BMW’s active front steering system, which will be standard on the upcoming 5-Series, slots neatly into the premise that BMWs are first and foremost “drivers’ cars”…
BMW does its utmost to stay at the cutting edge of technology. It strives to be innovative with design and prefers to lead the way rather than follow others. This, of course, means investing vast resources developing new designs on its own, or in partnership with other companies who may be specialists in a particular field. As an example of getting the jump on other manufacturers, one can look back at the late 1970s and early 1980s when fuel injection began to replace the less efficient carburettor.
While Bosch K Jetronic mechanical injection was working reliably on certain makes, BMW fitted LE Jetronic to the 528i. This system opens and closes the injectors electronically instead of using a continuous flow of fuel. Although some may have been wary of this “complicated” new technology, it soon proved as reliable as the mechanical system and quickly superceded it.
BMW’s latest step ahead in the high-tech stakes is Active Front Steering (AFS). This development slots neatly into the premise that BMWs are first and foremost “drivers’ cars” and there should be a fun aspect about getting from A to B. Together with performance and handling, a good deal of this enjoyment comes from responsive steering. Ideally, steering should not be too heavy or too light and should allow the driver to “feel” the road and receive feedback.
Hydraulic power assistance was the first major step towards reducing the effort required, simultaneously enabling the number of steering wheel turns to be reduced. Thereafter followed variable assistance, where one receives added assistance at low speeds but which reduces the pump pressure at higher speeds to avoid the frightening possibility of careering off the road should one sneeze.
Non power-assisted cars need about four turns to reduce the human effort required, while power-assisted cars usually have about three turns. CAR always measures and supplies these figures on the specifications page of a road test. When the new 5 series BMW is launched in 2003, this figure will no longer be a single number, but a range of numbers. What BMW has done with the latest development is to provide a variable ratio assistance that is also speed dependent to enhance driving pleasure.
What follows is a summarised description of how it works: First take your common or garden steering wheel (no joysticks yet), attach it via a collapsible tube or rod to the steering box, add hydraulic power assistance to reduce the arm effort, allow the pinion to turn the rack.
From there, simple mechanical geometry, using ball joints and rods, conveys input commands to turn the wheels. Now BMW adds a new ingredient to the recipe. An electric motor (what would we do without them?) at right angles to the shaft and pinion drives a worm gear that rotates a cylindrical gearbox housing via gear teeth machined into its outer casing.
The existing steering shaft runs through the centre of this gearbox housing and continues down to the rack and pinion, but the shaft is interrupted and straight cut gears are added to the input and output sides of the shaft. Power is conveyed from input to output via three planetary-geared shafts around this sun gear.
With no movement of the casing, the input sun-gear rotates the planetary gears, which rotate the output shaft as if the steering shaft ran straight through the gearbox. But, if the casing is rotated, the planetary gears rotate around the input shaft and turn the output shaft while the input side can remain stationary.
This means that with no effort from the driver, it is possible to turn the wheels by means of an electrical signal. Normally, if the driver makes a steering adjustment, a computer will determine just how much assistance is needed. The inputs to this “black box” come from speed, yaw and lateral acceleration sensors, already in use as part of the dynamic stability control system.
The point of this is that, should the car break away on a slippery surface, not only will the ABS brakes be available to correct the problem, and engine torque reduced to allow the tyres to regain their grip, but now the car’s “brains” can also calculate the ideal front wheel angle needed to correct over or understeer and will help the driver regain control. This was well demonstrated on a wet skidpan.
According to BMW engineers, the system can execute corrections faster than most drivers, although they also point out that an experienced driver will be able to control a car in similar fashion by intuition and practice. What this system will achieve is to make allowances for drivers who have not done advanced driving courses and who have not been scared (or excited) by the need to correct a car after skidding, usually on a very wet road.
To demonstrate the AFS, a fleet of identical BMW 530is, half with AFS and half without, were driven on BMW’s test track at Aschheim, 30 minutes outside Munich.
A number of test routes were driven to simulate town driving, parking, twisty country lanes, freeway, high speed lane changes and, of course, the cobbled skidpan. At the first turn of the wheel, one is surprised how rapidly the car changes direction. This is due to the slow speed assistance that gradually eases off as your speed increases.
We measured the number of turns, lock to lock, at crawling speed and arrived at a go-kart-like 1,7 turns, increasing progressively to 2,9 turns at freeway speeds. It takes only a minute or so to get used to the lightning quick response, after which tight corners and parking become a breeze with the benefit of being able to keep one’s hands in the same position on the steering wheel.
Once this system has been appreciated on the upcoming Five series range, there is no doubt that customers will be pestering BMW to extend it across the full spectrum of models. We wait in anticipation.
