|
|||||||
|
|
|
|||||
|
|
|||||||
|
|||||||
|
|
|
|||||
|
|
|||||||
The term "ply" refers to a layer of fabric
cords that is used
to reinforce the tire casing. The natural
rubber and synthetic
blends that are used would cause the tire
to inflate like a balloon
and make it difficult to retain the required
shape and structure
without the reinforcement. The cord
materials used are usually
nylon, rayon or polyester. Tires may
be marked as a "Four Ply
Rating", which means that the tires
do not contain four layers
of cords, but have only two layers with the
strength of four.
Bias Ply
In a bias ply tire, two or four layers of cords are wrapped at
a 30 to 38 degree angle from bead to bead to produce the
reinforced body casing. Layers must be an even number to
maintain stability.
Bias Belted
Bias Belted tires use two (or more) layers of body casings
made of nylon, rayon or polyester - plus two belt layers under
the tread section made of fiberglass, steel or kevlar that are
used to reduce tread "squirm" on the road surface. This
produces a tire that will corner flatter and produce longer
tire life because of the reduction of heat due to friction.
Radial Belted
Radial tires use cords that are wrapped directly across the
tire from bead to bead and result in cords that run at right
angles to the bead wires. As a result, many radial tires are
constructed with only a single layer of cords (or plies).
This
construction results in a cooler running tire that has more
sidewall flexibility and better directional stability. This,
in turn,
results in longer tire life and improved handling. The tread
area
must still be reinforced with two belts of fiberglass, steel or
kevlar to prevent the tread area from "rolling" during cornering.
Tire Designations
P = passenger car use
55 = tire sidewall height as a % of the width
(ie) the sidewall height is 55% of 205 or 112.75 mm
V = the speed rating
(ie) ratings are from B (31 mph or 50 kmh) to Z (over 149 mph or
240 kmh)
R = Radial Construction
D = diagonal (bias ply)
B = bias belted
16 = the rim diameter in inches
UTQGS - Uniform Tire Quality Grading System
Tires are rated under the three "T's" of tire performance:
Treadwear - is based on
a minimum of 100. If a tire is rated
at 300, it can be expected to last 3 times
longer than a tire
rated at 100 under the same driving conditions.
Temperature - is an A,
B or C rating. This indicates the amount
of heat generated in the tire under normal
driving conditions.
Because heat will soften the rubber and result
in more rapid
wear, the ideal would be to select a tire
with an A rating -
indicating that this tire would run cooler
than a B or C rating.
Traction - is an A, B
or C rating and indicates the holding ability
of the tire on a wet road surface. The
A rating indicates a better
traction than a B or C.
Load Rating
Tire load ratings at one time
used a letter designation
to indicate the ability of the
tire to carry weight under
a range of tire inflation pressures.
Today's tires carry
a specific maximum load at a
specified maximum
inflation - for example:
"Max Load 585 kg (1,290 lbs) at 240 kPa (35 psi)"
This rating is extremely important, and should
never be exceeded.
One of the causes of tire sidewall failure
is tire overloading, particularly
with trucks and recreational vehicles.
Tire purchasers should always
make sure that the tires they are buying are
suitable for the loads they
intend to carry. Overloading tires puts
extra stress on the sidewalls
resulting in overheating and cord and sidewall
failure.
Tire Rotation
Tires at different wheel positions will wear
at different rates.
As a result, tires should be switched to produce
even wear and
extend the life of the full set of tires.
Rotations should be done
every 8,000 km (5,000 mi) for best results.
The top diagram shows rotation patterns for Bias and Bias belted
tires - with or without use of a full size spare.
The bottom diagram shows the rotation pattern for Radial tires,
with or without the use of a full size spare. Radial tires
should
not be switched from one side of the vehicle to the other, because
the cords tend to develop a "set" due to the flexing of the cords
during driving. Reversing the direction by switching to the
opposite
side can result in cord failure and allow the rubber to separate
from
the cord body. When this happens, a bulge develops in the
tread
area that hits the road with more force resulting in "flat spot"
wear
and may cause the separated portion to break off the tread surface
resulting in a "blow-out" condition.
Wheel Balancing
No wheel and tire assembly is perfectly balanced.
If there is a
heavy spot in the tread area, a "bouncing"
or "thumping"
vibration is produced. This is known
as Static Unbalance.
If there is a heavy spot in the sidewall area,
as the tire rotates
it will continually try to correct its position
due to centrifugal
forces. This produces a "shimmy" vibration
that is referred
to as Dynamic Unbalance.
Both of these conditions can be
easily corrected with a computerized wheel
balancer. It is
important that wheels be balanced on a regular
basis, as
unbalance will not only cause excessive tire
wear, but can
also damage other vehicle components - such
as ball joints,
tie rod ends, radiator joints, etc. due to
the vibration.