► AWD can be specced on most models
► 4wd system usually 40:60 rear-biased
► Can redistribute drive within 0.1sec
Many premium manufacturers have their own version of four-wheel drive or all-wheel drive, and BMW’s is known as xDrive. Available on larger SUVs as well as smaller cars like the 1-series and 3-series, you can now get xDrive on most models from the German manufacturer.
It's a rival to Audi's Quattro system and the 4Matic set-up at Mercedes-Benz and is designed to provide extra traction on low-grip surfaces such as snowy roads or rain-dampened tarmac.
BMW claims that xDrive is lighter than rival systems, features a greater degree of traction splitting and can even respond to changes in grip faster than other all-wheel drive set-ups.
BMW xDrive: how it works
xDrive is a full-time all-wheel drive system, rather than a part-time drivetrain; systems like VW's Haldex differential only fires surplus torque to the opposite axle when the car’s on-board ECU senses wheelslip. So under normal conditions, BMW's xDrive is rear-biased, sending 40% of drive to the front wheels, and 60% rearwards.
However, this can be altered in one tenth of a second, with almost 100% of engine torque being applied to the front or rear axle, depending on grip available. This is allocated by an electronically controlled multi-plate clutch, rather than hydraulic fluid.
After normal driving conditions return, the system reverts back to its 40:60 split. More recent and high-end models combine this system with BMW’s Dynamic Stability Control to help rotate the car into the turn by braking individual wheels and cutting power across the axle.