Deceptively simple: the ZF concept’s rear axle combines twist beam suspension with twin electric motors mounted close to the wheel hubs.
Unveiled to a European public in Frankfurt in September and to an Asian audience in Tokyo this November, ZF’s Advanced Urban Vehicle (AUV) concept uses innovative axle technologies to dramatically boost agility and maneuverability in tight spaces. Most notably, its front axle design allows the wheels to swivel to remarkable angles to enable the vehicle to turn almost within its own length.
Germany-based ZF, which recently took over multi-faceted US supplier TRW, has developed the compact city car to “demonstrate the potential inherent in intelligently networking individual chassis/driveline and driver assistance systems.” The concept represents “an exemplary solution for urban individual transport in the compact and subcompact segments,” claims the company.
As an urban concept, the AUV is naturally battery powered and locally emission free. Drive is provided by a novel eTB (electric Twist Beam) rear axle design, which uses the suspension geometry of a torsion beam for maximum package efficiency and which places an electric motor at each end of the axle, close to the hub carrier and just forward of the coil spring unit. The arrangement gives a very effective use of chassis space, with generous room for a battery under the floor between the suspension’s trailing arms, and with the two-motor configuration opening up possibilities for torque vectoring to aid maneuverability and to tailor handling dynamics to fit the desired brand attributes.
In its current specification the axle is home to twin 40 kW electric motors, giving a combined torque of 1,400 Nm; with the motors running to a maximum of 21,000 rpm, the vehicle has been tailored for use in urban traffic and the motors’ reduction gearing gives a top speed of 150 km/h.
Sharp steering: front axle design allows unprecedented steering angles of up to 75 degrees for exceptional maneuverability in tight situations.
The suspension on the front axle employs a double wishbone set-up, but with a key difference: the relatively narrow-based lower wishbones enable greatly increased steering articulation, and the offset geometry of the steering axis substantially reduces the steering effort required during driving maneuvers and while parking. But the most spectacular feature of the axle is the 75 degree steering angle that its layout enables. This gives the car — which is based on a current production compact car platform — a turning circle of under 6.5 meters.
On a conventional car, such extreme steering angles would have been impossible. No front-drive driveshaft could possibly accommodate such a deflection, and a rear-drive solution would be very tricky as the forward push of the rear wheels would tend to slide the angled front wheels directly along the vehicle’s longitudinal axis. ZF solves the problem elegantly through torque vectoring on the rear axle. If the vehicle needs to turn sharply to the right, only the left rear wheel is engaged, thus creating a yaw movement around the right rear wheel; the inverse happens in sharp left maneuvers.
With most residential streets between 6 and 8 meters wide if they are on a bus route, this would allow the vehicle to perform a 180 degree turn in a two-lane road in a single maneuver without having to reverse, steer in the opposite direction, or mount the curbs.
The axle design is also of major benefit in allowing the concept to park in very confined spaces — and this is where the partial automation of the vehicle comes in. A Smart Parking Assistant maps out the vehicle’s immediate surroundings using an array of 12 ultrasonic and infrared sensors mounted on its perimeter. The control electronics are then able to assess whether a potential parking space is big enough, and then can operate the electric drive and electric power steering to carry out the maneuver.
Vehicle can be remotely parked using a smart watch; both parallel and perpendicular parking are possible, says ZF.
Both parallel and perpendicular parking are supported, and the driver can trigger the parking sequence either from within the car or remotely from the curb via a smart phone or smart watch — see below.
For the future, says ZF’s Harald Naunheimer, the Smart Parking Assist opens up potential scenarios which provide additional benefits for the driver. The driver can get out at the destination, leaving the vehicle to head off autonomously for a parking garage, thus saving valuable time in the process.
“When implementing the concept, we weren’t only looking at the benefits for the driver,” he explains. “If passenger cars in future park without a driver, parking space can also be used more effectively. As such, the door opening angles would no longer need to be taken into account in the parking garage — thus making the parking spaces smaller. All of which also takes the pressure off cities because the freed-up space can then be used productively as additional living and working areas.”