All-wheel drive v. 4-wheel drive

So what exactly is the difference?

The Subaru AWD works sorta like this: The car is “front wheel drive” the vast majority of the time. If the front wheels start to lose traction, power is transferred to the rear wheels. (You can sometimes feel it kicking in. Very cool.) The AWD system gives you most of the advantages of a front-wheel drive car (like good mileage) while giving you most of the advantages of a 4WD car (4-wheel traction). The disadvantage is that it is not full-time 4-wheel drive, so it is not really made for serious off-roading.

Now, 4-wheel-drive vehicles can come as either full-time 4WD (like a Range Rover) or part-time 4WD (like a Jeep Cherokee). Full time 4WD gives you the traction advantage all the time, but burns fuel like crazy. With the part-time 4WD, you can switch in on or off as needed. The disadvatage is that the 4WD might not be engaged when you need it. Obviously, 4WD means that all four wheels are powered. But the ratio of how much power goes to each wheel varies by car/truck model.

I’m sure someone will come along soon and explain it better.

Thank, Green Bean. That makes sense. I thought I had seen “permanent AWD” somewhere, but I was probably misread “permanent 4WD” instead.

Not bad, Green Bean. Some advanced AWD’s will even compensate in a skid, delivering traction to the wheels that will do the most good
A related, extra point (not a hi-jack) question;
I was at Lake Tahoe last weekend and almost had to chain up. Where do you put the chains on an AWD? Or even on a 4WD with street tires? 4WD is virtually usless in snow with those wide, pretty show tires.

Generally the main difference is the presence or lack of a dual-range transfer case.

Green Bean’s answer comes close but I think he is confusing 4WD/AWD with the type of differential in use.

Another typical difference is the type of chassis. A vehicle on a car chassis that turns all four wheels is usually called AWD. The only car I can think of that was marketed as 4WD was the old AMC Eagle. Others, like the various Subarus, Toyota Celica GT4/All-Trac, Audi Quattro, some Porsche 911 models (like the new Turbo, finally!) are marketed as AWD.

Trucks are nearly always marketed as 4WD, whether it be full-time or part-time. Additionally, these vehicles typically have a two range transfer case attached to the ouput of the transmission. High range is normal driving gear. Low range offers MUCH lower gearing which provides maximum traction when offroad. It also usually locks the front and rear axles. In 4WD High, the rear wheels usually get more torque than the fronts; In 4WD Low, torque is split equally. AWD cars never, AFAIK, lock the two axles together, torque-wise. There is almost always a bias though in some cars it is toward the front wheels (those that were derived from FWD cars) and toward the rear in others (those derived from RWD cars).

The kicking in GB describes in the Subaru is the surge of acceleration felt when the rear wheels start getting more torque when the fronts slip. They are always getting some torque, but usually far less than the fronts. (65F/35R is a traditional split for AWD cars like the Subaru. The new Porsche 911 Turbo AWD is probably going to have something like a 65R/35F torque split under normal circumstances. The bias is not the same for all cars. 4WD Low range on a truck would be a 50/50 split.)

I’d beg to differ. A good M/S truck tire will offer far better traction than the AS tires found on most AWD cars. Locking up the diffs on a 4WD helps greatly in snowy/icy conditions. With AWD, there is always some lag in getting torque to the tractionless axle. Locking it all up eliminates this.

The biggest problem facing 4WD trucks is not the tires, it’s the clueless drivers who equate 4WD with invincibility.

I’ll put myself up against any AWD car on snow given a HMMWV with Uncle-issue Goodyear Wrangler-II tires. I’ve done alot of off-road winter driving in one of those and never even came close to getting stuck. Well, there was one time when I was fording a stream and broke through the ice and came reeeaaallllly close… When the weather got really bad, they were the only things that always got through on the roads, too.

The Nissan Stanza wagon was available in a 4WD model (not marketed as AWD).

Are you sure? WRT the Subaru in particular, I don’t think the rear wheels are powered most of the time. (Like during highway driving.) I suppose I could be wrong, not that that’s ever happened before. :stuck_out_tongue: I looked at the Subaru site, and strangely enough, they do not describe their AWD system. Weird.

p.s. I’m a feeemale! I’m even wearing pink!

Picking a nit: Jeep Cherokees come standard with part-time 4WD, but full-time 4WD is an option. IIRC, the “full-time” can be disengaged and the Jeep can run with 2WD. The advantage of Jeep’s “full-time” 4WD is that the driver does not have to switch to 2WD on a hard, flat surface.

Unless you have a limited slip center diff instead of a transfer case, torque is neither applied equally to front or rears axles, nor are front and rear axles locked. Rather, the front and rear driveshafts are locked. That is why 4WD should only be used on slippery surfaces, unless you have a diff in the center (like a Jeep QuadraTrac) instead of a transfer case. If you have an open differential instead of a transfer case, you’ll apply equal power to the front and rear axles rather than torque. I think Subarus and Eagles had these for a while.

Most OEM transfer cases do nothing besides lower your effective gearing by a factor of 2-3 when you shift from 4H -> 4L, allowing you to exhert much more torque at lower speeds. About the only time I use 4L is to position very heavy trailers, such as goosenecks or the 5th wheel, or to inch slowly down slippery surfaces. Sometimes I used it on the trail when I was heavily into Jeeping.

Green Bean, you’re sort of right, sort of wrong.

