1.INTRODUCTION Four-wheel steering, 4WS, also called rear-wheel steering or all-wheel steering, provides a means to actively steer the rear wheels during turning maneuvers. It should not be confused with four-wheel drive in which all four wheels of a vehicle are powered. It improves handling and help the vehicle make tighter turns. Production-built cars tend to understeer or, in few instances, oversteer. If a car could automatically compensate for an understeer/oversteer problem, the driver would enjoy nearly neutral steering under varying conditions. 4WS is a serious effort on the part of automotive design engineers to provide near-neutral steering. The front wheels do most of the steering. Rear wheel turning is generally limited to 50-60 during an opposite direction turn. During a same direction turn, rear wheel steering is limited to about 10-1.50. When both the front and rear wheels steer toward the same direction, they are said to be in-phase and this produces a kind of sideways movement of the car at low speeds. When the front and rear wheels are steered in opposite direction, this is called anti-phase, counter-phase or opposite-phase and it produces a sharper, tighter turn. the vehicle body sways as the rear wheels again try to keep up with . the vehicle must again try to adjust by reversing the same forces developed by the turn. The car momentarily resists the turning motion. WHY FOUR-WHEEL STEERING SYSTEM? To understand the advantages of four-wheel steering. Steering input causes the front wheels to turn. The tires are subject to the forces of grip. This is referred to as rear-end lag. Each of these must be balanced against the others. and steering input when making a movement other than straight-ahead driving. When the front wheels are turned back to a straight -ahead position. it is wise to review the dynamics of typical steering maneuvers with a conventional front -steered vehicle. the rear end is always trying to catch up to the directional changes of the front wheels. the driver learns to adjust to these forces without thinking about them. As the steering is turned. When turning. Once the vehicle begins to respond to the steering input. As a normal part of operating a vehicle. causing a tire slip angle to form. momentum. These forces compete with each other during steering maneuvers. The vehicle sways as the rear wheels attempt to keep up with the cornering forces already generated by the front tires. The tires are subjected to road grip and slip angle. the driver is putting into motion a complex series of forces. Grip holds the car's wheels to the road. With a front-steered vehicle. This causes the vehicle to sway. because there is a time delay between steering input and vehicle reaction. cornering forces are generated. and momentum moves the car straight ahead.2. There is also less sway when the wheels are turned back to a straight-ahead position. when the driver turns the wheel slightly. The entire vehicle moves in one direction rather than the rear half attempting to catch up to the front. all four wheels react to the steering input. The idea behind four-wheel steering is that a vehicle requires less driver input for any steering maneuver if all four wheels are steering the vehicle. However. tire grip holds the four wheels on the road. . causing slip angles to form at all four wheels. The vehicle responds more quickly to steering input because rear wheel lag is eliminated.the cornering forces generated by the front wheels. As with two-wheel steer vehicles. Mechanical 4WS .1 Mechanical 4WS 3. TYPES OF 4WS There are three types of production of four-wheel steering systems: 3.3.3Electro-hydraulic 4WS 3.1 Mechanical 4WS Figure 1.2 Hydraulic 4WS 3. 5 to 10 degrees. This pin engages a second offset pin that fits into a planetary gear. two steering gears are used-one for the front and the other for the rear wheels. Further rotation of the steering wheel results in the rear wheels going counter phase with regard to the front wheels. . Turning the steering wheel to a greater angle. Proportionately. This means that the planetary gear can rotate but the internal gear cannot. The eccentric pin of the planetary gear fits into a hole in a slider for the steering gear. about 230 degrees. About 5. Further rotation of the steering wheel. past the 120degree point. finds the rear wheels in a neutral position regarding the front wheels. A 120-degree turn of the steering wheel rotates the planetary gear to move the slider in the same direction that the front wheels are headed. Mechanical 4WS is steering angle sensitive. The planetary gear meshes with the matching teeth of an internal gear that is secured in a fixed position to the gearbox housing. the rear wheels turn the steering wheel about 1. causes the rear wheels to start straightening out due to the double-crank action (two eccentric pins) and rotation of the planetary gear. A steel shaft connects the two steering gearboxes and terminates at an eccentric shaft that is fitted with an offset pin.3 degrees maximum counter phase rear steering is possible. It is not sensitive to vehicle road speed.In a straight-mechanical type of 4WS. 3. The rear wheels turn only in the same direction as the front wheels. They also turn no more than 11/2 degrees. both in components and operation. The system only activates at speeds above 30 mph (50 km/h) and does not operate when the vehicle moves in reverse. A two-way hydraulic cylinder mounted on the rear stub frame . Hydraulic 4WS The hydraulically operated four-wheel-steering system is a simple design.2 Hydraulic 4WS Figure 2. the