What is the difference between AWD and 4x4

[Gunta]

I am having a debate with a mate about the difference between 4x4 and all wheel drive ie Subaru vs hilux. My understanding is that an AWD and I might be mistaken sends power to all the wheels, but if in an axle twister and one wheel loses traction completely then the vehicle will be stuck as all the power is transferred to the wheel in the air. Now they may be fitted with a limited slip diff (Front or rear or both) which would be able to transfer some power to the other wheels. But the question is how does AWD perform against 4x4 (without low range and diff locks)

[OOOOMS]

Law of physics says power will go to least resistance.

Proof is in the pudding :wink2:

[jan30]

My opinion is that 4H and AWD is pretty much the same thing but obviously with a 4x4 you have the option of the little lollipop that sits next to your gearlever

[Mud Dog]

AWD vehicles are permanently in 4x4 and employ visco couplings on the half shafts to accommodate slippage on turns. They are diff locked.

4x4 is like our bakkies with open diffs and sometimes have diff lock. If you took one of our bakkies and fitted a LSD diff front and back then it almost becomes AWD.

[Gunta]

The term four-wheel drive typically describes larger passenger vehicles that may allow the driver to manually switch (sometimes with an automatic option) between two-wheel drive mode (if available) for streets and four-wheel drive mode for low-traction conditions such as ice, mud, snow, or loose gravel.

All-wheel drive (AWD) is often used to describe a "full time" 4WD that may be used on dry pavement without damaging the differentials, although the term may be abused when marketing a vehicle.[2] AWD can be used on dry pavement because it employs a center differential, which allows each axle to rotate at a different speed. This eliminates driveline binding, wheel hop, and other driveline issues associated with the use of 4WD on dry pavement. For vehicles with more than four wheels, AWD means all wheels drive the vehicle, to varying degrees of engagement, while 4WD means only four of the wheels drive the vehicle continuously. For example, an AWD vehicle with six wheels is often described as a 6×6, the M35 2½ ton cargo truck being one of the best-known examples (dual wheels on the rear axles are not counted as additional drive wheels).

Because all 4 tires in a full time AWD system are connected by a system of differentials, they are potentially very susceptible to torque reduction when a wheel loses traction. Without sophisticated traction control systems, they would become immobilized when any one of the four tires lost traction. A traditional part time 4WD system does not connect the front and rear via a differential, and therefore does not suffer any front/rear torque reduction—if a front tire loses traction, it does not reduce torque delivered to the rear tires, even without traction control systems.

Part-time 4WD systems are mechanically simpler and cheaper than AWD systems. Also, a part-time 4WD transfer case is usually equipped with a reduction gear setting that provides for higher torque at lower speeds, a vital feature for vehicles that will see much off-road use. In AWD systems a more expensive separate reduction gearbox is usually used. The main drawback of 4WD is that because it lacks a center differential, a part-time 4WD system can only be used in low traction situations where the wheels have the ability to slip as needed.

For these reasons, full-time AWD is appropriate for improving on-road handling and is seen on cars and car-based crossover SUVs, while traditional part-time 4WD systems without center differentials, or with locking center differentials, are better for heavy-duty use such as off-road or in deep snow and are commonly seen on trucks and truck-based SUVs.

Manufacturers often use these terms interchangeably, and the English word all is obviously equivalent to the word four when referring to a set of four wheels. In typical use, the terms are used as described above, but they are not fixed or legal definitions, and some manufacturers, such as Honda, often use the term "four wheel drive" when referring to systems that include center differentials. These terms are therefore not always very reliable for assessing the features and capabilities of a given drivetrain.

Identical drivetrain systems are commonly marketed under different names for upmarket and downmarket branding and, conversely, different drivetrain systems are commonly marketed under the same name for brand uniformity. Audi's quattro, Mercedes-Benz's 4Matic, BMW with the xDrive, Saab's XWD, and Volkswagen's 4motion, for example, can mean either an automatically engaging "on-demand" system with Borg-Warner ITM 3e magnetic or Haldex Traction hydraulic clutch, or a continuously-operating permanent 4WD system with a Torsen (torque-sensing) or other type of a differential.

