What's Out There in the World of Tires
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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
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.