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Nissan X-Trail long term update: Powering all four wheels explained

17.02.2017

For the last few months I have been spending quite a bit of time with our long term Nissan X-Trail. It has a 1.6-litre turbodiesel engine which is fantastic on the long road, light on fuel and it offers some decent torque.

Our car is also fitted with a toggle switch that says 4WD on it which makes it a 4x4 right? Well, it has come to my attention that not many people know the difference between four-wheel drive (4WD or 4x4) and all-wheel drive (AWD). Let me explain.

Let’s start with the oldest form of the technology, four-wheel drive. This is typically used on off-road vehicles or at least vehicles with all-terrain capabilities. Power goes from the transmission to what is known as a transfer case. This system then splits power between the front and rear axles so that maximum torque is going to each wheel.

When the transfer case splits power evenly, it ensures that each wheel turns at the same speed. This is deeply problematic when doing things like turning. You see, for a car to make a turn, the inside wheel has to turn more slowly than the outside wheel, which is covering more ground. If the vehicle can’t do this, the inside wheel loses traction and it spins freely.

There are a couple of ways that modern 4WD systems get around this. For starters, most modern 4WD systems are only on when you activate them. This can be done electronically like in the Nissan or by using that protruding lever that sits somewhere between the radio and centre console like in a Land Rover Defender.

That way, you can use 4WD at low speed in mud or over rocks but enjoy the drivability of two-wheel drive in normal conditions. Most 4WD vehicles have various gearing options for example High and Low range. Low range splits the power evenly between the front and rear wheels. This is best suited for extremely tough conditions.

So what is all-wheel drive then? Is it not the same thing? Well, sort of but not really. All-wheel drive is a much more recent innovation, and, as you might expect, much more complicated. It can be found on everything from supercars to your garden variety Subaru.

The biggest difference between 4WD and AWD is that an AWD system is on all the time, well mostly. You see, there are two types of systems; mechanical and electronic. The most common way of accomplishing a capable, mechanical AWD system is by using three differentials. In AWD, the mechanical system works to get power to the wheels with the most traction by splitting power between the front and rear axles on the centre differential and the individual wheels, by way of the front and rear differentials.

Electronic AWD systems makes use of sensors on each wheel that monitors traction, wheel speed, and many other data points hundreds of times a second. An Electronic Control Unit (ECU) then dictates where power is sent and to which individual wheel depending on whichever has the most grip.

This type of system, usually called torque vectoring, appears on everything from the Subaru WRX to the Volkswagen Golf R. Torque vectoring has allowed massive improvements in handling and all-weather capability.

So which is best? Well there are pros and cons for both systems. For example, 4WD needs many different parts which adds weight and lowers fuel economy. AWD is complex and difficult to repair if something goes wrong, it’s also limited however offers improved levels of safety.

I guess you need to ask yourself what it is that you need from your vehicle. If you enjoy the odd bundu bashing session, then maybe a 4x4 would suit you better. If you have an active lifestyle but don’t need anything extreme, a crossover with AWD could offer what you need.

Article written by Justin Jacobs
17.02.2017
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