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ZF showcases mSTARS electric axle system for axle hybrids or fully electric vehicles

ZF is using its plug-in Vision Zero concept vehicle to showcase a number of new safety and zero local emissions technologies. The vehicle features a 150 kW electric axle drive system for dynamic acceleration. The entire propulsion system, including the integrated power electronics, is housed in an innovative, space-saving modular ZF rear axle system called mSTARS (modular Semi-Trailing Arm Rear Suspension).

This modular axle system makes it easier to electrify volume production vehicle platforms—even existing ones as demonstrated in the Vision Zero Vehicle. The system offers vehicle manufacturers the opportunity to respond to a diverse range of market requirements using just one car platform variant.


mSTARS provides our customers with a basis for a wide range of applications in multiple vehicle segments. The solution is suitable for hybrid, fuel-cell and battery-powered vehicles as well as in combination with conventional all-wheel modules or our AKC active rear axle steering. The Vision Zero Vehicle demonstrates how quickly it could help the industry meet the challenge of producing high-performance e-cars or hybrids.

—Dr. Holger Klein, head of ZF’s Car Chassis Technology Division

In the Vision Zero Vehicle, which is based on a volume production platform for compact and mid-size cars, the mSTARS semi-trailing link independent suspension takes up no more installation space than the conventional rear axle installed originally.


The ZF system frees up installation space between the rear wheels as a result of its unique integral link design and separate spring damper configuration. This means that ZF’s electric drive fits effortlessly into the mSTARS system axle carrier.

The 150 kW drive unit houses not only the electric motor but also a two-stage, one-speed spur gear drive, a differential, and power electronics along with control software.

We’ve put our concept of an intelligent mechanical system and clean mobility into this multifunctional axle system for compact passenger cars and up.

—Holger Klein

Even in its basic version, the mSTARS axle also features a high level of driving dynamics and safety, delivering the performance of more expensive, conventional multi-link axles typically used for compact premium and sports vehicles, the company says.

ZF’s AKC active rear axle steering system, which can be combined with any modular axle configuration, improves agility, comfort and stability. Since it also controls the rear wheel steering, it allows advanced assistance systems to operate more reliably and effectively.



Make FWD CUVs hybrid AWD.


150 kW is a whale of a lot of power to add to any vehicle.  That's more than enough for anything but a sports car all by itself.


@EP, Indeed. 60-75kw would be plenty, IMO.
And 40 would probably do for 90% of situations.

Keep it simple and cheap enough to implement widely.

Liviu Giurca

Simpler and cost efficient solutions are described on:

Account Deleted

There are already many "through the road" all-wheel drive systems: The Volvo XC90 T8 Plug-in Hybrid, Acura MDX Sport Hybrid, the BMW X1 XDrive 25Le IPerformance Plug-in Hybrid, and Mini Cooper Countryman S E ALL4 to name a few. Front Drive Crossover/SUV converted to all wheel drive.
ZF has been a force in in promoting Hybrid conversions using their transmissions. The mSTARS electric axle system should also be successful. While 150 kW is a lot of power, maybe this will convince some manufacturers to use more electric power in their hybrids or move to all wheel drive EV. Imagine a 350 hp Chevy Volt Crossover that would challenge a Camaro!


Imagine a Ford Focus conversion with 40kw to the back axle and the ability to run as EV in cities, and sell for $1-2K more than the regular one.
You could do the 1.0 petrol or the 1.5 diesel, which would also have superb motorway performance.
Ditto the Mondeo.


Agree they are certainly making strides.
I wonder how their (induction) motor compliments this transmission as compared to various other motor types.

The requirement to match controller and associated algorithm to specific motor and the combination match to transmission has been shown to be important for dynamic and energy efficiency.
This has led to the understanding of benefits of two speed transmissions, control strategy to disconnect (from overspeeding) at highway speeds etc.

Multiple motors for example front and rear could either be smaller and so expect to spin to higher RPM or if larger supply more low speed torque for single gear acceleration. All very complex design so we trust such a large and respected manufacture is well equipped to make these calls.

From their descriptions and specs their preferred induction motor has a mix of advantages.


Induction motor power is proportional to the drive frequency and the applied voltage SQUARED.  The disadvantage of induction motors over switched-reluctance motors is that the former have losses due to slip (which is required to induce the currents in the rotor).  However, induction motors are stone-ax simple, rugged and quiet.  The switching transients for switched-reluctance motors leave some things to be desired in those departments, IIUC.

Induction generators can be self-energized with appropriate capacitors to supply reactive power.

I do not know of any limit to the speed of an induction motor besides its structural limits.  At some drive frequency, you'll have to switch from laminated stators to something like powdered-iron or ferrite stators to limit eddy-current losses.  Laminated rotors will be fine, as eddy-current losses are determined by the slip frequency rather than the drive frequency.  The smaller the slip relative to the drive frequency, the higher the efficiency.

What this comes down to is that physically small and light induction motors operating at high RPM could supply large amounts of power at fairly high efficiency.  Torque is roughly proportional to slip frequency in the region around synchronous (both positive and negative), so there is an "automatic transmission" effect where the back EMF from the rotor to the stator can be scaled down to match the drive voltage even as rotor speed goes up; efficiency goes up all the while.

If you read a textbook on this it will give you some amazing insights into the thinking of Nikola Tesla.  The man truly invented our modern energy infrastructure all by himself.

Henry Gibson

Many industrial induction motors use aluminum in their rotors but copper makes a smaller more efficient rotor. ABB has introduced the synchronous reluctance motor with even more efficiency and uses stators identical to induction motors. It is possible to build the highest speed electric motors with switched reluctance designs quite easily however.

An electric motor does not really have a horsepower limit except for continuous operation but the electronics does.

Hybrid automobiles using hydraulics can be the simplest and most cost effective and cheapest to build and can save half the fuel now used and the engines don't even need crankshafts. ..HG..

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