Audi quattro ultra
Bruce Newton30 Nov 2016
NEWS

Audi tests rear-drive quattro ultra

German luxury giant flips its new AWD system, but says too much overstseer is a problem. Really?

Audi has flipped its new quattro ultra all-wheel drive system and tested it as a performance-oriented rear-wheel drive set-up.

Ultra has been developed as a fuel-saver that decouples drive to the rear wheels when only front-wheel drive is required, drastically reducing friction losses as the propeller shaft and rear diff are no longer turning.

We explained the technicalities of the quattro ultra system here.

But Audi head of four-wheel drive development Dieter Weidemann confirmed testing had been conducted that reverses the philosophy; disconnecting the drive to the front wheels and keeping the rear wheel permanently engaged.

But for now, at least, he said, there is no scope for a performance version of quattro ultra in the Audi line-up.

The reason? It oversteers too much – which might not sound like a problem to some of us – and understeers too much too (which does sound more like a problem).

“It was interesting, but after some months when we did several tests with these cars it was absolutely clear we would go to the direct drive to the front axle [current ultra set-up] because in our cars with our concept it is a better concept and it gives us the better driving dynamics,” said Weidemann, speaking to motoring.com.au at last week’s Q5 global launch in Mexico.

Weidemann revealed the quattro ultra test was comprehensive, with three S4 sports sedans set up with different drivetrains. One had the current production Torsen mechanical centre-locking differential, another was fitted with front/all-wheel drive ultra and the third with the experimental rear/all-wheel drive ultra set-up.

“We compared them under several conditions, especially on low Mu (grip) on our proving ground near the polar circle,” he explained.

“We were checking out the driving dynamics of the systems and for our cars with our weight distribution between front and rear with our suspension it was absolutely clear the concept with the direct drive through to the front axle and hang-on rear axle has got far better driving dynamics than the opposite.

“The other one with the direct drive to the rear axle had much more tendency to understeer in the beginning, then when the torque is going to the rear axle you had much more oversteer.

“So it was far less neutral than what we wanted it to be and with our cars we want to avoid understeer from the beginning – this is very important for us – and the car with the direct drive to the front axle had far less understeer, because the front wheels are towing you inside the curve.

“You can with the all-wheel drive clutch close it and have a very good driving dynamics and have neutral to slightly oversteering tendency.”

Ultra is designed to work with the MLBevo architecture that places the engine longitudinally, so from the A4 on upwards. It is currently focussed below a torque limit of 500Nm.

It debuted with the A4 allroad, which launched in Australia in September and will be progressively rolled out across the MLBevo range.

However, the Haldex part-time AWD system is retained by transvere-engined vehicles that are based on parent Volkswagen’s MQB architecture, such as the A3, TT and Q3.

Weidemann said ultra was not torque-limited but there was no intention to replace the traditional Torsen centre-locking diff in higher-output models.

“We will keep the permanent all-wheel drive for over 500Nm of torque and the customers that buy cars with engines with more than 500nm of torque are not the same customers who buy an all-wheel drive because of the fuel consumption.

“So you always have to look what you develop and is there a market for it or not.”

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