English can be an ambiguous language when it gets into the hands of marketers.
You’d think the “all-wheel-drive” emblazoned on your crossover or SUV means that all four wheels are powered all the time.
Instead, as they say, it “ain’t necessarily so.”
All-wheel-drive (AWD) and four-wheel-drive (4WD) are sometimes used interchangeably by manufacturers, but these terms don’t necessarily mean the same thing.
Furthermore, one vehicle tagged by the term may function differently than another similarly named. It’s important to know how yours works, to avoid getting stuck and, in extreme cases, avoid damaging your vehicle.
So what’s ‘AWD’?
In most cases, ‘AWD’ is used to describe cars, crossovers, and SUVs that primarily operate in front-wheel-drive. That’s just a general rule, though, and there are numerous exceptions. The term ‘4WD’ is more often found on those with a rear-wheel bias, although again, that’s not always the case.
So how do these systems work? Quite simply, it depends. On many AWD vehicles, especially lower-priced ones, the system primarily powers the front wheels. If those wheels start to slip, the system will send power to the rear wheels. For this reason, it’s sometimes called “slip-and-grip” (amongst auto writers and owners, never by the car companies).
These systems are primarily meant for light duty such as wet or snowy roads, not for off-roading. Depending on how sophisticated the system is, it may also power the rear wheels on acceleration, or use sensors to determine the vehicle’s path and position, and proactively transfer power if it thinks there may be an impending loss of traction.
How much power gets sent to the rear depends on the system, and sometimes it’s difficult to know the exact numbers, since not that many owners’ manuals provide them. It’s presented as a percentage: a vehicle with a 60/40 system has the ability to divert a maximum of 40 percent power to the rear wheels.
Some AWD systems actually do provide power to all wheels, all of the time. This is the type that Subaru uses, for example – the rear-wheel-drive BRZ excepted – but you’re not alone if you (mistakenly) think that the company’s famous “symmetrical all-wheel-drive” means that each wheel gets equal power.
Instead, the name refers to the fact that the engine, transmission, and differential are symmetrical: they’re centered along the vehicle’s north-south axis for better balance, and each side is a mirror image of the other. Depending on the vehicle, and even on the trim line, the power split may not be even. The new Crosstrek with CVT, for example, normally has a 60/40 power split, although it can transfer up to 50/50 if it loses traction. Equipped with a stick shift, it’s 50/50 all the time.
You’ll notice that those numbers are for the front and rear axle only. Most systems are set up this way, although an increasing number have torque vectoring, which means they can also distribute power from side to side, as well as from front to rear. These systems are usually found on pricier vehicles. They provide even more traction when necessary, as well as improved handling, especially on corners, where they give more power to an outside wheel to “push” the car around the curve.
Again, though, terms aren’t necessarily carved in stone. Some automakers use clutches in the differential to distribute power, while other systems apply the brakes to one wheel to alter the power distribution, but both may be dubbed ‘torque vectoring.’
And 4WD? What’s that?
So what’s 4WD? We hate to keep saying “it depends,” but it does. Some companies use it to describe the type of AWD system we’ve explained above. It’s sometimes also used as the name for AWD systems that can be locked. By pulling a lever or turning a dial, the system switches to its maximum split – say, 60/40 or 50/50 – and stays in it, providing extra traction for getting through heavy snow or mud. These are typically low-speed only and will automatically unlock if the driver exceeds a specified speed.
The systems that will get you through the truly rough stuff, such as on pickup trucks or off-roaders such as Jeep’s Wrangler or Toyota’s FJ Cruiser, are called 4WD or 4×4. But even here, you have to know what your system is doing. There are two basic types, and how they work determines how they can be used.
Part-time systems run in rear-wheel-drive until you engage all four wheels, through a dial or lever. This locks the front and rear axles together, which gives you maximum traction. 4High simply locks the wheels, while 4Low further uses gears to increase the engine’s torque to the wheels. 4Low is for the toughest stuff and should only be used at low speeds.
Engaging part-time 4WD is only for loose surfaces, such as gravel or mud. On a hard road, vehicles always experience minor wheel slippage when they encounter bumps or wet areas. If the axles are locked together, the wheels can’t turn at different speeds to compensate, and you can potentially damage the vehicle’s components.
Full-time 4WD does allow the wheels to turn at different speeds, so the vehicle can be driven over any surface with all four wheels engaged. That’s simple enough, so we’ll throw in the confusing part. On some SUVs and trucks, a model’s lower-priced trim lines may have part-time 4WD for off-road use only, while the luxury lines have full-time AWD and can be driven in all conditions with all four wheels powered up.
So are you familiar with all of these systems now? If you’re not, you’re not alone: the nomenclature is often frustratingly confusing or vague, the automakers don’t always explain things clearly, and sometimes, even the person selling you the vehicle doesn’t fully understand the system.
If you’re buying a new vehicle and the salesperson doesn’t know, ask to speak to someone who does. If you already own one and you’re not sure, look in your owner’s manual, ask a dealership, or call the manufacturer’s information line. Knowing exactly how your all-wheel system works is essential if you’re going to maximize its performance in every driving situation.