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excellent article about audi's proposed new quattro - ....(coming from an mdx owner with sh-awd :-))
Byline: Bill Visnic
All-wheel-drive was viewed as nothing less than a metaphysical advance when Audi AG introduced its quattro technology for road cars in 1980.
In the quarter century since, adaptation for all types of vehicles has fostered countless engineering variations, but the goal essentially is the same: distribute power to all four wheels to augment traction and security.
AWD provides undeniable worth in foul-weather conditions (and off-road, of course), and is a key attribute in the meteoric growth of SUV sales. Now, as buyers expand their horizons beyond SUVs into cross/utility vehicles and other car-based architectures, they want AWD along for the ride.
Witness AWD's recent proliferation into just about every market segment above subcompact cars.
From the start, though, Audi insisted AWD had another advantage: improved handling - even on dry tarmac. The auto maker made its point over the years by dominating several forms of racing, sometimes thrashing 2-wheel-drive competition so emphatically that AWD was banned.
In the real world, engineers and enthusiasts agree AWD usually is superior to either front-wheel drive or rear-wheel-drive in wet, ice and snow, but they debate AWD's ability to "improve" handling or cornering behavior on dry roads.
Mechatronic technology, until now largely unheralded, advances AWD to a new level of dynamic ability far beyond its reputation as a traction enabler for foul-weather driving.
That technology has a name: torque vectoring. The engineering that enables torque vectoring is not uncharted territory, but a new showcase, the Super Handling All-Wheel Drive (SH-AWD) system that's standard for Honda Motor Co. Ltd.'s '05 Acura RL sedan, has grabbed the attention of driveline and chassis engineers and buyers.
Whether attributable to SH-AWD or not, the new RL is burning rubber on the sales figures of the prior-generation Acura flagship.
Torque vectoring - engineers at multinational engineering specialist Ricardo plc claim to have coined the term - is the practice of enabling the AWD driveline to actively increase the rotational speed of one axle - or in the case of Acura's SH-AWD, a single wheel, even - to enhance handling.
Think of it as the industry's now-ubiquitous stability control system - which selectively brakes an individual wheel to affect handling - in reverse.
In a corner, Acura's torque-vectoring SH-AWD system will "overspeed" the rear axle, then channel a disproportionate amount of that torque to the outside rear wheel, generating an inward yaw that pushes the car more resolutely into the corner - and helping to offset the tendency of AWD/FWD vehicles to understeer, or push away from the corner. The system also can stabilize oversteer situations, such as when the throttle is abruptly shut while cornering.
Whether in low-traction conditions or in the dry, many who have driven the Acura RL say SH-AWD's performance adds an eye-opening new dimension for AWD. Toss the RL into a corner and bury the throttle: The aggressive torque transfer between rear wheels creates the driving sensation of a car on rails.
The RL's torque-vectoring system has auto maker and supplier engineers buzzing. Some debate the technology's ultimate impact from a volume perspective, but most agree torque-vectoring is a formidable new advance in AWD development.
More telling, perhaps: Almost every auto maker and AWD-connected supplier is actively evaluating torque-vectoring AWD technology.
"There's a lot of interest around at the moment," says Ben Reynolds, chief engineer, AWD driveline systems, Ricardo Driveline and Transmission Systems. Ricardo, he says, currently is developing torque-vectoring AWD for a vehicle that will be in the market "sooner than later," and "several major Tier 1s are active in the field."
For Audi, the big daddy of AWD in the premium market, torque vectoring plays a definite role in future quattro development programs.
Ulrich Hackenberg, head of concept development and body engineering for Audi AG, says Audi is working to integrate torque vectoring with its foundation quattro AWD system. If you can selectively apply torque, "you can get the maximum" from AWD, he says.
"We are working with (torque vectoring) demonstrators internally," Hackenberg says, adding he has driven the Acura RL and found the SH-AWD technology impressive. "We will not stand still (with conventional quattro AWD). We have to improve ourselves."
BorgWarner Inc., one of the largest AWD/4WD suppliers in the world, also has an eye on torque vectoring, says Dave Bruder, chief engineer of the TorqueTransfer Systems division.
Bruder tells Ward's BorgWarner is in "a (torque-vectoring) development program with an OE," but declines to name the auto maker. The vehicle likely is a sport sedan.
Ricardo's Reynolds says one of the next production vehicles to sport torque-vectoring AWD is expected to be from a German auto maker in '07. That torque-vectoring system will not be Ricardo's design, he adds.
To appreciate torque vectoring's advantages, it's best to understand "conventional" AWD systems currently available. Disregarding their method of actuation and other system-specific peculiarities, most AWD systems rely on a couple of basic underlying principles.
First, many conventional AWD systems that always have some amount of drive being applied to all four wheels rely on gears, clutch packs or some other differential mechanism to generate a fixed torque split, or "bias," between front and rear axles.
Audi's quattro, for example, employs a Torsen center differential with a 50/50 torque bias. That means, in normal conditions, 50% of drive torque is applied to the front axle and 50% to the rear. This type of system often is referred to as "permanent" AWD.
