What is All-Wheel Drive (AWD)?
An all-wheel drive (AWD) vehicle is one with a powertrain capable of providing power to all its wheels, whether full-time or on-demand.
As the name implies, all-wheel-drive systems power both the front and rear wheels all the time. But in practice, there are actually two types of drivetrains that are called AWD. One does, in fact, drive all the wheels continuously, and some manufacturers refer to this as full-time AWD.
The second, often called part-time AWD or automatic AWD, operates most of the time in either front-wheel drive or rear-wheel drive, depending on the vehicle’s drive system. In these systems, power is delivered to all four corners only when additional traction control is needed.
All-wheel drive is added to a car by giving it three differentials, or gearboxes, in different parts of the car. You’ll find the three gearboxes on the front, center, and rear of your car, and these differentials mean your four tires can get traction independently of each other, allowing for superior traction and handling in all types of weather situations.
With AWD, torque is sent to all four wheels. The advantage of getting moving in slippery conditions is obvious. Since AWD turns four wheels instead of just two, there’s that much more grip, and when the available traction is very low as on snow and ice you can accelerate better, with less or even no tire slippage.
The vehicle feels stable and doesn’t slip or fishtail in a way that makes your heart beat faster. In almost any slippery situation, an AWD is able to accelerate from rest better than one with front-wheel drive or rear-wheel drive that’s rolling on identical tires though there are exceptions.
The most common forms of all-wheel drive are:
- 4×4 (also, four-wheel drive and 4WD). Reflecting two axles with both wheels on each capable of being powered.
- 6×6 (also, six-wheel drive and 6WD). Reflecting three axles with both wheels on each capable of being powered.
- 8×8 (also, eight-wheel drive and 8WD). Reflecting four axles with both wheels on each capable of being powered.
Vehicles may be either part-time all-wheel drive or full-time:
- On-demand (also, part-time). One axle is permanently connected to the drive, the other is being connected as needed
- Full-time (also, permanent). All axles are permanently connected, with or without a differential.
- Independent. The wheels are driven, but not dependent on a central mechanical power coupling.
How Does All-Wheel Drive (AWD) Work?
In an AWD system, torque is sent to all four of a vehicle’s wheels automatically. Drivers typically don’t need to act to start the process, though some systems offer selectable modes that allow drivers to determine how power is distributed.
There are two types of AWD: full-time and part-time. With a full-time system, torque is sent to all four wheels 24/7. With part-time AWD, power is typically sent to either the front or rear axle during normal driving. Sensors determine if there is a need for extra traction, such as when there’s rain, snow, or mud on the ground. Then, the part-time AWD sends power to both axles.
There’s one important note to consider: AWD (and 4WD) only helps you with acceleration. They can help you get started on slick surfaces, but neither one is able to brake better than a vehicle with just two-wheel drive, even on a slick surface. They also don’t increase the grip of your tires themselves, so they won’t help you if you’re coasting around a corner, although they can help you maintain traction when you’re accelerating around a corner.
The Advantages and Disadvantages of All Wheel Drive
All-Wheel Drive (or AWD) is a system in which all four wheels of a car operate simultaneously to improve traction and handling. While it is possible for a car to have continuous AWD capabilities, it is far more common for one pair of wheels to engage only when sensors detect that the other pair has begun to slip. There are both advantages and disadvantages to AWD systems:
Advantages of All Wheel Drive
- Traction. In intermittent AWD systems, the rear wheels engage when sensors detect slippage from the front wheels. Under these circumstances, the vehicle effectively detects and compensates for dangerous driving conditions such as standing water, snow, ice, or gravel that could otherwise compromise control of the vehicle. By engaging the second set of wheels, the vehicle experiences two additional points of contact on the surface of the road, allowing a greater likelihood that its tires will grip the surface and allow the driver to retain control.
- Performance. Performance and handling are additional reasons for some cars to be equipped with all-wheel drive. Steering is also improved in all-wheel drive vehicles, as the front wheels can be more devoted to steering (gaining sideways traction) than driving (gaining forward traction).
- Weight Distribution. All-wheel drive systems come with better weight distribution which provides more consistent handling and allows engineers to more evenly distribute the stress of a vehicle’s weight across its frame.
- Off-Road Capability. All-wheel drive vehicles are also more capable of moving on muddy surfaces or through shallow water.
Disadvantages of All-Wheel Drive
There are several disadvantages to all-wheel drive systems that should also be considered.
- Fuel Efficiency. Since power is being sent to all four wheels some or part of the time, all-wheel drive vehicles see significant drop-offs in terms of their fuel efficiency.
- Increased Cost and Complexity. All-wheel drive systems are also more costly to produce and manufacture, making the vehicles that use them more expensive than their two-wheel drive counterparts.
- False Sense of Security. Finally, all-wheel drive can lead to a false sense of security and encourage dangerous driving in extreme conditions.
- Braking Distance and Collision Avoidance. While the weight of AWD vehicles improves their handling, it also increases the distance they require to stop. In a scenario where the vehicle must make a sudden stop and cannot swerve or turn, a collision becomes more likely than with a lighter car. Under similar circumstances, but ones in which an accident can be avoided by turning, AWD vehicles offer superior collision avoidance than similar vehicles with less effective handling and turning capabilities.
