M xDrive Intro

One of the biggest changes in the F90 M5 was the introduction of a 4-wheel-drive system. This change was greeted with a lot of trepidation from the community because 4WD had a history of poor handling. However, because of the difficulty of getting the M5's torque to the road, and because of competitors who were smashing BMW's 0-60 times with 4WD, BMW was forced to follow suit.

As I explained in Horsepower:

In practice, 1g of acceleration (+22 mph per second) would never happen with the M5 because its road tires just aren’t that sticky. Generally a good rule of thumb for a decent road tire is that it can push forward with about the same force as the weight pushing down on it.

Since the M5 is a rear wheel drive car, and about half the weight of the car is on the rear wheels, it can only accelerate at about 0.5g to start. It gets a bit better than this in practice because as the rear wheels start accelerating, the weight goes more onto the back wheels resulting in an effective acceleration of about 0.7 g’s (about +15 mph per second) with good tires on good pavement.

The above implies that regardless of how powerful the engine, no 2-wheel drive car with 50-50 weight distribution can accelerate to 60 faster than in about 4s on road tires. However, with 4WD with road tires on a good surface, a car could achieve 1g of acceleration if the engine is powerful enough (2.7s to 60). Because the F90 M5 engine can't quite generate 1g of acceleration in 2nd gear (more like 0.9g), and you have to shift to 2nd at around 30mph, in practice it's about 3s to 60.

While 0-60 is the most visible metric dramatically impacted by 4WD, acceleration out of corners is also improved with BMW's M 4WD. This is because the traction of the tires is divided between turning and accelerating. If you put too much torque into a RWD car while turning, it will spin out ("oversteer"). If too much torque on a FWD car, it will not turn in ("understeer"). If you divide the acceleration amongst all 4 tires, the total acceleration available from all tires out of a turn can be more given the same traction available for turning on both front and rear tires.

This last point is crucial to the development of the 4WD system on the M cars (called "M xDrive"). The M xDrive can actively direct torque between front wheels and the rear right and rear left wheels. So during turning the system can exactly provide torque to where it's needed at any given moment in time in order to optimize the handling (which was the big bugaboo with 4WD systems in the past).

Quoting from the BMW site 5 Facts about M xDrive:

[The system] monitors real-time parameters from the accelerator pedal position, steering angle, road speed, slip at the front and rear axle, longitudinal and transverse acceleration, friction coefficient, yaw rate, etc. This information is analysed to establish the vehicle’s current situation. M xDrive can then support the driver in every set of circumstances.

When accelerating out of a bend, M xDrive successively puts more and more torque on the front axle when it detects decreasing lateral acceleration. At that moment, it becomes possible to transmit the longitudinal forces without understeering. This occurs so gradually that it’s possible to exit the bend with a slight drift, as one would expect from an M automobile. It feels as if you are coming out of the curve in a controlled catapult.

From my test drive of the car, I can tell you that this absolutely is a real thing and puts a huge smile on your face even in regular road driving. I was instantly sold! The faster 0-60 is a bonus.

We'll get into the development and the technical details in follow-up blog posts.