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GMT-900 Hits The Road

“Gas is so expensive SUV now stands for Sport Utility Victim.”—JAY LENO.
#Ford #Porsche #Nissan


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While Leno and his ilk are hamming it up about the rising cost of fuel, the folks at General Motors may not find comments like these humorous, especially since they are in the midst of the most aggressive truck and SUV launch in recent history. Last month, the world’s largest auto maker introduced the world to its new GMT-900 SUV family—Chevrolet Tahoe, GMC Yukon (Denali) and Cadillac Escalade—and while initial reaction has been going well (GM says more than 24,000 dealer orders have already been booked for the Chevrolet Tahoe alone), there’s a fair amount of concern the timing may be a bit off. The worries over fuel prices and a potential fallout in SUV sales are the underlying factors behind GM’s decision to forgo touting the GMT-900’s towing capability or massive cargo volume and concentrate on boasting fuel economy which, due to “Active Fuel Management” (which used to be known as “Displacement on Demand” until GM’s marketing department trademarked the new name), improves fuel economy by up to 7%, or 2 mpg, by switching between eight- and four-cylinder operation depending on load requirements (the Chevrolet Tahoe is rated at 16 mpg city/22 mpg highway in two-wheel-drive configuration and 15 mpg city/21 mpg highway in four-wheel-drive), besting the ’06 Nissan Armada, Toyota Sequoia and Ford Expedition and moving the Tahoe on par with the significantly smaller Kia Sorrento.



What’s most surprising about the fuel economy gains is the fact that engineers managed to meet the initial 20 mpg target while utilizing GM’s venerable small block V-8 (the architecture of which debuted in 1955), albeit in a revamped Gen IV version (this version debuted in late ’05). The Chevrolet Tahoe, which is the first of the trucks to be made available and which will be the volume leader among them, will feature the 5.3-liter version of the engine as standard equipment on all drivetrain configurations at launch. Later in ’06, it will be standard only on four-wheel drive models and optional on the two-wheel drive models; a 4.8-liter variant will be the standard for the two-wheel drive trucks. The 5.3-liter engine produces 320 hp (an 8% improvement compared with the Gen III version) and 340 lb.-ft. of torque, while the 4.8 produces 290 horses. One important thing to note here: Active Fuel Management is not available on the 4.8-liter engine. To accommodate the shift between four- and eight-cylinder operation on the 5.3-liter, engineers developed a stronger engine cradle and new hydraulic mounting system to help reduce vibrations transmitted through the chassis and into the passenger cabin. Other changes to the engine family include special friction coatings on the piston skirts and full-floating piston pins, as well as an acoustically-tuned engine cover and quieter alternator. The small-blocks are mated to an archaic Hydra-Matic 4L60-E electronically controlled four-speed automatic transmission. GM decided to launch the Tahoe with the four-speed and leave the highly-anticipated 6-speed transmission for its more luxurious stable-mates (the Yukon and Escalade), which seems like a very bad decision since the six-speed will undoubtedly provide even better fuel economy and comfort. Not to mention that Toyota and Nissan already have a five-speed transmission in their SUVs, as does GM’s cross-town rival, Chrysler in its Dodge Durango. “A six-speed will be a nice thing to do but it will not be a glaring omission in terms of performance and capabilities of the truck” says Gary White, GM North America vice president and vehicle line executive for full-size trucks.



