Audi Quells Understeer With Differential

In a bid to alleviate the understeering inherent in most of its nose-heavy all-wheel-drive cars, Audi AG introduces a new active sport differential to its trademark quattro AWD system.

Known as torque-vectoring and popularized by Honda Motor Co. Ltd.'s recent introduction of its Super Handling-All-Wheel Drive (SH-AWD) system, the new Audi technology incorporates an active rear differential, which can distribute up to 100% of the engine's output to either of the rear wheels.

Building on the updated 40/60 front/rear torque split of the latest quattro configuration, which debuted on the '07 RS4 performance sedan, the new technology bows with the all-new '08 A5 coupe and A4 sedan and will be integrated into all other ranges moving forward, the auto maker says.

Understeer is the tendency of a vehicle to push, or run wide in a corner, scrubbing its front tires in protest as the chassis and suspension fail to grip the pavement efficiently in response to the driver's steering inputs. The effect is multiplied in AWD and front-wheel-drive vehicles, as the front tires must cope with providing both forward and lateral grip.

With most of its powertrains situated atop the front axle in many of its vehicles, understeer has long been an unfortunate side effect of the added grip provided by Audi's quattro system.

With the new active sport differential, handling response and overall grip are significantly improved, as the outside rear wheel can be “over rotated” to pitch the car more effectively into the corner. This helps in canceling the resistance to changes in motion from the front wheels.

In addition, the system works both under power and when coasting through a corner, Audi says, improving handling response and overall stability.

Specifically, the active sport differential works with Audi's electronic differential lock and electronic stabilization program, along with a host of other chassis inputs, to react and distribute power in less than 100 milliseconds through its multi-plate clutch pack.

In addition to steering angle and vehicle speed, other determining factors for torque distribution include inputs from twin yaw-rate sensors and accelerometers.

For the most effective use of the technology, Audi incorporates the active sport differential into its new 3-stage Audi drive select dynamic driving program. Incorporating preset mappings for the engine, transmission, steering and suspension, as well as the new differential, Audi drive select allows for various configurations of the vehicle personality, with settings including comfort, auto and dynamic modes.

Center console-mounted buttons provide general control over the three modes, but Audi says the system can be configured to suit the driver's personal preferences, as well.

Also aiding in the sharpening of vehicle responses is the introduction of a new dynamic steering system, which features an electrically assisted, compact design that vastly improves steering precision.

Similar to BMW AG's active steering technology, the dynamic steering setup is speed-sensitive, yet can widely vary the steering ratio for easier parking and better high-speed control. Essentially, the system allows the wheels to be turned all the way in one direction at low speeds with minimal steering input. At higher speeds, the ratio relaxes to provide minute control of the vehicle's direction.