What exactly is th edifference between all wheel drive and four wheel drive.
Typically, 4WD is selectable between 2WD and 4WD. 4WD vehicles also usually have a low range 4WD selection. AWD is like high-range 4WD all the time.
4WD vehicles usually have 2 differentials, front and rear; AWD vehicles generally have a single, centrally located differential.
It varies, as there is no “official” definition that car companies are required to follow. Having said that, in general, people mark the distinction by whether you can shift into 4WD or if it is always on (AWD).
The differentials are a factor, but on Subarus for example the Legacy has only one differential while the Outback version of the same car has 2, but they are both referred to as AWD.
So, if a car (the Chrysler 300C, for example) has AWD, then it technically has 4WD, too?
It depends upon how you define your terms. If you mean it applies driving force to all four wheels, then yes. If you mean 4WD drive is driver-selectable and AWD is not, then no. Using the specific term “4WD” to refer to AWD systems has the disadvantage of muddying up the already less-than-crystal-clear distinction between the two terms, even if you can logically justify it.
To further complicate things, some AWD systems are full-time, always driving through all four wheels, and some are on-demand, normally driving through two wheels but engaging the other two the moment any slippage is detected from the first two.
Well, ‘technically’ All-Wheel Drive and Four Wheel Drive are the same. If a car has one, it also has the other. The difference comes in the form of selectability (generally).
AWD tends to be “automatic AWD”. Where, for instance Subie, uses that as a gimmick “Transfering power from the wheels that slip to the wheels that grip”. The automatic versions of their cars come with automatic power transfer. Manual cars (like the WRX) is a 50/50 full time AWD system. It’s still called AWD even though it’s Full time 4WD.
“Full (aka ‘real’) time 4WD” is all wheels are being driven 100% of the time regardless if it’s slipping or not. Mostly you’ll see this as a selling point on trucks and off-road vehicles.
Many vehicles seem to follow the general rule that if it’s “automatic” 4WD it’s called AWD where as large SUVs/Trucks tend to use 4WD when speaking about full or real time AWD.
YMMV of course.
My Envoy XUV has both and I think the terms they use make the most sense.
I have a transfer case switch on my dash. It’s labeled: 2HI, A4WD, 4HI, 4LO.
2HI - The front axel is not engaged. I use this for most highway and street driving.
A4WD - Automatic Four Wheel Drive. The front axel is engaged but normally power is only sent to the rear wheels. If the car senses a loss of traction it will engage the front two wheels at its discretion.
4HI - Or fulltime four wheel drive. Power is sent to all four wheels at the same time.
4LO - As above with extra torque.
I think if everyone else used a term similar to “Automatic Four Wheel Drive” instead of “All Wheel Drive” there’d be a lot less confusion.
That should really be:
Power is sent to all four wheels all of the time.
Can someone elaborate on the configurations of the two-differential versus one-differential systems? If there’s one differential, is it on the shaft that connects the two axles, or is it generally between the rear wheels? If there are two, where are they? Also, what are the practical differences (driving feel, situations where it matters) between the two-diff and one-diff setups?
In a front or rear wheel drive car, usually only one wheel is really getting power. If your wheels start slipping, the one with the least traction will get power, not necessarily a good thing. This is why limited slip differentials were created, to give power to the wheel with the most traction.
IIRC, in a normal AWD car with a single differential, again, only one wheel is really powered. If things start slipping the single wheel with the most traction gets power. With a second diff, things are divided in pairs, so one of the front and one of the back will always get power.
My description is probably way off. Here are some links that might help:
It used to be (and possibly still is) that with selective 4WD, you did not want to be in 4WD on dry pavement. I know that the 86 Isuzu Trooper has big warnings about not driving in 4WD on normal paved road conditions (put, of course, on the visor, along with the instructions on how to get into and out of 4WD.) However, I was never clear on whether this was just because of a waste of gas, or whether it would wreck something. I’m not sure what it’d wreck, either. My first guesses would be the differential or the drive train. It’s a moot point now anyway, as the 4WD no longer works and it hasn’t been driven off pavement in probably at least half a decade.
On the other side of the coin, my manual AWD Subaru is explict on not allowing for uneven tire wear and a couple of other things because they claim something as little as 0.25" of difference can ruin the differential (or something like that.) Anyway, I know that when towing it, my options are to put it in neutral and tow with all four wheels on the ground or load it onto a flatbed or trailer, so that no wheels touch the ground. With the automatics, you don’t even have the wheels on the ground option.
Actually, it’s the flaming death thing.
In a fixed 4WD situation, all wheels will turn exactly the same time. On dry pavement, going in a straight line this isn’t a problem, the car will continue to go straight.
Turning on snow or mud is also not a problem. During a turn the tires all want to travel the same distance even though the outside tires should be going further to follow the arc of the turn. Fortunately, the tires can slip on the mud or snow and it won’t effect your handling.
Turning on dry pavement is a big problem. Your tires are all spinning the same speed but they are trying to travel different distances. This can cause serious handling problems, and can commonly cause the vehicle to flip or lose control, which is generally considered a bad thing.
Thanks, I think I understand it better now. And Telemark’s last post answered another question that I have had.
I’ve never heard “flipping” as being a reason, but driving on dry pavement with all four wheels engaged (and no “slipping differentials”) can cause things like drivetrain bind-up and severe gear wear.
I’ve never heard of the flipping either. I have forgotten to switch out of 4-HI a few times and you’re entirely correct about drivetrain bind-up. That’s the first thing that happens. If you notice it quickly, you can usually either stop and back up to reduce the bind or pull off the pavement to allow tire spin, and allow the transfer case to shift back to 2-HI.
Some 4WD indicator lights can cause problems because they don’t actually indicate whether the transfer case has actually shifted or not. They will lead you to believe it has shifted when it hasn’t. Luckily mine is actually triggered by the transfer case and not the lever.
If you continue to drive on dry pavement in 4-HI (unless it’s a full time 4WD) the bind will continue to get worse until something breaks. Luckily the time that I did it, the rear U-joint was the first thing to go. It was twisted almost in half.
I’d like to hear about this single differential set-up, too. I was always under the impression that any 4WD had to have at least 2 differentials. One for front wheels and one for back wheels that allow differential in wheelspeed from side to side on each axle (yeah, I know IFS doesn’t have an actual axle). I thought that the 3rd differential (located in the transfer case) would allow difference in front/rear wheelspeed and would be what’s considered a full-time 4WD. Am I wrong?