TIRES AND THE MARKETING OF AMERICA

TDRAdmin4 · Jan 11, 2002

[RAW] TDReprint 
TIRES
AND THE MARKETING OF AMERICA 
(Issue 31, pages 126 29)<o

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Thought Provoking Discussions with
Automotive/Motorcycle Journalist Kevin Cameron<o

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Tire Construction<o

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All the recent
talk about mysterious tire defects suggests it?s time for some background on
how tires have developed to their present technological level, how tires are
made, and how they respond to their conditions of use.
Progress in
tires has always dealt with the twin problems of strength and temperature
management.
A tire is
basically a flexible rubber-impregnated fabric structure, given rigidity by the
tensioning of its carcass of cord fabric by inflation pressure. Applied over
this carcass is the part that rolls on the road he rubber tread. The
flexibility of the tire allows it to lay down a flat footprint on the road,
large enough to generate useful traction.
The earliest
tire carcasses were made of cotton fabric very much like heavy canvas, with
interwoven fibers. Rubber didn?t stick to this fabric very well, and the
weakness of cotton required many plies of fabric to make an adequately strong
tire.
Rubber is
elastic, but not perfectly so. When you stretch or otherwise deform a piece of
rubber with 100 units of energy, then release it, it returns to its original
shape, giving back not 100 units of energy, but some lesser amount ay 70
units. The rest hat other 30% of the deformation energy ppears as heat
in the rubber. Flexing rubber generates heat.
Because this is
so, as a tire rolls and the tread and carcass rubber flexes to lay a flat
contact patch on the road, heat is generated. The more rubber there is in the
tire and tread, and the faster it rolls, the more heat it generates. The lower
the inflation pressure, the bigger the flat footprint laid down on the road, and
the more sharply the rubber must flex as it enters and leaves that flat
footprint. The lower the inflation pressure, the more heat is generated as the
tire rolls. Because applied load also increases footprint size and rubber
flexure, the more load the tire carries, the more heat it generates.
Back in 1920,
pneumatic truck tires were impractical because the heat they generated in the
necessary 15 or 20 plies soon destroyed the tire?s strength. This, and the
absence of good highways, were the reasons why there was no long-distance
trucking before about 1927. In-city trucks used solid rubber tires in that
period, and these were limited by flex-driven heating to low speeds like 20 mph.
Racing cars at Indianapolis actually had their pneumatic tires catch on fire
from high-speed heating.
Interwoven tire
fabric had to be abandoned very early on because, as the tire flexed, the
interwoven fibers of the fabric sawed at each other until they broke. This
caused the adoption of so-called cord fabric, which has all its fibers going in
only one direction here are no interwoven fibers crossing them. To get
strength in all directions, these cord plies were applied at an angle to the
tire centerline ne ply angled to the right at 45 degrees, the next to the
left, and so on. Each ply was embedded in a thin skim layer of rubber, so that
when the plies became part of a tire, they were separated from each other by
this rubber, and so were unable to saw against each other.
The rubber in
these plies was ?green?, that is, uncured, and in a slightly sticky
condition. This stickiness, called tack, is what holds the parts of the tire
together during the building process. Early tires were built on a tire-shaped
metal form, on which they were cured by heat in wrapped stacks, inside
steam-heated autoclaves. Later, tires were built as flat bands on a building
drum, then given shape by being driven into a heated female tire mold by the
inflation of a ring-shaped bag. After the required number of fabric plies were
built up, the tread was applied as a long, extruded belt of rubber, carefully
applied so as to trap no air between carcass and tread, rolled into place with
rollers as the building drum rotated. The right and left edges of the cord plies
were rolled over two hoops of high-strength steel wire called the beads, in
alternating directions. In the finished tire, these beadwires provide the
tensile strength to prevent inflation pressure from forcing the tire?s edges
up and over the rim flanges.
The green
rubber contains curing agents, accelerators, and cure modifiers, so that when
the green rubber enters the hot mold at 315 degrees F, it cures to produce
rubber of the desired properties, in a reasonable length of time. Curing is a
process by which the soft, putty-like green rubber is transformed into a tough,
elastic solid. The long rubber molecules are cross-linked to each other during
curing by sulfur bonds process driven by heat. Once the tire is cured
few minutes he mold opens in clamshell fashion and the finished tire is
pulled out. It is then placed on a dummy wheel and inflated to tension its cord
fibers in the positions they will occupy in use.
To make rubber
