Audi chalks up 40 years of quattro

The Geneva motor show was the venue for the introduction of a very different type of passenger car back in March 1980. That was the year that the Audi Quattro coupe arrived on the scene, blazing the trail for all-wheel drive rally rockets to come, at a time when ‘all-wheel drive’ simply didn’t appear in automotive lexicons.

And the acknowledged rally rocket up to that point was the Ford Escort RS1800 – a rear-wheel drive small car developed from humble, shopping-trolley origins, but powered by the mighty DOHC BDA four-cylinder engine.

Within a few short years, the Escort was consigned to history, as the Quattro swept all before it on the WRC (World Rally Championship) stage. It wasn’t long before the Quattro was joined by other turbocharged all-wheel drive passenger cars that had been purpose-designed to win rallies.

But Audi’s plans for all-wheel drive went well beyond sticking it to rally rivals. The company intended for all-wheel drive to be offered in more prosaic models right across its range. In Europe and other northern-hemisphere markets, rear-wheel drive cars had been the norm, but were often fallible in icy conditions.

While the Quattro was for quite a while the standard bearer for Audi in motor sport, the company has also relied on rear-wheel drive in motor sport categories such as DTM (German touring car series), Le Mans sports car races and GT3 .

Initially, Audi reserved quattro for sporty, high-performance models on the road, but as customers have become increasingly aware of the traction benefits, different iterations of the branded all-wheel drive system have trickled down to variants in the range that are purchased by families rather than cashed-up hoons.

According to William Wijts, a spokesman for Audi on the subject of vehicle dynamics, “Over 45 per cent of our customers have gone for a quattro, so nearly every second car we sell has a quattro powertrain…”

A lot of development work has been carried out along the way since the introduction of the original Quattro (or ‘Ur-Quattro’, as it is known in its home market of Germany).

What Audi road car customers wanted was a centre diff that could think for itself – one that would lock when required, but would remain open at other times – says Audi’s head of all-wheel drive development, Dieter Weidemann.

“For example, it's not something for… parking the car, and brake-control systems that entered the automotive industry in the 80s were not to be combined with that fully-locking centre diff,” he explained in an online presentation last week.

“So in the middle of the 80s we started to develop other technical solutions, and that's the self-locking centre diff. These self-locking centre diffs… they simply have an inner locking mechanism.”

These were ‘torsen’ (torque-sensing) centre differentials. They provided the best of both worlds – an open diff when traction wasn’t in doubt and for ease of cornering, but a locking function when one axle was drawing more torque than the other.

Finding a way to lock the centre diff when required wasn’t the sole focus of Audi’s quattro engineers at this time, however. The 50:50 torque split to front and rear axles was fine for the rally-winning car, and could be locked manually for very slippery conditions, but did introduce some handling vagaries at times, according to Wijts.

“With the 50:50 split, there's a bit of a disadvantage here; if you accelerate in the corner, you never quite know exactly, is the car going to understeer or oversteer? So, if you have… a little less to the front, a little more to the back axle, the consequence is the car is either very neutral or has just a tiny bit of oversteer – which on snow and ice is simply much better behaviour, handling for the car, and for the customer.”

The first quattro centre diff to move from the straight 50:50 torque split to something a little more refined for improved vehicle dynamics was what Audi names ‘Torsen-C’. This system, introduced with the RS 4 in 2005, uses two drive shafts, each driven by the same planetary gear set on an annulus gear connected to the transmission.

One driveshaft, with a wider diameter and an internal ring gear that meshes with the planetary gears drives to the rear axle and delivers more torque. The other driveshaft features an external ring gear that meshes with planetary gears to drive the front axle, supplying less torque. The planetary gears allow the drive shafts to turn at varying speeds, but the default ratio is 40:60 front/rear.

Weidemann says that Torsen-C “was really the first solution where we had a 40:60 load or torque split. That was more geared towards drive dynamics, and now with Torsen-CSM that we use at the moment… superb traction and super drive dynamics, but also small and compact build…”

Audi is not jogging on the spot when it comes to further development of its various quattro systems.

The company subjects its cars to pretty tough tests in Sweden and around the Nurburgring, where the quattro drivetrains are tested for traction and driving dynamics. In a singularly demanding test, a car is loaded with 500kg of ballast in addition to the driver and set off uphill using launch control for optimal engine output.

The front climb is on a bitumen road surface punctuated by slippery tiled surfaces at intervals. Audi expects each and every quattro model to complete this climb smoothly and seamlessly during the transition from front to all-wheel drive operation. There should be no thumps and bumps as the centre diff redirects drive to the rear wheels as the front wheels begin to lose grip.

And as if that isn’t enough, the centre diff and the rest of the quattro drivetrain is expected to last up to 300,000km in normal operation, the company says.

The development path for quattro these days is focused primarily on reducing weight and improving efficiency. To the latter end, Audi has formulated ‘quattro ultra’, an on-demand system that runs initially in all-wheel drive, but drops into front-wheel drive mode if the conditions permit.

The system has been around for a few years, and is available in Audi models with longitudinal engine mounting and either manual or s tronic (dual-clutch) transmission. That limits the system to cars of A4 size or larger, but nothing fitted with a torque-converter (epicyclic) automatic.

It’s a clever system that has a ‘claw clutch’ at the aft end of the transmission to isolate the driveshaft from the rear wheels, and ‘separator’ clutches on the rear axle to isolate the rear diff. This adds a little weight, but more than compensates by the reduction of parasitic losses when the driveshaft and rear differential are brought to a halt – rather than being dragged around and around (and in a bath of oil, in the instance of the differential crown gear).

Dieter Weidemann says that the initiative to develop quattro ultra was pushed by customer feedback.

“A number of years ago, we conducted a major customer survey, with thousands of customers, and we asked – especially those who had not yet decided to buy a quattro.

“They didn't know what the added value of all-wheel drivetrains was, because they hadn't experienced it. But by the same token, a consistent answer was also that they felt quattro all-wheel drivetrains would [consume] extra fuel.”

Addressing this perception became a pressing need for Audi.

“We decided to go for a solution with a multi-plate clutch at the end of the transmission and on top of that… a separator clutch at the rear axle drive,” Weidemann explained.

“Almost 80 per cent of any [drivetrain efficiency] losses are to be found on the rear axle… namely, the crown gear… that is sloshing through the oil, but also the … pinion shaft."

Isolating the rear differential and the driveshaft from the rest of the drivetrain promised to reduce fuel consumption, but at the cost of extra weight. Weidemann reckons that the extra weight over a constant quattro system is somewhere between three and five kilograms, depending on the model and variant of car.

Another important consideration was the demand for rapid but smooth transitions. Weidemann says that quattro ultra can reconnect its entire drivetrain in as little as 200 milliseconds, and he claims furthermore that drivers “will notice nothing of the change between two- and four-wheel drive”.

While quattro ultra is the most recent iteration of the brand’s all-wheel drive system, there’s more work ahead for Audi’s drivetrain engineers, with the company now embarked on quattro drivetrains for alt-energy vehicles, such as the high-performance e-tron S electric SUV.

And that leads Audi into at least another decade developing drivetrain systems that lie at the very heart of the brand’s slogan, ‘Vorsprung durch Technik’.