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 The
tires on your car would seem to be simple and
straightforward but every year manufacturers spend
millions of dollars on research and development to
improve the quality and reliability of tires. In
this article we’ll examine some of the things that
tire makers must be concerned about.
Tires have two main sections, the tread which is the
part of the tire that rolls along the ground and the
sidewalls that connect the tread to the wheel.
Although there are many different sizes and tread
styles, there are really only two major
manufacturing methods for tires. The bias ply tire
was used almost exclusively for cars and trucks in
the U.S. until the 1970’s. Although it was used in
Europe for many years, the radial ply tire did not
gain acceptance in the U. S. until its fuel saving
capabilities were recognized in the 1970’s. Each
type of tire has unique characteristics that make it
desirable for certain applications.
The
rubber like compound that tires are made of is a
blend of many different substances chosen to produce
the exact combination of qualities that the
manufacturer wants for a particular tire. Some of
the things that tire makers are concerned about are
tread life (how long will the tire last), traction
(will the tire resist skidding under any conditions)
and durability (can the tire resist damage from
running over objects or being exposed to heat,
chemicals, etc.). Creating a quality tire requires
reaching a good compromise on these features because
a tire with exceptional tread life may not have good
traction and vice versa.
Bias ply tires consist of layers of nylon or
polyester mesh encased in rubber. These sheets of
mesh run from one side of the tire to the other at a
slight angle. This angle is called the “bias” and
each layer is called a “ply” thus creating a “bias
ply” tire. These layers of mesh are placed in a mold
and hot liquid tire compound (what we call the
rubber) is injected. The outside edge of the mold
has the pattern that forms the tread of the tire.
The injection, heating and molding process is
sometime called vulcanizing. When the tire compound
cools and solidifies, the mold is opened and the
tire is ready for use.
The construction process for a radial ply tire is
very similar except that belts of nylon, polyester
or even steel mesh are wrapped around the tire under
the tread area. Because these belts go around the
circumference of the tire they are called “radial
plies”. The injection and molding process is
essentially the same.
Now we’ll look at the different features of tires
and examine the differences between bias and radial
ply tires. The first feature is tread life. When we
buy new tires, one of the first things we ask about
is tread life. How many miles will this tire travel
before it has to be replaced? There are two main
factors that affect tread life, the hardness of the
rubber and “squirm”. If you were able to watch a
tires tread as it moves down the road, you would see
that it doesn’t remain straight and flat on the
ground. It moves from side to side and moves up and
down. This is called “squirm”. The effect on the
tire is the same as the effect on a rubber eraser
when you rub back and forth on a piece of paper. The
rubber is scrubbed off. If you make the tread from
hard rubber, it is more difficult for it to be
scrubbed off but you pay a price in loss of
traction. Tires with hard rubber tread skid much
more easily than ones with soft rubber. Put an
eraser on a table and push it sideways. Now put a
checker or domino on the table and push it sideways.
The checker moves easier because it is harder and
has less traction. The other way to increase tread
life is to prevent squirm. This is an advantage of
the radial ply design. The belts under the tread in
a radial ply tire hold it steady and minimize
squirm. A typical bias ply tire will last 10 to
20,000 miles whereas radial tread life starts at
about 40,000 miles. An added advantage is that
reducing squirm increases gas mileage significantly.
The next important tire feature is traction. Here
again there are several things that affect traction.
The first is size of tread and design. Under the
ideal conditions of a straight, hard, dry road,
maximum traction is achieved by putting the largest
possible amount of rubber in contact with the road.
This is why race cars use wide tires with no tread
(slicks). Unfortunately on a wet road these tires
act like water skis, lose contact with the road and
skid uncontrollably. Passenger car tires must
perform well on wet, dry, muddy or snow covered
roads and treads are designed to accommodate all of
these. Another thing that affects traction is rubber
hardness. As we discussed above, softer treads have
better traction but lower tread life so the tire
designer must compromise. And the last factor is
squirm. Here again radial tires have the advantage
since the belts hold the tread in better contact
with the road and therefore provide better traction.
The last major design consideration is resistance to
road hazards. As a tire travels down the road, it is
constantly flexing. This movement creates heat.
Carefully feel of your car tire after you driven
several miles and you’ll see what I mean. Tires must
withstand heat, cold, gas, oil, chemicals and being
driven over bumps and through potholes. By varying
the construction techniques and the formulation of
tire compounds, manufacturers are able to create
tires whose characteristics vary so that the right
tire can be used for each particular application.
There is much more to be said about tires but this
should give you an idea of how amazing those tires
on your car are.
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