J. MARTIN KOHLER

For the trailering enthusiast, when it comes to stability, maneuverability and over the road handing, "every little bit helps." Be it high speed lane changes, navigating around corners or backing up and positioning a towable where it’s needed, those who tow know that when it comes to control, more is definitely better. And now for the first time ever or anywhere, a full-size truck is being offered by GMC that utilizes an onboard computer to control and direct the articulation (the turning left and right) of the rear wheels. The objective is to facilitate and enhance steering performance that until now has been solely a function of the front wheels. Though the visual differences (watching the rear wheels turn) appear to be modest, the magnitude that the wheels do turn (up to 12-degrees) yields dramatic performance dividends.

Right out of the gate, a couple of things need to be mentioned. In addition to the state-of-the-art electromechanical Quadrasteer system which was a joint development effort between GM and Delphi Automotive Systems (the first time ever to be used in a full size truck), the Sierra Denali presents a host of features that makes the Sierra Denali both a "drop dead" gorgeous vehicle and one of the most useful trucks on the road today. Case in point, the Sierra Denali is the first-ever full size pickup with full-time, all-wheel drive. Through GM’s 4L65E heavy duty automatic transmission and a viscous-coupled transfer case, the Denali senses slippage vs. traction (front and rear axles) and responds by delivering torque to the front or rear wheels as required to maintain the best traction possible. Imagine the importance of this feature when roads are wet and slippery and especially when the roads are wet and slippery and you’re towing. "This is good-stuff."

Also new with this truck is the 6.0L Vortec V-8 that yields 325 horsepower and develops 370 lbs.-ft. of torque. For improved cooling efficiency, lightweight aluminum cylinder heads are featured. And for top performance, GMC employs coil-near plug ignition. The Dana 60 Series rear axle features a 4.10 axle ratio and a 9.75 inch ring gear (standard for the Sierra 1500 is an 8.6 inch gear). Along with this and the Z82 standard heavy duty-trailering package, the Sierra Denali comes with a towing capability of up to 10,000 lbs. GMC’s tires of choice (the Michelin P265/70R17) are mounted on 17-inch polished aluminum wheels.

OK, "so how does the Quadrasteer System work? Through a central controller, several pieces of data are received and processed. First, the controller determines if the Sierra Denali is in 2WS, 4WS or 4WS Tow?" The driver has already decided this and has made their selection by the touch of a button on the dash. A selection light on the panel indicates what steering mode has been selected.

The onboard computer continuously receives data telling it what degree of turn (left or right) the front wheels are experiencing. From the trucks speed sensor, data is continuously fed to the controller about truck speed. If nothing is being towed, 4WS is the selection GMC recommends. With this selection, if the vehicle is traveling at speeds less than 45 mph, the rear wheels turn in the opposite direction to the front wheels. At speeds above 45 mph, the rear wheels turn in the same direction as the front wheels. Forty-five mph is considered the neutral zone where the rear wheels will remain straight when the front wheels turn in either direction. All of this geometry has been calculated by GM/Delphi and designed into the Quadrasteer System.

When the truck is in the 4WS Tow mode (GMC recommends this for trailering), the same turning relationship exists between the front and rear wheels. However, the maximum amount the rear wheels turn in response to the front wheels and the speed of the truck, is 9 degrees and not the 12-degrees as programmed in the 4WS mode. Also in the 4WS Tow mode, the controller reduces the neutral point from 45 mph to 25 mph. With the rear wheels turning in the same direction as the front wheels (at speeds above the neutral point), the improved performance can be characterizes as being much like having a steering axle on what ever is being trailered. For example, when changing lanes or passing on the freeway, the trailer doesn’t follow the truck, it moves with it, as if the two were one. At speeds below the neutral point (25 mph), no longer does the driver need to make the dramatic wide swings to clear a curb when turning a corner. And when backing the trailer, the high degree of maneuvering previously required to position the trailer where it is needed is reduced.

Unbeknownst to the driver (and on a real time basis) a microprocessor is making all the calculations and decisions about how aggressive the rear wheels need to steer and in what direction they need to turn. Based upon this computation, a signal is sent to an electric motor that drives the rear steering rack, via a planetary gearset. One caveat GMC offers is, "To prevent damage when changing a tire, never put a jack under any of the rear wheel steering gear."

FM and Delphi have designed and built sophisticated safeguards into the Quadrasteer System. Should any of the sensors, switches or the microprocessor malfunction, the Quadrasteer System by design automatically defaults back to 2WS Mode. When it comes to selecting the steering mode, GMC suggests 4WS when not towing and 4WS Tow when towing. They say, "Set it and forget it." However, at anytime while operating the Sierra Denali, the steering mode can be changed from one to another mode (even on the fly). However, if the steering mode is changed, the new selection will not take affect until all four wheels return to the straight-ahead position. The only time GMC recommends using 2WS is if (for example) you are parked too close and parallel to a wall. The 2WS Mode lets you gradually pull away from the wall, thereby preventing the rear of the truck or what ever you are trailering from swinging out and clipping the stationary object.

One of the significant performance indicators that demonstrates how dramatic turning performance is improved with Quadrasteer System, is the fact that when the Sierra Denali is changed from 2WS to 4WS, the turning diameter is reduced by 21 percent. Yes, it drops from 46.2 ft. down to 37.4 ft. The turning diameter of the Sierra Denali is just shy of being equivalent to that of a Saturn Coupe. The one caveat we would offer is that when trailering, if a sway control device is installed on the tongue of the trailer, making a turn this tight might possibly damage the sway control device. You really need to check this out with the sway control manufacturer or the service center where you purchase and have the device installed. Just a "heads-up."

We would be remiss if the subject of exterior styling and interior luxury appointments were not addressed. Regarding the first item, GMC has incorporated roof and box markers (exterior amber lights) that give the Sierra Denali a tough, brawny demeanor. Because of the added steering gear and size of the Dana Series 60 Axle, the rear wheels have a five-inch wider track than the standard 1500 Sierra. Because of this and because additional space is required so the rear wheels can turn, GMC has appointed the rear wheel well area with flared fenders. These flared fenders are a composite material that is dent and scratch resistant. They are fabricated using an injection mold process. It’s important to note that even with the new design changes, the standard dimensions of the bed box have not changed. Standard are chrome tubular assist steps and the grill has its own unique styling.

Interior appointments include leather surfaces (seats), power lumbar supports in the front seats, heated front seat cushions and back rests and power reclining seatbacks. The audiophile will appreciate the console mounted 6-CD changer and the 8-speaker system with rear seat audio controls and headphone jacks. GMC has incorporated an additional audio control that is steering wheel mounted, which means the driver will never again have to take his or her eyes off the road when seeking a change of stations or when adjusting volume. A Driver Information Center displays operating data, offering instant information on several operating parameters, including current miles-per-gallon and up to 15-days of operating history.

For us, after driving the 1500 Sierra Denali around the block, we felt we were "good to go." And during the several hundred miles of driving and towing (that encompassed a broad spectrum of conditions - winding mountain roads, city traffic, freeway speeds and backing up), we were pleased and completely at ease with how the Sierra handled. The Sierra Denali represents the next level in full-size trucks and for those who tow, this is one you will want to look at. Check it out.

J Martin Kohler is a freelance writer based in northern, California, and author of several Lifestyles and Holidays features.