If you're in the market for new tires, all of the variables
in tire specifications and the confusing jargon you might hear
from tire salesmen or "experts" might make your purchase
rather stressful. Or maybe you just want to fully understand
the tires you already have, the concepts at work, the
significance of all of those sidewall markings. What does all
this stuff mean in regular terms?
In this edition of HowStuffWorks,
we will explore how tires are built and see what's in a tire.
We'll find out what all the numbers and markings on the
sidewall of a tire mean, and we'll decipher some of that tire
jargon. By the end of this article, you'll understand how a
tire supports your car, and you'll know why heat can build up
in your tires, especially if the pressure is low. You'll also
be able to adjust your tire pressure correctly and diagnose
some common tire problems!
How Tires are Made As illustrated below, a
tire is made up of several different components.
The parts of a
The Bead Bundle The
bead is a loop of high-strength steel cable coated with
rubber. It gives the tire the strength it needs to stay seated
on the wheel rim and to handle the forces applied by tire
mounting machines when the tires are installed on rims.
The Body The body
is made up of several layers of different fabrics, called
plies. The most common ply fabric is polyester
cord. The cords in a radial tire run perpendicular to the
tread. Some older tires used diagonal bias tires, tires
in which the fabric ran at an angle to the tread. The plies
are coated with rubber to help them bond with the other
components and to seal in the air.
A tire's strength is often described by the number of plies
it has. Most car tires have two body plies. By comparison,
large commercial jetliners often have tires with 30 or more
The Belts In
steel-belted radial tires, belts made from steel are
used to reinforce the area under the tread. These belts
provide puncture resistance and help the tire stay flat so
that it makes the best contact with the road.
Cap Plies Some tires have
cap plies, an extra layer or two of polyester fabric to
help hold everything in place. These cap plies are not found
on all tires; they are mostly used on tires with higher speed
ratings to help all the components stay in place at high
The Sidewall The sidewall
provides lateral stability for the tire, protects the body
plies and helps keep the air from escaping. It may contain
additional components to help increase the lateral stability.
The Tread The
tread is made from a mixture
of many different kinds of natural and synthetic rubbers. The
tread and the sidewalls are extruded and cut to length. The
tread is just smooth rubber at this point; it does not have
the tread patterns that give the tire traction.
Assembly All of these
components are assembled in the tire-building machine. This
machine ensures that all of the components are in the correct
location and then forms the tire into a shape and size fairly
close to its finished dimensions.
At this point the tire has all of its pieces, but it's not
held together very tightly, and it doesn't have any markings
or tread patterns. This is called a green tire. The
next step is to run the tire into a curing machine,
which functions something like a waffle iron, molding in all
of the markings and traction patterns. The heat also bonds all
of the tire's components together. This is called
vulcanizing. After a few finishing and inspection
procedures, the tire is finished.
What All the Numbers Mean Each section of
small print on a tire's sidewall means something:
To see the different
sidewall markings, run your cursor over the tire.
Tire Type The P
designates that the tire is a passenger vehicle tire. Some
other designations are LT for light truck, and T
for temporary, or spare tires.
Tire Width The 235
is the width of the tire in millimeters (mm), measured from
sidewall to sidewall. Since this measure is affected by the
width of the rim, the measurement is for the tire when it is
on its intended rim size.
Aspect Ratio This number
tells you the height of the tire, from the bead to the top of
the tread. This is described as a percentage of the tire
width. In our example, the aspect ratio is 75, so the tire's
height is 75 percent of its width, or 176.25 mm ( .75 x 235 =
176.25 mm, or 6.94 in). The smaller the aspect ratio, the
wider the tire in relation to its height.
Two tires with different
aspect ratios but the same overall diameter
High performance tires usually have a lower aspect ratio
than other tires. This is because tires with a lower aspect
ratio provide better lateral stability. When a car goes around
a turn lateral forces are generated and the tire must resist
these forces. Tires with a lower profile have shorter, stiffer
sidewalls so they resist cornering forces better.
Tire Construction The
R designates that the tire was made using radial
construction. This is the most common type of tire
construction. Older tires were made using diagonal bias
(D) or bias belted (B) construction. A separate
note indicates how many plies make up the sidewall of the tire
and the tread.
Rim Diameter This number
specifies, in inches, the wheel rim diameter the tire is
Uniform Tire Quality
Grading Passenger car tires also have a grade on
them as part of the uniform tire quality grading (UTQG)
system. You can check the UTQG rating for your tires on this
page maintained by the U.S. National
Highway Traffic Safety Administration (NHTSA). Your tire's
UTQG rating tells you three things:
Tread Wear: This number comes from testing the
tire in controlled conditions on a government test track.
