When shopping for a car, you often hear the terms “four-wheel drive” and “all-wheel drive” thrown around, frequently interchangeably.
If you aren’t a boorish car nerd like me, and kudos to you if you aren’t, you may not know that these two terms aren’t interchangeable. They actually refer to very different systems, which can produce radically different results.
So just what the heck is the difference and why should it matter to you? If you stick with me, I promise to explain it without even having to explain things like chain drive and planetary gears!
You know what? Forget I even mentioned those last two things.
There are as many versions of 4WD as there are words in this article, so I am going to stick to the basics in explaining it.
Power goes from the transmission to what is known as a transfer case. This snarl of gears splits power between the front and rear axles so that maximum torque is going to each wheel. It was good enough to beat the Nazis, and it’s still good enough to hurl your Jeep over a big pile of rocks. Despite its heroic heritage, however, it has some problems.
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. This, as you might be able to guess, isn’t great for moving forward efficiently.
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 weird secondary lever that usually sits forgotten next to your coffee cup. That way, you can use 4WD at low speed in snow or mud, but enjoy the drivability of regular two-wheel drive in normal conditions.
The other, more refined 4WD systems are activated with buttons or switches, rather than a rudimentary lever, and include multiple settings for the 4WD system. These systems usually have a 4WD ‘High’, which 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 to a point. Typically, however, High 4WD is recommended only up to around 60 mph. Flip these into ‘Low’, and they act much the same as old, locked systems.
– Best traction in off-road conditions
– Can be turned off to improve fuel economy
– Proven, rugged technology
– Adds weight and complexity to cars
– Can’t be used in all conditions
– More expensive than two wheel drive models
All-Wheel Drive is a much more recent innovation, and, as you might expect, much more complicated. It appears in everything from supercars with out-of-this-world performance like the Audi R8 to family crossovers and SUVs like the Volvo XC90.
The biggest difference between 4WD and AWD is that an AWD drive system is on all the time. Well, mostly. But we’ll get to that, as there are two types of all-wheel drive: mechanical and electronic.
The most common way of accomplishing a capable, mechanical AWD system is by using three differentials. A differential is a box of gears, and engineering magic, that can take power from the transmission and split it at different levels between two wheels or the front and rear axles.
In AWD this system works to get power to the wheels with the most traction by splitting power between the front and rear axels on the center differential and the individual wheels by way of the front and rear differential.
This is useful either in slippery conditions when different wheels might be getting different amounts of grip from moment to moment. The Mercedes-Benz E63 AMG 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.
AWD isn’t quite as robust as 4WD and it can’t match the same levels of traction in extremely low-speed off-roading that the older 4WD systems provide. But AWD does have some clear advantages.
In the godfather of all AWD systems, Audi’s Quattro, all torque redistribution was done mechanically. Quattro allowed Audi to dominate rallying for nearly a decade. But heaven help you and your bank account if it went wrong. Audi should have included instructions on how to file for bankruptcy in its owner’s manual.
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 dictates where power is sent and to which each wheel depending on which wheel 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 inclimate weather capability.
– Provides increased grip and control under all road conditions
– Gives sportier handling and traction to a broader range of cars.
– Works all the time
– Reduces Fuel Economy
– Increases the weight and complexity of vehicles
– Not as good in extreme off-road conditions
Which Should I Get?
As the pros and cons show, your four-wheel drive system decision depends on what you need the system for. If you plan on using your vehicle off-road often, 4WD is definitely the best bet. If you’re keen on wheelin’, though, you probably already knew 4WD was your only option. 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 also aids in traction.
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, unless of course you drive like a lunatic. In that case, Mr. AMG, you’ll definitely need some AWD traction.
Update 1-15-2015 by Brandon Widder: This article has been updated since it was originally published to reflect recent car models and technology.