There is a time and a place for everything.  After wheeling out to your favorite nature escape for the day, miles from paved roadways and with dusk on your tail, it occurs to you that this is not the time or place to ponder the difference between an AWD and a 4WD system while you inch your vehicle along through the muck.  

Long before you find yourself in this type of situation – ideally during the vehicle selection process – you want to have made the right decision based on your own set of priorities and needs, so spending a few minutes reviewing the differences between vehicles equipped with Four-Wheel Drive (4WD) versus those with All-Wheel Drive (AWD) will be time well spent.

There are a ton of common misconceptions around the differences between (as well as recommended applications for) these systems.  Often, a quick internet search only adds to the confusion, as there are many explanations highlighting the differences between the ideal application of each (e.g. “4WD is better off road, while AWD makes for better on-road handling…”), but these answers merely address the differences in the result of each approach, rather than the specific details of how the approaches are mechanically different.

The only commonality between the two systems is that power from the engine is routed to both the front and rear wheels of the car, but the similarities end there.

TL;DR: Quite simply, the difference lies in what ratio of power is routed to each end of the car, and how that ratio is managed.

The differential or transfer case in the center of the driveline only controls how engine power is routed to the front and rear of the car; differentials at the front and rear axles determine how that power is routed from side to side, and are independent in definition from 4WD or AWD systems.

In either case, to get power to both ends of the vehicle, the engine power (torque) is first transmitted to a device that is responsible for managing how much of that power ultimately makes it to either end. In 4WD cars, the device is a Transfer Case, while in AWD cars that device is referred to as either a Differential or a Transaxle.

The Transfer Case:  Pillar of the 4WD System

The 4WD Transfer Case is a real dummy; primitive and unpretentious, but darn is it reliable! It can’t be bothered with making any kind of decisions as to how much power should go to the front or the rear axle, so it evenly splits the power 50/50 between the axles when engaged. The moniker of “Transfer Case” comes from the requirement that the vehicle operator must choose to transfer power to one or both sets of axles manually and as they see fit, since the device doesn’t do it for them. Later it came to include the fact that the operator also has the option to engage a reduction gearing (“Low range”), which both slows a vehicle’s speed (handy when descending a steep hill or inching over a boulder) as well as multiplies the torque produced by the engine. These latter two features are what lend a proper 4WD system to perform better off-road; because when you are going up, over, or through the nasty stuff, you want to be sure that power is going to both axles, and that you are transferring as much power to the ground as possible when in an off-road situation, such as when a wheel lifts from the ground.

In many cases, marketers or salespeople will try to pass off an AWD vehicle as 4WD if the vehicle also has other features that enhance off-road driving, like under-body skid-plating, increased ground clearance, or aggressive tires. Don’t be fooled.

The reason you cannot drive around in a 4WD vehicle with the transfer case engaged full-time is because the old, simple T-case cannot accommodate different wheel speeds between the front and rear, so while turning something must break: Either traction between the tires and the road (which is why they chirp and hop) or the vehicle itself.  If traction doesn’t break at the tire, the transfer case and/or drive-/prop-/half-shafts will implode, as something has to give. In low-friction scenarios like snow or dirt, this is less of an issue.

Central Intelligence:  How the Differential Enables AWD

On the other hand, the AWD Differential certainly pretends to be smarter than the transfer case, and employs mechanical and/or computer-controlled electronics to decide when and how much power should be sent fore and aft. However, the reality is that the overall performance of the AWD system depends heavily on the specific type, and in some cases programming, of a given differential.

The “Differential” is very aptly named, as its job is to accommodate the differences in speeds that all four wheels of a vehicle may be traveling at any given time. Unless a vehicle is traveling in a perfectly straight line, all 4 wheels are rotating at a different speed. In a turn for example, the front wheels are traveling at a different speed than the back. Because AWD systems adjust to each wheel’s varying speed, such systems are able to be full-time.

There are three types of differentials:

• Open: These differentials send power along the route of least resistance. If the front of a vehicle is on an ice patch and the rear is firmly on solid, dry ground, this differential will send all the power to the front, and is therefore essentially useless as a center differential. Only when the front and rear wheels encounter the exact same level of friction would power be sent to both axles. On some modern cars, ABS braking systems are used to try to maintain such parameters and “trick” an open differential into acting like a limited slip differential.

• Limited Slip (LSD): As the name suggests, these differentials will permit some speed differential between front and rear axles to facilitate smooth driving, but at a certain threshold will engage both front and rear axles. There are multiple sub-classifications of this type of differential that vary in how they govern slippage and distribute power, but we won’t get into that here (torsen, viscous couplings, haldex, etc.).

• Locked: Also referred to as a “full-time” or selectively lockable differential or “locker,” this type of differential apes a traditional transfer case in that it either always or selectively (at the operator’s discretion) routes torque to the front and rear or side to side in equal measure; e.g. it is binary – either on or off.

This video from 1937(!) does a remarkably simple and effective job of explaining how differentials work, and is actually quite entertaining, even for non-gearheads!