[CasKru]

4WD - four-wheel drive


Also known as part-time all-wheel drive, this system has an open differential on the front and rear axle and a transfer case on the output from the gearbox. Typically 4WD is normally driving the rear axle with the front axle only coming into play in 4WD mode. The transfer case is the device that splits the torque between the front and rear axles. It typically has some sort of selectable internal differential or viscous coupling to allow the front and rear drives to turn at different speeds if need be. Some trucks and SUVs have a selector with 2H, 4H and 4L on it - it looks like a second gear shift. This is actually controlling how the front and rear outputs of the transfer case get locked together. In 2H mode (2-wheel drive, high), it essentially disconnects the front output completely and only drives the rear axle. In 4H mode (4-wheel drive, high), it engages the front output via the viscous coupling so that the axles can turn at different speeds, and now sends torque to both open differentials. In 4L mode (4-wheel drive, low) it engages a second set of reduction gears and locks the front and rear axles together so they must spin at the same speed. This would be bad for on-road driving because it does not allow any difference in speed between the front and rear wheels, so you'd often get dragging and slipping which would make the car essentially unsafe to drive. However, locking everything together like this and reducing the gear ratio makes perfect sense for off-roading, which is why it's an option. However, with open differentials, it's still entirely possible to get stuck with a 4WD vehicle. If you're off-roading and the front-left and rear-right wheels both leave the ground together (for example), then the torque will all be sent to those wheels and they'll spin helplessly in the air. Locking, limited-slip or Torsen differentials solve this but add weight, complexity and cost to the system.
Locking hubs On older 4WD systems, the front wheels could only be engaged to the transfer case by locking hubs. Essentially the transfer case was always sending torque to the front driveshaft and had no viscous coupling. To get into 4WD mode, the driver had to stop and get out, and lock the front wheels to the axles so they could be driven. In newer 4WD systems, the lockable hubs are still present on some models, but are designed more for mechanical sympathy and fuel economy than anything else. With the hubs onlocked, the whole front part of the drive system isn't being dragged along for the ride, which causes mechanical wear and a drop in fuel economy.


AWD - all-wheel drive type 1

Finally, all-wheel drive or full-time 4WD. Found mostly on sportier cars, but also on some SUVs, there are two types of AWD, both designed to try to overcome the problems with 4WD. The simplest form has two open differentials - one on each axle - and a viscous coupling between. The engine drives the gearbox which drives two output shafts. One goes to the front open differential and the other goes to the viscous coupling, the output of which is connected to the rear open differential. Under normal conditions, this type of AWD system functions exactly like a 2WD car, driving only the front axle (unlike a 4WD which normally drives the rear axle). Lower end Subarus and some of the Honda trucks use this system. The front wheels turn at a certain rate, and the rear wheels are dragged along for the ride. Both halves of the viscous coupling are spinning at the same speed so no torque is sent to the rear axle. If the front wheels begin to slip and spin, the input to the front of the viscous coupling begins to spin faster than the rear and because of its torque-converter-like design, this causes the rear output to want to speed up. At this point, the drivetrain is now transferring torque to the rear axle and the car starts to function in AWD mode. Actally, AWD is a bit of a misnomer at this point, because unless the car has limited-slip differentials front and rear, it's still only really driving two wheels in this mode - the one on the front and the one on the rear axles that have the most traction. That leads us nicely on to.....


AWD - all-wheel drive type 2


This is the other type of AWD found on higher-end Subarus, rally cars, expensive sports sedans and such. Very similar to the type 1 AWD, it replaces the viscous coupling with a Torsen differential, and replaces the open differentials front and rear with either Torsen or limited-slip differentials. This is the only true all-wheel-drive system because it will always drive all four wheels. It's also bloody expensive and it saps gas-mileage because of all the extra drag induced in the driveline. But then if you're into performance off-roading, gas-mileage really isn't your primary concern.

[Gunta]

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