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AWD 101: Analyzing all-wheel-drive systems

A quick primer on some of the most prevalent all-wheel-drive systems on the market.

By James_Tate Jan 10, 2014 1:33PM

The sales say it all: Americans are increasingly opting for all-wheel-drive vehicles.


Nearly one third of new vehicles sold in the United States last year were equipped with some form of all-wheel drive, and the rise of brands such as Audi means that those numbers will likely grow larger.


To better understand how all-wheel drive works, we're taking a brief technical dive into some of the best-selling all-wheel-drive vehicles on the market.


Subaru Symmetrical AWD


Another brand on the rise closely associated with all-wheel drive is Subaru, whose Symmetrical AWD is found in nearly its entire lineup (save for the BRZ coupe). Symmetrical AWD has a base 50/50 torque split, but can send up to 80 percent of the vehicle’s torque to either axle when needed — and in the case of some models with automatic transmissions, up to 90 percent.


Mitsubishi S-AWC


Subaru isn’t the only Japanese builder with strong AWD credentials; Mitsubishi has Super All-Wheel Control, which involves a host of technologies and systems including an Active Center Differential, Yaw Control, and Stability Control. From a 50/50 torque split, S-AWC can send up to 100 percent of torque to the front axle, but the rear axle never receives more torque than the front.


Audi quattro

The quattro name has been synonymous with Audi for over 30 years — the brand has been at the forefront of all-wheel drive road cars. Most quattro systems employ a planetary center differential with a 40/60 torque split between the front and rear axles. Quattro can actively transfer the majority of torque to either axle (up to 70 percent to the front and, in RS model variants, 85 percent to the rear), and can apply individual brakes to help aid the car in sharp turns.


If you add the “sport differential” on selected models, the rear axle uses actual torque vectoring to slow the inside wheel speed and accelerate the outside wheel speed, thus helping the car resist understeer (the tendency of the front of the car to “push” straight, despite steering input).


Mazda Active Torque Split and Acura SH-AWD


Mazda’s Active Torque Split AWD takes a different approach, operating with 100 percent of power going to the front wheels as its default setting, then sending torque to the rear axle as needed (up to a 50/50 split) through the use of an electromagnetic center differential. Acura’s Super Handling All-Wheel Drive sends 70 percent of torque to either axle, but cleverly uses torque vectoring on the rear axle, which has been especially beneficial to the handling of its big MDX sport utility. SHAWD can send 100 percent of the rear axle's torque to one wheel.


BMW xDrive and Mercedes-Benz 4MATIC



Performance cars and supercars are increasingly employing AWD systems. BMW’s xDrive is becoming more renowned as sales of its SUVs, crossovers and sedans increase. The xDrive system has a 40/60-rear bias, but can send up to 100 percent of torque to either axle. In many cases, it will only send up to 50 percent of torque up front to counter oversteer (the tendency of the rear of the car to step out of line during cornering). Mercedes-Benz 4MATIC, meanwhile, integrates the transfer case and center differential into the transmission to save weight and relies heavily on the traction control system to brake slipping wheels. Up to 70 percent of torque can be sent to the front or rear, but normally it is fixed at 45/55.


Nissan ATTESA-ETS



The Nissan GT-R offers one of the more interesting systems on the road, the very specifically named “Advanced Total Traction Engineering System for All-Terrain with Electronic Torque Split”—even the acronym (ATTESA-ETS) is long. ATTESA-ETS uses two driveshafts: from the engine, power is sent down one driveshaft to the rear axle before the system sends up to 50 percent of it back to the front axle through the second.


Ferrari 4RM


More interesting still is the 4RM all-wheel drive system Ferrari debuted on its FF. 4RM uses two gearboxes — a 2-speed unit is attached to the front of the engine, sending up to 20 percent of the power to the front axle. On top of that, torque vectoring is applied between the two front wheels. At the back of the engine is Ferrari’s 7-speed dual clutch gearbox: the two gearboxes work in tandem through the FF’s fourth gear, past which the FF operates as your run-of-the-mill screamingly fast rear-wheel-drive Italian supercar. Not only is the 4RM system a heck of a mechanical innovation, it’s also about half the weight of a standard AWD system at just 90 pounds.


Jeep


On the other side of the spectrum, hardcore AWD has advanced, too. As the nameplate that made off-roading famous, Jeep’s AWD systems were historically capable and simple. In the modern era however, it seems there are as many Jeep AWD systems as there are models in its lineup. Variants of Freedom Drive, Quadra-Drive, Quadra-Trac, Selec-Trac, Command-Trac, and Rock-Trac can easily lead to confusion over what the differences might be, and in turn confusion for the consumer over what Jeep model is the right one to buy. Modern Jeep AWD is largely built around Brake Lock Differential (BLD), used in models without a locking differential. BLD uses automatic braking to make sure the shared wheels of an axle are spinning at the same rate, handling wheel slippage in wet/snowy/icy conditions.