stick better to the cotton fabric, it was first run through baths of rubber
thinned with solvents, to drive the rubber deep into the fibers. This brought a
great improvement in the strength of the tread-to-carcass bond, and in tire
integrity as a whole.
Since tires
generate heat as they roll (and more as they roll faster), at some high speed a
tire may generate enough temperature to threaten its structural integrity. Such
failures are familiar to anyone who has an interest in motor racing. The two
major types of failure are blistering and chunking.
In blistering,
oily or waxy elements of the tread rubber, added to enhance the softness and
grip of the tire, begin to boil and generate gas within the rubber. As a result,
the affected part of the tread turns to foam and swells up, causing thumping and
vibration.
In chunking,
heat deteriorates the bond between tread and carcass, allowing pieces of tread
to separate and fly off. I have seen pieces of thrown tread penetrate heavy
fiberglass seats on racing motorcycles, and we know from the recent Concorde
aircraft disaster that thrown tread (in that case moving at between 200 and 300
feet per second) can penetrate fuel tanks and destroy hydraulic and electrical
connections.
Anyone who has
driven a car has seen plenty of separated truck tire treads by the roadside.
Checking tire pressures on an 18-wheeler takes time, which is why it?s usually
done by bonking each tire with a tire iron. If it sounds like the others, it?s
assumed to be okay. Sometimes the checks aren?t done, and tires come apart.
Tires for the
fastest of all applications acing at Bonneville ave the thinnest
possible tread. This reduces heating from rubber flexure, and it relieves the
rubber-to-carcass adhesive bond of most of the centrifugal load created by the
mass of the tread. In track racing, whenever a tire shows excessive operating
temperature (as read by a thermocouple needle, carefully pushed down to the
tread/carcass interface), two remedies may be tried. First, inflation pressure
is increased to reduce flexure. Second, some of the tread thickness may be
pared, or ?skived? off the tire, to remove some of the source of the
heating.
Something needs
to be said about how rubber creates traction. By being elastic, it is able to
take a print of all the asperities on the road surface, creating a kind of
?key? between tire and road. Other, more complicated, phenomena also
contribute. Grip increases with the total surface area of rubber in actual
contact with the road, which is why tires that require the highest possible grip
in dry conditions have no tread pattern at all. They are slicks. The whole
purpose of tire tread patterns is to provide drainage pathways for water in
wet-road operation. Race tires for moist conditions have just a very few wavy
lines cut into them. So-called full rain tires have extensive drainage, and
resemble ordinary auto tire tread patterns. The more cuts and channels in a
tread, the less stiff it becomes, the more it flexes in use, and the hotter it
runs. When rain tires are used in a race, and the rain stops, the tires promptly
overheat and must be exchanged for intermediate or slick tires. The reason
Formula One racing car tires now have five grooves is a decision by the F1
governing body to reduce tire grip for race marketing reasons. The edges of tire
tread patterns do not generate traction by cutting into the road he road is
much, much harder than the tire.
At the end of
the 1920s, tire technology had advanced enough that truck tires could be built
with some chance of survival on the roads of the time. In a well-publicized PR
stunt, Goodyear filled a convoy of trucks with tires and drove them across the
whole US, incidentally using up all the tires in the process. The point was
made; tires were ready for long-distance truck service.
During and
after WW II, cotton as a carcass material was abandoned for the much stronger
nylon, pioneered in aircraft tires. There were problems in making rubber stick
to the new material, but these were overcome. Because of the strength of nylon,
fewer plies were needed to achieve a given strength, so tire casings became
thinner. Less rubber flexing meant less heat generated, so tread wore more
slowly. That, in turn, allowed use of thinner tread for equal mileage, leading
to less heating, and so on, in a cycle of improvement that continues to this
day. Other types of tire carcass fiber have replaced nylon ayon, polyester,
steel, and aramid. The constant improvement in the strength of tire fibers has
allowed a steady decrease in the number of plies necessary to achieve mechanical
strength. This, in turn, has reduced heat generation, making tires in general
much safer and longer-lasting.
The crowning
achievement of tire technology is the radial-ply tire, which requires only one
carcass ply, and therefore operates with the least heat generation. Radial tires
for heavy trucks were viewed with suspicion by operators when they were
introduced in the early 1970s, but the outstanding durability and long life of
these tires soon made believers of them.
Bias-ply tires
are built as I described above y laying on plies with their fibers at an
angle to the tire centerline, the first angled one way, the next one the other
