Audi’s Quattro permanent all-wheel drive is about to get a huge overhaul: a front-wheel drive-based system to be used on longitudinal-engine cars and SUVs like the A4 and Q7. It’s called “Quattro with Ultra Technology.” Here’s how it works, and why it might make Audi’s biggest fans blow a gasket.
When you think Audi, there’s a good chance you think “Quattro.” It’s the all-wheel drive system with the strongest cachet, partly because it’s been underneath some seriously badass historic race cars and because Audi markets the crap out of it.
The four-ringed brand’s all-wheel drive system has seen plenty of changes throughout the years, and next year, one of the biggest changes yet will make its debut on the Audi Allroad Quattro: a front-wheel drive-based system will replace the permanent 60:40 rear/front all-wheel drive system for which Quattro is so well known. Say goodbye to that center differential.
Of course, this won’t be the first time Audi puts a front-drive based all-wheel drive system in their cars; the Audi A3, S3 and TT all use a Haldex front-drive-based setup, and have for a long time. These days, Quattro, usually spelled with a lowercase “q” by Audi, is a branding tool that encompasses both systems.
But what makes Audi’s new “Quattro with Ultra Technology” so notable is that, unlike in the A3 and TT, this system will make its way into applications with longitudinally-mounted engines, including the larger sedans and SUVs.
That’s something likely to upset Audi’s most hardcore fans, who have long hailed previous Quattro as a superior all-wheel system—all while looking down on the front-drive-biased Haldex systems in other cars. They claim the Haldex systems aren’t as responsive, feel like a front-drive car with more traction and are more prone to understeer.
However, this new system, which Audi claims shows “no discernible differences to permanent systems with respect to traction and driving dynamics,” is meant as a fuel savings measure, and Audi claims it saves 0.3 liters for every 100 kM when tested in Ingolstadt traffic.
That number means nothing to me, but intuitively, it makes sense. If you can reduce bearing drag, gear frictional losses and driveshaft windage losses, you should expect at least some fuel economy benefit.
How It Works
The video above, with its overly dramatic and inspiring theme music, shows how Audi’s new “Quattro with Ultra Technology” functions. It’s not much different from a typical Haldex system, but let’s break it down.
Hardware
First off, let’s have a look at the hardware comprising this system. The two major components are the multiplate clutch up front and the decoupler in the rear.
Multiplate Clutch
The multiplate clutch sits right at the tail of the transmission, and acts to couple and decouple the transmission and the rear driveshaft. As described in the video above, the clutch pack works similarly to a conventional clutch in a manual transmission, in that it uses slip between pairs of friction surfaces to gradually connect a stationary component with one that’s rotating quickly.
A multiplate clutch is made up of many pairs of clutch discs, five or seven in the case of Audi’s Quattro with Ultra system, depending upon the vehicle application.
The benefit of a multiplate clutch over a conventional one in your old Ford Ranger is that it can have a smaller diameter and fit in a more compact package.
One of the two discs in each pair rotates along with the clutch basket, which spins with the transmission output shaft. The other disc is connected with the driveshaft and remains stationary until an electric motor spins a worm, which rotates the worm wheel.
Using a power screw, a drive that converts rotational motion into linear motion (just like when you spin a nut onto a bolt, the bolt moves linearly down the bolt), the worm wheel moves axially and forces the clutch discs forward until they grab the transmission output clutch discs. When that happens, the rear driveshaft becomes coupled with the transmission.
To disengage the driveshaft from the transmission, the worm simply spins the opposite direction, pulling the worm wheel back and disengage the clutches.
Rear Decoupler
But there can’t be only a single decoupler in the front of the car, because even when that is open, the rear wheels will have a tendency to drive the driveshaft, and that creates drag. So you need to have a decoupler in the rear so the driveshaft can remain stationary, and windage and bearing losses can be avoided.
So Audi uses a rear decoupler in their rear power takeoff unit. The primary component of the decoupler is a claw clutch, also called a positive clutch or dog clutch, which splits the rear right halfshaft int two stub shafts. This type of clutch is different from a conventional or multiplate clutch in that it relies on interlocking of jaws to make a connection, not on pure friction.
The left stub shaft half of the claw clutch connects to a spider gear in the differential, and the right one, which has a worm gear on part of its outer diameter, connects the wheel hub.
To get into two-wheel drive, a pin—which Audi describes as “electromechanical,” (so probably solenoid-based)—pushes a separating arm (my term, not Audis) against a worm gear. As the right wheel spins, the worm gear spins, and moves the separating arm towards the center of the car, compressing its spring, and forcing the right stubshaft away from the left one, thus opening the coupler.
Once the coupler is open, the separating arm is locked in place, and the right stubshaft spins freely with the wheel, while the left stub shaft spins freely with the spider gear in the differential.
The electric motor-driven worm at the front of the car then disengages the clutches in the multiplate clutch, so that the driveshaft, carrier and pinion no longer rotate. The only components in the rear drivetrain still rotating are the halfshafts and spider gears, which which are now driven by the left wheel.
To go back into all-wheel drive, the front clutch pack must already be engaged, and the driveshaft should be spinning. Then in the rear, the electromechanical pin releases a locking lever, a spring relaxes and pushes the separating arm back to its original position away from the worm gear. This closes the claw clutch, and all wheel-drive is activated.
Control Strategy
Audi says that, in terms of pure handling, the system works as well as their permanent system, and part of that has to do with their control system, which they say analyzes sensor data every 10 milliseconds to decide when to activate the front multiplate clutch and rear claw clutch.
Audi breaks their control strategy three main strategies: proactive, predictive and reactive.
Proactive
Audi says that with their proactive strategy, “the all-wheel drive system is activated before the driver needs it.”
The system takes data from accelerometers, yaw sensors, steering angle sensors, wheel speed sensors, throttle positions sensor, and a bunch more to detect how quickly the car is reaching its lateral grip limit, and activates all-wheel drive approximately 0.5 seconds before that limit is ever reached.
Predictive
Predictive mode, referred to in Audi’s press release as “forward-looking,” is meant to put the car into all-wheel drive based on driver’s style.
The computer looks at the stability control status, drive mode and driving behavior (likely looking at throttle position, RPM and later acceleration), to make sure you can go full opposite lock around a turn instead of plowing into a guardrail.
Reactive
Reactive all-wheel drive occurs when you’ve already slipped. You might call this Oh Crap-mode, because it’s essentially the car saying “oh crap, we just slipped on ice, let’s activate all-wheel drive.”
But Audi says this mode “rarely occurs in practice,” which is their way of saying that the system is so good at predicting road conditions, that it rarely has to say “oh crap.”
So Audi’s system is pretty fascinating, but will the Audi purists line up for it—or will they feel the brand has abandoned one of its strongest attributes in the name of fuel economy?
We’ll have to wait and see how good it really is, and whether the four-ring faithful can get behind it.