farther it moves. . The higher the pressure on the spool. The faster and farther the steering wheel is turned. the front steering pump sends fluid under pressure to the rotary valve in the front rack and pinion unit. this system limits rear wheel movement to 11/2 degrees in either the left or right direction. This forces fluid into the front power cylinder. As mentioned earlier. The fluid is also fed under the same pressure to the control valve where it opens a spool valve in the control valve housing. Fluid for this cylinder is supplied by a rear steering pump that is driven by the differential. When the steering wheel is turned. and the front wheels turn in the direction steered. the more fluid it allows through to move the rear wheels. the greater the fluid pressure. A tank in the engine compartment supplies the rear steering pump with fluid. As the spool valve moves. The fluid pressure varies with the turning of the steering wheel. The pump only operates when the front wheels are turning. The farther it moves. it allows fluid from the rear steering pump to move through and operate the rear power cylinder.turn the wheels. the rear wheels of this system are not considered a dynamic factor in the steering process. Electro-hydraulic 4WS Several 4WS systems combine computer electronic controls with hydraulics to make the system sensitive to both steering angle and road speeds.3 Electro-hydraulic 4WS Figure 3. By processing the information received.3. the ECU commands the hydraulic system steer the rear wheels. At moderate road speeds. In this design. a speed sensor and steering wheel angle sensor feed information to the electronic control unit (ECU). the rear wheels are steered . At low road speed. a swing arm. Two electronic sensors tell the ECU how fast the car is going. a set of beveled gears. and a control valve with an output rod. the rear wheels turns only in phase with the front wheels. through neutral. Stepper motor action eventually causes a push-or-pull movement of its output shaft to steer the rear wheels . The yoke is a major mechanical component of this electrohydraulic design. The arc of the swing arm is transmitted through a control arm that passes through a large bevel gear. a stepper motor. which results in the rear wheels steering in the counter phase (opposite front wheels) direction. the rear wheels steer in phase with the front wheels. The stepper motor moves the control yoke. The basic working elements of the design of an electrohydraulic 4WS are control unit. A swing arm is attached to the control yoke. a control rod. The position of the yoke determines the arc of the swing rod. the control yoke swings up through a neutral (horizontal) position to an up position. and the quickness of the steering wheel turn are interpreted by the ECU to maintain continuous and desired steer angle of the rear wheels. These three factors . In the neutral position. For example. At high road speeds. at speeds below 33 mph (53 km/h). As road speeds approach and exceed 33 mph (53 km/h). The ECU must know not only road speed.road speed. The position of the control yoke varies with vehicle road speed.momentarily counter phase. then in phase with the front wheels. amount of steering wheel turn. but also how much and quickly the steering wheel is turned. the yoke is in its downward position. The electronically controlled. within certain parameters. thereby producing enhanced stability and. agility. This speed-sensing system optimizes the vehicle's dynamic characteristics at any speed.up to a maximum of 5 degrees in either direction. . 4WS system regulates the angle and direction of the rear wheels in response to speed and driver's steering. 4. a solenoid valve that disengages the hydraulic boost (thereby activating the centering lock spring in case of an electrical failure) is included. a power cylinder. Additionally. and an output rod. hydraulically powered by the main pump. The rear steering is comprised of the input end of the rearsteering shaft. . The rear-steering shaft extends from the rack bar of the frontsteering assembly to the rear-steering-phase control unit. ACTUAL 4WS The actual 4WS system consists of a rack and pinion front steering that is hydraulically powered by a main twin-tandem pump. The system also has a rear-steering mechanism. A centering lock spring is incorporated that locks the rear system in a neutral (straight-ahead) position in the event of hydraulic failure. and steering-phase control unit (deciding direction and degree). vehicle speed sensors. and the position of the rear wheels. Specifically. and converts the entire steering system to a conventional two-wheel steering type. the system automatically counteracts possible causes of failure: both electronic and hydraulic. draw a line . it would be detected by a self-diagnostic circuit integrated in the four wheel-steering control unit. On any 4WS system. which neutralizes hydraulic pressure. the rear-wheel-steering mechanism is automatically locked in a neutral position. the position of the front/rear wheels is checked with respect to steering wheel position. For example. the position of the front wheels. The failure would be indicated by the system's warning light in the main instrument display. thereby alternating the system to two-wheel steering. The control unit stimulates a solenoid valve. It is usually recommended that the car be driven about 20 feet (6 meters) in a dead-straight line. FAIL-SAFE MEASURES All 4WS systems have fail-safe measures. When the wheel is positioned dead center. activating a low-level warning light.5. Then. if