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BMW Explains the M Active Differential Used on the new M4

Part of a massive in-depth guide to the new M3 and M4 models, the guys from BMW released a new interview with one of the key people involved in the development of those cars, Mr. Bernd Jacob. He was one of the people that worked on creating the M Active Differential everyone loved on the new cars.
BMW M active differential 1 photo
Before we explain what makes the M Active differential so special in comparison with other differentials, we should explain what a differential actually is.

When a car goes around a curve, the drive wheels spin at a different speed. That means the one inside the curve will turn slower than the one on the outside. In order to compensate, cars use a differential that makes sure the engine torque is transferred to the axle gearing equally between the two wheels, balancing out the differences in rotational speed.

That’s just the basic stuff though. Over the years, people started wanting rear-wheel drive cars to have lockable differentials that would make sure the two rear wheels spin at the same time. That’s because by doing so you could drift, drive faster and have more control over your car, in various situations.

BMW always had lockable diffs on their cars. Some would lock up to a certain degree (or percentage if you will) while others would lock up to 100% meaning there’s no rotational difference between the wheels.

On the new models, BMW used what they call an Active M differential. What does that mean? According to Mr. Jacob “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 is the steering angle and the yaw rate, plus a few additional factors,” he added.

The aim of creating this new diff was to make sure that the driver benefits from the best combination of vehicle controllability and traction at all times. This way, whether it’s a professional driver behind the wheel or a complete amateur, things are slightly evened out, allowing them to cover a track as fast as possible without losing control.


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