Four-wheel steer

Four-wheel steer is back
Renault dusts off an old idea that can improve both safety and driver appeal

This is the dilemma that manufacturers now face: how do they distinguish themselves from the competition in an era of what feels like across-the- board five-star Euro NCAP scores and advanced standard-fit safety systems? Renault thinks the answer lies in an innovative (and currently unique) feature that Frances active safety and dynamic response: an all-new four-wheel steering system branded Active drive.

There are few completely fresh ideas in the motor industry, and this isn't one of them. We've had lots of actively steered rear axles in the past. The most recent UK examples are Honda Prelude and Nissan Skyline. In America, buyers of some of GM's bigger pick-ups and SUVs have for several years been able to specify four-wheel steering, to help manoeuvre their gargantuan vehicles at low speeds.

But Renault claims the Laguna's new system is the most advanced yet. The benefits of steering both ends of a car are undeniable from an engineering point of view. Steering the rear wheels in the opposite direction to the front wheels reduces the turning circle at low speeds. An actively steered Laguna has a turning circle of 10.8 metres, compared with 12.05 metres for a normal one. And turning the rear wheels in the same direction as the front ones at higher speeds sharpens the chassis reaction time and reduces its tendency to yaw or oversteer.

The downside has always been extra weight and mechanical complexity; the rear axle requires some kind of actuator, linkages and steerable hubs. The systems available until now also haven't been able to access the sort of computing power necessary to provide a natural feel to the driver, something Renault has tackled with a separate high-speed ECU to control the rear wheels.

On the test track, it certainly seems to work impressively, dramatically sharpening responses and stability during a high-speed lane change.

"ESP will be standard at some point soon. It will no longer be a point of differentiation," explains Christian Steyer, Renault's VP for customer specification. "The benefit has to be clear or there is no point trying to market a system like this. The most important thing is the feeling of safety. Average drivers will rarely feel the ESP system working, or even anti-lock brakes, but this is something they will experience every day."

Renault's Active Drive will initially be offered only on top-spec versions of the Laguna and the forthcoming Laguna coupe, but its inclusion o n other models within the line-up. hasn't yet been ruled out. The price hasn't been confirmed, but Renault has indicated a premium of about 1000 (Rs 55,000). If the market reacts favourably, we can expect to see many similar systems on the market by 2010.

HOW IT Works
On the new Laguna, the rear steering system gets its own Ecu, connected into the rest of the car's data architecture by a high-speed CAN-BUS system (the automotive equivalent of broadband web access).

This enables the Laguna's steering to use both a static model of the car's behaviour the theoretical values that should be applied for given speeds and steering wheel inputs - and an advanced dynamic model. This takes data from yaw and wheel speed sensors and the car's own ESP system and can react in as little as 10 milliseconds. An electric actuator is then used to steer the rear wheels by up to 3.5deg. The system adds 19kg to the weight of a standard Laguna.