All-wheel-drive systems and how they work (photos)
The truth behind the badge
We see all-wheel drive (AWD) badges on cars, crossovers, and SUVs all of the time. But what does that badge mean? The answer, as it turns out, varies by automaker and even by model. We've rounded up a few of the more common AWD configurations and a few of our favorite uncommon setups to help you figure out the truth behind the badge.
Consider this more of a primer than an all-inclusive roundup, as we've mostly steered clear of the world of 4x4, 4WD, manual drive select, and locking center differentials that populate the world of off-roading and truck transmissions.
Photo by: Ford/Lincoln
Mazda Active Torque Split All-Wheel Drive
Seen on: Mazda Mazdaspeed6, CX-7, CX-9
How it works: The Mazda Active Torque Split All-Wheel Drive system's resting state sends 100 percent of torque to the front wheels. However, depending on the vehicle's grip needs, an electromagnetic center differential can divert power to the rear wheel for up to a 50/50 torque split between the front and rear axles. In the case of the Mazdaspeed6, the torque that makes it to the rear axle is once again split between the rear wheels with a grip, increasing limited-slip differential.
Photo by: Mazda
Other on-demand all-wheel-drive systems (FWD-weighted)
Seen on: Honda Real-Time 4WD, Hyundai TorqTransfer, Ford/Lincoln Intelligent AWD, Lexus Active Torque Control AWD, Volvo AWD system
How they work: The overwhelming majority of passenger car AWD/4WD systems are on-demand in nature. In their resting and cruising states, these systems operate in a standard front-wheel-drive configuration, sending 100 percent of power to the front wheels for more-predictable handling and increased fuel economy. As necessary, these systems can send power to the rear axle to counteract slip or to boost handling. Although the amount of power that gets sent to the back axle varies between manufacturers and models, they rarely approach and almost never exceed a 50/50 split.
Volvo's AWD system is technically a real-time system (which we'll discuss in a bit), but since it only send 5 percent of its power to the rear axle in its default state, we're categorizing it as a front-weighted system.
Advantages: Improved fuel economy during normal driving conditions, sending more power to the front makes them potentially more stable than rear-weighted vehicles
Photo by: Honda
Hybrid on-demand AWD systems
Seen on: Lexus RX400h and 450h, Toyota Highlander Hybrid, Ford Escape Hybrid
How they work: While the all-wheel-drive system in most crossovers is a standard on-demand type, a few hybrid crossovers take advantage of their electrification to add AWD in a novel manner: with a second electric motor. Lexus' RX400h was the first to play this trick, adding an electric motor to the rear axle to add on-demand all-wheel drive without a physical connection to the gasoline motor.
Advantages: Increased fuel economy, increased braking regeneration from second motor, lack of center driveshaft potentially opens more cabin space
Photo by: Corinne Schulze/CNET
Seen on: Various BMW vehicles
How it works: BMW is best known for its rear-wheel-driven sports car handling. So when the automaker designed its xDrive AWD system for use of its sport sedans, crossovers, and SUVs, it stuck with what it knew and started with a RWD platform. In its resting state, a clutch outputting to the front axle sits opens, sending 100 percent of power to the vehicle's rear axle. However, under most driving conditions, that clutch is activated and maintains an actual torque split that sits closer to a more stable 40/60 front to rear split. Power distribution is variable, so if the vehicle detects understeer while cornering, it may send more power to the rear axle to help rotate the vehicle. Likewise, the xDrive system may send up to 50 percent of power to the front axle to counteract oversteer. From the driver's seat, it's highly unlikely that you'll notice any of this happening.
Photo by: BMW
Other rear-weighted all-wheel drive systems
Seen on: Lexus All-Wheel Drive, Infiniti Intelligent AWD, Dodge Charger R/T AWD, Mercedes-Benz 4Matic, Porsche Traction Management
How they work: These all-wheel-drive systems are found on vehicles based on a rear-wheel-drive chassis and, consequently, tend to be more performance oriented. The default power split varies between manufacturers. Lexus starts with a 30/70 front-to-rear split for its IS, GS, and LS sedans. Meanwhile, Infiniti's Intelligent AWD has a resting state of 100 percent rear axle power with the front wheels only getting torque on-demand. Some Mercedes-Benz 4Matics can be locked into a RWD mode or a 4WD mode with a 35/65 front-to-rear axle torque split. What all of these systems share is that they're limited to a max 50/50 split, which means the front axle is never more powerful than the rear, maintaining the rear-wheel-drive pedigree.
