Carl F. Thelin

FORD MACHO PRIDE

I was stunned when attorney Jefferson told me why he would not trade his Ford Explorer for a Ford Windstar minivan. Jeff met me at the airport in Jackson, Mississippi in the first Ford Explorer I had been able to ride in. The interior was a big improvement over the basic Ford Ranger pickup truck. This vehicle, not yet called a SUV by the popular press, was just the right replacement for the station wagons being discontinued by the American big-three carmakers. Shortly after Ford introduced these things a co-worker bought a four door Explorer, but I had not been inside one yet.

While I was telling Jeff how I admired the Explorer I mentioned that the ride comfort on the roads in rural Mississippi was harsh. He agreed that it was. “If you needed all this station wagon room for your business, did you consider the Ford Windstar minivan? It has even more room, front wheel drive and a ride that is more like a passenger car.”  “Oh no, the van is a women’s car. This is more macho, and I can put up with the hard ride.”

Really? Why did he have to do that? Soon the answer came out in an automobile recall that danced around the true cause of the problem. Ford Explorers were crashing because the tires were falling apart. Litigators blamed Ford and Firestone companies. More than 200 deaths and severe injuries occurred in rollovers that occurred shortly after the tire tread came off the tire body. Ford Motor Company said the crashes were the fault of poorly made Firestone Wilderness AT 15-inch tires made specifically for the Explorer. Ford had to offer truck owners replacements of other brands of tires.

I am an engineer who began in auto chassis design and testing at General Motors. Later I worked in tire design. Later I supervised a test track for car-handling exercises. With that background, and my experience in that ride with Mr. Jefferson, I knew what was wrong. While I was a research engineer at Uniroyal, I learned something that no one at General Motors told me. In Europe, the switch to radial ply tires began long before the US car manufacturers even considered them. My experience at GM was that the car engineers told the tire engineers what characteristics they wanted of the tires for a particular model. Tire companies kept modifying the tire construction of samples until the car company accepted them.  Anti-trust commerce laws required that a car company get tires from at least three sources and limited purchases to not more than 40 percent from one tire company. That explains why US tire manufacturers made all OE tires almost exactly alike regardless of the fancy sidewall logos.

It was not the way in Europe. The tire companies made the best tires they could. Their engineers advised the car company engineers how to adjust the springs and shock absorbers to accommodate the characteristics of the tires.

The Ford Explorer chassis was fundamentally a pickup truck. The tires on a pickup truck required 35 psi inflation pressure. Initially, those were the type of tires used on the Explore. Tire life and vehicle handling were just fine, except for people like Mr. Jefferson. Eventually complaints from early buyers about the harsh ride got to the Ford engineers. To quiet the grumbling, they had to recommend using lower tire pressures – as low as 26 psi. Softer tires did make for a more comfortable, quieter ride – at speeds below 55 mph.

However, when families loaded their “tall station wagons” (Explorers) with vacation supplies, and took off on a long route over the Interstates in the summer – bad things began happening. High loading and high pavement temperatures are going to make all tires run hotter. The rear tires are channeling propulsion energy from the engine to the pavement through the tread. That adds to the build up internal temperatures. Then, low tire inflation pressure allows the sidewalls and tread to bend more when they cycle through the contact with the pavement. The part of the tire that goes through the contact area changes from round to flat and back at least 12 times per second at 65 mph – about 700 times per mile. The tire tread gets so hot that the bond between the tread rubber and the radial-ply carcass fails to hold them together. At first, just a small portion of the thick tread comes loose from the underlying steel-belted radial plies. That little bit of separation causes the surfaces to rub against each other – very rapidly, as noted above. The length and width of the separation area grows until the tread breaks free all the way across its width. Then the thick tread peels off. Sometimes it wraps around the axle. The steel radial ply carcass does not fail. It simply is not made to retain the inflation pressure without the overlaying material.

The failure of one front tire is much less hazardous than one on the rear axle of a four-wheel vehicle, especially if the truck has a solid rear axle. The driver can turn the steering wheel to change the direction of both front tires. The other front tire still has plenty of authority, we say. It is very different when a deflated rear tire causes the axle to tip down at one end. That motion usually cocks the axle at a small angle in the horizontal plane. That realignment causes the whole vehicle to self-steer from the rear end. The driver, who normally has not practiced recovery from this kind of surprise, does not quickly respond. The yawing does quickly become oversteer – when the yaw rate progressively reinforces itself. At some point it has turned the vehicle into a sideways slide. The vehicle may go off the road or dig the wheel rim into the pavement. Overturning follows.

Bridgestone is one of the world’s leading tire manufacturers. They have the typical Japanese (think Toyota) commitment to high quality design and manufacture. They took terrible public relations exposure because they are the owner of the Firestone brand.  They survived, still highly respected.

I think Ford got off easy. Maybe next time the domestic car manufacturers will learn from the Europeans and listen to the tire engineers.