While the powertrain configuration may leave some doubt, GM engineers have to be given credit for developing a chassis that is world-class. Utilizing the same hydroforming technology found on the GMT-800, the new foundation receives a 35% increase in bending frequency, while overall torsional stiffness jumps 50%. Front track has been increased 3.2 in. to 68.2 in. and rear track increased nearly 2 in. to 67 in., while overall width of the chassis has been increased by 3 in., all in an aim to give GM’s SUVs improved stability over their predecessors. Along with the new chassis, GMT-900 SUVs receive a revamped front and rear suspension, with a new coil-over-shock configuration (replacing the aged torsion bar) in the front and a five-link in the rear. “We needed to package a coil-over-shock in the front to get the weight down,” says Terry Woychowski, chief engineer for the GMT-900 program. “We also have a cast-forged aluminum lower control arm, which helps take an additional 10 lb. of unsprung weight out of each front corner.” The rear suspension is mostly carried over from the previous iteration, although engineers revamped the frame mounts and installed variable-rate spring and monotube shock absorbers while boosting payload capacity. The Tahoe will be available with three different suspension settings (ZW7 premium ride, Z55 and Z71 off-road), with unique bushing, spring and material characteristics. To improve steering performance, Woychowski’s team threw out the old reciprocating ball system, replacing it with a modern rack-and-pinion steering unit, which is now mounted directly onto the engine cross member, reducing vibration. To round out the key ride and handling changes, the GMT-900 features larger, four-wheel disc brakes with 17-in. rotors and revised calipers that are 50% stiffer than the previous generation. “We made another change to the mounting of the master brake cylinder, which is now mounted to the front of the dash,” says Woychowski. “Typically, the cylinder mounts to the front of the vacuum reservoir, which is then mounted to the front of the dash, which means as you apply the brakes, not only are you pushing the piston through the master cylinder, you are also bending that cam. Now, all the work you do as you push the pedal is going directly to the master cylinder.”



Improving overall quality of GM’s trucks was a key goal for the GMT-900 development team, which spent a fair amount of time focused on both perceptual and real-build quality. “We wanted to give a feel of precision on both the inside and outside of the vehicle. We did things like reduce the body gaps on the outside to 4 mm, where it was 30 mm on some areas before. We also did things like introduce a faster windshield and incorporated stylized running boards and took the lip off the fuel door and moved the antenna from its previous mast design and incorporated it into the windshield,” says White. “It all worked together to reduce overall drag down to 0.36, similar to where Porsches and Corvettes were several years ago. We also wanted to do a lot of shrink wrapping with the sheet metal to give the vehicle a more tailored look, which also helps when it comes to perceived quality.” The team used the Toyota Sequoia and Nissan Titan as benchmarks in the ride-and-handling department, while interior fit-and-finish bogeys were set by the Acura MDX for the Tahoe and the Audi Q7 and Volks-wagen Touareg for the GMC Denali and Cadillac Escalade. The attention to detail can be seen just about everywhere in the ’07 Tahoe, as the exterior design looks a bit slimmer than the previous model, while the interior is leaps and bounds ahead of the outgoing models. The only problem is will these advancements be enough for GM to fend off Toyota’s next-generation of pickups and SUVs? Engineer Woychowski says he is eager to see what Toyota has up its sleeve with the next Tundra, but he is equally confident that GM’s pickups will “surprise and delight” industry watchers and consumers when they arrive later this year. The GMT-900 team has scored major points for the improvement in ride-and-handling for the 900, especially when it comes to steering feel. There’s no need to constantly adjust the wheel, just point and the Tahoe goes where you want it. GM needs to get rid of the aged four-speed automatic quickly, while the small-block V-8 also needs some additional tweaking to keep the noise levels down.



GM is producing the GMC Yukon, Cadillac Escalade and GMC Yukon Denali at its Arlington, TX, and Janesville, WI, plants; the Chevrolet Suburban and GMC Yukon XL are also in Janesville. White says GM will begin production of the Chevrolet Avalanche and Cadillac Escalade EXT, along with heavy-duty pickups, in April, and production of the full-size Chevrolet Silverado and GMC Sierra pickups will begin in October. “We’re going to meet the new launch targets set last summer, which called for six weeks pull ahead for the SUVs and 13 weeks for the pickups. Our total development schedule will be 42 months for this platform,” says White. By the way, Bob Lutz cannot take any credit for the design of these trucks, as White says the program was “pretty well lock-and-loaded” before Lutz landed at the General.