Q: I'm looking to buy an AWD sports car and was told today by the dealer that they offer extra grip because they drive four wheels instead of two. Is this right? How are they different to a two-wheel drive car?
A: This is a great question Tony and in this instance, the dealer is wrong about the benefit of an all-wheel drive car.
Let's start with the basics. A two-wheel drive car, whether it be a front-wheel drive or rear-wheel drive, will only ever supply torque to one or two wheels. In the instance of a front-wheel drive car, the same wheels that steer the car are the ones being forced to turn.
In the instance of a rear-wheel drive car, the front wheels are able to do the turning, while the rear wheels do the driving. In these two examples, you are afforded grip from four wheels but traction from up to two when accelerating.
With an all-wheel drive car, the vehicle decides which axle (whether it's the front or rear) needs torque and it sends it there. What you gain from all-wheel drive is traction, as opposed to grip. Grip levels are theoretically the same between two identical cars that only differ in terms of two-wheel or all-wheel drive modes.
The traction benefit comes in the form of four wheels potentially receiving drive as opposed to two.
To demonstrate this to the extreme, we went to the Southern Hemisphere Proving Ground in New Zealand with the new Holden Insignia VXR, which uses an all-wheel drive system called Adaptive All-wheel Drive.
In the first test, we disconnected drive to the rear axle and made it a front-wheel drive car. When driving in a circle on the snow with the throttle on, the car would understeer and remain generally uncontrollable when throttle was applied.
This is because the driving wheels are also the steering wheels and as a result, traction becomes unavailable and the car just goes straight instead of turning with accuracy.
When we turned the rear axle back on, the Insignia VXR become an all-wheel drive car again. The intelligent all-wheel drive system in the Insignia can send between 0 and 100 per cent of torque between the front and rear axles.
It also uses an electronic rear differential lock to further optimise torque distribution on the rear axle. As you can see in the video, the results are quite clear with the car now easily controlled and driven in a circle.
So, now you know that while an all-wheel drive car will provide better traction, it won't improve your grip levels.