A set of MaxTrax will keep you sane on sand
All-wheel and four-wheel drive vehicles are not all created equal. Their vastly distinct capabilities stem from purpose-driven design cues that cater to a variety of purposes and a range of users. The kind of vehicle you need is dependent on the type of terrain you intend to tackle and the frequency with which you intend to do so. The truth is, there’s no such thing as the perfect solution, so you’ll need to weigh up the options and figure out what works best for you, and you’ll probably have to make a few compromises.
What is 4WD?
Four-wheel drive (4WD) refers to a drivetrain system that can deliver power to both axels simultaneously and in turn, provide torque to all four wheels. Different types of four-wheel drive vehicles are distinguished by the mechanical systems that make it happen; the drivetrain components that connect the engine to the wheels.
All four wheels are not required to rotate in perfect harmony. When turning a corner, for example, the outside wheels need to cover a greater distance than those on the inside, while front and rear axles also vary for several reasons. Components known as ‘differentials’ are used to enable two wheels the same axle to rotate independently, while the front and rear axles are linked and managed by a transfer case, may or may not include a differential (more on that shortly).
Different systems have been developed by each manufacturer for a range of purposes, and since they’ve all put their own marketing spin on what makes theirs so special, it can be difficult to draw hard lines between various terms. The main things we’re focussing on when discussing these systems are the common considerations surrounding off-road use.
Rising popularity of mid-sized SUVs has made all-wheel drive (AWD) increasingly common, which makes defining it all the more difficult. For instance, both a Lamborghini Aventador and a Subaru Outback are classed as AWD and glean performance benefits from increased traction but are quite clearly not built for the same purpose.
Furthermore, there’s no technical standard that separates from 4WD (assuming both have four wheels). Both deliver torque to all four wheels and both have differentials on two axles. When it comes to vehicles built for leaving the bitumen the distinction seems to have evolved as a marketing tool that distinguishes AWD vehicles as road cars that are capable of some rugged terrain, as opposed to dedicated off-road 4WDs.
AWD vehicles will most likely have some ability to limit slippage between axles when they lose traction – by way of a differential known as a viscous coupling – but will not be equipped with any other off-road specific systems.
Part-time 4WD is the original and the most common drivetrain style for off-road use. It can be found in vehicles such as the Ford Ranger, Jeep Cherokee, Kia Sorento, Nissan Patrol and Toyota Hilux. These vehicles deliver power to only one axle (two wheels) as standard. They generally operate as rear-wheel drive on the road, requiring the driver or a smart electronic system to engage 4WD mode when they’re heading off. Once this is done and the front hubs have been locked to prevent the wheels spinning freely, the drivetrain will deliver equal power to all four wheels.
In these systems, the transfer case does not contain a differential. When 4WD is engaged, the front and rear axels have a rigid coupling. Since this won’t allow the front and rear axles to rotate at different speeds it can lead to the drivetrain experiencing a phenomenon known as ‘wind-up’ if it’s engaged on high-traction surfaces. If left unchecked, excessive windup will exert unnecessary stress on various drivetrain components and can lead to damage or failure. If any sign of wind-up becomes apparent (shuddering, tyre skipping and heavy steering) you should stop the vehicle, take it out of 4WD mode and reverse slowly; this can help to release the tension.
Vehicles with full-time 4WD deliver constant power to both axels (all four wheels). It’s less common than part-time, usually found in dedicated off-roaders such as the Land Rover Defender, Mitsubishi Pajero, Toyota Land Cruiser 200 and Prado. The transfer case includes a differential that prevents transmission wind-up, in most cases this differential can be locked to deliver equal power to all wheels. With the centre differential locked a full-time 4WD will function the same as a part-time with 4WD engaged.
This system introduces an extra level of mechanical complexity, but its most frequent criticism has to do with the fact that it burns more fuel on-road than a part-timer, though in reality it’s most often a negligible amount. Full-time 4WD is preferable for towing on-road, thanks to the increased traction.
An on-demand 4WD vehicle typically operates in two-wheel drive but will direct torque to the second axle by engaging a coupling in the transfer case. Strictly speaking, an on-demand system may require the driver to engage 4WD, meaning it’s the same thing as a part-time 4WD. In the current market, however, it tends to refer to self-managed system that can detect wheel slip or torque and engage the coupling by way of an electronic or hydraulic control system. In most cases there will be a provision within the system to avoid wind-up.
Open, Limited-Slip and Locking Differentials
Another factor that determines how a drivetrain performs is the kind of differentials it uses. Open differentials are by far the most common in the wider automotive market but can render a 4WD system completely useless. They’re designed to direct more power through the path of least resistance, which is all well and good on high-traction surfaces such as bitumen. Off-road, however, not so much.
If a vehicle with an open differential loses traction on one wheel, that wheel becomes the path of least resistance and the receiver of the bulk of the power available. In an off-road scenario, one wheel may hit a loose patch or lose contact with the ground entirely. There’s no point directing power to the slipping wheel when it’s the remaining three that need to be doing the work at that point.
Limited-slip differentials limit the maximum amount of power that can be assigned to a single wheel so that some remains with the wheel that still has traction – this is done without driver intervention. Without getting too bogged down in the technical stuff, it’s worth noting that these systems can take many different forms and are not the same as traction control.
Locking differentials distribute equal power to all wheels, turning them in sync. This is sometimes managed by a computer so that the driver need not worry, but in dedicated off-road 4WD vehicles it will usually need to be activated by the driver before they head into uncertainty.