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.
3.Electronic control unit 4.Yaw sensor 5.Oil resevoir 6.Hydraulic pump 7.Cooler
The twin planetary gear sets seamlessly blend active steering inputs supplied by the motor with the driver's commands. The direction of the motor's rotation determines whether the driver's steering inputs are amplified or reduced. Transitions between the 10:1 and 20:1 steering-ratio extremes are so smooth that the driver can't detect them. The nominal steering ratio (with no contribution from the electric motor) is 18:1.
When BMW's Dynamic Stability Control (DSC) system senses a yaw rate (rotation about a vertical axis) verging on a spin-out, Active Steering goes to work to save the day. The front wheels are automatically countersteered up to five degrees to placate the maneuver. These steering corrections are available in 150 milliseconds versus the 300 milliseconds needed to build hydraulic pressure for a one-wheel brake application. That's why Active Steering is now BMW's primary defense against loss of control, with brake applications saved for truly dire circumstances.
There is no Active Steering off switch. Disabling DSC cancels countersteering, but only a system error will stop the variable-ratio function. Downsides are minimal. Active Steering adds a small weight penalty, and the 530i's optional Sport package is an additional $3300 expense, partly because it also includes Active Roll Stabilization. With eight more gears between your hands and the front tires, friction is eight percent higher than it is with a conventional Servotronic-2 system.
Active Steering also will be available on the 2004 6-series, which goes on sale this spring. The 2006 Audi A6 is probably next in line. GM, Delphi, and Honda also have experimental variable-ratio steering systems under development. What started out as an intermediate step on the path to steer-by- wire guidance ended up as the greatest leap ahead in handling since anti-roll bars.