a hydraulic defect should reduce pressure level (by a movement malfunction or a broken driving belt). there must be near-perfect compliance between the position of the steering wheel. with the electro-hydraulic setup. The base reference point is a strip of masking tape on the steering wheel hub and the steering column. In the event of an electrical failure. such as repositioning the steering wheel. Run the car a short distance straight ahead to see if the reference line holds. corrections are needed. Even severe imbalance of a rear wheel on a speed sensitive 4WS system can cause problems and make basic troubleshooting a bit frustrating. If not. .down the tape. vehicle maneuvering on narrow roads and during parking becomes easier. . Therefore. The vehicle's response to steering input becomes quicker and more precise throughout the vehicle's entire speed range. Stability in lane changing at high speeds is improved. By steering the rear wheels in the direction opposite the front wheels at low speeds. the vehicle's turning circle is greatly reduced. Negative effects of road irregularities and crosswinds on the vehicle's stability are minimized. ADVANTAGES OF 4WS The vehicle's cornering behavior becomes more stable and controllable at high speeds as well as on wet or slippery road surfaces.6. The vehicle's straight-line stability at high speeds is improved. The vehicle is less likely to go into a spin even in situations in which the driver must make a sudden and relatively large change of direction. QUADRASTEERTM by Delphi is an electronic four-wheel steering system . The system works spectacularly well with the Suburban and the turning circle diameter drops down from 44. in contrast.1 Chevrolet Suburban 2500: The purely electronic system works so that.5 feet to 35. At higher speeds all four wheels turn in the same direction for better stability in lane change maneuvers. the rear wheels turn the opposite direction of the front wheels. at low speed.2 feet.7.2 GM Concept Truck: QUADRASTEER TM (four-wheel steering system) by Delphi is featured on General Motor Corp. thus shortening the turning circle. Hopefully as the four-wheel steering system becomes more ubiquitous across the GM range of products the cost of the system will drop. 7.'s GMC Terradyne concept vehicle. But the very worst thing about the fourwheel steering system is its $4495 option cost. There is a switch to turn the system off and the Suburban drives like a regular two-wheel steering machine and. APPLICATIONS OF 4WS Some of the vehicles in which the 4WS is applied are: 7. it feels quite ponderous. Unfortunately the four-wheel steering system also pushes the width of the Sub out past 80 inches. which in turn determines the appropriate angle of the rear wheels.3 Jeep Hurricane: The Jeep Hurricane.4 feet and a Saturn Coupe at 37.7 litre V8 engines features a turn radius of absolutely zero. one full-size pickup's turning radius was reduced from 46. Based on tests with full-size SUVs and pickup trucks.1 feet.4 feet. The first is traditional with the rear tires turning in the opposite direction of the front to reduce the turning circle. a radical off-road machine with two 5. Algorithms are then used to determine the correct phase of operation. steerable solid hypoid rear axle. making it comparable to a Nissan Ultima at 37. the vehicle features two modes of automated four-wheel steering. In fact. In addition.2 feet to 37. The system has four main components . electric motor-driven actuator.that enables vehicles to significantly improve handling and maneuverability in full-size vehicles. The QUADRASTEERTM by Delphi Systems also provides a controlled return to regular two-wheel steering if the four-wheel steering system is damaged. Hand wheel position and vehicle speed sensors continuously report data to the control unit. QUADRASTEER by Delphi reduces the minimum turning circle diameter by an average of 19 percent. using skid steer capability and toe steer: the ability to turn both front and rear tires inward. The second mode is an . QUADRASTEER TM by Delphi combines conventional frontwheel steering with an electrically powered rear-wheel steering system.a front-wheel position sensor. and control unit. 7. This allows the vehicle to move sideways without changing the direction the vehicle is pointing.innovation targeted to off-road drivers: the vehicle can turn all four wheels in the same direction for nimble crab steering. The multi-mode four-wheel steering system offers killer performance and maneuverability. Figure 4. Ford Suburban 2500 . Jeep Hurricane Figure 5. lane changing and lowspeed maneuverability. . As the systems become more commonplace the cost of four wheel steering will drop. Even though it is advantageous over the conventional two-wheel steering system. Four wheel steering is growing in popularity and it is likely to come in more and more new vehicles. 4WS is complex and expensive. Currently the cost of a vehicle with four wheel steering is more than that for a vehicle with the conventional two wheel steering. straight-line stability. steering response. CONCLUSION Thus the four-wheel steering system has got cornering capability.8. com 7. 4.howstuffworks. Repair”. http:\\www. Tim Gilles.com . http:\\www.com 5. http:\\www.REFERENCES 1. 3.thecarconnection.delphiauto. “Automotive Suspension and Steering Systems”.com 8. Maintenance.howhurricaneworks. “Automotive Technology-A Systems Approach”.theautochannel.com 6. http:\\www. Thomas W Birch. http:\\www. Jack Erjavec. “Automotive Service-Inspection. 2.
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