Both cars drive all four wheels, so in one sense there is no difference except that AWD has become an accepted description for a car that drives all of the wheels, all of the time. 4WD is generally accepted as a car or more typically a larger SUV (Sports Utility Vehicle) that uses a selectable driver system that mechanically engages the drive to all four wheels
4WD is normally used on large SUV Four-Wheel Drive (4×4) vehicles designed to use the extra traction of 4WD in off-road situations. These vehicles are predominately truck-based platforms with large wheels, and off-road tyres that combined with a manually selected and a locked 4WD driveline enables the vehicle to venture off-road and negotiate very difficult terrain.
A locked 4WD driveline means that there is a direct mechanical link between the front and rear axles with no mechanism to allow any difference in the number rotations of the front and rear axles. This means that when the 4×4 vehicle turns a corner because the radius of turn is different for front and rear axles, the tyres on the axle with the smaller radius of turn must be able to slip on a loose, slippery ground surface. If the ground surface is not slippery and the tyres do not slip, then the driveline (axles and propeller shaft etc) will twist, and stress will be induced. This is known as ‘wind up’ and ultimately if the twist cannot dissipate the vehicle will no longer be able to move as it becomes ‘locked up’. This will generally only happen at lower speeds on ground surfaces with no slip. At higher speeds or on slippery road surfaces, the tyre is able to slip, and the ‘wind up’ is released. This means that when 4WD vehicles are driven on normal road surfaces, 4×4 must be deselected and the vehicle is driven in two-wheel drive.
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What Is All-Wheel Drive?
As the name implies, all-wheel-drive systems power both the front and rear wheels all the time. But in practice, two types of drivetrains are called AWD. One does 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 two-wheel-drive mode, with power delivered to all four corners only when additional traction control is needed.
How Does All-Wheel Drive Work?
AWD systems, both full-time and part-time, generally operate with no input from the driver, although some offer selectable modes that allow a degree of control over how much power goes where. All the wheels get torque through a series of differentials, viscous couplings and/or multi-plate clutches, which help distribute power to the wheels so that the car’s traction is optimized. The vehicle still operates smoothly under normal conditions.
In full-time AWD, both the front and rear axles are driven all the time. On dry pavement, this kind of AWD can help the vehicle handle better and ensure that full power gets to the road. And in slippery conditions, such as ice, snow or mud, it provides additional traction for safer, more confident handling.
In normal operation, part-time AWD sends torque to two driven wheels, either the front or rear, depending on the make and model. The system then automatically engages the other two wheels when road conditions demand extra traction. Modern part-time AWD uses an array of electronic sensors that feed information to a computer, which controls the amount of power directed to each wheel.
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Electrified all-wheel drive
Many electric vehicles (like the Jaguar I-Pace and the Audi e-tron) use what’s called a through-the-road all-wheel-drive system.
Each axle gets its own electric motor, so the four wheels are always powered, but there’s no mechanical connection between the front and the back of the car. This improves traction and performance while helping clear up passenger space in the cabin because there’s no need for a transmission tunnel.
Some plug-in hybrid vehicles use a blend of technologies to achieve all-wheel drive. Take Volvo’s XC90 T8, for example. The 2.0-liter, four-cylinder engine spins the front wheels while an electric motor mounted over the rear axle spins the rear wheels. It’s front-wheel-drive when the four-cylinder works on its own, rear-wheel drive in electric-only mode, and all-wheel drive with both power sources up and running.
All-Wheel-Drive Pros and Cons
The best thing about AWD is that the driver doesn’t have to make any decisions about engaging the system. Either all the wheels are being driven full time, or the system itself is designed to sense a loss of traction and send power where it’s needed. AWD is available on a wide variety of vehicles, from compact sedans to performance models to all sizes of SUVs, giving you a broad range of choices.
While AWD is able to work well in a variety of conditions, from rain to snow to light off-roading, it’s generally considered a lesser choice by serious off-roaders. This perception is changing somewhat as modern AWD systems get better and more capable, but many drivers who like to venture far off the beaten path still prefer to decide for themselves when to engage four-wheel drive. AWD also increases the cost of a vehicle and, in most cases, will reduce fuel economy.
What Is Four-Wheel Drive?
