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How Does All-Wheel Drive Work?

The past few years have seen an explosion in the amount of vehicles offered with all-wheel drive. From crossovers and SUVs to compact cars and flagship sedans, all-wheel drive is becoming a must-have feature for those who live in inclement climates.

All-Wheel Drive Basics

Let’s get a basic understanding of how all-wheel drive works before we dive into the configurations of this system. For a vehicle to have all-wheel drive, it has to be equipped with a center differential. A center differential is a set of gears that splits power from the transmission to the front and rear axles. Helping out the differential out are wheel sensors, which detect traction loss, wheel speed and other data points.

Now let’s look at how all of these components work together. Let’s say you are stopped on a snowy road and you decide to its time to move. You press down on the accelerator and power begins flowing to the wheels, but one wheel is spinning in the snow without gaining any traction on the road. This is where all-wheel drive comes into play. Sensors detect the loss of traction and begin feeding information to the vehicle’s computer. From there, the computer will adjust the amount of power going to each wheel, sending more to the wheels with better grip to get the vehicle moving.

All-Wheel Drive vs. Four-Wheel Drive

All-wheel drive is often interchanged with four-wheel drive in advertising, which causes many people to think they are the same thing. However, there are some key differences between all- and four-wheel drive systems. One of the big differences is how often these systems are on. All-wheel drive is running all the time, varying the amount of power to each wheel. Traditional four-wheel drive systems in many vehicles are classified as part-time systems, which means the vehicle operates in rear-wheel drive until four-wheel drive is selected by the driver. You’ll find part-time four-wheel drive systems in many trucks and off-road SUVs. However, certain models built by automakers such as Jeep and Land Rover have full-time four-wheel drive systems that are always on.

While all- and four-wheel drive systems both have front and rear differentials, four-wheel drive systems usually feature a transfer case instead of a center differential. This part has a number of gears encased in it that split power to the front and rear axles, providing both sets of wheels with maximum torque.

Transfer cases also usually provide high- and low-range gearing. High range will allow you to drive at normal speeds, and is typically used for icy conditions or snow-covered roads. Using high range might also make sense on dirt roads, or in light off-road situations. Low-range gearing will limit your top speed, but it also provides a lot more power for more intense off-road conditions such as rock climbing, deep sand or muddy trails.

All-Wheel Drive Considerations

All-wheel drive provides a number of benefits, including improved traction, better acceleration in poor weather conditions and handling that’s more confident on dry roads. If you live in the Snow Belt it could also mean a higher resale value when it’s time to sell your car.

On the flip side, it can lead to higher upfront costs since all-wheel drive can be a costly option. Ownership costs are also higher, as all-wheel drive cars typically use more fuel and require maintenance for their additional drivetrain components.

Types of All-Wheel Drive Systems

On-Demand All-Wheel Drive: This is a system found on many vehicles that are based on front-wheel drive models. The key trait of an on-demand system is that it sends all of the power to the front wheels in normal driving situations. This contributes to better fuel economy, as well as handling that’s similar to the related front-wheel drive model. When slip is detected, an on-demand all-wheel drive system sends power to the rear wheels to improve traction. Crossover SUVs like the Honda CR-V and Chevrolet Equinox are examples of models that use an on-demand all-wheel drive system.

Symmetrical All-Wheel Drive: As the name suggests, symmetrical all-wheel drive systems send power to the front and rear axles at all times. When the system detects a loss of traction, more power is sent to the axle that needs it. Subaru’s Symmetrical All-Wheel Drive is the most recognizable of these systems, and it can send up to 85 percent of power to the axle that needs it if slip is detected.

Torque-Vectoring All-Wheel Drive: Like other systems, torque-vectoring all-wheel drive can provide additional power to the front or rear axles when it is needed. However, these systems can also split power between the wheels at the rear axle, sending more of it to the outside rear wheel to improve handling. Acura’s Super-Handling All-Wheel Drive (SH-AWD) is a torque-vectoring system that can be found on models like the MDX and TLX.

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