What is the difference between 4WD and AWD?:
- Three basic systems available: full-time 4WD, part-time 4WD and all-wheel drive
- Differences in how and when torque is supplied to each wheel
- Important to choose the correct system for the typical vehicle use
Most mainstream manufacturers have released 'offroad' vehicles to take advantage of the public's obsession with four-wheel drive. But it's not just the traditional SUV and 4x4 vehicles that have contributed to the boom. More and more cars have been offering all-wheel-drive drivetrains from high-performance sportscars to luxury sedans and family-oriented peoplemovers.
But why do manufacturers use different terms. Isn't four-wheel drive (4WD) and all-wheel drive (AWD) the same thing? Let's have a look at the different terms in use and what each of them should mean.
Although each manufacturer tends to use its own name for its chassis systems for branding reasons, we are generally able to separate 4WD/AWD drivetrains into three basic groups.
- full-time 4WD
- part-time 4WD
- all-wheel drive (AWD)
Full-time 4WD operates as the name suggests. At all times torque is supplied to all four wheels, all of the time. The driver usually has several options available to them which affect the operation of the drivetrain depending on the conditions that are encountered.
In normal operation (such as driving on a bitumen surface) the front and rear axles are split by a differential which allows them to operate at different speeds when required -- for example, when cornering.
In most vehicles there will be the option of 'diff lock'. This locks up the centre differential and restricts any rotational difference between the front and rear axles. It's a feature that is commonly used when offroading to gain maximum traction.
There may also be the option of 'low range'. Usually used in extreme offroad conditions and on steep inclines, this changes the gearing of the drivetrain to provide a torque multiplying effect similar to the lowest gearing on a mountain bike.
Part-time 4WD is the original 4WD system and the most basic.
As with Full time 4WD there are several options available to the driver. In normal conditions the vehicle is driven in two-wheel drive, usually powering only the rear wheels. 4WD mode can be selected either by a separate mechanical lever or by an electronic switching system.
When 4WD is selected, torque is split evenly between the four wheels.
Part time 4WD vehicles tend not to have differentials between the front and rear axles. This limits the use of 4WD to offroad use because use of this feature on bitumen surfaces (where tyres have good traction) can cause excess stress and damage to the drivetrain (often called wind-up).
Most vehicles of this type have the option of 'low range' for extreme off-road conditions.
All-Wheel Drive is in some ways similar to the full-time 4WD system in that it also sends torque to all four wheels constantly.
These systems never have the option to operate in two-wheel drive, and unlike the 4WD systems, the differential between the front and rear axles cannot be locked. The differentials do, however, have ability to limit slippage between the axles if a low traction situation is encountered.
Usually this ability is provided with a differential known as a viscous coupling although more recently a similar outcome has been achieved by using electronically-controlled hydraulic or electro-magnetic 'clutch' systems.
AWD systems also lack the 'low range' feature which is common in most 4WDs.
The AWD system is less effective and more fragile than the 4WD system in a situation where high torque is required. Indeed, the viscous coupling units simply cannot satisfy high torque demands when the vehicle is offroad. AWD vehicles tend to be more 'car' like than obvious offroaders.
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