April 6, 2006
Photo: BMW. Click image to enlarge
By Paul Williams
Arjeplog, Sweden – BMW’s winter test facility at Arjeplog, Sweden had only been officially open for a few days before the company invited international journalists there to experience its revised xDrive four-wheel drive system. That system is now available on 11 vehicles sold in Canada, and vehicles so-equipped comprised 50% of Canadian BMW sales in 2005.
The 27-hectare test centre is located about 50 kilometres below the Arctic Circle, and even the Canadians had to admit that at this latitude, there was a nip in the air.
The Arjeplog facility comprises several tracks, circles, and special courses that put BMWs through their paces on surfaces ranging from dry asphalt to glare ice. There are also steep inclines and circuits that are built with heating and refrigeration technology beneath the surface, so that the drive routes can simulate a range of winter driving conditions. The nearby public roads can also put the vehicles through their paces, especially when off the main highways.
The star of the show was the xDrive system, updated from its introduction two years ago, and now featuring an electronically controlled, multiple-plate clutch and intelligent management. This “power divider” is located behind the transmission, and serves to divide power and torque from the engine to the front and rear axles.
In normal driving conditions, 40% of the power is distributed to the front and 60% to the rear, preserving BMW’s traditional rear-wheel drive bias. However, in more challenging conditions, the flow of torque can be varied all the way from 100:0 or 0:100, front-to-rear. If a rigid connection is required, the system can lock the front and rear axles, providing all-wheel drive with 50:50 power distribution.
The xDrive system on the cars is not quite the same as that fitted to trucks. On the cars, the system uses a spur gear to direct power to the front axle, and on the X3 and X5, a toothed chain is used. Perhaps more importantly for car drivers, xDrive in the 5 Series and 5 Series vehicles features Dynamic Traction Control (DTC) that intentionally enables the drive wheels to spin slightly in the interest of enhanced traction. A further benefit, according to BMW, is that DTC “enables the skilled driver to drift around bends in a controlled process, enjoying greater driving dynamics.” It’s a sportier driving experience, in other words.
BMW’s winter test facility at Arjeplog, Sweden. Photo: BMW. Click image to enlarge
The xDrive system also features a hill-holder function that prevents the vehicle from rolling back when starting on a steep incline, and Trailer Stability Control that recognizes the potentially dangerous swinging of a trailer, and applies the brakes on the towing vehicle to re-stabilize the car and trailer.
But the performance of xDrive is fully realized with the addition of BMW’s Dynamic Stability Control (DXC) program, with which xDrive is fully integrated. In fact, if xDrive was the star of this show, DSC was definitely the best supporting actor.
Several exercises were prepared to demonstrate the usefulness of these stability technologies, and the day was spent flinging the assembled X3s, X5s, 3 Series and 5 Series BMWs around the Arjeplog facility to do just that.
In one of the simpler exercises, xDrive permitted the vehicle to start from a standstill when the front wheels are on glare ice, and the rear wheels are on asphalt or snow, or when the rear wheels are on glare ice, and the front wheels have the traction. In these cases, all of the power is moved to the front, or the rear, permitting stable and straight acceleration.
Another notable feature of the system is its ability to shift power from side-to-side. This was demonstrated by situating a vehicle so that wheels on one side were on glare ice, and wheels on the other side were on dry asphalt (the manoeuvre was made more difficult by starting on a steep incline). The power distribution at each axle is then forced to the side of the vehicle with traction (dry side) via brake intervention at the slipping wheels, as directed by the xDrive/DSC system. The result was that the vehicle was able to move forward with power only available to one side.
When taking corners at speed, the multiple-plate clutch minimizes oversteer or understeer by continuously modulating power between the front and rear axles. This it does transparently to the driver, actually anticipating the propensity of the vehicle to rotate through the use of yaw rate and steering angle sensors, and applying minute corrections to maintain the driver’s desired course. It’s this pre-emptive, corrective application that BMW regards as “intelligent” all-wheel drive. Should additional control be required, the DSC system subsequently intervenes by pulsing the brakes, further preserving the vehicle’s stability.
The system was ably demonstrated on one of the driving circles that was polished to a flat sheet of glare ice; much the same surface as you’d find in a hockey rink. In this situation, with careful steering and throttle inputs, we were able to drive the vehicle to the threshold of its control, at a speed of about 45 km/h. At this speed, we were able to hear the DSC’s gentle chattering as it worked hard to keep the vehicle on course. Turning off the DSC in this situation immediately caused the vehicle to slide sideways, and off the driving circle.
There is no system that can negate the laws of physics, and maintain control of a vehicle if it’s travelling too fast for conditions. However, the xDrive all-wheel drive system, in combination with Dynamic Stability Control, is a formidable technology that BMW now proposes is the best all-wheel drive system available. Already, the system is integrated into the optional active steering technology available on its vehicles, and in the future, the company’s goal of full-scale integration will be realized in its Integrated Chassis Management (ICM) technology, designed to control the longitudinal, latitudinal and vertical dynamics of a vehicle at the same time. The experience at Arjeplog suggests that this goal is close to realization.