Auto Academy: What’s the Difference Between AWD and 4WD?


Source: Subaru
Source: Subaru

In many parts of the country, a cold winter is a reality that many must face. Where I live, it’s dark by 4:30 p.m. and there are road signs asking drivers if they have installed their winter tires yet. Many folks up here are not fazed by the looming prospect of winter; no matter how bad it is, it seems like they can always tell you about one that was worse. They simply point to their buffalo flannel shirts and say “between this and my Four-Wheel Drive (4WD), I’ll get through anything.” While I admire their gumption, they usually say this while standing next to a Subaru Outback wagon (in green, of course). Subaru offers their vehicles with All-Wheel Drive (AWD), not 4WD. When I tried to explain this to one driver, the grizzled woodchuck scoffed at me, “Typical flatlander! Thinks he knows everything. My cars only got four wheels and it’s driving all of ‘em so it’s four-wheel drive.”

No, my woodchuck friend, it’s not. Here, drink this maple syrup while I explain.

Public service announcement: 4WD and AWD systems are great, but they will not exempt your car from the laws of physics. If the traction between the road and tire is lost, as it can be on ice or during hydroplaning, you can still lose control. Traction during winter driving can be improved using winter tires, but the best safety features in any car are awareness and common sense.

Let’s start with a typical Rear-Wheel Drive (RWD) car. Power leaves the back of the engine, goes through the transmission, down the driveshaft, and into the differential. A differential is a device that allows wheels to spin at different speeds. This is important because the wheels will rotate at different speeds any time a car goes around a corner. The wheel on the inside of the corner will travel a shorter distance than the wheel on the outside. Given that they are connected and will cover the distance in the same amount of time, the outside wheel needs to rotate more quickly.

Back in 1893, Brahmah Joseph Diplock patented a 4WD system for a traction engine. It wasn’t until a decade later that the first 4WD car emerged: It was known as the Spyker 60HP and was also the first car to feature a six-cylinder engine. 4WD systems, like the one used on the Spyker, allow the engine to drive all four wheels simultaneously (clearly they spent a long time coming up with a creative name).

To provide power to the front wheels as well as the rear, a transfer case is used. The driveshaft that is coming out of the engine mates to the input shaft of the transfer case. This causes the internals of the transfer case to rotate. If the system is in two-wheel drive mode (2WD), then the power will continue out the back of the transfer case, via the rear output shaft, to the rear differential as usual. If 4WD is selected, the internals of the transfer case will connect the input shaft to the front output shaft and some of the power will be directed to the front wheels.

It’s very important that the transfer case be able to switch between 2WD and 4WD modes. When a vehicle is in 4WD, the engine is rotating all four wheels. In theory, this means that they would all rotate at the same speed. However, because of things like the cornering speed changes mentioned above, that’s not the case: When powered wheels start turning at different speeds, it stresses the drivetrain and can cause transmission wind-up, which can destroy the transmission.

Source: ReTech

If the vehicle is on a limited traction surface (ice, snow, sand, gravel, etc.), then the different wheel rotation speeds will not be an issue as they are able to slip to release the internal tension. A transfer case will allow the user to select between 2WD and 4WD based on the situation. Most transfer cases will also have high and low ranges: The high range is typically 1:1, meaning that the torque applied to the input shaft is equal to the torque provided by the front output shaft, whereas the low range is often 2:1 or higher. This multiplies the torque going to the front wheels, which is useful for low-speed crawling situations.

An inherent flaw to the 4WD system that I have described is that it requires the user to manually switch from 2WD to 4WD. Usually, the vehicle needs to be at a stop (or at a very low speed with electronic systems). Some systems require that the user get out of the car to engage manual locking hubs that connect the hubs, and therefore wheels, to the half-shafts that power the wheels on the front axle.

All of this means that it is less likely that you will be able to benefit from 4WD systems during daily driving. Subaru developed a part-time 4WD system that aimed to solve this problem, but it wasn’t until the AMC Eagle in 1979 that the AWD system reached the state that still exists on many AWD cars today, with the change from a transfer gear system to one that used a viscous-coupling center differential. A center differential should make sense at this point, but what on earth does viscous-coupling mean?

Source: Subaru

The “viscous-coupling ” part of the name refers to the fact that there are plates connected to the input and output shafts that are manufactured with perforations on them. These plates are bathed in a dilatant fluid, which becomes more viscous when it is subjected to shearing force.

If the front and rear wheels begin to turn at different speeds (during a low-traction situation when the rear wheels are spinning), the plates rotating in opposite directions cause a shearing force in the fluid, which increases the viscosity, effectively locking the plates. This transfers power to the front wheels and causes all four wheels to be driven without the need for driver input. Additionally, if the wheels resume rotating at similar speeds, the viscosity drops and the front wheels stop being powered, which prevents the driveline damage mentioned prior.

It should be noted that Subaru only uses a viscous coupling system on cars equipped with manual transmissions. They have several other setups that provide power to all wheels at all times, rather than as a reaction to a loss of traction. Regardless of the setup, powering more wheels comes at the expense of added weight and complexity, plus decreased fuel economy, but that is often a small price to pay for the potential to improve performance.

As a thank you for making it to the end of this dense article, here is the most ’90s car video (covering 4WD versus RWD) I have ever seen.

Like classics? It’s always Throwback Thursday somewhere.