Because the ratio of fore & aft power can be varied on the fly, a differential can be setup to meet different goals. For example, in a modern sports car, vehicle sensors can use yaw, pitch and wheel-speed sensors to determine if a vehicle is beginning to “under-steer”, and then may invoke the center differential to decrease the power ratio to the front and ramp it up to the rear, in effect helping to “push” a car through a corner. (Under-steer is where the front end is over-loaded and washes out, resulting in diminishing ability to turn the vehicle through a corner). The inverse approach may be seen in sports cars with a rear-wheel bias, wherein torque may be diverted to the front axle on occasion to “pull” a vehicle through corners when it detects “over-steer.”

Alternatively, in a vehicle with “all-weather” or fuel-efficiency goals like a compact SUV, the differential may route all or most of the power to the front wheels only, and when – and only when – mechanical or electrical sensors detect wheel slippage, does it route any power to the rear. Even in these cases it may not be an even split, or even much power at all that gets routed.

Another common misconception between the two systems is around how, or even if, these systems vary in controlling traction from side to side of the vehicle. To clarify, 4WD vs. AWD has absolutely zero relevancy regarding the distribution of power to each side of the vehicle. Even on a vehicle equipped with a traditional transfer case (e.g. pickup truck, Jeep, etc.), many believe using the “4 Wheel Drive Low” range option will lock all 4 wheels together; it does not.  The wheel speeds side to side – even on the same end of a vehicle – are different, which is why the driven axles on all cars are also equipped with differentials, regardless of whether the vehicle is 4WD, AWD, Front wheel drive, or – as God intended – Rear wheel drive.

These front and rear differentials come in the very same flavors as described above (open, limited slip, and locking), and therefore manage the very same attributes of torque distribution, but they do so from side-to-side, rather than front-to-rear as a center differential does.

Faking It:  Brake-Based Limited Slip Differentials and Electric Propulsion

In addition to those 3 types of traditional differentials, some modern vehicles utilize a combination of open differentials and brake-based traction systems to “mimic” the effects of a limited-slip differential, such as on full-size US-market Toyotas like the Tundra and Land Cruiser (“Auto LSD” as they call it).  While brake-based systems can viably be called an “alternative” to the mechanically-based limited-slip differentials, they are not often equal in terms of actual effectiveness.

Since a brake-based system is typically applied to an open differential, power still follows path of least resistance. The idea with this approach is that the brake on one wheel can be applied to generate greater resistance on the braked wheel than what the wheel with traction is experiencing, therefore routing power to the wheel with traction. However there are some factors which limit a brake-based system’s effectiveness as compared to a traditional, mechanical limited-slip or locking differential:

• It is possible that based on the logic in the system, the brake force resistance on the slipping wheel doesn’t actually end up exceeding resistance on the traction wheel, so it ultimately doesn’t send power to the desired wheel.

• In certain conditions like traversing mud and deep snow, a certain amount of wheel-spin is desirable, to both maintain momentum as well as clear the voids in the tire of debris (the latter more in the mudding scenario). The brake-based approach kills wheel spin, so it also kills momentum.

• It is not exactly an alternative to a true locker, where BOTH wheels are driven at exactly the same speeds & power. Unless the vehicle’s computer algorithms behind the brake application can constantly adjust braking force to the slipping wheel in a way which PERFECTLY matches the resistance of the gripping wheel 100% of the time, power is going to favor one wheel over the other, not equally like a locker does.

 

Electric Company:  Replacing Differentials and Transfer Cases With Electric Motors

Another approach to all-wheel traction emerging in the industry is a setup that ditches mechanical transfer cases or differentials to apportion engine power front and rear. In these cases, a traditional internal combustion will drive one set of wheels (typically the rear in the applications today), while electric motors drive the other (again, typically the front today). This adds even more possibilities, such as FWD-only electric cruising or all-wheel full-power blasts. This setup is seen today on “hypercars” like the Ferrari LaFerrari, and “supercars” like the new Acura NSX.

While performance-oriented calibrations of this technology are fairly nascent, the concept is not really all that new or exotic.  When the Lexus RX400h first debuted in 2005, it powered the front wheels with the engine and engaged the rear wheels via electric motors to achieve AWD at the computer’s discretion.

Finally, some of today’s offerings from Tesla ditch all engines and differentials entirely, and achieve AWD instead with dedicated electric motors driving the front and rear axle independently.  This approach is currently on offer in Tesla’s Model S, X, and (perhaps eventually) 3 vehicles.

In all of these cases, the control over fore and aft torque distribution must be 100% governed electronically by on-board computers.

Therefore, the AWD vs. 4WD element is just one of many that will determine a vehicle’s on- or off-road performance and traction. Front and rear differential type is another element, as is the tire itself. In fact, the latter two have a far greater impact on traction and performance than either AWD or 4WD (for example, a RWD vehicle with dedicated snow tires is proven to be more effective at handling wet, icy and snowy roads than an AWD or 4WD vehicle with all-season tires).

Armed with this information, go forth and conquer the type of terrain you are most interested in, and do not let the marketing-speak distract you with factors irrelevant to the mechanicals. Happy wheeling!