If you demand total steering compliance from a tow vehicle, along with stability and maneuverability from what's being towed, the GMC Sierra Denali with Quadrasteer is just the ticket. Developed jointly by GM and Delphi Automotive Systems, Quadrasteer (a rear-wheel-steering system for trucks) is a standard feature on the '02 Sierra Denali pickup and soon to be on other full-size GM products. By using various sensors around the vehicle, a microprocessor computes the correct rear-wheel position and actuates an electric motor to turn it accordingly.
Below 45 mph, the rear wheels can steer up to 12 (left or right). At these speeds, the rear wheels turn in the direction opposite the fronts. Above 40 mph, the rear wheels turn in the same direction as the fronts to significantly limit the pivoting a trailer feels at the hitch. Forty-five mph is a neutral zone, where the rear wheels maintain a fairly straight-ahead position even when the front wheels are turned. When in the 4WS Tow Mode (recommended for towing), the speed of the neutral zone is reduced by the microprocessor to 25 mph. Visually, rear-wheel turn appears modest, but the amount they do turn yields dramatic performance dividends. The Sierra Denali, with its Vortec 6000 V-8 and strong all-wheel-drive system, is capable of towing up to 10,000 lb.
Hitching the Sierra Denali to a Holiday Rambler Alumascape travel trailer, we set out to test the setup. With the Alumascape having a dry weight of just under 5000 lb and a hitch weight of about 500 lb, we augmented our tow setup with a load distribution hitch and a sway-control device. We also added an electronic brake controller that actuates the trailer's electric brakes when the brake pedal is applied. Fully prepared, we headed up California Highway 74, which begins at sea level and climbs and winds a few thousand feet up before plunging down the backside of some rough desert mountains to Lake Elsinore below.
Developed by Delphi Automotive...
Developed by Delphi Automotive Systems, the rear-wheel-steer application uses a heavy-duty Dana axle with a dedicated computer controller under the bed, protected by the spare tire. The system has a heavy-duty spring-loaded fail-safe preset in case power is lost, pushing both wheels into straight alignment. (Below) The advantages of the system are easily seen when compared to other vehicles.
The trailer tracked well, exhibiting minimum trailer sway, and, with the exception of being visible in the side mirrors, for all intents and purposes its presence was transparent to us. Powering through mountain esses and tight hairpin turns, this twosome exhibited good synergy. Returning via the Interstate, we found that transitioning from one lane to another at freeway speeds was noticeably quicker and smoother, yielding less trailer sway than when we duplicated these same maneuvers using the 2WS Mode.
Standard features of the Sierra Denali include a Vortec 6000 that develops 325 hp and 370 lb-ft of torque. Standard is a full-time all-wheel-drive system that employs a viscous coupling center differential in the transfer case that senses slippage front and rear and provides torque as needed, where it's needed. All by itself, this improves towing performance, especially under slick-surface conditions. The heavy-duty Dana 60 rear axle features a 4.10:1 ratio and a 9.8-in. ring gear (standard for the 1/2-ton Sierra is an 8.6-in. gear). The transmission is GM's newly revised and strengthened four-speed automatic 4L65E.