Subaru’s AWD is different depending on which type of transmission you get. According to the unofficial outback FAQ, the manual is 50-50 (with a viscous coupling to transfer power), and the automatic is 90F-10R most of the time, with the microcontroller switching the power (probably slightly better time lag). There is a fuse you can pull out to force it into FWD (automatic only).

So unless you pulled the fuse on an automatic, there is some negligible amount on the rear tires.

As for putting on chains, the owner’s manual may be of some (but not all help). As far as I can tell on the Subaru Legacy manual, you can put them on the front two. On the automatic, you need to put them on all 4 (otherwise you might confuse the sensors). Also, they recommend that you should be towed with all four tires off the ground, unless you pull the fuse.

sewalk. I think we’ve mis-communicated here a little. As a matter of fact, we may even be in agreement. By “show” (notsnow) tires, I was referring to those wide, low profile tires that a lot of people put on their 4WD’s to make them sexy. At least they do down here in the flats. And when they do get up in the snow, they get in the way of that ‘62 VW Bug puttin’ on through. Wide and smooth is not a good combination for snow.
Look at the tires on the military Humm Vee. Tall and skinny, right? Just like your’s? :wink:
BTW; I don’t know what an “AS” tire is.

Four-wheel drive is for rugged outdoorsmen who need to haul a boat up a muddy gorge

All-wheel drive is for soccer moms who don’t like to drive in the snow.

or for couch potatoes who want others to believe they are rugged outdoorsmen when in reality the closest they get to needing 4wd is driving over the speed bump on the way to get another 6 pack :wink:

Yep. My Cherokee has the standard-issue part-time 4WD system(CommandTrac), which has no limited-slip differential and locks the front and rear wheels so that they travel at the same average speed, IIRC. So even when engaged, my part-time 4WD is not true 4WD or AWD, although it is still pretty useful in snow, sand, loose dirt, etc.

One other comment: When I was buying my Cherokee, I test-drove it and made the mistake of putting it into “4WD” and trying to turn around in the parking lot. I was treated to the indescribable sound of the axles and/or driveshaft trying to break in two; my own personal “moron moment”. Definitely not recommended.

First of all…Audi’s quattro rules!

Here’s the definitive article on AWD systems:

The main difference between Audi’s quattro and systems like on the new BMW 330xi is the BMW brakes the wheel that is slipping while quattro tranfers power to the wheels that aren’t.

99.5 A4 2.8qms

man I hate it when I’m late answering a Subaru question. :slight_smile:

Anyway, Subaru actually has three distinct Systems depending on the type and transmition of the car. One equal, one near equal, and one heavily weighting to the front, I don’t think any AWD system does all front, 90/10 is the usual weigthing of a front-wheel predominant AWD system.

anyway from the subaru site.


Well someone posted this on the audiworld forum so I take a crack at it.

Remember, the entire point of either AWD or 4WD is one of three reasons:

  1. offroading
  2. bad weather driving
  3. performance driving

The line is blurry, but usually AWD is a car term and 4WD is a truck term.

And then there are 2 ways to implement 4WD or AWD generally, a limited-slip system (differential, clutch) or a transfer case.

A transfer case locks driveshafts are equal speeds unlike the action of a differential which lets wheels spin at different speeds. Generally, this is best for offroading and when your stuck in snow or ice. Manly because, your mobility is limited, turning is prohibited when your turning wheels are locked at the same speed. But when you want all the torque spread even to get out of that ice or go over a rock, a transfer case is what you need. (i’m no truck expert so correct me if I’m wrong)

A limited slip system used to be only used in cars but nowadays, a few SUVs are starting to use them. The highend Cherokee system uses one for instance. The good thing about this type of system proactive rather than reactive. Torque is transferred on the fly when needed, so it is especially good at changing with the situation say in snow conditions or when your doing exiting out of a high speed corner.

The 3 main types of limited slip systems are viscous fluid systems (viscous coupling differential or viscous coupling differential lock), multi-plate clutch systems, and Torsen differentials. Now remember, each of these can be placed at any of the 3 points: 1) the center to transfer torque to the front or back or 2) the front to transfer torque left and right in the front axel or 3) the same for the rear. Subaru uses a viscous coupling differential lock in the center for manual trannies and a clutch system for the auto trannies. I also believe that the Subbie system has a limited slip for the rear axel too (probably viscous). Audi uses a Torsen in the center and traction control in the front/rear axels.

More advanced systems are clutch based systems like the Porsche 959’s NSK system and the Skyline GT-R’s ATTESA E-ETS Pro which basically engage multiplate clutches to transfer torque via a computer. The computer in both these systems take into consideration loads of parameters like traction, throttle, boost… etc etc

I could go on and on but I’m getting long winded.

unwashed brain enthuses;

Saving it up here, boss!
Maybe 2 more years and I’ll be trading in my Windstar. :slight_smile:
Hope I haven’t forgotten how to drive. This is not the car for “rugged outdoorsmen who need to haul a boat up a muddy gorge” though. :wink:

I’d describe the Goodyear Wrangler II as fitted to the HMMWV as tall and wide, with at least 10 inches of tread width.

AS, as relates to tires, stands for All-Season, a notch or two below M+S (Mud and Snow) in traction during inclement winter weather.
As far as my statement about 4WD systems locking the axles, I meant to state more clearly that the axles were locked in relation to one another, not to imply that each wheel on a particular axle was locked to its partner. That is a different (pun intended) subject.