The higher the number, the longer you can expect the tread
to last. Since no one will drive his or her car on exactly
the same surfaces and at the same speeds as the government
test track, the number is not an accurate indicator of how
long your tread will actually last. It's a good relative
measure, however: You can expect a tire with a larger number
to last longer than one with a smaller number.
Traction: Tire traction is rated AA,
A, B or C, with AA at the top of the
scale. This rating is based on the tire's ability to stop a
car on wet concrete and asphalt. It does not indicate the
tire's cornering ability. According to this
NHTSA page, the Firestone Wilderness AT and Radial ATX
II tires that have been in the news have a traction rating
Temperature: The tire temperature ratings are
A, B or C. The rating is a measure of
how well the tire dissipates heat and how well it handles
the buildup of heat. The temperature grade applies to a
properly inflated tire that is not overloaded.
Underinflation, overloading or excessive speed can lead to
more heat buildup. Excessive heat buildup can cause tires to
wear out faster, or could even lead to tire failure.
According to this
NHTSA page, the Firestone Wilderness AT and Radial ATX
II tires have a temperature rating of C.
Service Description The
service description consists of two things:
Load Ratings: The load rating is a number that
correlates to the maximum rated load for that tire. A higher
number indicates that the tire has a higher load capacity.
The rating "105," for example, corresponds to a load
capacity of 2039 pounds (924.87 kg). See the chart on this
page for all the ratings. A separate note on the tire
indicates the load rating at a given inflation pressure.
Speed Rating: The letter that follows the load
rating indicates the maximum speed allowable for this tire
(as long as the weight is at or below the rated load). For
instance, S indicates that the tire can handle speeds
up to 112 mph (180.246 kph). See the chart on this
page for all the ratings.
Calculating the Tire
Diameter Now that we know what these numbers mean,
we can calculate the overall diameter of a tire. We multiply
the tire width by the aspect ratio to get the height of the
Tire height = 235 x 75 percent = 176.25 mm (6.94
Then we add twice the tire height to the rim diameter.
2 x 6.94 in + 15 inches = 28.9 in (733.8
This is the unloaded diameter; as soon as any weight is put
on the tire, the diameter will decrease.
Traction There are a lot of different terms
used today in the tire industry. Some of them actually mean
something and some do not. In this section, we'll try to
explain what some of the terms mean.
All-Season Tires with Mud and Snow
Designation If a tire has MS, M+S,
M/S or M&S on it, then it meets the Rubber
Manufacturers Association (RMA) guidelines for a mud and
snow tire. For a tire to receive the Mud and Snow designation,
it must meet these geometric requirements (taken from the
bulletin "RMA Snow Tire Definitions for Passenger and Light
Truck (LT) Tires"):
1. New tire treads shall have multiple pockets or slots
in at least one tread edge that meet the following
dimensional requirements based on mold dimensions:
a. Extend toward the tread center at least 1/2 inch
from the footprint edge, measured perpendicularly to the
tread centerline. b. A minimum cross-sectional width of
1/16 inch. c. Edges of pockets or slots at angles
between 35 and 90 degrees from the direction of travel.
2. The new tire tread contact surface void area will be a
minimum of 25 percent based on mold dimensions.
The rough translation of this specification is that the
tire must have a row of fairly big grooves that start at the
edge of the tread and extend toward the center of the tire.
Also, at least 25 percent of the surface area must be grooves.
Severe winter traction
idea is to give the tread pattern enough void space so that it
can bite through the snow and get traction. However, as you
can see from the specification, there is no testing involved.
To address this shortcoming, the Rubber Manufacturers
Association and the tire industry have agreed on a standard
that does involve testing. The designation is called Severe
Snow Use and has a specific icon (see image at right),
which goes next to the M/S designation.
In order to meet this standard, tires must be tested using
Society for Testing and Materials (ASTM) testing procedure
described in "RMA Definition for Passenger and Light Truck
Tires for use in Severe Snow Conditions":
Tires designed for use in severe snow conditions are
recognized by manufacturers to attain a traction index equal
to or greater than 110 compared to the ASTM E-1136 Standard
Reference Test Tire when using the ASTM F-1805 snow traction
test with equivalent percentage loads.
