Q: What’s the difference between 2-Wheel-Drive and 4-Wheel Drive?
A: Well, that one’s easy. In 2WD, two of the wheels are getting power. In 4WD, all four wheels are getting power. Gimme another one.
Q: OK. What’s the difference between All-Wheel-Drive and 4-Wheel-Drive?
I can’t tell you how many countless times I’ve been in that conversation, and how many times that conversation must happen on a daily basis. I’ve decided that I’ve had enough of it. I was going to answer the question, once and for all, what the difference was between using all (four) of your wheels and using four (all) of your wheels. My search for an answer got messy real quick.
First off, there’s the kind of traditional view of what 4WD means: a vehicle – typically an SUV or truck – that sits fairly high off the ground, and whether for recreational purposes or not, occasionally ventures off the beaten path through mud, snow, or sand. These cars can basically climb straight up a vertical cliff if they need to.
And then there’s everything else, which falls neatly into the 2WD category: your boring sedan that dad/mom drives through parking lot traffic to work every day, and the sports car for when he gets tired of commuting and decides to let loose and run his savings account balances into the ground. Oh, and all of those SUVs and trucks that fall into the 4WD category, when not being used for crazy things, are also just regular old 2WD.
That’s the way it seems to work on the surface.
Introducing: All Wheel Drive
Several years ago, car makers introduced options that allowed you to run a car in 4WD mode all the time, leading to a new distinction among 4WD vehicles: those that run some of the time in 4WD (like those people scaling cliff faces) and those that run in 4WD all the time (like soccer moms/dads driving crossover SUVs). This was helpful because it gave you better traction control, and helped drivers maintain control of their vehicles during adverse conditions, like heavy rains, snow, or if the river flooded and left three inches of mud across the road. These can be called (quite neatly): Part-time 4WD and Full-time 4WD. These names apparently weren’t clear enough, and people started to think that their full-time 4WD cars could scale cliffs (which I’m sure some of them can), so car makers replaced that name with a nice new, refreshing name: All Wheel Drive.
So now there are three nice neat categories that cars fall into: 2WD (boring commuter cars), 4WD (the part-time, exciting, climbing Kilimanjaro type), and AWD (the full-time, safety and security for my boring commute car type).
Now it gets complicated.
Vehicle engineers said to themselves, “What if someone is driving on an icy road and only one wheel is slipping? Or if there’s a large puddle, which first reduces traction on the front tires, then reduces traction on the rear tires? Shouldn’t there be a way to redirect power only to those wheels that have the most traction?”
Well, to some extent, this was already solved in regular old AWD using something called a differential (if you’re hazy on how differentials work, watch this video, then watch this one to see how they work in most AWD vehicles). Differentials have been in use for a long time in automobiles, but apparently this wasn’t good enough for the engineers. They needed precision – exact control over every tire. And so they invented:
Intelligent AWD. This is where an on-board computer is used to track how much traction each wheel has, using sensors that check wheel slippage hundreds of times in a second, and then redirects power to only those wheels that have traction, whether that is all four wheels, just one side or the other, just the front, just the rear, or even just a single wheel. Don’t ask me how it actually works. I assume its either magic or fairy dust. All I know is that it works, and when you’re picking the kids up in the middle of winter and a foot of snow falls during your commute, having Intelligent AWD could be a real (and sometimes literal) lifesaver.
Some types of AWD – and this depends heavily on the manufacturer and the model of the specific vehicle you are in – have the default (normal driving conditions) set as primarily having the engine power split evenly between front and rear wheels, or favoring to various degrees the front or rear wheels. Typically, favoring the front wheels happens on more family/commuting focused cars, while favoring the rear wheels, which gives a sportier feel, happens on sportier cars.
This wasn’t enough for vehicle engineers though, and so they came up with new and more exciting questions, like, “If you are driving through heavy snow and ice, you definitely need 4WD, but shouldn’t it be a different 4WD from the one used for going through mud? or from the one used to drive over something like sand? and while we’re at it, what about something to help you in descending a hill? Like Kilimanjaro?”
Introducing Ford’s Terrain Management System Intelligent 4WD, which comes as an option on the Ford Explorer. With this system, rather than the traditional (and somewhat confusing to the uninitiated) 4WD High and 4WD Low that are options on those cliff-ascending types of cars, there is a knob with options for Normal, Mud/Ruts, Sand, and Snow/Gravel/Grass, as well as a button in the middle called Hill Descent which helps with… well, descending hills. With this system, it takes some of the guesswork out of how to go off-roading. What it does is control wheel slippage using a computer/sensor system similar to Intelligent AWD, but in this system, the various settings are designed to respond in a way that maximizes control in the conditions that they are designed for. It takes some control away from the driver, and might not be great for the truly extreme off-road enthusiasts, but for anyone who ranks less than hollywood-stunt-driver in skill level, it’ll definitely help with your ability to maintain control in adverse conditions.
Oh, and then there’s something called Shift-on-the-fly AWD, which is where you can manually change between 2WD and engaging all four wheels without having to be stopped, though you do generally need to be going under about 60 MPH. It doesn’t really fall neatly into any of the previous dichotomies (part-time/full-time, static/intelligent, AWD/4WD), but kind of just sits in its own invented category halfway in between all of them. Also, this is only available on some cars from some manufacturers, so anything more detailed about the nitty-gritty details of how it works is probably proprietary.
That pretty much sums it up. What seems like it should be a nice simple dichotomy (2WD vs 4WD) is actually somewhat of a continuum from basically using two wheels to basically using all of the wheels, with new and exciting grey areas constantly being filled in. If you had to try and quantify it into some kind of easy to understand list, I woudn’t. Trust me. I tried.
Moral of the story: if you’re looking into buying a new car, and you want to know what exactly the AWD or 4WD label means on any particular car, just ask somebody who works in the dealership. Even if they might not know right away, they definitely know where to look. And knowing is better than not knowing.
Here are some more resources to help shed some light on the issue:
In these videos, what I called Intelligent AWD, he calls Automatic AWD, just so you know.
There. Clear as mud.