The Active M Differential in Detail

BMW M5

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.

BMW M5

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)
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  • Adam

    Fascinating, gives Ferrari’s E-Diff a run for it’s money.

  • Dr Obnxs

    What’s worthy of note here is that integration of the electronics of the rest of the car means that predictive capacities of the car control system can be used, as opposed to reactive only, like all the mechanical only systems that we gear heads typically bolt into a car. Opens the door to torque vectoring and all that other stuff that goes with the whiz-bang technology.

    The E-Diff and this are just two ways to skin the very same cat. The Ferrari unit uses electrohydrolics to control the clutchpacks, BMW uses a gear reduced electric motor. Both are integrated into the DSC system to provide predictive torque distribution, only thing is, Ferrari owners had the opportunity to access this technology 7 years ago.

    If you’ve driven a Lancer MR, or heck, even an Acura MDX, (I haven’t gotten to drive a Ferrari 430 though…) you can really tell that these elctromechanical torque distribution systems really increase the performance envelope of pretty much any chassis they are deployed on. The end result is a vehicle that feels lighter than it really is, giving more confidence through turns.

    • The X6’s system is a good example of this as well.

      • Dr Obnxs

        It’s coming to pretty much every car, eventually. Heck, even the little Nissan Rogue has a torque vectoring rear diff! I don’t think it will have as large an effect in safety as DSC systems do, but it will provide a marginal increase in safety as well as performance.

        But really, it’s just a question of price points now. As more and more cars deploy it, the unit costs will drop and it will penetrate more and more of the performance car market. Should make good inroads into larger SUV and heavy luxury cars too, as vehicles equiped with the tech really do drive “smaller” than they are.

    • Anonymous

      This tech has been around for a long while- for one reason or another BMW M did not vet it for the M3. I know they spent countless hours making changes to what the supplier offered up to meet their standards.

      The Jag XKR (IF memory serves me right) uses a similar unit to this and it is by no means built as well. M over builds everything so there is nonquestion that the car can perform and be reliable.

      There is nothing that I am aware if that is off the shelf.

      It is insanity at it’s finest- we’d all fit in just fine.

      -M

      • azz

        M over builds everything so there is nonquestion that the car can perform and be reliable.

        Like subframes LOL