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I'm curious as to what are the particular difference between the various drive systems available in modern vehicles. FWD and RWD are pretty straightforward - Front Wheel Drive cars put the power to the front wheels, Rear Wheel Drive puts it in the rear. However, the remaining variants are less clear to me.

What is the difference between 4WD and "Part-Time" 4WD? How is AWD different from any 4WD? Are there any systems I've missed?

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To expand a little on what Eric said...

I consider the 4WD versus AWD to largely be marketing terms differentiating between vehicles with additional ground clearance and plating underneath to protect sensitive components while going over rough terrain off-road travel (4WD) from systems that are targeted more towards on-road travel (AWD). Though this is confused more by companies making their own names for all wheel drive systems like quattro, Syncro, SH-AWD, Hydratrak, etc...

I think it's worth differentiating between these flavors of drive:

  • Full time AWD. These systems are designed to be left engaged even when on dry roads.
  • Part-time AWD. These systems usually have a locked center differential AKA transfer case, which is will cause excessive wear on the drivetrain and/or tires if left engaged on dry roads. It's meant only to be engaged in reduced traction situations like snow, ice, gravel.
  • Hi/Lo AWD. This is probably the most likely to be called 4WD, because it's usually fitted on vehicles meant for off-road use. This will either be part time or full time, and on older vehicles would also have "locking hubs" up front. This has an additional set of gears after the transmission, which can increase the gear ratio, particularly for off-roading or other high torque applications.
  • RWD. Only the rear wheels are driven.
  • FWD. Only the front wheels are driven.

However, beyond AWD, FWD, and RWD, there are the front and rear differentials that can make a dramatic difference in how well a vehicle can handle low traction conditions. The differential is what connects the drive-line to two output shafts, say the ones going to the wheels on the left and right sides. It allows the wheels to turn at different rates as you go around corners. The inside wheels will describe a smaller circle, and therefore travel less distance than the outside wheels.

Not all differentials are created equal. Normally, differentials are of a type called "open". A sad artifact of these differentials is that power goes to the wheel with the least traction. So if one wheel is on ice, and one is on dry pavement, you will sit there spinning your wheels.

There are also a huge variety of "limited slip" or "locking" differentials which either allow you to manually lock and unlock them, or automatically distribute power to both wheels. These are more frequently found on sports cars, sometimes as additional-price option packs or upgrades, sometimes standard, in RWD and FWD cars. They are also used in better AWD systems in the center, front, and/or rear differentials.

There are also some hybrid systems, such as Audi's quattro system. Some models of this use a Torsen center limited slip differential, and open differentials in the front and back, but they do this trick where they use the ABS sensors to detect wheel spin, and then apply brakes to the spinning wheel, forcing it traction to go to the other wheels. While not literally a limited slip differential in the front and back, this is still extremely effective.

Sadly, the manufacturers don't really make it clear what exactly their system is. In short:

  • If there is a lever or button to turn on AWD/4WD, it is a part-time system.
  • AWD versus 4WD are basically the same, but 4WD tends to be used on off-road oriented vehicles.
  • Not all AWD systems are created equal, so you'll have to research if you want specific functionality out of the system..
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I recall someone saying that with 4WD only the wheels on opposite corners spin, but with AWD all the wheels spin. Is there truth to this, or is 4WD/AWD just marketing and the deciding factor about which wheels will spin is the differential? For this reason, I've always thought of 4WD as open differentials with the option to lock, whereas many AWDs have limited slip (but no option to lock). Just curious. –  Brian M. Hunt Mar 24 '11 at 21:17
    
@Brian, There isn't really an absolute answer here, it really depends on what the marketing department of a vehicle vendor decides they want to use. In general though, it seems to be common for off-road oriented vehicles to be called 4WD, and road-only to be called "AWD". The lines have really been blurred by marketing departments applying the labels inconsistently. I'd agree that a vehicle with optionally locking differentials is more likely to be called 4WD, but many Audi and VW vehicles have had locking differentials in street cars (for example). –  Sean Reifschneider Mar 27 '11 at 20:13
    
