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01/11/2004
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A rack-and-pinion steering system for heavy duty trucks has been designed with the prospect of independent front suspension (IFS) transferring at last from the bus and light commercial vehicle sector, writes Roger Bishop.
The modular system, developed by ZF Lenksysteme, has been integrated with a ZF Sachs airspring and damper module in a proof-of-concept double wishbone axle design.
To meet the kinematics requirements of the IFS, the mechanical and hydraulic elements of the steering system—called Servoline—are arranged in a parallel design. This allows the system to comply with the emergency steering requirements (ECE R79) using a single circuit.
As an IFS integrated unit, the system makes obsolete relay levers, linkages and couplers associated with conventional systems and opens up valuable space at the left front side next to the frame. This allows a larger radiator to be fitted which may help manufacturers to achieve Euro 4 and 5 emission limits. According to ZF, for most applications the anti-roll bar is superfluous while the elimination of the axle beam and its required clearance for ‘bump travel’ enable the engine and transmission to be mounted in a lower position. In addition, the IFS expands the wheel turning angle.
The Servoline rack-and-pinion steering has advantages of its own. The short overall length of the design reduces its (unsprung) mass, gives it great rigidity and reduces the required operating forces. The short distance between ball joints permits long track rods and, consequently, minimal Ackerman and deflection errors. Forces are direct from track rod to wheel and the reduced number of joints reduces play. Steering is therefore said to be more precise even under conditions of axle bounce and rebound.
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Author
Roger Bishop
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