ZF’s Torque Vectoring rear axle drive
June 14, 2007 Driveline and Chassis Technology company ZF invests five percent of its sales (2006: EUR 600 million of EUR 11.7 billion) in Research and Development and in so doing, has developed a very interesting new Torque Vectoring rear axle drive which it claims considerably increases driving dynamics and safety reserves of all-wheel drive and rear-wheel drive vehicles. The system distributes the drive torque individually to the rear axle wheels, generating a yaw moment around the vertical axis of the vehicle, which can be used both to improve agility and stabilize the vehicle. Generation of the wheel differential torque is independent from the drive torque, so it is also generated during coasting, when the driver is not accelerating. The system is ready for volume production and is already planned for volume application at BMW.
ZF's Torque Vectoring rear axle drive, which, leads to noticeable added value, both for drivers of all-wheel drive and rear-wheel drive vehicles. The drive torque is distributed individually to both wheels of the rear axle, generating an additional yaw moment (veering-in) which supports the steering motion of the vehicle, particularly when cornering. This way, the vehicle can also be stabilized in the case of quick swerving maneuvers without having to brake. Vehicles equipped with the Torque Vectoring rear axle drive provide better handling in critical situations. The steering system responds more directly – with less effort and fewer required corrections. This increases safety without restricting driving dynamics.
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When driving straight, the Torque Vectoring rear axle drive acts like an ordinary transmission with an open differential: The drive torque is distributed equally among the drive shafts of the wheels. The torque is only distributed individually among both drive shafts during cornering. It is controlled by the electromechanically actuated multi-disk brake of the superimposed axle drive.
Major new feature: The Torque Vectoring axle drive also generates a wheel differential torque independently of the drive torque, i.e. also when cornering downhill without additional acceleration; in this case, in the bend the outer wheel receives more drive than the inner wheel.
This is achieved by one superimposed axle drive in a planetary design on both sides of the axle drive, respectively.
The system, which is based on a planetary design - unlike a high-ratio transmission design - is more efficient. The gears of the planetary gear set do not turn when driving straight on. Therefore, the system power losses are limited to oil shearing in the released multi-disk brake and the churning of the planetary gear set rotating without relative gear rotation.
The new rear axle drive also features the familiar benefits of locking differentials, as the torque can be targeted to the wheel with the higher friction locking potential. Thus, drive wheel spin can be avoided, in particular when both wheels of the drive axle are on different road surfaces when starting off. This leads to improved vehicle propulsion; moreover, fewer and less intense brake interventions to reduce wheel spin are required. There is less wear on the brakes and also a positive effect on fuel consumption.