Audi ditches the rear differential

The Audi e-tron S and its low-roofed counterpart, the Audi e-tron S Sportback, promise to deliver buyers plenty of driving thrills and more than just a little tail-out sliding action.

In a live-streamed presentation to journalists last week, Audi executives revealed what had been long suspected – that the sportier versions of the e-tron and e-tron Sportback would employ software and two electric motors at the rear axle to provide torque vectoring – and the system does away with the rear differential altogether.

Each motor will drive one wheel and the torque-vectoring software can operate the two motors in three different modes, according to Marc Baur, Audi’s project manager for electric torque vectoring.

“The two motors act entirely independently of one another, without any mechanical connection, which means we are replacing the differential with software,” Baur explained through an interpreter.

“This twin-motor set-up allows us to bring together the best of three worlds: Controlled rear-axle differential, a sports diff and an open diff – all simply through software.”

Baur says that the weight penalty for using the two electric motors to drive the rear axle, rather than one larger motor and a conventional differential, is in the “low double-digit kilogram range”, which sounds like a figure between 10 and 15kg.

Audi was also constrained to using two smaller motors by the packaging space available. A single motor offering the same output as the two smaller motors would have occupied more space than is available – especially with the need to incorporate a differential as well.

So the manufacturer has turned a potential problem into a better solution. The two smaller motors at the rear are complemented by a larger motor at the front, producing 150kW and 355Nm.

Each of the motors at the rear generate 132kW and 300Nm, which Audi claims will add up to 370kW and almost 1000Nm in total (including the front motor’s output). This results in a 4.5-second 0-100km/h sprint time.

Baur said the “interconnected control units and the electronic chassis platform” are at the heart of the torque-vectoring system and the broader operation of the whole final drive system.

“It's the centrepiece of integrated longitudinal and transverse torque distribution. It manages and regulates wheel-selective torque control, electric torque vectoring and the all-wheel distribution.

“This allows us to shift over 2000Nm of torque, which the customer notices at every roundabout, at every cross-country road... really, everywhere.”

Unlike many conventional torque-vectoring systems, the Audi set-up allows one rear wheel to spin freely – not driven – while all the available torque goes to the rear wheel on the other side, helping the vehicle tip-toe through a slippery and treacherous section of road, or setting up the car for high-speed drifting.

Audi will offer the e-tron S and Sportback with a ‘range pack’ to extend the range of the car on just one charge. This will involve uncoupling the large motor from the front axle, plus revised brakes, thermal management and aerodynamics.

The Audi e-tron S and e-tron S Sportback will be identifiable by wheel-arches that are 23mm wider on each side than those of the standard e-tron.

In addition, black brake callipers and a black tailgate spoiler will also distinguish the high-performance variants. Cars ordered without the camera-based rear-view system will be supplied with silver mirror caps.

Inside, the Audi e-tron S and e-tron S Sportback will be fitted with a three-spoke sports steering wheel, sports seats (and the option of very aggressive ‘super’ sports seats), plus carbon-fibre decorative trim throughout the cabin.

Audi plans to bring the e-tron S twins to global markets from Autumn, which would be our Spring.

Unfortunately, they will likely take a while longer to reach us in Australia. Certainly the e-tron S models are not expected here before customer deliveries of the regular e-tron commence, to be followed by the Audi Q4 e-tron and the Audi e-tron GT. Since the presentation, an Audi Australia spokesman has revealed that the e-tron S and e-tron S Sportback are scheduled for local launch during the second half of 2021.

The assembled Audi execs would not admit the existence of an RS version of the e-tron, but nor did they explicitly rule it out.

And while noting that the e-tron S was “an S model on steroids”, one spokesman – possibly caught out in translation – observed that “the e-tron S and e-tron S Sportback are at the moment the spearhead of electric driving at Audi”.

'At the moment' until when?