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Torque Vectoring drive technology, with 1st application in a future BMW model
New ZF intelligent torque distribution system

ZF Torque Vectoring drive: when cornering downhill without additional acceleration, the outer wheel receives more drive than the inner wheel. Photo: ZF.

When cornering downhill without additional acceleration, the outer wheel receives more drive than the inner wheel. Photo: ZF.

APN,
22 June 2007,
last update 18 Dec. 2007.

ZF announced earlier this month the readiness of its new Torque Vectoring rear axle drive for volume production. The first application will be in future BMW vehicles (X6 as the Dynamic Performance Control), as announced by the German supplier.

The ZF Torque Vectoring rear axle drive system can be used in all-wheel drive as well as rear-wheel drive vehicles. It distributes the drive torque individually to the rear axle wheels, to improve agility and stabilize the vehicle. Generation of the wheel differential torque is independent from the drive torque (torque coming from the engine): for example, it is also generated during coasting, when the driver is not accelerating.

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. In critical situations, the steering system responds more directly with less effort and fewer required corrections.

ZF Torque Vectoring drive: A superimposed axle drive in a planetary design on both sides. Photo ZF.

A superimposed axle drive in a planetary design on both sides.

How it works - 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 (opposite photo).

A major advantage comes from the fact that 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 (upper photo)

This is achieved by one superimposed axle drive in a planetary design on both sides of the axle drive, respectively (lower photo).

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.

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