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What
All the Numbers Mean
Each section of small print on a tire's sidewall
means something:
How
To Read A Sidewall:
P-Metric (example 1) P215/65R15 95S
P = Passenger Car
215 = Section width measured in millimeters
(25.4 millimetes per inch)
65 = Aspect Ratio, which means the sidewall
height is 65 percent of the section width.
R = Radial Carcass Construction
15 = Rim Diameter
95 = Load Index
S = Speed Rating - View Speed Rating Chart
Below
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A.
Passenger car tire.
B. Width of tire diameter.
C. Ratio of height to width.
D. Radial.
E. Diameter of wheel in inches.
F. Load index & speed symbol
G. U.S. DOT safety standard code.
H. Max. cold inflation & load limit.
I. Treadwear, traction and temperature grades.
J. Tire ply composition and materials used.
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Speed
Rating Chart:
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Speed
Symbol
A7
A8
B
C
D
E
F
G
J |
Speed
(km/h)
35
40
50
60
65
70
80
90
100 |
Speed
(mph)
22
25
31
37
40
43
50
56
62
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Speed
Symbol
R
S
T
U
H
V
W
Y
ZR* |
Speed
(km/h)
170
180
190
200
210
240
270
300
Open Ended |
Speed
(mph)
106
112
118
124
130
149
168
186
Open Ended |
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Contact Les Schwab Tire Centers for top speed
capability. |
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"):
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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: |
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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. |
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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 icon
The 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 an American 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.
Hydroplaning
A tire designed to help prevent hydroplaning
Hydroplaning 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 when hydroplaning.
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.
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 problems).
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 illustrated
below.
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
pressure.
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 bigger.
A
properly inflated tire and an underinflated or
overloaded tire
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.
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 tire
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 torn appearance.
Tustin Ranch Tire & Auto Center Phone:
714-573-2727
2541 El Camino Real (In Tustin Ranch)
(At the corner of
El Camino & Tustin Ranch Rd. In Costo,
Sears Shopping Center) |
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