All-Wheel-Drive Systems Explained (2024)

From the August 2016 issue

All-wheel-drive systems are proliferating through the car market like so many tribbles on Kirk’s Starship Enterprise. These systems promise all-weather assurance as well as dry-road dynamic benefits, and many car buyers believe them to be essential to any new-vehicle checklist. But not every all-wheel-drive system is created equal. Herewith, splitting some hairs over how all-wheel-drive systems split torque.

Torque, despite its industrious reputation, is lazy. Left undirected, like toddlers or teen­agers, it will frustrate, always preferring the path of least resistance. And in automotive terms, that most frequently means spinning tires. Not that we mind spinning tires, but since an engine’s job is to get us where we want to go, harnessing its torque to accomplish that task is only pragmatic. Thus, all-wheel drive, which divides the work of moving us not among two wheels but among four. How today’s all-wheel-drive vehicles direct torque is varied, but doing it well means distributing the right amount of torque to the right wheels at the right time.

Notice that we wrote all-wheel drive, not four-wheel drive. It’s a distinction that matters in these pages. By our definition, four-wheel-drive vehicles, mainly trucks, can only lock their front and rear drive­shafts so that each axle always turns at the same speed. And they do it that way whenever they’re driving all four wheels. It’s a ­little elementary, sure, but quite often so are the problems trucks aim to solve. Like crawling up steep, rocky trails. Or yanking boats up moss-covered launch ramps. Or our favorite, jumping over cars in beer-doused arenas.

If your goals are more ambitious—like turning, for example—there are more-­effective ways of dividing torque to the front and rear axles than simple transfer cases. One of them is to forgo a mechanical connection altogether and power one axle with electricity. By powering its front wheels with an electric motor, Porsche’s 918 Spyder recalibrated not only our definition of rapid but also our definition of all-wheel drive. Yet it’s not alone in the world of the electrically driven axle. Appearing at the other end of the performance spectrum is Toyota’s RAV4 hybrid crossover, which powers its rear wheels exclusively with an electric motor.

Gas/electric all-wheel-drive systems, which are still in their infancy, range wildly in cost and purpose, and e-axle vehicles are outliers. Though their popularity is increasing, only a handful are sold in the U.S. today.

Many of today’s all-wheel-drive vehicles rely on the far more common center differential, which is a proven means of controlling torque delivery to both axles. Most, however, are on-demand systems based on front-wheel-drive powertrains. What follows is a closer look at the most popular hardware used by today’s all-wheel-drive vehicles to direct power to the ground:

Open Differential

The humble open center differential­—simple, reliable, cheap—has been driven to near extinction by electromechanical alternatives that offer more control and greater efficiency. An open differential, a variation of the common planetary gearset found in automatic transmissions, splits a single torque input (the transmission) into two outputs (the front and rear axles) but allows them to rotate at different speeds. Yet open diffs have no means of limiting the speed variation between the two outputs, so torque is free to follow the path of least resistance. Hence, it’s possible for a vehicle to become stuck with one wheel spinning furiously while the others remain stationary. Most modern vehicles compensate with a cheap but effective combination of software and existing hardware that uses the brakes to create a reaction torque at the slipping wheel, closing the path of least resistance and thus increasing the torque applied to the wheels with more traction.

Open differentials also can be paired with driver-selectable lockers, as in the Mercedes-Benz G-class, which can lock together both the front and rear axles, as well as the left and right wheels. A locked differential is akin to having no differential at all, establishing a solid link ­connecting axles and wheels to the transmission. But the drivetrain will bind and buck once the vehicle reaches high-grip surfaces such as paved roads, where it needs its differentials back for the reason they were invented: to compensate for significant differences in wheel speeds while turning.

[+] Simple, inexpensive
[-] Limited control over torque distribution
Found in: Jeep Grand Cherokee Laredo, Mercedes-Benz G-class

Torque Split: The Gripping Truth

Whenever manufacturers talk about their all-wheel-drive systems, there’s always blather about where the torque is going and in what proportion. This is all theoretical, based on assumptions that are rarely true. When wheelslip occurs in the real world, torque distribution is ultimately determined by the available traction at each tire. That makes torque split a function of load transfer and the friction of the road surface, as much as it is a consequence of the differential configuration. When a manufacturer talks about a 50/50 torque split, it assumes equal grip at each axle, a condition that is unlikely to occur in any situation where you most need all-wheel drive. Likewise, the ability to send 100 percent of the torque to a single axle typically does not mention the caveat that the opposite axle must be spinning its wheels on a surface with almost no friction. (On-demand couplings are the exception to this, as some can send 100 percent of the torque to one axle by decoupling the other.) Because both grip and weight distribution are constantly changing, the quoted torque splits become largely meaningless in the real world. Think of manufacturers’ torque-split talk as akin to promises from presidential candidates: When reality sets in, results may vary.

