Tire Construction
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—the 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 – say 70 units. The rest—that other 30% of the deformation energy—appears 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—there are no interwoven fibers crossing them. To get strength in all directions, these cord plies were applied at an angle to the tire centerline—one 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—a process driven by heat. Once the tire is cured—a few minutes—the 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— racing at Bonneville—have 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 – the 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 – rayon, 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—by 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.
Marketing Versus Physics
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—if 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—there 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—but 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 × 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—treads 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:
• Choosing tires appropriate to the speeds and loads contemplated
• Being aware of conditions—i.e. not driving at excessive speeds in very hot weather or when carrying heavy loads
• Setting and frequently maintaining tire pressures at the values recommended for current loads and speeds.
• 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.
By Kevin Cameron – from TDR Issue #31.
ABOUT KEVIN CAMERON:
From the Motorsports Hall of Fame website: “Cameron is one of motorcyle racing’s foremost authors and historians. He is renowned for his ability to convey technical information understandably and with a dry sense of humor.” A graduate of Harvard, he studied physics. He went on to become a motorcyle racer, tuner, journalist, and author. Kevin’s technical knowledge and passion for motorsports and mechanics has grown and expanded into multiple automotive disciplines. In 1998, the editor of the TDR asked him to author a regular column in the Turbo Diesel Register magazine and so he has since then, in the aptly named, Exhaust Note column.
This article was first published in Turbo Diesel Register Issue 31.
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