Advantages: Usually more performance oriented
Photo by: Infiniti
Acura Super Handling All-Wheel Drive
Seen on: Acura TL SH-AWD, Acura MDX
How it works: Acura's full-time all-wheel-drive system uses an electromagnetic clutch to vary the torque split, sending up to 70 percent of available power to either the front or real axle. However, the SH-AWD system's torque vectoring on its rear axle is what transformed the large TL and the downright massive MDX into handling gems. Up to 100 percent of available rear axle power can be sent to either rear wheel via a second electromagnetic clutch-actuated rear differential. During cornering, the TL can send about 160 pound-feet of torque (estimated with drivetrain loss) to the outside rear wheel to help rotate the vehicle around its center, reducing understeer and improving handling significantly.
Photo by: Acura
Other torque vectoring all-wheel drive systems
Seen on: Saab Cross-Wheel Drive (XWD), Nissan Juke Torque Vectoring AWD, Audi Quattro with Sport Differential
How they work: A vehicle with torque vectoring can start as an on-demand or full-time all-wheel-drive system. What sets these cars apart is the magic that happens on the rear axle. Like any other AWD system with a rear differential, whatever power is sent to the rear axle is then split between the rear wheels. Unlike a standard limited slip differential, which reacts to increase traction, a torque vectoring system's active rear differential proactively sends power to the wheel where it can best be used--usually the outside rear wheel in a turn. Saab's XWD system can send as much as 85 percent of available engine torque to a single rear wheel if its ECU deems necessary.
Advantages: Dramatically improved handling, even faster cornering than standard RWD and RWD-weighted AWD systems
Photo by: Nissan
Subaru Symmetrical All-Wheel Drive
Seen on: Practically every Subaru vehicle
How it works: Subaru built its reputation on its Symmetrical All-Wheel Drive system, so it's no surprise that nearly every vehicle that wears the automaker's Pleiades-inspired badge is underpinned by it. Subaru's system starts with a 50/50 torque split between the front and rear axles (hence the symmetrical moniker), but can send as much as 80 percent of power to either end of the vehicle in the event of slip or divert the majority of its power to a single wheel if that's all the grip it has. Some automatic transmission-equipped models can send even more power (up to 90 percent) in either direction, but the center differential's design prevents Subaru's system from ever locking completely in front- or rear-wheel-drive modes.
Photo by: Subaru
Other full time all-wheel drive systems
Seen on: Audi Quattro, Mini All4, Land Rover/Range Rover
How they work: Vehicles that fall into this category are usually built from the ground up as all-wheel-drive vehicles. These all-wheel-drive systems are always engaged and differ from on-demand systems with a resting torque distribution that sits somewhere around a 50/50 split and the ability to send the majority of torque to either axle as needed. Rare cases, such as Mini Cooper Countryman's All4 system, are even able to send 100 percent of available torque to either the front or rear end.
Advantages: Increased grip in slippery situations, faster reaction to road conditions, increased safety, and best overall safety
Photo by: Antuan Goodwin/CNET
Seen on: Various Audi vehicles, also known as Volkswagen 4Motion
How it works: No doubt, you've probably noticed Quattro popping up all over this roundup. Audi vehicles have been wearing the Quattro badge since 1981 and in that time the system has gone through six distinct generations. (If you count the additions of the sport differential to the fifth-generation system, then increase that number to seven. If you also count the Volkswagen-developed transverse engine systems like the one in the R8, go ahead and add a few more.) Most new Audis use the fifth-generation Quattro systems, which uses a Torsen planetary gearset center differential that defaults to a 50/50 split, but can actively transfer up to 80 percent of torque to either axle. On models with the sport differential, torque vectoring is added to the rear axle. We like to call that version 5.5.
The newest sixth-generation Quattro system is currently only available on the RS5, which can alter torque application by sending up to 70 percent of available power to the front axle or up to 85 percent to the rear. This new "Crown Gear" differential is more rugged than the Torsen type of the fifth generation and is meant to eventually replace the older system.