@BrianM.Hunt There is no truth to 'opposite corners spinning'. 4WD = locked center differential/"transfer case", meaning at worst only two wheels will spin (one front, one rear). AWD = open center differential, at worst only one wheel will spin (though it's rare that three wheels are on perfect ice). The differentials do not 'prefer' one side of the axle over the other, nor does the front diff know anything about what the rear diff is doing (so the opposite corners thing can't really happen). –  Ehryk Dec 26 '13 at 6:47
    
@SeanReifschneider Though some marketing departments may disagree, I consider any system with a lockable front/rear axle to be "4WD", and open/limited slip center differential to be "AWD". Within this there are selectable ones that blur the lines a bit (Jeep and Subaru mainly, with selectable locked/open center differentials), and certain other nuances. FWIW, wikipedia would agree with my summation: en.wikipedia.org/wiki/4wd#4WD_versus_AWD –  Ehryk Dec 26 '13 at 6:51
    
I would suggest that 'opposite corners spin' may have more truth than expected. Imagine your vehicle is on flat ground except that one tire is on a block. This would cause the other tire on that side of the vehicle to support less weight. Same for the other tire on that axle. Make the height of that block extreme such that one (or two) tire(s) is(are) off the ground. The vehicle can now teeter on two tires, so they could both spin easily. –  Les Feb 28 at 20:04
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In regards to most pickup trucks

  • 2WD - RWD — only the rear wheels turn. If the diff's have limited slip they will allow some variation across axles in tire speed.
  • 4WD High — all 4 wheels are driven. If the diff's have limited slip they will allow limited variation across axles in tire speed.
  • 4WD Low — all 4 wheels are driven. All diff's are locked. This forces all 4 wheels to turn at exactly the same speed. This is meant for low speed, limited operation only. EX. off-roading, or pulling someone out of a ditch.

In regards to most SUV's

  • Full time - AWD — Like 4WD High. May not have limited slip. All 4 wheels are always driven.
  • Part time - AWD — Like Full time, however may have a switch to disengage or more likely will engage or disengage based upon speed variance between axles. IE. From a stop, 4 wheels are driven until x speed and all wheels are turning at the same rate. Generally the front wheels are always powered.

Acura's SH-AWD — This is a special case where they have devised a system to actually speed up the outside wheel in a corner.

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How is SH-AWD different to what a differential does by default? It allows an outside wheel to rotate faster than the inside wheel. –  Rory Alsop Mar 8 '11 at 14:37
    
@Rory, @dilbert kind of oversimplified it. SH-AWD will "overdrive" the rear-end by 2% (one model) or 0-6% faster than the front wheels. It can also transfer up to 100% of the rear-end power to the outside wheel. Wikipedia has a detailed article about it. –  Sean Reifschneider Mar 13 '11 at 1:06
    
@dilbert789 In your SUV section: Part Time AWD is pretty similar to 4WD High. They are selectable and link the front and rear axle with a transfer case, in what could be considered a 'locked center differential'. However Full-Time AWD is a different animal, it will distribute power to the front and rear axles through an open/limited-slip center differential, but will only force one wheel to spin under the worst conditions, where 4WD / Part-Time AWD (another name for 4WD) will force two wheels to spin. –  Ehryk Dec 26 '13 at 7:06
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4WD is distinguished from "part time" 4WD in that you can't turn it off - the vehicle is always in 4WD. Vehicles that have this usually have a differential built into the transfer case to allow for full time 4WD even on dry pavement.

AWD is almost the same as full time 4WD, except that AWD vehicles generally don't have the high-torque low gear option (4L) that you get with a 4WD vehicle.

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Most 4WD systems allow you to turn them off, usually via a lever to the Transfer case. AWD has nothing to do with the low torque option, which can be present on AWD systems (like some Subarus) or missing from some 4WD systems (some Jeeps). Please read more here: en.wikipedia.org/wiki/4wd#4WD_versus_AWD –  Ehryk Dec 26 '13 at 7:08
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