A look inside the Four Wheel Steering (4WS)

The steering system's primary function is to direct the vehicle towards the desired direction as per the driver's input by steering the wheel. This system includes a few mechanical linkages from the steering wheel to road wheels. In the power steering system, hydraulic/electric support has integrated with those linkages to reduce the driver's effort in critical turnings. Moreover, Steer-by-wire is also a notified technology, which evolves without or less mechanical connections, that seems to be easier to alter the steering ratios. Whatever the innovations have done on the steering system, it has always adapted with front wheels. 

 

Why don't in rear wheels? (rear wheels alone)
Possibly plenty of reasons and logic are behind that, notably safety and stability concerns. Almost it made benefits in the low-speed environment, while in high speeds, it is more inferior. But the better stability and precise steering response have been actualized in the "Four Wheel Steering" concept. Absolutely yes, four wheels are steered together.

The Four Wheel Steering (also known as 4WS, All wheel steering, Quadrasteer, Activesteer, or rear-axle steering) is a system, which makes the rear wheels turn for certain angles reference with the front wheels turning response. 4WS affords smooth steering with improved maneuverability. The predominant thing is, it reduces the turning radius as low as possible, which is up to a 21% reduction.

                                

In Volkswagen Touareg, 4WS reduces the turning radius from 12.19 to 11.19 m. Also, in Jeep Hurricane, this system achieves on-spot turning (i.e.,) zero returning radius.

Effortless steering is achievable in risky U-turns and sharp corners. The enhanced cornering stability plus precise steering response comfort the driver to steer easier in tightened areas, especially in parking lots, also provides straight-line stability in high-speed and makes smoother lane-changing.

This system has also evolved from a mechanical to hydraulic/electric system.

Mechanical Rear Axle Steering:

In the mechanical 4WS, the steering system includes a rack and pinion-type gearbox in front and an epicyclic gearbox in the rear. The linkages are as usual for the front wheels. For the rear wheels, another separate pinion (for output) has meshed with the front rack. The output pinion has linked to an eccentric shaft inside the epicyclic gearbox via a long center shaft with Hooke's joints at the ends. This eccentric shaft has attached to the planetary gear via an embodied offset peg. The planetary gear has been placed and rotated inside the fixed annular ring gear that is internally toothed. An offset peg on the other side of the planetary gear connects the slider in the output shaft. 

 

While turning the steering wheel, the input pinion moves the rack, so from there, the output pinion transmits the rotary motion to the eccentric shaft. The shaft allows the planetary gear to rotate on the annular ring. It makes the offset peg roll in an odd path that tends the slider to move in horizontal and vertical directions. Whereas the fixed forks in the stroke rod guide the slider to move freely in vertical (up and down) but make constraints in horizontal (left and right), that's why when the peg moves left and right side, it makes the entire stroke rod move in the transverse direction that allows the split tie rods to turn the rear wheels. This mechanism has employed in the 1988 Honda Prelude.

Hydraulic Rear Axle Steering (with electronic assist):

In the hydraulic system, the power steering concept has interlinked with the rear-axle with the help of components includes an electric motor, fluid tank, hydraulic pump, steering cylinder with a position sensor, valve system, and a control unit with a modernized electronic assist. (In the early hydraulic mechanisms, this system has integrated with the front steering pump accompanying with a phase control unit and so on)
 

In a modern system, the control unit fetches the data from the steering angle sensor and wheel speed sensor about the steering response on the notified speed. Then it commands the electric motor to operate the hydraulic pump to increase pressure in the cylinder until the perfect steer attains in rear road wheels. Along with this, the control unit took data about the position of the piston inside the hydraulic cylinder using a position sensor. The valve system modulates the expected pressure inside the cylinder. By syncing the acquired data, the pump supplies the fluid from the tank to the cylinder with the required pressure to the demanded side as per the requisite level  (minimum turning angle) to turn the rear road wheels reference with turn angle of front wheels. Mazda, Mitsubishi, Nissan, Toyota and Honda employed Hydraulic rear-axle steering in their vehicles with some unified mechanisms.

Electric Rear-Axle Steering:

This system is a clean mechanism, which uses an electric motor centralized with rack and pinion type or recirculating ball-type assembly near the rear axle. The rotation of an electric motor has been controlled by the Electronic Control Unit (ECU) intelligibly by receiving the data from the steering angle, wheel speed, lateral acceleration, yaw rate with the help of sensors. This rotation has transformed into linear motion, so the tie rods are directed the rear road wheels to move transversely to and fro.

All these various types of mechanisms have turned their rear wheels a maximum of 5 to 10 degrees. The working of this 4WS is quite attractive and intelligible as well (i.e.,) up to the speed of 35kmph (22mph), the rear wheels turn to an angle opposite to the front wheels. Whereas at above 35kmph (22mph), the rear wheels turn in the same direction as the front wheels. This system is more advantageous to 2WS because the maneuverability and steering response has increased at both low and high speeds.

If any failure or inactive response takes place, automatically, the system cut off the functioning of the 4WS system (i.e.,) the rear wheels are reverted to neutral (straight ahead position) without interrupting the front wheel steering.

Recently, Mercedes Benz launched their luxury sedan S-class 2021 with world-class specifications. This car has a 4WS feature with a smaller turning radius from 5.5m to 4.5m. Also, GMC Hummer EV has a CrabWalk feature. It works similarly to the concept of the 4WS system.

 

 

References: Advanced Vehicle Technology by Heinz Heisler
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