tech

All-Wheel Steering

Control Under Pressure

By Bill Senefsky

All-wheel steering is neither revolutionary nor a new concept. Offered in select racing cars and high-end passenger car platforms for decades, the system has also been incorporated on large off-road trucks and specialty vehicle platforms.

Kaiser demonstrated a passenger sedan in the early '50s that used a version with the rear wheels operating for parking chores and tighter maneuvering at low speeds. Mitsubishi offered its 4WS in combination with traction control, electronically controlled 4WD, and ABS a decade ago on its high-end sports cars. In addition, Honda, Mazda, Toyota, and Nissan all had FWS at some point. What is new however, is that all-wheel-steering technology has added electronic controls and push buttons for ease of operation.

What's the Big Deal?

Four-wheel-steering systems function by including an additional steering system at the rear of the vehicle. Older versions were controlled mechanically. In both cases, when the front wheels are directionally pointed at certain angles, the rear wheels are equally adjusted to angled positions, to provide superior platform control while engaged in handling and turning maneuvers. Industrial three-wheeled vehicles traditionally have used rear-wheel steering that can turn sharply in tighter spaces. The downside of these platforms is that they lack platform stability and require extensive operator training and experience.

With four-wheel-steering systems, the rear wheels are turned in the same or opposite direction as the fronts, depending on platform speed or the angle at which the steering wheel is turned. All of this occurs the instant the steering system is used, allowing a tighter sight and turning line into the corners. At a predetermined speed, when the steering wheel is turned to the desired angle, the rears will turn in the same direction as the fronts. Over that steering angle or speed, the rears will either move to a straight-line position or turn in the opposite direction.

These modern systems are either controlled by a set of driveshafts or electronically, depending on the type of vehicle platform. Obviously, additional components are added to the underside of these platforms to monitor and control the rear steering mechanical components. Usually, an additional steering gearbox that is similar to the front unit is used to control the pre-determined rear-wheel angles. A safety device or fail-safe unit will lock the alignment of the rears in the conventional straight-ahead mode if a problem were to develop.

With regard to consumer truck and SUV platforms, Delphi Automotive Systems designed and developed the Quadrasteer setup, a four-wheel-steering system that is currently offered on select GMC and Chevrolet SUV, and fullsize pickup platforms. Since Delphi was once part of the General, it should be no surprise that GM currently has exclusive bragging and marketing rights. The big deal of course, is that this modern update allows larger platforms a shorter turning radius for tight maneuvering and better road handling manners, especially under loaded and towing conditions. Regardless of the initial sticker shock for this option, you can bet the other manufacturers have a system in the works to take advantage of the continued truck and SUV market war being waged worldwide. The Quadrasteer System

This unique Delphi system allows a 21 percent reduction in large-platform SUV turning radius. In other words, if your SUV currently makes a U-turn in 27 feet, this option will cut 5 feet off that radius. Additionally, the system is electronically based, with three buttons on a dash-mounted switch to control its three modes.

A two-wheel-steer mode allows the platform to operate traditionally. The four-wheel steer mode allows the rear wheels to operate (out-of-phase steering) until the platform reaches 45 mph, when the rears (in-phase-steer) turn in the same direction as the fronts. A four-wheel-steer tow mode operates in the same way the 4WS mode does, with the exception of the rear-wheel angle, which is set tighter to better control the rear-mounted trailer. An internal on-board computer uses four main components: a front-wheel position sensor, a steerable solid hypoid rear-axle assembly, and an electric motor-driven actuator.

Sensors located throughout the platform continually generate ongoing operational data to the controller, which automatically adjusts the amount and direction angle the rear wheels should move, and further, whether they should turn in the same direction as the fronts or move in the opposite direction. Algorithms are used to determine how much steering input should go into the rears.

Delphi has identified three principal parameters for the system, depending on platform speed. As expected, in the Negative phase, when the vehicle is moving at slow speeds, the rears turn in the opposite direction of the fronts, making the parking chores easier. On the pavement, at moderate blacktop speeds, in the Neutral phase, the rears now move in the same straight direction as the fronts, because neither affect the platform's maneuverability.

In the Positive phase, with the vehicle moving at higher rates of speed, the rear wheels turn in the same direction for easier and crisper lane changing. What comes into play here is the reduction of vehicle yaw or sway, the rotational motion noted to ccomplish a maneuver. Stable platform response during evasive maneuvers or under adverse road conditions is a definite plus. When pulling a trailer, the appreciation is immediate, and stability to both vehicles is notably enhanced. Positive rear steering lowers the articulation angle between vehicle and trailer, reducing the lateral forces pressed on the rear of the tow vehicle.

Your SUV can now reduce yaw velocity gain and increase yaw damping of the trailer and tow vehicle. Increased trailering stability, combined with reduced sway and lower corrective steering maneuvers, tackles the external annoyances such as wind gusts, curvy on-ramps and cloverleafs, and lane-change maneuvers without the caterpillar sway-motion. Trailering with this system also improves SUV true vehicle path, as well as making it easier to spot large trailers into position. Although the General's legal department will not allow direct endorsement of the system's ability to handle rollover, this system has got to improve SUV stability.

Aside from the unusual wheel degree appearances noted by some bystanders, the other notables are the wider rear track and expanded rear fender design that accommodates the system. We've heard comments that suggest its outer appearance looks like a semi-dualie without the extras. In theory, what's not to like about a system that enables larger SUV platforms to enter tighter parking places, renders them more stable at higher speeds with less body roll, handle better in cornering, make it easier to lane change on the Interstate, and make towing a breeze?

Getting Testy

In 3,500 miles of driving with two distinct platforms, we tested the Quadrasteer on the blacktop, highways, byways, thruways, and freeways. When the vehicles came equipped with all-wheel drive, we also included the sand and gravel stuff. Quadrasteer performed as advertised. The road feel is different, and your operational approach is strange at first for conventional users.

Some felt initially that the road-feel issue was a problem. As more miles were piled on, confidence levels rose, and the system won majority approval. Two other issues, price and availability, were noted. Checking an option box for $4,500 can in some cases cause innovation to lose some of its luster. GM limits this option to a couple of heavier SUV platforms, and a few lighter-bodied fullsize pickups.

GM's rationale is a wait-and-see approach for the numbers. That's one view, but we bet a broader offering would reinforce the handling and safety attributes, and generate broader appeal. Brand issues also are noted, as the General likes to pick and choose who gets the action. Cadillac Escalade was mentioned as a natural for this system, but it's not offered. Expansion to other platforms would distribute the development costs, generate the numbers, reinforce control and safety issues, and bring in more buyers to lower the cost. Since the SUV and truck market continues to dominate sales, this system should be a slam dunk for both GM and the customer. We're waiting.