2004 Technology of the Year

Don Sherman
0402 Bmw Steering 01

1.Planetary gear sets 2.Electric motor

A better mousetrap.

Why fiddle with the one car control system that's nearest and dearest to every enthusiast's heart? The only rational motive is to build a better mousetrap--specifically, a means of steering the car that's markedly superior to existing power rack-and-pinion systems.

At moderate speeds, a quick steering ratio is desirable to keep from banging elbows or shuffling the wheel hand-to-hand around a 90-degree corner. The problem is that ultrafast ratios make the car twitchy when you're cruising 10 mph over the limit. Sneeze, and you're in the ditch.

The solution to the dilemma is called Active Steering, an integral part of the 2004 BMW 5-series Sport package. Three cooks stirred this stew of existing and innovative equipment: A joint venture between Bosch and ZF called ZF Lenksysteme engineered the hardware, while BMW chassis experts were responsible for fine-tuning how it works.

Active Steering delivers three benefits:

  • At speeds of less than 45 mph, it provides a 44 percent quicker steering ratio, improving agility.
  • At speeds above 75 mph, it provides an 11 percent slower steering ratio, enhancing stability.
  • At the ragged fringes of adhesion, Active Steering helps out with rapid countersteering corrections to keep the tail from wagging the dog.

    The hardware consists of a ZF Servotronic-2 speed-sensitive hydraulically-power-assisted rack-and-pinion system with a few additional parts. At the base of the steering column, between the power-assist control valve and the pinion gear, there's an aluminum housing containing two linked planetary gear sets inside a single carrier. An electric motor (2 in the cutaway above) mounts to this housing to rotate the carrier when commanded to do so by a control computer (3).

    The driver's steering-wheel movements turn the uppermost sun gear. That motion is relayed to the lower sun gear by three pairs of planet gears (1). As the lower sun gear turns, so does the pinion gear. To minimize system lash, the sun and planet gears are held in tight mesh by small springs.

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