Many Japanese AWD systems, as well as the Haldex system used by Volvo Cars, Ford Motor Co. and Volkswagen AG, typically are not designed to drive all four wheels when extra traction is not required.
These "part-time" systems usually place a differential on or ahead of the axle not normally employed as a drive axle, i.e. the rear axle of a Toyota RAV4, which in non-AWD format is front-drive only. Driving down the straight, dry road, a RAV4 is sending no drive torque to the rear axle. A Haldex-equipped AWD vehicle might be the same, or perhaps transfer a nominal 5% to the rear axle.
But what happens when a wheel, or wheels, start to spin? It's important to realize today's standard AWD systems - regardless of how their differentials work and regardless of their default torque split - are designed only to equalize the speed of both axles and the wheels on those axles.
Let's say one of a quattro-equipped vehicle's front wheels hits ice. The system deduces that wheel is spinning faster than the others. The center differential shifts torque from the front axle to the rear axle, until the rotational speed of each wheel is equalized. Limited-slip differentials or traction control also can aid the AWD system in equalizing the speed of a spinning wheel or wheels.
This action is passive in comparison to a torque-vectoring AWD system, which actively increases the rotational speed of a wheel, or axle, to offset a loss of traction or to improve handling performance.
Acura's SH-AWD employs a collection of electromagnetic clutches and planetary gears in an elegant and complex rear differential unit that effectively handles both the front/rear torque split (70% front/30% rear is default) and the side-to-side apportioning of torque (either rear wheel can accept from 0 to 100%) that plays a critical role in achieving the torque-vectoring effect.
It is not an inexpensive unit.
And cost is one of torque-vectoring's unavoidable drawbacks. Price is what will keep the technology in the realm of premium vehicles.
"It's definitely a niche-type product," says Vern Jones, director of business development at BorgWarner's TorqueTransfer Systems.
He says any torque-vectoring system is going to be "complex and expensive."
Grip And Grin
Ward's Auto World, April 1, 2005 by Bill Visnic
<< Page 1 Continued from page 2. Previous | Next
"AWD on its own isn't enough any more," to be a differentiator in upscale market segments, says Jonathan Wheals, chief engineer, Ricardo Driveline and Transmission Systems.
He says one of Ricardo's chief development goals is to design a torque-vectoring system with "actuation technology at a reduced cost." Ricardo is working on a novel design that uses a geared system with clutches and magnetorheologic fluid.
James M. Baumbick, chief vehicle engineer, Medium/Large FWD/AWD platforms at Ford Motor Co., says Ford currently regards torque vectoring as an extremely cost-prohibitive system, for which buyers would be unwilling to pay, in higher-volume applications where Ford is seeking to proliferate AWD.
He points to the RL's comparatively ambitious price tag - $49,100 - as evidence the cost of the system is being passed along to the buyer.
Audi's Hackenberg says any decision to invest in torque-vectoring technology will be based on a careful cost-benefit analysis, but the company that's built its reputation on AWD engineering ultimately "will do the right thing for the customer."
He says Audi sees much of the cost potentially coming from the need for increased onboard data-handling capability. A torque-vectoring AWD system could demand a significant upgrade in data-handling architecture, he says.
Matt Karaba, vehicle controls performance engineer for General Motors Corp.'s Cadillac Sigma series, says torque vectoring might add 20%-30% more integration effort.
Ricardo's Wheals says software controls are important for robust integration with the vehicle. One chief concern, he says, is in how software would be "de-commissioned" if overheated, say on the racetrack, or in the event of a component failure.
As torque-vectoring applications make their way from development to production, the technology likely will see a variety of engineering and marketing efforts.
An Acura spokesman says the auto maker is backing its torque-vectoring SH-AWD system as a market differentiator. Acura confirmed its upcoming RD-X compact CUV will have SH-AWD, and the spokesman says Acura will "seriously consider SH-AWD for other products as well."
Expect to see torque vectoring emerge in the next few years for high-end or performance vehicles, says Karaba, adding the technology is "on our radar screen," but Cadillac has yet to commit to a production system.
Torque vectoring is an important innovation for either type of vehicle because "you can affect vehicle dynamics without cutting power," he says, referring to contemporary stability-control systems that influence handling through a combination of curbing drive torque and applying the brakes.
Ricardo's Reynolds agrees torque vectoring's best advantage is its active nature.
"(Stability control) is very interventionist." He says when stability-control systems activate to affect handling, "it doesn't necessarily feel good. It's a common complaint" that doesn't go down well with premium-vehicle customers.
Grip And Grin
Ward's Auto World, April 1, 2005 by Bill Visnic
<< Page 1 Continued from page 3. Previous | Next
"Brakes have the capability to apply a lot of torque," adds BorgWarner's Bruder. "It's hard to do that in a seamless fashion." Torque vectoring, he says, is "stability control turned upside down."
The ideal application, he adds, probably is an AWD performance car - where torque vectoring can help generate a more entertaining "oversteer tendency."