This is the more traditional system that comes to mind when most people think of drivetrains that power all four of a vehicle’s wheels. It isn’t surprising since the principle goes back almost to the beginning of motorized transportation. The stereotypical picture of a 4WD vehicle is of a truck with high ground clearance, a shielded underbody, tow hooks and big, knobby tires. And, indeed, this system is still found primarily in large trucks and SUVs.
But through the years, 4WD engineering has become increasingly sophisticated. Although it generally remains capable of more serious off-road use, it can now be found on a wider variety of comfortable, even luxurious, models. 4WD systems deliver torque through a series of the front, rear and centre differentials, transfer cases and couplings, which allow the vehicle to operate at maximum traction under a variety of conditions.
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How Does Four-Wheel Drive Work?
Like AWD systems, 4WD is designed to send torque to all four of a vehicle’s wheels to increase traction when needed. But 4WD systems tend to be more robust than AWD ones and can generally handle more rugged terrain. And they, too, come in two types: full-time and part-time.
Many 4WD systems also have low and high ranges that can be selected by the driver, either with an electronic switch or a floor-mounted mechanical lever. The low setting provides maximum traction in an off-road environment. In contrast, the high setting is the default configuration, useful for slippery on-road conditions, such as packed snow, ice, loose sand or gravel.
Full-Time Four-Wheel Drive
Full-time 4WD powers all four wheels, all the time. To get around that problem of transmission wind-up, mentioned above, the system employs a centre differential (or simply diff), which allows a different speed for each axle.
Even though the transfer case is constantly engaged to power the front and rear wheels, the diff allows for different rotation speeds. This means that, on the road, the four-wheel-drive system won’t try to hold each wheel at a fixed speed, avoiding the potential for transmission wind-up.
In full-time systems, the diff can be locked, forcing the wheels to rotate at the same rate, and thus providing the same gravel-grappling off-road ability as its part-time counterparts.
Locking a diff, rear or centre, and engaging low-range* is used for when the going gets supremely tough off-road and you to need optimum traction from your wheels and maximum torque from your drivetrain.
Part-Time Four-Wheel Drive
In most traditional, off-road-focused 4WDs the drive from the engine is sent to the rear wheels by default through the transfer case. The transfer case houses two cogs, which can be connected by a chain. You disengage the chain for a two-wheel-drive – just the rears – and it’s operational in 4WD; this locks the speed of the front axle to the speed of the rear one.
Four-wheel drives run in 2WD on the road, a high-grip surface, because you don’t need all four for optimum traction the way you would on gravel back roads or tracks.
In part-time 4WD systems – sometimes referred to as on-demand 4×4 or 4WD systems – engaging the transfer case gives you maximum drive in slow-going off-road scenarios. The wheels will still slip and scrabble due to the loose surface, though, which ensures that any overwinding on a wheel will resolve itself by spinning to release the tension.
On the road, however, wheels need to spin independently to drive around corners. If the 4WD system constricts each wheel’s rotation, cornering will cause tyres to slip or spin to try and maintain a constant rotation rate.
If you did use 4WD on the road for extended periods you’re asking for strife: it will increase fuel consumption, inflict unnecessary wear and tear on your vehicle and, worse still, cause serious damage to it, due to transmission wind-up (aka driveline binding).
That’s a situation in which your off-roader’s drivetrain cops massive amounts of stress due to the extreme torque forces involved in forcing your vehicle, locked in 4WD, to take on bends and turn corners when all four wheels are still turning at that constant rotation rate.
If the tyres can’t slip to relieve the pent-up energy, that ‘winding up’ stresses the wheel hubs and driveline to breaking point, which can be, in the least, very expensive to fix and, at its worst, very dangerous.
Low-range 4WD explained
Part-time and full-time 4WDs will generally have a dual-range transfer case, and this gives you even more freedom when it comes to just how far you can go off the beaten track.
First, high range: In 2H (two-wheel drive, high range) two wheels, usually the rear, are driving your vehicle. You use 2H for normal road driving.
In 4H (four-wheel drive, high range), all four wheels are driving your vehicle. You use 4H for surfaces which you may need greater traction than you would for bitumen; think firm sand, dirt roads, gravel tracks and the like.
Next, low range: In 4L (four-wheel drive, low range), all four wheels are driving your vehicle, and a low gear ratio is being used. Your vehicle’s wheels will turn much more slowly than they would in high-range, so slower speeds and much more torque are the go.
