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First Drive Dodge Ram T-Rex: Real-world 6x6 with A 26,000-Pound GCWR

Ram 6x6 Front Three Quarter
Mark Williams | Writer
Posted July 1, 1997
Photographers: Greg Grasmehr

Prehistoric Future

Sometimes, to see the future, you have to look into the past. And when the concept team at Chrysler was given the go-ahead for a 6x6 full-size Dodge Ram, they went all the way back to the Jurassic Age . . . to create "T-Rex'' (Technology Research Experimental vehicle). "We wanted to make something no one could turn away from," said Leon Dong, manager of Advanced Chassis & Drivetrain for Dodge Truck. "But we also wanted to make something to exist in the real world-something that could eventually get made."

That may sound strange, but given the number of freaky concept vehicles at the different auto shows, this particular attitude is refreshing. Leon and the team were given the problem in August 1995, and told they needed to have the working solution in 14 months. Needless to say, they scrambled.

Tall And Mean
The T-Rex is founded on a 133-inch regular-cab 1-ton chassis with the front half of the frame being identical to a standard Ram; however, the back half of the frame is fully boxed and dead straight (no bends to allow for dual rear wheels are necessary). With three working axles, the chassis design took some thought.

Two huge crossmembers under the 8.5-foot bed support the air suspension's air bags, one at each wheel. The six Firestone-designed air springs were used by Delphi Chassis (formerly a division of General Motors, now an independent contractor) to suspend the full-full-size truck weight. The springs are damped by six dual-rate adjustable shocks, which control the front and dual rear Dana 60 motions. Delphi designed the computer and electronics for the suspension with individual sensors on both air bags and shocks to provide input about load leveling and shock rebound speeds. Four vehicle position sensors-two near the front axle, two on the frame above the third-also keep the computer informed, while input from the brake system, speedometer, and dash-mounted manual switches are constantly monitored as well.

With all these sensors, the suspension can be adjusted to any degree preferred. A control switch adjusts shock valving between "Comfort" and "Sport," while a five-position dial allows for close to six inches of adjustable ride height: Max Off Road, Off Road, Auto Adjust, Entry, and Disable.

"Max Off Road" raises the air bags approximately 3.5 inches from the "Auto Adjust" baseline, and "Entry" deflates the bags almost to the bumpstops, lowering the air springs 2.5 inches. The system was originally designed several years ago by Delphi Chassis for Chrysler as an option for the '99 Grand Cherokee, but the project was scrapped two years ago. Our guess is even if the system doesn't make it on a full-size Ram, where the design seems ideally suited, the Grand Cherokee or the next-generation full-size Grand Cherokee will require this type of system to compete with the air-bag suspensions found on the Ford Expedition/Lincoln Navigator and Range Rovers.

We found the prototype Delphi suspension system on the T-Rex to be surprisingly responsive to ATV-sized dirt trail swells, allowing the front axle, and both rearends, to stay locked to the ground where previous leaf- and coil-spring experience had us waiting to go airborne. The system seems to react immediately to soften the compression side, while stiffening the bags on the rebound side to prevent the typical stiff-suspension full-size truck bounce. We found the system tuned to handle the lower-speed axle motions with ease, while higher speed, fast-frequency responses were more similar to stiffer-riding 3/4- and 1-ton suspensions when hauling close to a full load. That's quite an accomplishment for a truck that weighs 700 pounds more than a BR3500 standard-cab dually.

Dueling Transfer Cases
From the beginning, the idea was to create a 6x6 with tandem-driven axles; i.e., a traction monster. "We'd seen a lot of tandem-axle trucks where the third axle was dead, purely for load carry capacity," says Nick Rosenthal, the only full-time staff Project T-Rex engineer for the concept: "We wanted a way to drive both rear axles for maximum off-pavement capability." Many designs were entertained, mostly looking at heavy-duty military trucks and carriers from around the world. However, in keeping with their feet-planted-on-the-ground parameters, Nick and Leon eventually decided on off-the-shelf components. Knowing they needed something strong enough to manage the huge torque and horsepower numbers of the built V-10 (more on that to follow), a heavy-duty Hummer NVG 242 full-time transfer case (with 2.72:1 low-range) was selected and modified to allow for part-time duty as well, as in the Jeep Cherokee, Grand Cherokee and Dodge Durango four-wheel drive system. Various internal parts were heat-treated to increase their overall strength, while the chain drive itself was increased in width by two full chain links, also significantly adding to its capacity. The end result is called an NVG 244 HD. The 'case offers four separate four-wheel and six-wheel drive options, all controlled by a rotary dial on the dash, and includes a driveline-releasing Neutral position.

The first transfer-case option is called "4WD Full-Time," in which only the rear output shaft of the 244 is turning (similar to conventional part-time two-wheel drive); however, T-Rex uses a dual transfer-case setup whereby the rear driveshaft out of the first transfer case is connected straight to the second 'case (a Dana unit with a 1:1 gear ratio) that's mounted onto the first rearend pinion yoke. The chain-drive output shaft of the Dana transfer case runs over the first Dana 60 rearend to a carrier bearing that sends power through a second driveshaft to the back Dana 60 rearend. Basically, when power is sent to the rear axles, on high-traction surfaces, all four wheels will turn. This gives new meaning to the conventional idea of "full-time four-wheel drive." Confused? Think of it as a regular two-wheel (i.e., rear-wheel) drive, only with four (rear) wheels turning.

Next, the transfer case offers "6WD Full-Time," where the center differential and all axles are open, functioning in the standard high-range. In this case, power is sent to the front Dana 60 via the open center diff. Next in line is the "6WD Full-Time Locked" position, also in high-range, that mechanically locks both the center 244 differential and electro-magnetically locks the Dana transfer case-to be used only in low-traction situations. (There is a "Neutral" selector on the dial for emergency towing, along with a deactivation mode on the load-leveling air spring suspension.) And finally, the "6LO" position locks the 244 'case in low-range, locks the Dana 'case, and automatically locks up the three Torsen Dana 60 differentials for mega-maximum traction.

We liked the ease of operation of the twin dash dials to customize the suspension and transfer-case settings to any terrain or towing need. Being able to choose, and experiment with, the settings depending on the circumstances occurs to us to be a darn smart use of technology. At a time when many manufacturers are trying to make systems invisible, idiot- (as well as litigation-) proof, it's nice to see a design that puts a little faith in the abilities and skills of the driver-even if it is just a concept.

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