What is All-Wheel Drive?
All-Wheel Drive (AWD) vehicles send power to all four of the wheels. This can be done in a number of ways but the end goal is improved traction and performance for the vehicle. While All-Wheel Drive is a more expensive option and uses more parts (more things that can break), there are some huge benefits. These include:
Better acceleration: With all four wheels putting power down (usually), gaining speed is easier.
More stable acceleration: With the power spread out between two axles there is less wheel-spin and consequently accelerating becomes more consistent.
Better grip in slippery conditions: Whether there is snow on the ground or heavy rain coming down, All-Wheel Drive will make the wheels grip more when accelerating or maintaining speed. All-Wheel Drive also makes the car much less likely to be stuck in mud or snow.
There is a slight distinction to be made between All-Wheel Drive and Four-Wheel Drive. In the US, in order for a vehicle to be labeled All-Wheel Drive, both axles must be able to receive power and rotate at different speeds simultaneously. If a vehicle has a transfer case, meaning that if both axles are receiving power then they will be forced to spin at the same speed, then it is Four-Wheel Drive, not All-Wheel Drive.
Many modern SUVs and Crossovers use All-Wheel Drive systems that are labeled Four-Wheel Drive. This gives axles the ability to spin at different speeds and has many practical applications, meaning manufacturers often reserve true Four-Wheel Drive for heavy-duty and off-road vehicles. They can be labeled as Four-Wheel Drive because they technically are, allowing all four wheels to drive the vehicle forward. Labeling an All-Wheel Drive drivetrain as Four-Wheel Drive also makes it appear more rugged and seem more like a dedicated off-road vehicle.
How Does All-Wheel Drive Work?
If the vehicle has a center differential, then the arrangement of the drivetrain resembles a Rear-Wheel Drive setup. The engine runs into a transmission and then back to the differential. Usually the engine is longitudinally mounted. Instead of connecting to the rear differential, like in a Rear-Wheel Drive vehicle, the driveshaft connects to the center differential.
A center differential acts just like the differentials in either axle. When one side of the differential is spinning at a different speed than the other, it allows one side to slip and the other side to receive more power. From the center differential, one driveshaft runs straight back to the rear differential while another runs to the front differential. Subaru uses a system that is a variation on this type of All-Wheel Drive. Instead of having a driveshaft go to the front axle, the front differential is built into the transfer case along with the center differential.
If the vehicle does not have a center differential, then the arrangement probably resembles that of a Front-Wheel Drive vehicle. The engine, likely transversely mounted, sends power to a transaxle. Instead of sending all of the power to the set of wheels under the engine, some is also sent to a differential on the opposing axle via a driveshaft extending from the transaxle. This functions similarly to an arrangement with a center differential, except that the transaxle almost always gets more power than the opposing axle. This allows a vehicle to use the All-Wheel Drive only when more traction is needed. This type of system provides improved fuel economy and is generally lighter. The downside is less All-Wheel Drive performance on dry roads.
Different Types of All-Wheel Drive
There are two main types of All-Wheel Drive used in vehicles today:
Full-Time All-Wheel Drive: This type of drivetrain uses three differentials to spread the power effectively between all four wheels. The wheels are all receiving power all of the time in this arrangement. Very well-liked All-Wheel Drive systems that have this arrangement include Audi’s Quattro All-Wheel Drive and Subaru’s Symmetrical All-Wheel Drive. Rally racing vehicles and their road-going equivalents use this type of All-Wheel Drive setup almost universally.
Automatic All-Wheel Drive: There is no center differential with this type of All-Wheel Drive. A transaxle powering one set of wheels directly puts most of the power into the front or rear axles while a driveshaft moves power to the differential on the opposing axle. With this type of system, the driver only gets the benefits of All-Wheel Drive in low-traction situations. This setup takes up less space than the alternative and allows the car to run more efficiently when acting as a Front or Rear-Wheel Drive.
Where is All-Wheel Drive Best Used?
Vehicles that see lots of weather: It’s easy to see why people living in very snowy or rainy areas would prefer All-Wheel Drive vehicles. They are less likely to get stuck and are more likely to get themselves unstuck if they do. When paired with weather-appropriate tires, All-Wheel Drive is near unstoppable.
Performance applications: Traction is important in high-powered vehicles. Solid traction allows a vehicle to be faster off the line and faster accelerating out of the turns. Every Lamborghini and Bugatti uses All-Wheel Drive. While there is a heightened risk of understeer (front wheels losing traction in a turn), modern engineering makes it largely a non-issue.
What Are The Drawbacks to All-Wheel Drive?
Sending power to both axles makes the vehicle less fuel-efficient. It has to use more power to get all of the wheels turning and more to make the vehicle accelerate.
The handling characteristics are not universally loved. While All-Wheel Drive allows consumers to get some of the best benefits of both Front-Wheel Drive and Rear-Wheel Drive vehicles, it can also exhibit the negative characteristics of both. Some cars may understeer when the front wheels get too much power in the corners while another may get oversteer when the rear wheels get too much. It’s really a matter of the driver’s taste and the particular vehicle.
More parts means more weight. Weight makes the vehicle perform worse and use more fuel. More parts also means more things that can break. On top of the fact that All-Wheel Drive vehicles generally cost more to start with, services and repairs may cost more down the road as well.
Is All-Wheel Drive Right for Me?
For people living in areas with lots of snowfall annually, All-Wheel Drive cars make sense for everyday use. The higher cost and worse fuel economy is worth the ability to go down the road in heavy snow or drive over a snowbank left haphazardly by a plowing truck. In such areas, All-Wheel Drive vehicles also have great resale value.
That said, many traction problems can be solved by season-appropriate tires. Most roads in most places are drivable often enough that All-Wheel Drive is rarely needed. All-Wheel Drive does not improve brake or steering performance in slippery road conditions, so the vehicles using it are not necessarily safer.
This article originally appeared on YourMechanic.com as How All Wheel Drive Works and was authored by Ian Swan.