All- wheel steering is a technique in motor vehicles in which all wheels are steered to change direction; In contrast to the common steering of only one axle, where usually only the wheels of the front axle are steered. In fork lift trucks , harvesters , front dumpers (haulers) and the like, the wheels of the rear axle are steered.
Only special vehicles that have to maneuver in a confined space are normally equipped with all-wheel steering. Examples are four-way forklifts that can move sideways instead of taking a curve and sweepers that can turn in the smallest of spaces.
- Turning circle reduction when maneuvering by steering the rear wheels in opposite directions with relatively large steering angles (several degrees)
- Increased agility at low and medium speeds by steering the rear wheels in opposite directions with small steering angles
- Increased stability at higher speeds by steering the rear wheels in the same direction with small steering angles
The transition between the area of agilization and stabilization depends on the tuning of the vehicle; it is in the range from approx. 50 to 100 km / h. When steering in the opposite direction, an increase in the yaw angular velocity is achieved, whereas when steering in the same direction this is reduced and the transverse acceleration builds up more quickly. Furthermore, the float angle is also reduced here, which also benefits driving stability.
Active rear-axle steering in cars use various technical implementations. A steering system similar to that on the front axle can be used ( rack and pinion steering, etc.), which adjusts both rear wheels via tie rods . In another implementation, the passive toe links on the rear axle are replaced by active actuators, so that both rear wheels each have their own drive. Furthermore, special steering linkages are used, which in turn are adjusted via a single drive. The systems used today are all steer-by-wire systems, i. H. it is controlled by an electric motor and there are no mechanical or hydraulic couplings to the front axle steering. This places special requirements on functional safety , in particular in accordance with ISO 26262 .
Various integration scenarios are conceivable for the actual steering function, i.e. the calculation of the rear axle steering angle from the vehicle parameters. All active systems known today have their own electronic control unit (also called “ECU” according to the English word) to control the system (adjustment function, diagnoses, etc.). The steering function can be placed on this control unit. Alternatively, the function is integrated elsewhere, e.g. B. in the control unit of the front axle steering or the vehicle dynamics control .
The above Steering function can be represented by various algorithms . Input variables are i. d. Usually the steering angle of the front axle and the driving speed . In the simplest case, it is a pre-control which, on the basis of the input variables, determines the so-called steering factor, i.e. the quotient of the rear axle to the front axle steering angle.
Various automotive suppliers have corresponding rear axle steering systems in their range:
- Active Rear Axle Kinematics (ARK) from Continental (formerly Continental Automotive Systems , CAS)
- eRAS rear axle steering system for commercial vehicles from Robert Bosch GmbH
- Active Kinematics Control (AKC) from ZF Friedrichshafen AG
- Active Rear Steering from Aisin Seiki
In 1907 the Daimler-Motoren-Gesellschaft developed the Dernburg car with four driven and steered wheels. The Società Automobili Brevetti Angelino designed a prototype with all-wheel drive and all-wheel steering in 1927 , but it did not go into series production. In the 1930s, the Mercedes-Benz 170 VL and G5 off - road vehicles had all-wheel steering, as did two types of standard Wehrmacht cars up to 1940 .
The first mass-produced road car with all-wheel steering was the third generation Honda Prelude , which appeared in 1987. The steering worked mechanically with a planetary drive: with a small steering angle, the rear wheels were turned in the same direction as the front wheels, with large ones in opposite directions. Other cars with all-wheel steering include the Mitsubishi Sigma and Mitsubishi 3000 GT, as well as the Nissan 300ZX and the Nissan Skyline . The Mazda 626 in its third generation, the GD series, and other mostly Japanese cars were and are available with all-wheel steering on request. At BMW , between 1992 and 1996, the so-called active rear axle kinematics (AHK) was partly built in as standard (in the 850 CSi) or at an additional cost in the 8 series ( BMW E31 ) .
A new generation of all-wheel steering systems has been in use since the late 2000s . For example, BMW first introduced all-wheel steering as an option in the 7 Series at that time (integral active steering) ; this is also available in the new generation of the 7 series , as well as - also optionally - in vehicles of the 5 and 6 series . At Renault all-wheel steering is available as an option since 2009 (4CONTROL) , first in the Renault Laguna GT , below (also optional) in the model series Talisman , Mégane . and Espace In 2013 the Porsche 911 Turbo went . for the first time with standard all-wheel steering in series; The Porsche Panamera has also been available with all-wheel steering as an option since 2016 Audi has been equipping the Q7 with a system since 2014 . Japanese manufacturers also offer vehicles with all-wheel steering in the luxury segment, such as Infiniti in the QX70 (RAS - Rear Active Steering) model . and Lexus in the GS model With the Ferrari F12tdf , the Ferrari GTC4Lusso and the Lamborghini Aventador (S Coupé) , the Italian manufacturers also introduced this technology in the 2016/2017 model years.
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