Despite the increasing number of online resources and shifting dealer strategies, car shopping is still a headache. There’s a long list of confusing topics when choosing the right one, but all-wheel drive (AWD) and four-wheel drive (4WD) seem to consistently confound salespeople and car shoppers alike. Often, these terms are used interchangeably, but are they really the same? In the motoring world, they actually refer to very different systems, which can produce radically different results on and off-road. But let’s get to the point: what’s the difference between four-wheel drive and all-wheel drive, how will each impact your daily driving life, and which badge belongs on your car?
Four-Wheel Drive (4WD)
Lets start with the old-school version. 4WD, sometimes referred to as Four by Four or 4×4, is typically used on off-road vehicles — or at least vehicles with all-terrain capabilities.
Unfortunately, 4WD doesn’t fit neatly in a one-sentence explanation, but we’ll stick to the basics.
Power goes from the engine, to the transmission, to what is known as a transfer case. This system splits power between the front and rear axles so that torque is evenly applied to each wheel. This process is nothing new, and is still used in modern Jeeps to tackle just about any off-road obstacle. It isn’t perfect, though.
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 spins freely. This, as you might be able to guess, isn’t great for maintaining momentum.
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 or by using that protruding lever that sits somewhere between your radio and the center console. That way, you can use 4WD at low speeds when traction is at a minimum (for example, in snow or mud), but you can enjoy the efficiency of two-wheel drive in normal conditions. When left in 2WD, there are fewer moving parts, and therefore fewer restrictions to forward motion. Said a different way, you’ll save fuel when don’t need to engage 4WD.
More contemporary 4WD systems are activated with buttons or switches rather than a manual lever, and include multiple settings for the 4WD system. These systems usually have two 4WD gears. 4WD ‘High’ splits power less evenly and allows what’s called ‘limited slip’ between the inside and outside wheels. This corrects the locked, spinning inside wheel problem by channeling more power to the wheel with traction (in our example, the outside wheel). 4WD High limits available power to the wheels so you can move quickly over slippery surfaces (up to about 60 mph). For the most available power, however, you’ll want 4WD ‘Low.’ The Low gear limits wheel speed but is perfect for arduous terrain. A word of advice: you really don’t want to go too fast in 4WD Low … things start breaking.
|4WD Pros||4WD Cons|
|Best traction in off-road conditions||Adds weight and complexity to cars|
|Can be turned off to improve fuel economy||Can’t be used in all conditions|
|Proven, rugged technology||More expensive than two wheel drive models|
All-Wheel Drive (AWD)
All-Wheel Drive is a much more recent innovation, and, as you might expect, much more complicated. It crops up on everything from supercars like the Audi R8 to grocery-getters like the Buick Encore.
In fact, a good rule of thumb might be to think of AWD as the “car” system while 4WD is the “truck” system. This isn’t always the case, as some vehicle segments overlap with drivetrains, but if you’re asked the question on Who Wants to Be a Millionaire, this rule might win you some money. Consider crossovers like the Honda CR-V, Toyota RAV-4, and Mazda CX-3 tend to fall under the “car” category while SUVs like the Chevrolet Tahoe and Toyota 4Runner fall under the “truck” category. Here’s how to better understand the differences between crossovers and SUVs.
The biggest difference between 4WD and AWD is that an AWD drive system is on all the time. Well, mostly.
There are two types of all-wheel drive: mechanical and electronic. The most common way of creating a capable, mechanical AWD system is by using three differentials. A differential is a box of gears, a.k.a. engineering magic, that can take power from the transmission and split it at different levels between two wheels (the front or rear axle) or four wheels (the front and rear axle).
In AWD, this system works to get power to the wheels with the most traction by splitting torque between the front and rear axles on the center differential, and to the individual wheels by way of the front and rear differentials.
This is useful in slippery conditions when different wheels might be getting different amounts of grip from moment to moment. The Mercedes-AMG E63 is a perfect example. It is now sold only in AWD in the United States because its power can overwhelm the traction of the rear wheels alone. Even when we aren’t talking about 500+ horsepower cars, splitting power evenly means added stability in all types of weather.
AWD isn’t quite as robust as 4WD and it can’t match the acute power delivery necessary for low-speed off-roading (i.e. rock crawling). However, AWD does have some clear advantages.
The pioneer and industry standard for AWD systems, Audi Quattro, distributes torque mechanically. Quattro allowed Audi to dominate rallying for nearly a decade in the 1980s, but heaven help you and your bank account if it went wrong.
These days, computers are involved in most AWD systems. Sensors on each wheel monitor traction, wheel speed, and several other data points hundreds of times a second. An ECU (engine control unit) 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 Dodge Charger these days. Torque vectoring has allowed massive improvements in handling and all-weather capability.
But wait, there’s more. Electrified powertrains add another option when it comes to AWD systems, namely the ability to use an electric motor to power a set of wheels without any mechanical connection to the other set.
In hybrids and plug-in hybrids, an internal-combustion engine can be used to power one set of wheels, while an electric motor powers the other. This is called “through-the-road” AWD, and it’s currently used on vehicles such as the Volvo XC90 T8 plug-in hybrid and the Toyota RAV4 Hybrid.
All-electric cars can take this concept a step further. AWD versions of the Tesla Model S and Model X use two electric motors — one for each axle. That’s why these cars get a “D” suffix, for “dual motor.” Besides the packaging advantages of not having to physically connect both axles to an engine, the power output of electric motors can be precisely controlled via software, helping to maximize traction.
As more automakers get serious about electrified cars, it’s possible that these AWD systems will become more common. Given the current popularity of SUVs and crossovers, automakers will have to offer AWD in order to take hybrids, plug-in hybrids, and electric cars into the mainstream.
|AWD Pros||AWD Cons|
|Provides increased grip and control under all road conditions||Reduces Fuel Economy|
|Gives sportier handling and traction to a broader range of cars||Increases the weight and complexity of vehicles|
|Works all the time||Not as good in extreme off-road conditions|
So, which do I want?
As the pros and cons show, your four-wheel drive decision depends on your driving needs. Again, we’ll bring up that crossover vs. SUV debate, because it probably addresses the two types of vehicles you’re shopping right now. Also, though you’ve probably concluded that you need some type of four-wheel drive system, it might be worth freshening up on the differences between front-wheel drive (FWD), rear-wheel drive (RWD), and all-wheel drive (AWD) configurations.
If you plan on using your vehicle off-road often, 4WD is definitely your best bet. 4WD appears on pickups and truck-platform SUVs that have the durability to match the ruggedness of a 4WD system. For most people, however, AWD makes more sense.
In the sort of winter road conditions that most drivers experience, it’s nice to have a drivetrain, like a modern AWD system, that responds instantly without the driver having to toggle any switches. In addition, most vehicles featuring AWD tend to have better weight distribution, which improves traction and performance.
The reality is that for many drivers, you don’t need either. If you live in an area that doesn’t get real wintery weather, you probably would only notice the difference a couple of times a year, and in many cases, a good set of winter tires will make the biggest difference. Seriously. Tires can do far more than AWD or 4WD on all-season or summer rubber.
Update: Added information about the all-wheel drive systems in hybrids and electric cars.
Published at Thu, 17 Aug 2017 17:15:37 +0000