Hybrid AWD


The future of AWD, however, is emerging in hybrid powertrains. More and more a second electric motor is being used to provide on-demand or seamless all-wheel capability. Acura is taking Hybrid AWD systems a step further in its 2014 RLX Sport Hybrid SH-AWD. The 2014 RLX Sport Hybrid uses three electric motors — one attached to its 7-speed automatic transmission, and a pair of electric motors on its rear axle where a traditional differential would normally be. The rear motors control power distribution to the rear wheels and provide torque vectoring; essentially creating a hybrid version of Acura’s already renowned Super Handling All-Wheel Drive.


James Tate cut his teeth in the business as a race team crew member before moving to the editorial side as Senior Editor of Sport Compact Car, and his work has appeared in Popular Mechanics, Automobile, Motor Trend and European Car. When not writing, Tate is usually fantasizing about a vintage Porsche 911.

129Comments
Jan 12, 2014 8:39AM
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I had two Audi Quatros and loved driving them both just hated the cost of repairs. I now own a new Subaru Outback and these things are bulletproof and go anywhere. Great gas mileage and plenty of room, plus they are so incredibly reliable. I love German cars but they have such a high initial price tag and when they require any parts they cost far too much.
Jan 11, 2014 4:24PM
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it seems that Jeep is the only suv mentioned in this comparison. Why? Chrylser offers awd 300 sedans and Ford also offers a awd Taurus. Why leave these cars out? Not to mention Cadillac sedans offered with awd systems.

Jan 22, 2014 2:37AM
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All wheel drive is great but you did not even start to explain it to the readers.  For people that have not owned or looked into owning one your article did nothing to help them.
Jan 11, 2014 2:56AM
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we have been driving Subaru for two years now super car {outback}  30 mpg  summer time I pull a 17 ft boat with ease  winter time fantastic in snow we live northern Wisconsin best all wheel drive

I found

Jan 12, 2014 11:27AM
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No mention of the "Auto" mode in today's 4WD trucks. It is AWD mode and can be put in 4WD for heavy snow or off-road driving. It's kind of like 4WD-lite, but great on light snow or wet slippery roads with better gas mileage.
Jan 14, 2014 12:03PM
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I like AWD on passenger vehicles but depending on where one lives really determines just how much the extra cost is worth.  For people living in the south it can be a very expensive badge on the back of the vehicle.  AWD means extra parts that will require additional maintenence and eventual, possible repairs.  It also means the vehicle typically gets slightly lower gas mileage than the same model without AWD.  YES, there are many of us who can justify the extra cost for any number of reasons.  There are also many of us who really don't need AWD but choose to pay the premium for it.
Jan 12, 2014 7:59AM
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I drive a 05 Subaru baja turbo and my family has had the car since it had 3 miles on it. I drive it now as my daily driver and has over 151,000 and we have never had a problem with the AWD system in it. I live in FL and it's nice to have the AWD when going off road mudding and even with mostly bald tires or brand new sport tires the AWD will pull me out of anything. The new forester gets just as good of gas millage as the CR-V and if you get the turbo one you only lose like 1 mpg but get 250hp. Tires Are also a major factor in traction in snow. Good snow tires can get any type of car thought just some better than others. I will stick with my subarus 
Jan 10, 2014 8:16PM
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KWATRO1

Been driving Audi Quattros since 1988.....hands down the best engineered & most durable system out there. Put 210K miles on one.....currently 3 in the family w/140K+ miles on them.A good set of winter tires really brings out the best in these cars. The good part is how these cars communicate road conditions to the driver through the steering wheel and the seat through the chassis. I'll buy another one.....make mine an  3.0 TDI quattro in an A5,A6 or A7!

Jan 17, 2014 6:22AM
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I have a 2007 Chevy Trailblazer that has 2wd, 4wd(L), 4wd(H), and Awd.  It also has a separate traction control function.  Last year, and so far this year, we've gotten a lot of snow, and so far, it has never let me down. 

I used to think the 4wd option was more or less, a gimmick to get you into the showroom, but now that I've had it for several years, I'll never own another vehicle that doesn't have it. 

Jan 11, 2014 6:08AM
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Nice article but I would've liked a quick this is how it compares to 4WD.  I personally find FWD handles most of the bad weather driving I encounter and should I ever move to a southern state, or can afford to have 2 vehicles, I will get RWD. 
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