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, e v e r y o n e s u e s e v e r y o n e e l s e . 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
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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 heatcured 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.