Goodrich Open Trail; ContiTrac.

Street/Sport All Season tires are designed to replace OEM models and offer increased traction and handling. Because they are wider than OEM tires their traction on ice and snow is slightly less but still adequate. Examples include: Michelin Latitude; Sumitomo HTR Sport; Dunlop Grandtrek; Goodyear Wrangler

Highway Rib Summer tires are designed for highway use in dry conditions. They provide good wear and very low noise levels but are not intended to be driven in winter weather. Example: Michelin XPS

Street/Sport Summer tires are designed for large rim diameters and low profile appearance. They improve dry weather handling but are not intended for use in winter. Examples include: General Grabber UHP; Michelin 4x4 Diamaris

Off-Road All Terrain tires are designed with special tread patterns to provide traction on dirt, sand, mud and snow, as well as on paved surfaces in all temperature conditions. Examples include: Goodrich All-Terrain; Pirelli Scorpion A/T; Firestone Destination A/T

For maximum off-road traction, tires designed with very aggressive tread patterns are offered. These tires bite into loose surfaces for the most effective traction and emphasize off-road use. However, newer models can be used on highways, at the cost of tread wear and noise. Examples include: Goodrich Mud-Terrain; Goodyear Wrangler MT/R.

Do Your Part

Proper tire tread depth and inflation are extremely important things for owners to monitor regularly, but studies show few people do so. A tire is considered to be seriously under-inflated when it has 8 psi (pounds per square inch) or more less than specifications. When a tire is under-inflated it can overheat, a condition that can result in premature failure, tread separation, blowout or a loss of control of the vehicle due to instability in turns. Also, tread life is shortened and fuel consumption is increased.

Tire Pressure Monitoring Systems

The National Highway Traffic Safety Administration (NHTSA) enacted legislation that requires the installation of tire pressure monitoring systems in new passenger cars and light trucks, buses and multipurpose vehicles weighing 10,000 pounds or less. The most common monitoring system consists of a pressure-measuring device, along with a transmitter, located in the tire’s air-filling stem. It transmits pressure readings to a gauge or warning light on the dash. Other systems use tire-deflection or tire rotation-measuring devices that are monitored by the wheel sensors in the ABS system and analyzed through computer algorithms.

How To Read Your Tire’s Vital Information

Please remember that tire information placards and vehicle certification labels contain information on tires and load limits and these placards and certification labels are permanently attached to the vehicle door edge, door post, glove-box door, or inside of the trunk lid. You can also find the recommended tire pressure and load limit for your vehicle in the vehicle owner’s manual. These labels must show:

Recommended tire size
Recommended tire inflation pressure (usually given in PSI cold)
Gross vehicle weight rating (GVWR):
The maximum occupant and cargo weight a vehicle is designed to carry
Gross axle weight ratings (GAWR) for front and rear axles:
The maximum weight the axle systems are designed to carry

All tires today are stamped with an alphanumeric code that is federally mandated to contain certain information about the tire’s size compatibility. Also, another stamped area, referred to as the Uniform Tire Quality Grading System (UTQGS), contains key information about tread wear, traction and temperature capabilities.

Looking at a tire you might see the alphanumeric code: P205/70R14 93S. Deciphering the code is easy. The “P” stands for passenger vehicle tires, “205” refers to the nominal width of the tire in millimeters, “70” refers to the aspect ratio (ratio of the tire’s height to its width), “R” means radial and “14” refers to the wheel rim’s diameter. The final number and letter represent the load index and speed rating. Here’s what they all mean:

Nominal Width
This three-digit number gives the width in millimeters of the tire from sidewall edge to sidewall edge. In general, the larger the number, the wider the tire.

Aspect Ratio
This two-digit number, known as the aspect ratio, gives the tire’s ratio of height to width. A "70," for instance, means that the tire is 70% as high, from the rim to the outer diameter, as it is wide. Another term for aspect ratio is "profile." The lower the profile, the better the steering response and overall handling on dry pavement will be.

R
The "R" stands for radial. Radial ply construction of tires has been the industry standard for the past 30 years.

Rim diameter code
This two-digit number is the wheel or rim diameter in inches. If you change your wheel size, you will have to purchase new tires to match the new wheel diameter.

Load index
This two- or three-digit number is the tire’s load index. It is a measurement of how much weight each tire can support. You may find this information in your owner’s manual. If not, contact a local tire dealer. Note: You may not find this information on all tires because it is not required by law.

Speed Rating

The speed rating letter indicates the range of speeds at which the tire is certified to carry a load at continuous speed. Ratings range from A to Z, with A as the lowest rating, although “H” is out of sequence and refers to 130 mph. Tires with a “C,” for example, can only run up to 37 mph, but no passenger car tire with a rating lower than 80 mph is legal in this country. The speed rating denotes the speed at which a tire is designed to be driven for extended periods of time.

Traction

Traction grades are an indication of a tire’s ability to stop on wet pavement. A higher graded tire should allow a car to stop on wet roads in a shorter distance than a tire with a lower grade. Traction is graded from highest to lowest as "AA", "A", "B", and "C".

Treadwear

Treadwear grades are an indication of a tire’s relative wear rate. The higher the treadwear number is, the longer it should take for the tread to wear down.

A control tire is assigned a grade of 100. Other tires are compared to the control tire. For example, a tire grade of 200 should wear twice as long as the control tire.

Temperature

Temperature grades are an indication of a tire’s resistance to heat. Sustained high temperature (for example, driving long distances in hot weather), can cause a tire to deteriorate, leading to blowouts and tread separation. From highest to lowest, a tire’s resistance to heat is graded as “A”, “B”, or “C”.

Max Pressure is the last – and one of the most important - stamping you should be aware of. If a given tire states: Max Press 280 kPa (41 PSI), those numbers mean exactly what they say. The tire will fail if it’s overloaded or overpressurized. The most important thing to remember here is: When a tire is under-inflated it can overheat, a condition that can result in premature failure, tread separation, blowout or a loss of control of the vehicle due to instability in turns. Also, tread life is shortened and fuel consumption is increased.

The Max Pressure stamped on a tire is not the pressure it’s supposed to be filled to! It is the maximum amount of air the tire can hold without failing. Consult your car’s owner’s manual or door sticker for the proper everyday inflation pressure.