Last month, we talked about some of the background issues regarding the Firestone tire situation. We know that certain Firestone Wilderness tires, which were fitted primarily to Ford Explorers over the past 10 years, have been experiencing tread separations at a higher than normal rate. As this is written in late October, no one yet knows why these Firestones have been failing.

For those of us who have experienced tire failures, however, there is an even more interesting question. Why are so many of these Explorers crashing after they sustain one of these tread separations?

It's interesting because during our countless miles on public roads, test tracks, and racetracks, many of us on the staff have also experienced tire failures. We've also suffered broken wheels and had complete tire-and-wheel assemblies fall off. These incidents have occurred in a wide variety of vehicles, and occasionally at very high speeds, but we've never had any trouble maintaining control.

Control is the key here because the fatal accidents related to Firestone tires mounted on Explores have, in the vast majority, involved rollovers. And the overwhelming majority of these rollovers occurred after the vehicle left the pavement. If you maintain control of your vehicle, however, why would you leave the pavement? In our numerous tire and wheel failures, our vehicles have never left the pavement.

Is there something about a Ford Explorer that somehow renders it unstable during a tread separation? We resolved to conduct a test to find out.

Our first step was to procure a suitable test mule. We found a '94 five-door, four-wheel-drive Explorer XLT at C&S Auto Sales, a tiny used-car lot in Redford, Michigan, about 30 miles from our office. The ad said the XLT had 125,000 miles on it. The odometer read 37,273. Whatever the true mileage, it was clearly a well-used Explorer. But it had shiny blue paint, drove down the road reasonably well, had four brand-new Goodyear XL/T tires, and was listed for just $3800. We didn't even haggle.

We then had a roll cage and competition seatbelts installed at Big Sky Motorsports near Detroit Metro airport. Then we obtained a test rig that would allow us to dump, virtually instantaneously, the air from one of the Explorer's tires at our command.

Causing a tread separation would have been a more realistic simulation, but we couldn't figure out how to make the tread peel off a tire at the instant of our choosing. Besides, the accident reports have confirmed that in many of these tread-separation incidents, the tires have remained inflated even after the treads departed.

Clearly, the remaining rubber donut is less durable than an intact tire, but such an inflated carcass means that the vehicle remains almost perfectly balanced and level. We felt that by fully deflating a tire, which would allow the corner on which it was mounted to sag by about six inches, we would produce a condition that was far more destabilizing than just a tread separation.

So we fitted the rig to our test Explorer at the left-rear wheel position (statistically, the most likely position for the reported separations). It starts with a modified Explorer wheel with three one-inch holes in the middle of the rim leading to three short lengths of welded-on pipe. Using short lengths of rubber hose, we connected these pipes to a diaphragm valve that protruded from the wheel. On the outside of this valve was a swivel fitting that connected to an air hose from the Explorer's interior.

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