All Wheel Drive (AWD) Demystified
A quick primer on some of the most prevalent all-wheel-drive systems on the market.
The sales say it all: Americans are increasingly opting for all-wheel drive (AWD) 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 Splitand 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-systemFerrari 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 as an all-wheel-drive system 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.
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