AWD vs. 4WD - can someone edjimecate me?

[Green96GT]

Can someone explain the difference b/t AWD and 4WD (like in a truck, when you have that little shifter that shifts you into 4WD)?

 

And I mean the practical difference. I think I understand the mechanics, but maybe not.

 

TIA!!!!

[dfortin556]

Here is a pretty good explanation that I found at http://www.delalbright.com/Products/products_awd.htm

 

ALL WHEEL DRIVE:

 

Definition: A vehicle where all four wheels are driven, but there's no transfer case like a four-wheel drive rig. Most AWD setups are full time systems for year-round driving, and use a viscous fluid coupling center differential instead of a transfer case to route drive torque to all four wheels. This allows the front and rear wheels to turn at slightly different speeds when turning on dry pavement. Most folks do not consider this the same as four-wheel drive. It can be useful (and more fuel economical) in pavement driving where you're mainly negotiating bad weather conditions.

 

FOUR WHEEL DRIVE

 

Definition: A method of driving a vehicle by applying engine torque to all four wheels thru the use of a transfer case, differentials and hubs. Various schemes are used for 4WD including part-time, full-time and variable four-wheel drive. To help cut the drive train drag (and reduced fuel economy) that most 4WD's have, a transfer case is included that allows the driver to select either two- or four-wheel drive depending on driving conditions. Some performance cars have full-time variable four-wheel drive and use a computer-controlled transfer case to route power between the wheels.

 

It's important to note that even in 4WD, you still have only two driving wheels; one front; one rear. A normal passenger car is essentially one wheel drive, because the other one can slip. So a 4WD rig, with the front axle engaged, now has two wheels driving. Then if you add a locker to the rear, you've added one more wheel, to make your rig three-wheel drive. Add a locker to the front, so no tires can spin, and you have TRUE, 4-wheel drive, all four wheels driving.

 

[cannonball]

Although there are many configurations of both systems, I think the most basic difference is that with a 4WD you can lock at a minimum the center differential so that at least a front and rear are recieving equal amounts of torque through directly locking the gears(and front and rear drive shafts). AWD is more forgiving in that the center diff allows slippage between the front and rear. Thus as previously posted you can make a turn without the vehicle bucking like mad. There is a good thread here about locking the diff on auto subies. AWD has limited slip diff's working all the time instead of mechanical lockers used only when needed.

 

I would also like to add that I believe the true definition of AWD should be that all the wheels are receiving power at all times. It should not be confused with what we call around here "rear wheel assist" systems like the Volvo Cross Country and Honda Pilot which only delivers power the the rear wheels after the front begin to slip.

[Setright]

It's all marketing babble.

 

"All" and "Four" mean the same on a normal car with four wheels.

 

You have to look at each vehicle's mechanics to define further.

 

 

 

Volvo XC and VW "4-Motion" are part time 4WD/AWD. Subarus are permanent.

[Green96GT]

It's all marketing babble.

 

"All" and "Four" mean the same on a normal car with four wheels.

 

You have to look at each vehicle's mechanics to define further.

 

 

 

Volvo XC and VW "4-Motion" are part time 4WD/AWD. Subarus are permanent.

 

And I'll bet you $100 that no salesman will be able to explain it accurately on any given vehicle he sells.

[NoahDL88]

It's all marketing babble.

 

"All" and "Four" mean the same on a normal car with four wheels.

 

 

 

 

 

Not true.

 

AWD has a differential in the center, that distributes power to all 4 wheels all the time.

 

4WD has a Transfer case, which puts power to 2 wheels only, and when 4wd is engaged, power goes to all 4 wheels.

[Suzam]

Not true.

 

AWD has a differential in the center, that distributes power to all 4 wheels all the time.

 

4WD has a Transfer case, which puts power to 2 wheels only, and when 4wd is engaged, power goes to all 4 wheels.

 

Well it distributes power to the front and rear sets of wheels, unless there is closed a differential on either end, both AWD and 4WD will be stuck.

 

My friend had 2 blowouts on a jeep CJ10 pickup, one front and one back on the same side during a heavy snow storm (hit a stand pipe while 4 wheeling in a construction site). He could go nowhere. The 2 rims just spun inside the tires, if 1 tire on the front and one on the back don't have traction with open differentials you are stuck even with a locking transfer case.

