March 28, 2013
LOCAL
By John Bridge
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n early February Stewart Motors, local dealers for BMW pulled out all the stops to highlight the capabilities of their line-up of Sport Utility Vehicles by transforming Barbican Circle into an exclusive test track for the BMW X Challenge Off-Road Experience. For four days, media and the public had access to BMW ’s SUVs including the X1, X3, X5 and X6. All were encouraged to test their mettle against the tough obstacle course. From deceptively simple bumps to a treacherous hill climb and descent to a makeshift sandy riverbed, it was all designed to show those that participated the full benefits of BMWs xDrive permanent all-wheel drive system. Introduced in 1982 on the then new e30 model 3 Series, the basic xDrive technology was an option to enable buyers seeking all-weather operation, or those that lived in traction limited environments, the ability to still enjoy their vehicle. It continues to be an option on all of BMWs sedans, however as the company moved into the sport utility vehicle market with the X5, the development of xDrive expanded exponentially. Currently it’s standard on all X models.
The key to xDrive is its lack of a traditional center differential to distribute power to either front or rear wheels. The BMW system instead uses an electronically controlled multiplate wet clutch mechanism attached to the end of the transmission. The car retains a 40:60 power ratio, but can modulate this split quickly and infinitely between 0 to 100% as
needed for traction as determined by the vehicle’s Stability Control (DSC) and anti-lock braking system (ABS). Upon loss of traction xDrive moves the appropriate power to the correct axle and works in conjunction with the braking system to disable the spinning wheel. Off-road this keeps the wheels that have the most grip active and moving.
In normal driving xDrive helps to retain the BMW mantra of the Ultimate Driving Machine by invisibly reacting to normal over and understeer conditions for maximum cornering stability. At extreme low speeds, such as parking, the clutch opens reducing the drag associated with normal fourwheel drive systems, as well as driver effort.
The X6 prepares to enter a 60ft long trench of deep water to simulate a river crossing.
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