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BMW engineering VP Dirk Hacker with xDrive M5: "We want the driver to feel that he can steer the car on the throttle as well as with the steering wheel.” (BMW photo)

M xDrive: BMW’s power + grip solution

Channeling 591 hp (441 kW) and 553 lb·ft (750 N·m) through a pair of hand-sized contact patches was always going to be a challenge, even for the engineers at BMW’s M division.

“Four years ago we knew we had to find the right solution for the sixth generation M5 since it was going to have a considerable power hike,” Dirk Hacker, Vice President Engineering for the M division told Automotive Engineering at the 2017 Frankfurt Motor Show.

“The big challenge was to retain that rear-wheel drive feeling that the M5 is known for; we want the driver to feel that he can steer the car on the throttle as well as with the steering wheel,” he explained.

While the M xDrive’s physical architecture is largely based on that already seen on the 5- and 7-series xDrive with TRW (now ZF) DC slip control, apart from some stiffening to the transfer case and modifications to allow installation, it is the in-house-developed software that shuffles the power and torque between the wheels where the difference lies.

Although there is a slight weight penalty in the system, Hacker maintains the new 5-Series' lighter bodyshell more than compensates. “The additional weight, given the car’s extra capabilities, is more than acceptable,” he said, “especially when you think that’s it’s nearly a second faster to 62 mph (100 Km/h) which would only be possible with four-wheel drive.”

Data is taken from steering inputs as well as the throttle and brakes together with the driving situation, the loading on each wheel in addition to slip angles and other parameters.

“We know precisely what each wheel can transmit in longitudinal and lateral directions,” Hacker said, “as well as the tire’s behavior. So we can predict just how much power a tire can cope with in any given situation, irrespective of surface conditions.

BMW's philosophy "is to activate the rear axle first and then bring in the front wheels to deliver the optimum driving performance,” he noted.

The driver can select one of five configurations based on combinations of the DSC modes (DSC on, MDM, DSC off) and M xDrive modes (4WD, 4WD Sport, 2WD). In the basic setting with DSC (Dynamic Stability Control) activated and 4WD, the system permits slight slip through the rear wheels when accelerating out of corners.

In M Dynamic mode (MDM, 4WD Sport) M xDrive allows easily controlled drifts. The three M xDrive modes with DSC switched off have been conceived primarily for track use. Here, the driver can choose from three configurations up to and including 100% rear-wheel drive.

Future M xDrive applications depends on if it’s the right technology to deliver “safe, sporty performance” says Hacker. “We are discussing the next M3, M4 and M2 and whether we need to have M xDrive available in every car.

"By adding M xDrive you increase the load on the car and those platforms are one step below the M5," Hacker noted. "We have the technical solution; now we must discuss if we introduce it on other models.”

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