[jamal]

Exactly. Most 4x4s don't have locking front/rear differentials. Some have rear LSDs.

 

The advantage to a part-time 4wd is that it's mechanically locked. Clutch plates and viscous couplings can overheat/be overpowered and stop working. I like my friend's 4Runner. It has full-time 4wd with a locking center diff and low range.

[Setright]

Not true.

 

AWD has a differential in the center, that distributes power to all 4 wheels all the time.

 

4WD has a Transfer case, which puts power to 2 wheels only, and when 4wd is engaged, power goes to all 4 wheels.

 

PLEASE!

 

"AWD" doesn't mean "permanent". So your first statement is false.

 

Old style Land Rovers have a central diff with a selectable lock. Is that AWD or 4WD by your personal definitions?

 

And can you explain to me why my 1990 Legacy had a sign on the back reading "2.2 GX 4WD"???

[NoahDL88]

PLEASE!

 

"AWD" doesn't mean "permanent". So your first statement is false.

 

9 times out of 10, AWD is permanent, the new fangled "real time" and others aren't AWD or 4WD, there in a category of their own.

 

Old style Land Rovers have a central diff with a selectable lock. Is that AWD or 4WD by your personal definitions?

 

Like the STi, they are AWD with a 4WD switch

 

And can you explain to me why my 1990 Legacy had a sign on the back reading "2.2 GX 4WD"???

 

If you look closely, its an A not a 4

 

Thank you?

[jamal]

If you look closely, its an A not a 4

 

Looks like a "4" to me:

 

 

[otis]

Looks like a "4" to me:

The secretary in our dept said she was going to look at a used outback wagon and she was so happy about it. I told her that was a great choice considering all the things she was looking for. But then she started saying things that didn't make sense like how she liked the fact that it had 4wd instead of Awd. I told her I thought all outbacks had awd, but she corrected me and said this car had a 4wd button- further confusing me. Finally I saw the car and her "outback" ended up being an early 90's legacy wagon.

 

HOnestly after reading all these posts, I still don't understand what the "practical" difference is between the modern awd and 4wd is.

[johnceggleston]

HOnestly after reading all these posts, I still don't understand what the "practical" difference is between the modern awd and 4wd is.

 

to further confuse...the owners manual in my 97 obw a/t has driving tips for both the legacy wagon and the outback. 'off road' driving is not recommened for one and is (with limitations) for the other. this surprised me, my understandig was that there are no differences in the drive train except tire size. did i miss something?

[the_bard]

The manual's probably not recommending offroading the Legacy, due to its decreased ground clearance. They recommend limited offroading with the Outback, so if some idiot decides to take it up the Rubicon and trashes it, the guy can't point to the offroading line in the manual to Subaru and make some unreasonable demand.

[Martinjmpr]

Re: the 4wd/AWD question on Subaru, as I understand it the early Legacy's used the term "4wd" because "AWD" was not a commonly used (or understood) term in the automotive industry. Later on they changed the term to AWD. But the driving system is the same.

 

Interestingly, my very first vehicle was a 1957 International Harvester Travelall 4x4. On the front fenders there was a plate that read "ALL WHEEL DRIVE." Obviously it was not an AWD system as we understand it today (it was a standard part-time 4x4 with a two-lever transfer case and locking hubs) but this illustrates the point that Setright was trying to make which is that the terms AWD and 4WD do not have fixed and absolute definitions. They change with the times and are often used interchangeably.

[SakoTGrimes]

The secretary in our dept said she was going to look at a used outback wagon and she was so happy about it. I told her that was a great choice considering all the things she was looking for. But then she started saying things that didn't make sense like how she liked the fact that it had 4wd instead of Awd. I told her I thought all outbacks had awd, but she corrected me and said this car had a 4wd button- further confusing me. Finally I saw the car and her "outback" ended up being an early 90's legacy wagon.

 

HOnestly after reading all these posts, I still don't understand what the "practical" difference is between the modern awd and 4wd is.

 

 

Are you sure it was a Legacy? I though only Loyales had manually engaging 4WD (in the 90s)?

[otis]

Are you sure it was a Legacy? I though only Loyales had manually engaging 4WD (in the 90s)?

Actually, I'm not sure anymore since it's been months since I've seen it. It may be a loyale and I just got confused (see how confusing this whole issue is???), but it's too cold to run over to the parking garage to check.