way, and so on. In a radial-ply tire, the single carcass ply is applied with its
fibers at right angles to the tire centerline, so that in the finished tire,
these fibers run up the sidewalls in a radial direction, then straight across
the tread region at right angles to centerline. The radial tire was invented by
Michelin in about 1948, and has since been improved by many kinds of
modifications such as various types of under-tread stiffening belts and sidewall
stiffness modifiers. The radial tire was made possible by the development of
cord fabrics strong enough to make the concept workable.
In a sense, the
development of the radial tire was nearly suicide for the tire industry. Where
bias-ply tires had lasted 15,000-20,000 miles, radials immediately more than
doubled this. This made Akron, Ohio, formerly the tire capital of the world,
into a ghost town of empty brick manufacturing buildings.
<h1 align="center">Marketing Versus
Physics</h1>
We live in the
commercial world that some call ?The Megastore.? Marketing, image, and brand
recognition are everything, and quality is, at best, a secondary issue. We no
longer buy cars and trucks for value. We buy adventure, a rugged image, an
American icon. Vehicles marketed as part of the rugged, manly off-road
experience therefore must have tread patterns that suggest all those pioneer
virtues. These are invariably described in the marketing blurb as ?aggressive
tread design.? What this means is that they are rough, knobby-looking affairs,
with deep canyons cut between ranks of tall, sculptured, Gibraltar-like tread
blocks. You can be sure that plenty of focus-group time is consumed in
determining just what kind of tread pattern will light the public?s fire this
year. Never mind the fact that the only off-road mud likely ever to spatter
these SUVs comes from a spray can bearing the vehicle manufacturer?s accessory
part number.
Now here?s
the problem. By jacking the tire up on all these hundreds of little rubber feet,
by applying this thick, sculptured layer of tractor-styled tread rubber, the
tire designer is building a stove into his tire. Remember, the more rubber there
is in the tire, the more heat it will generate. The tire engineer knows all
these things better than I do, but as noted above, marketing is pretty important
in the Megastore.
The vehicle
manual tells us to check tire pressure monthly, and to increase tire pressure
when carrying heavy loads. It also provides speed warnings or even limits. But
in one review I know of, forty-percent of enthusiast vehicles checked at a
touring rally were found to have one or more tires underinflated by 5 psi or
more. The combination of tires burdened with excess heat generated by flexing,
thick ?aggressive? tread patterns, plus possible extra heat resulting from
underinflation, plus heat from operation in the American west and southwest,
appears to result in instances of tread separation. Tractor tires were never
intended for high-speed operation, but marketing found a special use for them.
In the press,
these tread separations are spoken of as if they were caused by some mysterious
agency, a ?sinister force? yet to be discovered. Nothing whatever is said of
the possible physical circumstances of underinflation, operation in hot climates
or at high speeds and loads, or the fact that the thicker tread is made, the
hotter the tire must operate. To the press, it?s all a mystery.
Could the
vehicles themselves somehow increase the probability of tire failure? This
question has to be asked because, in the game of corporate responsibility,
everyone sues everyone else.  Remember
the big rollover scandals that panicked SUV owners so recently? On the basis of
what she?d read in Consumer Reports, my sister went out and bought the
Range Rover, because it passed whatever rollover test CR used. My bet was
that Rover wisely fitted tires with harder, less grippy tread rubber, or
deliberately underinflated the tires, thereby reducing their cornering stiffness
enough to make the vehicles skid before they would roll over. Problem solved.
Many people are
confused about the effect of tire pressure on tire grip. When stuck in sand or
mud, it is useful to reduce tire pressure, thereby increasing the area of the
tire footprint and making the tire less likely to dig itself in. This makes it
easy to assume that lower pressure always equals more traction. On pavement, the
reverse is true. In this case, reduced inflation makes the tire casing less
stiff, allowing the footprint to distort and lift up from the pavement. This
causes reduced tire grip. Those of you old enough to remember the Corvair
handling controversy may also recall what was done to ?fix? it. The swing
axle rear suspension could, under certain circumstances, jack up and destroy
rear tire grip, causing the car to oversteer violently and spin out of control.
The answer? Chevy reduced the grip at the front by the simple expedient of
placarding front tire inflation at an amazingly low 12 psi.
It?s a law of
physics, not a mystery, that if you build a vehicle with a given track (lateral
distance between wheels), but with its center of mass raised high enough off the
ground, it will tip over before it begins to slide. The focus groups tell the