These tires, in addition to meeting the geometrical
requirements for an M/S designation, are tested on snow using
a standardized test procedure.They have to do better than the
standard reference tire in order to meet the requirements for
Severe Snow Use.
Photo courtesy Goodyear A tire designed to help prevent
can occur when the car drives through puddles of standing
water. If the water cannot squirt out from under the tire
quickly enough, the tire will lift off the ground and be
supported by only the water. Because the affected tire will
have almost no traction, cars can easily go out of control
Some tires are designed to help reduce the possibility of
hydroplaning. These tires have deep grooves running in the
same direction as the tread, giving the water an extra channel
to escape from under the tire.
How Tires Support a Car You may have
wondered how a car tire with 30 pounds per square inch (psi)
of pressure can support a car. This is an interesting
question, and it is related to several other issues, such as
how much force it takes to push a tire down the road and why
tires get hot when you drive (and how this can lead to
The next time you get in your car, take a close look at the
tires. You will notice that they are not really round. There
is a flat spot on the bottom where the tire meets the road.
This flat spot is called the contact patch, as
A tire showing the side
and bottom of the contact patch
If you were looking up at a car through a glass road, you
could measure the size of the contact patch. You could also
make a pretty good estimate of the weight of your car, if you
measured the area of the contact patches of each tire, added
them together and then multiplied the sum by the tire
Since there is a certain amount of pressure per square inch
in the tire, say 30 psi, then you need quite a few square
inches of contact patch to carry the weight of the car. If you
add more weight or decrease the pressure, then you need even
more square inches of contact patch, so the flat spot gets
A properly inflated tire and an underinflated
You can see that the underinflated/overloaded tire is less
round than the properly inflated, properly loaded tire. When
the tire is spinning, the contact patch must move around the
tire to stay in contact with the road. At the spot where the
tire meets the road, the rubber is bent out. It takes force to
bend that tire, and the more it has to bend, the more force it
takes. The tire is not perfectly elastic, so when it returns
to its original shape, it does not return all of the force
that it took to bend it. Some of that force is converted to
heat in the tire by the friction and work of bending all of
the rubber and steel in the tire. Since an underinflated or
overloaded tire needs to bend more, it takes more force to
push it down the road, so it generates more heat.
Tire manufacturers sometimes publish a coefficient of
rolling friction (CRF) for their tires. You can use this
number to calculate how much force it takes to push a tire
down the road. The CRF has nothing to do with how much
traction the tire has; it is used to calculate the amount of
drag or rolling resistance caused by the tires. The CRF is
just like any other coefficient of
friction: The force required to overcome the friction is
equal to the CRF multiplied by the weight on the tire. This
table lists typical CRFs for several different types of
Coefficient of Rolling
resistance car tire
0.006 - 0.01
0.006 - 0.01
Let's figure out how much force a
typical car might use to push its tires down the road. Let's
say our car weighs 4,000 pounds (1814.369 kg), and the tires
have a CRF of 0.015. The force is equal to 4,000 x 0.015,
which equals 60 pounds (27.215 kg). Now let's figure out how
much power that
is. If you've read the HowStuffWorks article How Force,
Torque, Power and Energy Work, you know that power is
equal to force times speed. So the amount of power used by the
tires depends on how fast the car is going. At 75 mph (120.7
kph), the tires are using 12 horsepower,
and at 55 mph (88.513 kph) they use 8.8 horsepower. All of
that power is turning into heat. Most of it goes into the
tires, but some of it goes into the road (the road actually
bends a little when the car drives over it).
From these calculations you can see that the three things
that affect how much force it takes to push the tire down the
road (and therefore how much heat builds up in the tires) are
the weight on the tires, the speed you drive and the CRF
(which increases if pressure is decreased).
If you drive on softer surfaces, such as sand, more of the
heat goes into the ground, and less goes into the tires, but
the CRF goes way up.
Problems With Tires Underinflation
can cause tires to wear more on the outside than the inside.
It also causes reduced fuel
efficiency and increased heat buildup in the tires. It is
important to check the tire pressure with a gauge
at least once a month.
The wear patterns of an underinflated,
properly inflated and overinflated
Overinflation causes tires to wear more in the
center of the tread. The tire pressure should never exceed the
maximum that is listed on the side of the tire. Car
manufacturers often suggest a lower pressure than the maximum
because the tires will give a softer ride. But running the
tires at a higher pressure will improve mileage.
Misalignment of the wheels causes either the inside
or the outside to wear unevenly, or to have a rough, slightly
For more information on tires and related topics, check out
the links on the next page.