[Setright]

Thank you, Martin

 

 

 

I have to add that Volvo calls their XC90 and the stationcar its based on "AWD".

 

Yet these both use a Haldex system, just like the VW's. No center diff, just a clutch pack that engages once the front wheels slip.

[kingbobdole]

Ok... This is what I know, and I'll try to keep it simple. The difference in 4wd vs awd is what happens in the center of the car. Think two driveshafts going from the front and rear to the center(even though that not 100% true)

Typically AWD has either a diff or clutch pack in the center allowing the front "driveshaft" to spin at a different rate then the rear, making up for a difference in all the tires speeds aka. cornering.

Typically the 4wd system has no such thing... its "locked" the front drive shaft to the rear driveshaft. so the power is put 50/50 front to rear.

 

As far as I know a true open diff AWD system, you could get only 1 tire to slip and be stuck where in a basic open diff 4wd system, you have to have 2 wheels slip...

 

But since the 4wd system doesnt allow the driveshafts to be at different speeds and since the rear of a car does NOT follow the front exactly, the rear will "hop" to make up the difference in tire speeds. AWD is better for the street and 4wd better for adverse conditions... like big mud pits:brow:

[cannonball]

Just out of curiosity what cars have or have had AWD with an open center diff?

[FirstSubaruGLwagon]

Now Green,,,, Aren't you glad we cleared that up!

[friendly_jacek]

Here is a pretty good explanation that I found at http://www.delalbright.com/Products/products_awd.htm

 

ALL WHEEL DRIVE:

 

Definition: A vehicle where all four wheels are driven, but there's no transfer case like a four-wheel drive rig. Most AWD setups are full time systems for year-round driving, and use a viscous fluid coupling center differential instead of a transfer case to route drive torque to all four wheels. This allows the front and rear wheels to turn at slightly different speeds when turning on dry pavement. Most folks do not consider this the same as four-wheel drive. It can be useful (and more fuel economical) in pavement driving where you're mainly negotiating bad weather conditions.

 

FOUR WHEEL DRIVE

 

Definition: A method of driving a vehicle by applying engine torque to all four wheels thru the use of a transfer case, differentials and hubs. Various schemes are used for 4WD including part-time, full-time and variable four-wheel drive. To help cut the drive train drag (and reduced fuel economy) that most 4WD's have, a transfer case is included that allows the driver to select either two- or four-wheel drive depending on driving conditions. Some performance cars have full-time variable four-wheel drive and use a computer-controlled transfer case to route power between the wheels.

 

It's important to note that even in 4WD, you still have only two driving wheels; one front; one rear. A normal passenger car is essentially one wheel drive, because the other one can slip. So a 4WD rig, with the front axle engaged, now has two wheels driving. Then if you add a locker to the rear, you've added one more wheel, to make your rig three-wheel drive. Add a locker to the front, so no tires can spin, and you have TRUE, 4-wheel drive, all four wheels driving.

 

 

Its all semantics.

If you read these above definitions, they are not entirely correct either. The following part from the above 4WD "definition" is exactly what Subaru calls AWD in its AT cars:

"Some performance cars have full-time variable four-wheel drive and use a computer-controlled transfer case to route power between the wheels."

[cannonball]

Setright kinda has a good point. It is definitley all marketing babble. I think it is up to us here at USMB to set it straight.

 

4WD= Lockable Center Diff.

AWD= Limited Slip Center Diff. at best

 

Now where it really gets sticky is when you have an AWD Subaru (auto) like AndyJo's Impreza that is modded to lock the clutch pack thus taking it from AWD to 4WD with the flip of a switch.

[nixon]

So! What's a 4x4 then? Aside from a dismissive term for a land rovery, jeepy type gas guzzler. 4wd? AWD? depends who the marketing people are targeting.

[mikkl]

Ok, related question. In my 2005 Legacy Sedan, how many wheels have to be slipping for me to get stuck? If I understand correctly, I believe the answer is three - one on the front and both on the rear as it is listed as having a rear limited slip differential.

 

But I am still trying to understand how this all works.

 

<edit>

So I have gone off and done some reading and found a very nice site explaining how this all works:

http://auto.howstuffworks.com/differential.htm

 

Now that I understand this all a bit better, three slipping wheels sounds right to me. (Well, actually, I prefer four in a sideways drift, but that is another subject)

 

mikkl