During our time with the M5 in Spain we were surprised by the new differential, which is called the Active M Differential (no surprise there). It was quiet, played nice and was seamless in its ability to shift power from the wheel with slip to the one with grip.

According to information we ‘ve received, it has limited parasitic loss and is just one of the many components that helps the new M5 achieve outstanding performance while improving efficiency. We have broken the complexities down as best we can. It is an amazing piece of hardware.

The Active M Differential is the next step in giving the rear wheel drive chassis more control and stability. Rather than requiring slip, like a viscous differential, the new system can be proactive thanks to the use of electronics. It interprets data from the Dynamic Stability Control to provide the optimum locking force to each wheel all before a wheel breaks free.

In addition to using DSC the system the Active M Differential can adapt based on pedal position, rotational speed of the wheels and yaw rate. All this happens extremely fast and precisely. This differential is able to shift power to the outside wheel in a tight turn as well as counteract the loss of traction during high speed lane changes. It can even decrease the tendency to understeer during full load cornering.

It uses some serious hardware and software to accomplish all of this. It all was evaluated and adjusted countless times to meet M ‘s high standards. With all the added performance there is also increased efficiency thanks to decreased mechanical drag over the outgoing unit.


How does it all work (Engineers and gear heads this is for you!)? It uses an electric motor and clutches to vary the power between the wheels- all controlled by sophisticated electronics.

– Current supplied to an electric-motor connected to a reduction gear directly rotates the cam gear of a unidirectional ball-ramp mechanism
– This rotational torque causes the balls to roll up the opposing ramps which pushes the front cam forward
– The cam thrust is transmitted through the pressure disc compressing the clutch
– The rotational torque from the outer plates (input) is then transferred to the inner plates (output) proportional to the electrical current
– An Electronic Control Unit (ECU) controls the clutch torque by adjusting the amount of current based on available vehicle inputs (DSC) and a custom control algorithm
– Optimum release response is achieved by back driving the electric-motor
– The low drag torque performance is achieved at all speed differences since actuation is independent of the speed difference (unlike in a viscous unit)
– Torque transfer is mechanically pre-emptive and does not require any speed difference to activate (electronics)
– Full and continuous lock-up is available (0.5 A current)