You use 4L for soft sand, sand dunes, steep hills and declines, deep mud or snow, and slow-going rock-crawling.
It used to be that you would have to engage high- or low-range using a little shifter (the stubby stick) near your main manual or auto shifter. Some of us from The Old Days even had to get out of our 4WDs and actually lock our manual locking hubs on the front wheels for off-road work, and then unlock them when switching back to 2H. Not any more; you can now switch to high- or low-range via a dial or knob in the cabin.
In many modern 4WDs you can switch from 2H to 4H without stopping, but still have to come to a complete stop to switch from 4H to 4L.
Four-Wheel-Drive Pros and Cons
4WD vehicles are generally best at handling adverse conditions, both on-road and off. Even though these systems are now available in well-appointed luxury trucks and SUVs, at heart, they still tend to be designed for ruggedness and maximum pulling power, and they are well-suited for work and play in difficult terrain.
These days, the 4WD design has become increasingly refined, as has the design of the vehicles that can be ordered with it. But, depending on the make and model, 4WD still often delivers a stiffer ride than 2WD. These systems also have a detrimental effect on fuel economy and increase the initial cost of the vehicle.
Which is Better: AWD or 4WD?
Is AWD better than 4WD, or is it the other way around? The truth is that it depends on your priorities and needs. In general, AWD vehicles usually prove to be the best option for most suburban drivers looking for extra traction in inclement weather and bad road conditions. In contrast, 4WD vehicles prove a better choice for those seeking out the most rugged off-road conditions or who are in need of onsite truck utility for a job.
Though the gap is narrowing rapidly between 4WD and AWD vehicles, in terms of car-like ride and handling, AWDs still generally pip 4WDs in the all-round comfort stakes.
But an all-wheel drive’s lower ground clearance and air intake, as well as its drivetrain and chassis, which are not as well-suited to off-road stresses as those in a 4WD, mean AWD vehicles aren’t anywhere near as universally bush-and-beach capable as a purpose-built 4WD.
If you have a large family and you love camping in out-of-the-way places that are difficult to reach in anything other than a LandCruiser, then you’ll need a 4WD. These vehicles have the drivetrain, gearing, suspension, ground clearance, air intake height, not to mention approach, departure and ramp-over angles to better traverse off-road terrain than an AWD.
There is also plenty of aftermarket equipment available for 4WDs – suspension upgrades, snorkels and more – to further boost their off-road capabilities.
AWD vs. 4WD in snow
A couple of things to keep in mind upfront: Whether you choose AWD or 4WD, any vehicle can lose traction and spin out if pushed hard enough. And while both systems are designed to increase traction by engaging the front and rear wheels, neither helps you stop better. Both AWD and 4WD can give you a significant advantage in snowy and icy conditions and may be worth the extra cost, depending on where you live and how you use your vehicle.
Driving in cold weather often means encountering a variety of rapidly changing road surfaces, from soft snow to hard-packed snow to glare ice. AWD systems, which deliver power to all four wheels all the time, or automatically engage four-wheel torque when needed, are best at dealing with these changing conditions. They take the guesswork out of the equation and can act more quickly than a driver to handle variable road surfaces.
On the other hand, 4WD is generally well-suited to navigating deeper snow or other more extreme winter conditions. It can get you unstuck from a snowdrift more easily, manage icy hills more effectively, and get you to work safely before the roads are ploughed.
One of the main benefits of AWD or 4WD is that a vehicle fitted with one of these systems will accelerate from a standing start with greater traction than a two-wheel-drive vehicle. This is especially the case when the road is wet or slippery as the traction is distributed equally between all four wheels.
AWD and 4WD also aid traction, depending on the conditions, when towing a boat, caravan or float. This is evident when towing on wet or slippery roads, at a wet boat ramp or on unsealed roads designed for off-roading. It is also handy when driving on sand.
The line between AWD and 4WD can be blurry. Understanding the advantages and disadvantages of each system will help you understand which one is better for your driving habits. Since each manufacturer’s AWD and 4WD systems may work slightly differently, make sure you understand the basics and pay attention to the details. Always remember that an AWD or 4WD powertrain can’t make up for unsafe driving, and neither will help a car with worn-out tires stop more quickly in slippery conditions.