From muscle power to autonomous drive Always steering toward the future

Four "contemporary witnesses" from 80 years of vehicle history came together for a family gathering with a twist: All equipped with steering technology from ZF, they illustrate the development from the heavy, pre-war classic car to an agile concept car for the urban mobility of the future.

Turning the vehicle in just a few moves – not a hope at the wheel of the Horch 830 BL cabriolet dating from 1939. What does nothing to diminish the majestic elegance of the classic car from the outside feels a lot less regal behind the wheel, demanding all the muscle power the driver can muster. Compared to today's standards, the easy handling of the ZF Ross steering system promised in the historical data sheet leaves a lot to be desired.

Nevertheless, the Ross steering system was a success. Under license from U.S. company Ross Gear & Tool Company, ZF produced what was known as the one-finger steering system from 1932 right up to the 1970s. What sounds like super-easy operation merely describes the transmission of the steering command, because the turning of the steering wheel is transferred to the front wheels via the worm gear on the steering column by means of a steering finger – although still without any hydraulic power assistance.

Elegant, but difficult to maneuver: the almost five-meter, 2.1-ton Horch 830 BL cabriolet.

Hydraulics finally come into play

Some 30 years later, hydraulic steering assistance is long established. That meant parking and turning in tight spaces were no longer a problem, even for the large saloon cars of the 1960s and 1970s. However, these easy-action steering systems proved problematic at higher speeds. Due to the constantly available servo power, there was always a danger of oversteering during cornering or lane changing. Then, reflexive counter-steering movements made the drive unnecessarily jerky.

Agile to maneuver, stable in curves: With its Servotronic for the BMW 740i E32, ZF launched a speed-dependent hydraulic steering system in the mid-1980s.

The solution launched by ZF in 1986 involved a control unit that linked the hydraulic power-assisted steering to the electronic speedometer: The lower the speed, the more assisted power is available for steering. At a higher velocity, the servo power is reduced. Termed Servotronic, this system was first used in the 7-series BMW, followed later by saloon cars from manufacturers such as Opel, Audi and Jaguar. Even today, the silver 740i demonstrates just how effective the system is: The car steers with a light touch at low speeds, yet steering at high speeds is precise and steady.

The rear axle steers too

The fact that this was still not the pinnacle of evolution is demonstrated by the Porsche 911 Turbo from 2014. Here, the AKC (Active Kinematics Control) from ZF supports the steering movement on the front axle by a few degrees in the same or the opposite direction – depending on the driving speed. In the 911, two electromechanical actuators installed on the right and left of the rear axle instead of the toe link are responsible for this. They control the steering angle of the rear wheels. The steering angle is a maximum of three degrees, yet it has an amazingly powerful effect: From a speed of around 60 km/h, the rear wheels steer in the same direction as the front wheels, supporting the vehicle's stability and agility. Even during fast cornering, the car stays solidly on track. However, if at low speeds the rear wheels steer in the opposite direction to the front wheels, the sports car's turning circle shrinks.

Almost invisible, but packing a real punch: In the Porsche 911 Turbo, the rear axle also steers thanks to Active Kinematics Control.

New front-axle concept

Turning miracle and parking perfectionist: Featuring a steering angle of 75 degrees on the front axle, the Advanced Urban Vehicle turns in just one go within the space of a soccer goal.

ZF attains even more agility with the innovative front-axle concept in its Advanced Urban Vehicle. A steering angle of up to 75 degrees substantially reduces steering effort during parking and above all increases the maneuverability of the subcompact car: Thanks to the modified wheel deflection, the turning circle diameter of the Advanced Urban Vehicle is reduced to under 6.5 meters – making a 180-degree U-turn easily possible on a standard two-lane road. In order to also utilize this wheel deflection from standstill, the all-electric rear-axle drive mounted close to the wheels features a torque vectoring function which assists the steering movements of each wheel on the front axle. That makes even the smallest parking spaces accessible – a clear advantage in urban traffic. Specifically, just 30 centimeters in front of and behind the car are required to enable the driver to park in a single move. Furthermore, the Smart Parking Assist function reliably maneuvers the vehicle into a free parking space – remote-controlled at the push of a button using mobile devices such as a smartphone or smartwatch. Thus, some 80 years later, parking with genuine fingertip control has finally arrived.

Pictures: ZF, Auto Motor und Sport.

Further related articles