manufacturers how high the vehicles have to be to look ?Baja-rugged? and
adequately manly, and that?s how tall they make them. There are no two ways
about it f you make vehicles taller, they tip over more easily.
Perhaps, as
some are saying, one or another of the SUV makers did write reduced inflation
pressures into their owner?s manuals, in the interest of avoiding the already
prickly rollover problem. Then the question was, will the tires give adequate
reliability at that pressure? The tire maker?s statistics probably looked
pretty good. Nothing?s perfect here are bound to be a few defects because
even fully-automated manufacturing cannot produce zero defects. Because tires
have to be heat-cured from their surfaces inward, the degree of cure decreases
with depth, and surely some zones in some tires will be to a degree undercured,
others slightly overcured. When plies, breakers, and tread are applied during
the build process, some air or even moisture may possibly be trapped between,
forming nuclei around which trouble becomes a bit more likely. This means there
will be some statistical scatter in the tolerance of a population of tires for
load, speed, temperature, and accidental underinflation. It is the job of
quality control to squeeze that scatter to an acceptable width.
The most
vulnerable tires at the edge of that scatter will not all belong to people who
travel loaded, at 90 mph, through Death Valley in summertime, underinflated for
conditions ut some will. And when those great thick treads get cooked off
of the tires and thrash around inside the wheel wells at a hundred feet per
second, some may damage steering linkage, and the sudden thumping and banging
are going to badly spook their drivers. Some will coast, shaken, to a safe stop.
Others will apply the universal remedy and jam on the brakes, compounding their
problems by locking the wheels and so losing control. Some will actually be
injured or killed, and we?re all sorry about it.
It may be that
there are more defects in a population of the subject tires than in some other
tire population, but the public debate on this business is not likely to give us
that information. Therefore, we won?t really learn anything useful. I suspect
that all tire makers try to achieve similar, industry-wide standards of tire
quality scatter, but now it?s the job of the courts and the teams of lawyers
to find out if this is indeed so in this case.
When the
Concorde supersonic transport had its Washington/Dulles tire incident in 1979,
fragments of a separating tire tread penetrated the aircraft?s fuel tanks in
more than ten places during take-off, but fortunately there was no fire that
time. Once a perceptive passenger alerted the flight crew to the existence of a
3 X 4 foot hole in the top of the wing, the machine was turned around and landed
safely. The important thing about this incident was what was changed because of
it, some of which is as follows;
(1) Air France
switched to another maker of tires
(2) Inflation
pressure was raised from 187 to 220 psi, in the interest of reduced flex and
heating (each tire carries 50,000 pounds of load at take-off)
(3) Much more
frequent checks of tire pressure before takeoff were mandated
(4) Strain
gauges were added to the main wheel trucks to detect and provide cockpit warning
of asymmetric strain resulting from a deflating tire
(5) In any case
in which wheel brake temperature had risen above a set level, the entire
assembly was to be stripped and inspected
(6) Pilots were
ordered to limit taxiing prior to takeoff (even rolling at low speed at full
take-off weight generates a lot of heat, and constant use of the brakes to
control taxiing speed generates even more)
Most of what we
can learn from this is obvious reads separate because heat destroys their
bond to the tire casing. The lower the tire pressure, and the more weight being
carried, the more heat is generated. Frequent tire pressure checks are necessary
to prevent accidental underinflation. Other possible sources of tire heating
must be controlled.
On a commercial
aircraft, all these safety matters are handled by those professionals who carry
that responsibility. Even with the exercise of great care, accidents are still
possible.
In the case of
privately-owned automobiles, matters like tire inflation, vehicle load and
speed, highway and ambient temperature, and the possibility of one or more
dragging brakes are the responsibility of the operator. Whatever the outcome of
the Firestone affair, any operator can greatly decrease his or her chances of
ever suffering a tire tread separation by doing the following;
(1) 
Choosing tires appropriate to the speeds and loads contemplated
(2) 
Being aware of conditions .e. not driving at excessive speeds in
very hot weather or when carrying heavy loads
(3) 
Setting and frequently maintaining tire pressures at the values
recommended for current loads and speeds.
(4) 
Sensing the abnormal. Experienced racers running at high speed on the
Daytona banking slow down instantly when they feel the sudden build-up of
vibration that signals blistering or chunking.[/RAW]

TIRES AND THE MARKETING OF AMERICA

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