Many people think that the terms 4WD and AWD are interchangeable, but in fact these two systems are quite different.
4WD stands for four wheel drive, and refers to a car or more often an SUV (Sports Utility Vehicle) which employs a driver-selectable system that mechanically engages the drive to all four wheels. AWD, on the other hand, stands for all wheel drive and refers to a car that drives all of its wheels all of the time.
A 4WD vehicle is designed for off-road situations because of the extra traction it provides in difficult terrain. A 4WD or 4X4 vehicle typically has a truck-based platform with oversized wheels, special off-road tyres, and a manually selected and locked 4WD driveline.
When 4WD is selected, what is known as the transfer case divides power between the front and rear axles, applying the maximum amount of torque to each wheel. This creates a problem when turning, however, as the inside wheels are turning slower than the outside wheels and so they begin to spin freely, making traction difficult. Modern 4WDs overcome this problem with what is called a ‘High’ setting, which splits the power less evenly between the inside and outside wheels and limits the amount of slipping.
An AWD vehicle drives all of the wheels all of the time, with the added advantage of having a limited slip differential or electronically controlled clutch, which allows a rotational difference between the front and rear axles. This system has twice the grip of a driver-selected 4WD system because it is always engaged, making it safer in slippery road conditions and circumstances where extra traction is needed.
And that’s the biggest difference between 4WD and AWD; the fact that the AWD drive system is on all the time, while the 4WD system must be manually selected. Modern AWD systems are computer driven, using sensors on each wheel to monitor traction and wheel speed at hundreds and even thousands of times per second. An ECU dictates which wheel the power is sent to, depending on which has the most grip.
Pros and cons of each
4WD and AWD systems both have their own advantages and disadvantages.
The pros and cons of a 4WD system include:
- It is more robust than AWD and has much greater levels of traction in low-speed offroad conditions.
- Because it is a selectable system, it can be turned off in normal driving conditions to improve fuel efficiency.
- It adds a lot of weight and complexity to a vehicle, increasing its cost factor in the showroom and at the bowser.
While those of an AWD include:
- It provides improved grip and control all of the time and in all road conditions.
- It can be installed in a wide range of different cars, giving them sportier traction and handling.
- Its permanently engaged system reduces fuel economy.
Which to choose?
Given that both systems have their advantages and pitfalls, how does one decide which, if any, to choose? The answer is, it really comes down to what you plan to use it for.
If you’re an enthusiastic off-roader, then a 4WD with rugged construction and superior traction would be a better bet for you. Bear in mind that 4WDs are associated with higher costs because of expensive oversized wheels, off-road tyres and more regular maintenance than normal vehicles, so you would need to be prepared to pay more for the privilege of being able to go almost anywhere.
On the other hand, if you live in a location where bad weather is a constant factor or even if you’re just looking for a safer car to drive on the road (and off), then an AWD might be the way to go for you.
Whichever you decide on, hopefully this article has at least clarified the difference between AWD and 4WD, so that when the salesperson tries to tell you they’re actually both the same thing, you’ll be able to set them straight once and for all.