Finally, there's a spectrum of methods to achieve the torque-vectoring effect. Acura's SH-AWD is a high-end approach, most say. Ricardo's engineers say they have worked with systems that deliver similar effects with a torque-vectoring device to overspeed just one axle, or with "cross-axle" systems that can increase the rotational speed of individual wheels.
Honda's fifth-generation Prelude (launched in the U.S. in '97) used an optional system dubbed Active Torque Transfer System (ATTS) that effectively generated cross-axle torque vectoring at the front axle. When cornering, the system could shift 80% of the drive torque to the outside front wheel, and overspeed that wheel by as much as 15%.
In markets other than North America, Mitsubishi's Lancer Evolution employs a torque-vectoring differential at the rear axle. Nissan's high-performance Skyline GT-R also has an AWD system that effectively vectors torque.
Wheals says Ricardo is focused on developing a torque-vectoring system that is less costly by employing low actuation torque and can be disconnected when not required, cutting cost and enhancing durability. But Ricardo has no particular preference on whether a torque-vectoring unit overspeeds the axle, or individual wheels.
"We're looking at both," says BorgWarner's Bruder. "We have a little more experience with side-to-side."
He adds that there's also nothing to stop torque-vectoring technology from being used in vehicles - largely fullsize pickups and SUVs - with traditional transfer-case AWD systems, invoking one additional prospect for torque vectoring: Its potential as a stability-control system, particularly for high-center-of-gravity vehicles typically already fitted with AWD.
"You've already got ESP (electronic stability control) to do that," asserts Wheals. But Audi's Hackenberg counters, "In the final stages (of development), it can be a safety feature. It's something we are thinking of."
Price and esoteric functionality, however, are likely to combine to relegate torque vectoring to narrow market segments. Many engineers say a vehicle has to be driven hard to derive torque-vectoring benefits - so unless the system is fitted to some type of sport sedan (such as the RL) or performance car, the benefit would be lost on a typical customer.
"It's a performance-enhancing device," Bruder sums up.
"It can improve limit-handling capability," says Cadillac's Karaba.
Ricardo's Reynolds is more bullish.
"In a decade's time, (torque vectoring will be as common as) air conditioning," he says.
COPYRIGHT 2005 PRIMEDIA Business Magazines & Media Inc. All rights reserved.
COPYRIGHT 2005 Gale Group
Not necessarily to the outer wheel but with the wheel with the most grip, unlike an open diff where it goes to the wheel with the least grip.
With a mechanical system, there is no way to 'override' or change the behaviour of this system. With the torque-vectoring, you can, so this all gets tied into the stability system for that extra bit of safety.
MDX and/or RL don't make good race cars;-) Actually the concept has been put into practice
to excellent effect by Alfa almost 15 years ago. I've driven a 164 Q4, and it feels a bit like an old school QTR car, but with much more willingness to change directions quickly. It can be made to over-steer or under-steer at will, or if driven below threshold, is utterly neutral.
Horribly complex (albeit reliable by all accounts) and expensive, it's taken cheap integrated electronics systems to make it viable for mass market cars. In the end, it depends on how a system is set up to perform. A vectored system going into an Audi will probably yeild more sporting dynamics than say a budget luxury car maker like Acura.
"Alfa Romeo brought the incredible technical experience it had built up over the years to bear when it produced a four wheel drive version of its range leader, the 164. In December 1993, customers were able to buy a 164 Q4 powered by the legendary 231 bhp V6 engine. In this case, the four wheel drive layout had been further developed to ensure maximum performance and peak driving comfort. The heart of the system was the central Viscomatic viscous coupling developed exclusively by Alfa Romeo in conjunction with Steyr-Puch. The Viscomatic was managed by an on-board electronic system that communicated in real time with the engine control unit and ABS control unit. Moment by moment, the system detected and processed information on four different parameters: total drive torque requested, speed, steering angle and slip difference between front and rear axles. It was able to adjust drive torque distribution between the axles with incredible speed on the basis of vehicle speed, cornering radius, engine rpm, throttle opening and closure and ABS parameters. This guaranteed improved torque distribution at any moment and in any situation. In this case too, the Q4 drive system was based on a Torsen self-locking rear differential. This rear differential was responsible for the important task of redistributing the torque allocated to the rear end (in real time) between the wheels on the rear axle: this benefited traction and also car handling over mixed routes. An epicyclic unit was also fitted between the coupling and rear differential to amplify speed differences between coupling input and output. This made it faster and more sensitive while reducing the level of torque managed by the coupling. "
Actually, it is the outer wheel that absorbs the torque with an open diff
it is the inner wheel that gets starved of torque until the bosy roll unloads the wheel enough when, all of a sudden, all the torque goes to the inside and spins it (well technically half the torque that can be put down on the road but the driver feels this as if it is all the available torque).
VC works extremely well with a torsen rear to allow complete torque adjustemtn to the wheels that can utilize it. The Subaru produced by Pro drive has a really advanced version of this set up.