Limited-Slip Center Differentials

Still relatively simple, these passive center differentials react to changes in torque—either at the wheels or from the engine—to redirect the engine’s motive force to the axle with more grip. They drive all four wheels all the time and rely only on physics, a predictable ally in our experience, to do their job. Forgoing sensors, actuators, and driver intervention means that they are an effective way to couple the front and rear axles while still maintaining the ability to vary the front-to-rear torque distribution. It also means that they keep cost, weight, and complexity relatively low. They come in several varieties:

Viscous Center Differential

These differentials couple the front and rear driveshafts via a series of plates submerged in synthetic fluid inside a sealed housing. When wheelslip causes one shaft’s speed to vary significantly from the other, the fluid’s properties change, allowing the two shafts to rotate at, or closer to, the same speed.

[+] Inexpensive, lightweight, smooth engagement
[-] Requires wheelslip to produce locking force
Found in: Subaru WRX and Crosstrek with manual transmissions

Helical Center Differential

Helical limited-slip differentials, commonly called by the brand name Torsen, are more complicated. These units use carefully tuned planetary-gear arrangements with teeth cut in a helical spiral pattern (think DNA) that bind up or push against friction discs to limit wheelspin and alter the torque distribution. Increasing torque from the engine creates more friction to enhance the locking action. The locking rate of this type of differential is determined by the angle at which the gear teeth are cut: Steeper angles produce more locking force. When used as center differentials, helical limited-slip diffs are often designed to offer an unequal torque bias­—an effect determined by the ratio between the gears that drive the front and rear axles.

[+] Reacts to torque changes from both the engine and slipping wheels
[-] Nonadjustable—locking force determined by gear angle and torque applied, requires resistance at the wheels to produce locking effect
Found in: Audi A8, Bentley Continental GT, Land Rover Range Rover Sport

Electronically Controlled Limited-Slip Center Differential

Functioning similarly to passive limited-slip differentials, these use electric or hydraulic actuators (or both) to engage a clutch that limits slip between driveshafts. The primary advantage here is the ability to function independent of engine torque or friction at the wheels. Using inputs from a series of sensors and computer controls, these diffs offer the full range of operation from fully open to fully locked whenever needed to best suit driving conditions. In recent years, manufacturers have been getting creative in their command of electronically controlled differentials, adding algorithms to predict when more slip is helpful or when preemptively engaging the clutch will ­prevent wheelspin before it occurs. Similarly, electronic controls make center-differential behavior tunable across various drive modes, which optimizes their performance for different surfaces and different levels of driving aggression.

[+] Highly adjustable
[-] Complex, costly
Found in: Subaru WRX STI

On-Demand Coupling

Up to this point we’ve been talking about systems that drive all four wheels all the time through a center differential. On-demand systems operate differently in that they primarily drive only one axle until the coupling engages the opposite axle for assistance. Clutch packs are commonly used here, but so are toothed couplings called dog gears. Often the hardware sits just ahead of the secondary axle, though some systems decouple on both sides of the driveshaft to improve efficiency. Wherever the coupling is located, its mission is the same: Engage the assist axle progressively as needed.

A clutch-pack coupling increases the torque routed to the assist axle by increasing the clamping force on the friction discs, but these systems typically use lighter-duty hardware than full-time systems use because they only drive the second axle a small percentage of the time. Defaulting to two-wheel-drive operation also improves efficiency, which is why on-demand systems have become so popular in this age of strict fuel-economy regulations. Moreover, they offer most of the benefits of electronically controlled limited-slip differentials since they can be programmed to dispatch torque to the secondary axle before slip is detected.

[+] Improved efficiency relative to full-time all-wheel-drive systems
[-] Not full-time all-wheel drive
Found in: Mazda CX-5, Volkswagen Golf R

On-Demand Twin Rear-Axle Couplings

These systems build on the concept of on-demand couplings with a dedicated clutch pack for the left and right rear-axle half-shafts. The rear axle houses conventional ring and pinion gears but no differential. With the clutches fully engaged, these systems function like truck-based four-wheel drive. However, because the clutches offer both rapid and partial engagement, these systems can avoid the binding common to four-wheel drive. Progressively and independently actuating the clutch packs mimics a vehicle with conventional center and rear limited-slip differentials.

Another advantage to twin-clutch systems is that torque vectoring is easily achieved by changing one axle ratio. Ford’s Focus RS, for example, uses this type of system with a rear-axle final drive that is 2.7 percent shorter than what’s used on its front axle. The effect is a rear torque bias and an increased “push” from the back. Anytime the rear clutch packs are engaged, the rear wheels receive more torque and try to turn faster than the front wheels. Either the clutches compensate for the speed difference or the wheels spin. But the urge to spin certain wheels faster creates a motive force that pushes the car from the back. Torque can be split left to right across the rear axle as well. Put it all together and you get the Focus RS’s drift mode, and we can hardly think of a more compelling argument for all-wheel drive than that.

[+] Inherent left/right torque biasing, torque vectoring is possible with a gear-ratio offset
[-] Heavily burdened clutches require careful thermal management
Found in: Acura TLX, Ford Focus RS