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Caterpillar 789D Mining Truck: Test Drive

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Is driving one of the world's largest mining trucks on your personal bucket list? If it's not, perhaps it should be.

Standing as tall as a two-story house, the Caterpillar 789D is a touch larger than the average pickup truck. Armed with a quad-turbocharged diesel engine that cranks out 2,100 horsepower and 6,400 pounds-feet of torque at a lazy 1,800 rpm peak, it's a bit more powerful, too.

The mining hauler may not be the largest dump truck on the planet, but it is sizable with 200 tons of payload capacity available in the rear-dump bed (the largest trucks can haul double that). Mining companies regularly use these trucks to haul copper ore, coal and other commodities on round-the-clock schedules. Keeping these multimillion-dollar rigs moving is critical: If they aren't going down a haul road, loading up or dumping, they aren't making any money.

When fully loaded, the Caterpillar 789D weighs up to 715,000 pounds. Put in perspective, it would take more than 165 Ford F-150 4×4 pickups to equal the same mass as a 789D hauling rock. Standing next to this giant, the king of pickup trucks looks like a mere plaything.

The mining brute is smart, too. The 789D packs a wide range of technology like the F-150 — and in some areas offers more intelligence, as we recently discovered during a test drive at Caterpillar's 6,500-acre Tinaja Hills Demonstration & Learning Center near Tucson, Ariz. Example: A camera mounted on the dashboard watched our facial expressions and eyelids — through our sunglasses — and reminded us on more than one occasion to keep our eyes on the road.

Before each shift begins at a mine, a haul-truck operator typically performs a brief walk-around inspection, which Brad Cook, a Caterpillar instructor and veteran mine-equipment operator, guided us through. He pointed out that tires are one of the most expensive maintenance items on these machines, so good treads are critical. This rig had six 57-inch rims holding 12-foot-tall Michelins — two in the front and four in the back — each costing upward of $50,000 to replace.

We climbed up a tall flight of stairs crossing in front of a giant radiator core that cools the truck's howling V-16 diesel engine — all 78 liters of it. Standing on the canopy, where the operator's cabin sits, we got a good look at Caterpillar's 2.25-mile off-road course. Cook pushed a button at the top of the railing, and a motor retracted a section of stairs so we could get on with our desert run.

Taking note that the cab's doors were hinged at the rear, we ducked inside, buckled our seat belt and shut the door. The roar of the diesel subsided into faint background noise that enabled us to have a normal conversation.

"It's just like driving a pickup," Cook said from the passenger seat. He instructed us to hit the horn, "in case somebody snuck up on us," and gestured toward the parking-brake release.

Mining trucks have several ways of stopping, which is a good thing when you are hauling 200 tons of earth. Components within the oil-cooled, multiple-disc brakes work together to provide parking, main, secondary and retarder brake capability at each wheel.

A traditional brake pedal next to the accelerator activates the main service brakes, while a third pedal — which is not a clutch — hits the secondary brakes. A lever to the right of the steering wheel allows drivers to retard speed more precisely. We found this handle useful in slowing down the truck predictably under normal driving. The engine also had an automatic retarder that kicked in during downhill runs; this feature is intended to save brake life.

Cat's haul truck uses a six-speed powershift transmission that allows drivers to preselect their top gear before pulling away. We started out with the preselector in 5th gear, then threw it into 6th as we approached the 789D's top speed of about 35 mph. Each gear change was as crisp as a well-tuned muscle car's. Some mining trucks have electric motors at each wheel, but this truck is an example of an all-mechanical monster, with double-reduction planetary gear sets driving power to the ground.

In terms of engine speed, 2,000 rpm was the end of the world. When really hustling, we averaged between 1,600 and 1,800 rpm during our lap around the desert. Did we mention that this truck had an optional 1,000-gallon fuel tank? The standard tank is 550 gallons. That's how much diesel you'd burn during a typical 12-hour shift. Under normal conditions, you'd be in good shape if you were getting 0.3 mpg.

Blind spots were everywhere. Camera screens provided a 360-degree view around the truck, and proximity sensors alerted us to nearby objects. Cook reminded us to keep to the left of the road, "just like driving in Australia," he said. Because it is such a long way to the other side of the truck — the 789D is more than 25 feet wide — judging distances is easier near the cab, he noted. Many mines run on the left-hand side of the road so truckers are less likely to hit each other.

Our test track felt like a mine, with rocky outcroppings and precipitous drops at the road's edge. Steering was surprisingly sensitive. Wrenching on the wheel too much at first, it felt as though the Earth moved. U-turns took a bit more space than a long-bed pickup: Caterpillar boasts that this truck has a tight turning circle of more than 90 feet, which felt to us like turning on a dime. Perspective changes when you are driving a giant truck.

"The novelty does wear off," Cook said. photos by Tudor Van Hampton



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