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AWD on adapted front-drive platforms

Driveline suppliers say cost pressures, common components and architectures, and the desire to provide improved performance with all-weather capability are behind the ascendance of AWD on adapted front-drive platforms. Critics say this drive strategy—no matter how advanced—can't compete on an image basis with rear-drive platforms. Illustration courtesy of American Axle & Manufacturing.

Acura's 2005 RL

Acura's 2005 RL ditches the stodgy sheetmetal of its predecessor, and adds a highly competent all-wheel-drive system to improve its image.

Engine is still transverse

The engine is still transverse, but drives both the front and rear wheels. A combination of three planetary gear clutch sets and a pair of direct electromagnetic clutch units are used to vary continuously vary torque between the rear wheels.

The Allure OF AWD

The battle between all-wheel-drive and rear-drive has just begun. Adapting front-drive to propel both the front and rear wheels would seem to be the smart answer for many automakers, but it may not be enough to pull their luxury models even with the image leaders.

Acura is the latest automaker to adapt an all-wheel-drive system to a front-drive architecture to produce a performance sedan that can turn as well as it accelerates. The 2005 RL's Super Handling All-Wheel-Drive (SH-AWD) system apportions power to the wheels individually, managing torque such that the car tracks with an almost otherworldly tenaciousness. And, as such, it answers the question whether a transverse front-engine vehicle can acquit itself when paired with its all-wheel-drive (AWD) and rear-drive competition.

The question it can't answer, however, is if AWD has the image, balance, and dynamic purity necessary to place it ahead of rear-drive in this market segment. AutoPacific (Tustin, CA) president George Peterson says his research firm routinely questions consumers in its Future Vehicle Survey, as well as in focus groups, whether drive configuration is an issue. The conclusion? "American drivers really don't know or care whether they have front-wheel-drive or rear-wheel-drive. When you add all-wheel-drive," he says, "they select that because 'more' sounds better than 'less'." Not surprisingly, those who live in the Snow belt are biased toward front- or all-wheel-drive, a predisposition not lost on DaimlerChrysler. It went to great lengths to show just how good the rear-drive Chrysler 300 and Dodge Magnum are in snow with the latest traction control system, and have AWD versions for those who remain unconvinced.

Cadillac has followed much the same path, jettisoning its front-drive focus for a predominately rear-drive future that has the occasional AWD entry. It is a pattern initiated by BMW and Mercedes, followed by Lexus and Infiniti, and now adopted by two of Detroit's Big Three. Conversely, Ford has chosen to follow the path trod by Honda/Acura, VW/Audi/Bentley, etc., and adapt AWD to what is basically a front-drive architecture for its future Lincolns.

"It's easier and less costly to add all-wheel-drive to an existing front-drive architecture," says John Bellanti, v.p. Engineering and Chief Technology Office at American Axle & Manufacturing (Detroit). "Unlike rear-drive performance/luxury vehicles," he says, "adding all-wheel-drive to a front-drive vehicle is a decision based on function, not performance, though the ability to manage the torque output to each wheel is so great that what you give up in dynamic purity can be compensated for with the combination of increased agility and all-weather ability."

With the increased focus on engine output, front-drive vehicles have begun to test the limits of market acceptability. "The manual transmission Acura TL is at the handling limit of current technology if you want to do more than travel quickly in a straight line," says Tom Elliott, executive v.p. Automotive Operations, American Honda. During development of the 2005 RL sedan, Honda had two choices: develop an AWD system for the "FF" platform, or create a unique rear-drive sedan from scratch. "Adding AWD was an easy choice," says Elliott.

As demand for all-wheel-drive continues to grow–Ford claims that consumer awareness of trends in the luxury and SUV markets is driving the desire down through the ranks–one of the greatest restrictions facing automakers is space. "In order to adapt it to a vehicle," says American Axle's Bellanti, "there has to be enough room around the steering gear, catalyst and front subframe to fit the requisite power take-offs and other driveline bits necessary for the transformation to all-wheel-drive." The other limiting factors–imagination and budget, according to Bellanti–are driven by space limitations, but often have an outsized effect on whether or not AWD is adopted on current platforms. As with all things automotive, starting from scratch with AWD factored into the plans makes the transition much easier.

Therefore, we can conclude that the industry's intense cost pressures, drive toward common component sets and architectures, and need to produce as many variations from these common pieces puts the path followed by Honda/Acura, VW/Audi and Ford in the driver's seat, right? Not necessarily. Despite admitting that, "Tragically, American consumers don't have the knowledge or sophistication to select the drive configuration of their car or truck," AutoPacific's George Peterson says automakers intent on meeting the driving dynamic demands of luxury consumers view rear-drive as the drive configuration of choice. "They know this is the only way to provide the steering dynamics and weight distribution luxury consumers demand." And the image rear-drive luxury sedans (think BMW) have with buyers will continue to separate "real" luxury cars from the "poseurs" for the foreseeable future.



 

An Opposing World View: The 2005 AcurA RL

 

Honda says the all-wheel-drive (AWD) versus rear-drive battle is far from over, and points to the SH-AWD system on the latest Acura RL as proof of savvy technology's ability to level playing field. The drive unit takes power down the centerline of the car through a carbon fiber composite driveshaft to a rear drive unit with three planetary gear and clutch sets. The first of these, called the "Acceleration Device" by Honda, uses a compact planetary gearset to increase output shaft speed by up to 5% during cornering. Identical direct electromagnetic clutch units on ether side of the rear drive unit's hypoid gear control the amount of torque that reaches each rear wheel, and act as the limited-slip differential. Torque to each rear wheel can vary continuously between zero and 100%, and is most often seen when the system counteracts understeer by spinning the outside rear wheel faster than the average speed of the two front wheels. According to Honda, it makes the RL more responsive, neutral, and predictable.

The 3.5-liter V6 produces 300 hp at 6,200 rpm, 260 lb-ft of torque at 5,000 rpm, and drives through a five-speed automatic transmission. It has an 11.0:1 compression ratio, VTEC variable valve timing, a two-piece dual-stage intake manifold, variable flow exhaust system, drive-by-wire throttle, 32-bit RISC ECU, and meets California's LEV-2 ULEV emission standard. Compared to its 3.5-liter predecessor, horsepower is up by 75 (15 from the intake manifold design, 40 from internal engine efficiencies, and 20 from the exhaust system).

The handsome new body was designed in Japan, is shorter, taller and roomier than its predecessor despite a 4.4-in. shorter wheelbase, and boasts an aluminum hood, fenders, trunk lid, front and rear subframes, suspension arms, and bumper beams. Blow forming–using a heated mold to warm the aluminum to 500ºF before forcing them into the mold with high-pressure gas–is used for the hood, trunk lid and fenders. Grade 80 high-tensile steel is used in the side sills, under-floor spars, and in the crush zones of the front and side frames. Grade 45 steel is used on the front shock mounts, and parts of the front bulkhead, front subframe mounts, and rear structure. The rest of the body is made of Grade 60 steel.

Active Noise Cancellation (ANC) is used to reduce low-frequency exhaust "booming" and operates whether the sound system is on or off. Two headliner-mounted microphones (one behind the front overhead console, one ahead of the rear headliner light) send signals to the ANC unit, which sends a reverse-phase signal through the amplifier and speakers. Five millimeter side glass, underbody acoustic covers, lightweight damping materials, and specialized mounts for the mechanical systems round out the RL's noise reduction strategy.

Impressive technology all, but the industry will be most interested in how an image-conscious public responds to the drive layout as each automaker decides which path to follow.