But doesn't the use of a differential lock increase the weight of the vehicle?
Yes, it weighs around 10 kg more than a conventional, open differential but it’s placed at the right position, at the rear and the bottom, which means it has a positive effect on the centre of gravity while optimising the vehicle’s traction.
M automobiles have been fitted with locking differentials for several generations but the BMW M3 and BMW M4 are equipped with an Active M differential. What’s the ‘active’ part of the differential?
Active means that a whole range of sensors have been built into the car that are able to identify the road conditions, calculate the optimum locking degree, and activate the lock accordingly by means of an electric motor. The central input variables are the torque, the individual rotational speeds of the wheels, the lateral acceleration and the driving speed. Then there’s the steering angle and the yaw rate, plus a few additional factors.
What do you mean by ‘optimum’?
Optimum means that the driver benefits from the best combination of vehicle controllability and traction. Whether a professional driver or a relatively inexperienced amateur, the aim is to enable all drivers to drive as fast as possible along a racetrack while maintaining good control of the vehicle, of course always subject to their individual skill and ability. Moreover, the demands placed on a locking mechanism like this under adverse conditions such as ice and snow are quite different to those on a dry racetrack.
So passive systems are less effective at this?
Passive systems primarily react to one input variable. Depending on their construction, this is either the differential speed of the rear wheels or the engine torque. However, the Active M Differential calculates the locking torque on the basis of considerably more input variables. It is therefore possible to react far more specifically to a certain road situation. In addition, an active system is not only able to react but can itself also act. The Active M differential takes into account so-called pre-steering components, which means that it not only responds to an event but actually sets the locking torque even before an event occurs. This results in enhanced transparency in terms of vehicle behaviour and also in better controllability.