Photo by: Audi
Subaru Driver-controlled Center Differential
Seen on: Subaru Impreza WRX STI
How it works: The hottest Subie of them all differentiates itself from lesser Imprezas with a standing 49/51 front to rear torque split. Its center differential is upgraded to an electronically managed Driver Controlled Center Differential (DCCD) unit, which essentially acts like both an open and a limited-slip differential, depending on what it's being asked to do at the moment. For example, when entering a turn, the DCCD opens up for a 35/65 front to rear split, but when accelerating hard out of the turn, the system locks at a 49/51 split for maximum traction. Additionally, both the front and rear axles gets limited-slip differentials of their own for maintaining grip from left to right.
In addition to the DCCD--which features three driver selectable modes, automatic modes, and a manual torque split selector--there's also a Vehicle Dynamics Control (VDC) traction control system with three modes of its own and Subaru's Intelligent-Drive (SI-Drive) throttle and power control system with yet three more modes. That's a whole lot of settings!
Photo by: Subaru
Mitsubishi Super All-wheel Control
Seen on: Mitsubishi Lancer Evolution VIII, IX, and X models, Outlander GT
How it works: The mighty Mitsubishi Lancer Evolution's hilariously named Super All-wheel Control is actually a suite of technologies that includes Active Center Differential (ACD), Active Yaw Control (AYC), and Active Stability Control (ASC). Starting with ACD, this center differential adjusts torque distribution between the front and rear axles. Being based on a FWD vehicle, the Evolution is able to send anywhere between 100-percent of power to the front axle and a 50/50 split from front to rear. Contrary to popular belief, the Evolution's rear axle never gets more torque than the front.
However, it does make good use of the power that does make it back there. Active Yaw Control was one of the first torque vectoring systems available on a mass-produced vehicle and allows the Evo to actively distribute torque across the rear axle for enhanced cornering performance. Working in conjunction with these systems is the Evolution's Active Stability Control, which ties in the ABS and traction control systems to enhance grip at all four corners on acceleration and deceleration.
Mitsubishi's Outlander GT features its own S-AWC system with a trick up its sleeve that sets it apart from the Evo: torque-vectoring at the front differential, rather than the rear.
Photo by: Mitsubishi
Nissan ATTESA-ETS Pro
Seen on: Nissan GT-R
How it works: Winning the longest acronym award for this roundup is the Nissan GT-R's Advanced Total Traction Engineering System for All-Terrain with Electronic Torque Split (ATTESA-ETS Pro). Versions of this system have been in use since 1989 on various generations of the Skyline GT-R, but the most recent iteration of the Nissan GT-R is dramatically different from any of its predecessors.
Power flows out of the engine, down longitudinal driveshaft to the transmission located on the rear axle. Here the ATTESA-ETS decides on the power split and can send as much as 50 percent of available torque (or as little as 2 percent) back to the front axle via a second driveshaft. At both ends of the vehicle, torque is also split laterally between the left and right wheels by a differential: an open differential on the front axle and a computer controlled limited-slip differential at the rear that's integrated into the body of the transmission for maximum responsiveness.
Essentially, the GT-R is a RWD car that can become AWD when needed. It also carries the distinction of being the only rear transaxled AWD car in production today.
Photo by: Nissan
Ferrari 4RM dual gearbox all-wheel drive
Seen on: Ferrari FF
How it works: A two-speed automatic gearbox, called a Power Take-off Unit (PTU), sits on the front end of the engine, directly connected to the crankshaft and sending up to 20 percent of available power to the front axle. The PTU features a pair of electromagnetic clutches that operate as a torque vectoring system to actively split power between the front wheels.
But it doesn't stop there, because at the back of the engine is the seven-speed double clutch semi-automatic transmission that you'd expect to see on a Ferrari sending the other 80-plus percent of available torque to the rear axle where an electronic differential uses torque vectoring to split power between the rear wheels. Check out Ferrari's video to see the system in action.
The two-speed PTU features tall gearing, so its first gear operates through the driver's first and second shift on the rear gearbox, while the PTU's second gear operates through the driver's third and fourth. Beyond fourth gear, the PTU disengages and the FF becomes a purely RWD affair. Interestingly, the PTU features a reverse gear as well, so you even get AWD while going backward. With two gearboxes and two torque vectoring differentials, Ferrari's 4RM is likely one of the most complex all-wheel-drive systems on the road today. Interestingly, Ferrari also claims that its system is about 50 percent lighter than a standard AWD system.