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Rear-axle steering

Increased maneuverability

On the IAA 2018 Commercial Vehicle tradeshow, Bosch will present its eRAS electric rear-axle steering system. It is designed for commercial vehicles.

The automatic rear-axle steering system consists of two parts communicating via CAN (Photo: Bosch)

The eRAS system lets commercial vehicles with three or more axles use the leading and trailing axles to steer. This makes the turning radius smaller and reduces wear on the tires. This is not a new idea: firefighter trucks with rear-axle steering function were launched already more than 100 years ago. But they needed a second steering person at the rear. Bosch’s electronic system does it now automatically.

Based on the steering angle of the front axle, transmitted via CAN, the steering system determines the optimum steering angle for the rear axle. After driving around a bend, the system also assumes the task of straightening out the wheels. Only when the vehicle is actually being steered the eRAS system consumes energy. This results in an additional fuel consumption of 0,6 l per 100 km.

Horse-powered ladder truck with rear steering in 1903 (Photo: Steve Hagy Collection)

The rear-axle steering system consists essentially of two parts: the cylinder unit with integrated position sensor and valve system, and the power unit, consisting of a motor-driven pump and control unit. The ever more demanding requirements faced by commercial vehicles in today’s transportation industry can only be satisfied on the rear-axle through use of an innovative steering system with electronic interface, explained the supplier. With the introduced system, the linking of the rear-axle steering to the vehicle’s electrical system is now possible. The available vehicle signals (e.g. steering angle of the front axle) on the CAN are transmitted to the ECU (electronic control unit) controlling the steering. From these signals, the rear-axle steering unit determines the desired steering angle of the rear-axle and controls the pump by means of the electric motor on the basis of the target-actual comparison. The volume-flow generated acts on the particular piston surface of the cylinder unit and moves the cylinder’s piston rod in or out, until the integrated position sensor gives the return signal, that the target position has been reached. The wheels on the axle are turned to the desired angle by coupling the cylinder unit to a control arm, which transmits the steering forces via the tie rods. With its freely programmable and plug-and-play capability, the independent and compact construction makes it possible to steer several axles on a vehicle through use of several rear axle steering systems.

To protect the steering system against overloading when the wheels are turned to the full-lock position, an electronic stop can be programmed. The control unit then decreases the flow from the pump as soon as the programmed end stop is reached so that the mechanical stops are not damaged.


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