[Ahmad Pervez]

Ahmad Pervez

April 25, 2008 at 3:13 pm #44

Part-Time? Full-Time? Automatic?… What About AWD?
The common assumption is that 4×4 means all four wheels are driven at the same time. However, this isn’t necessarily true. When a vehicle negotiates a bend, the differential in the axle compensates for the fact that the outside wheel travels further than the inside one does. Thus, it allows a speed differential to exist between the two wheels.
Being a mechanical device, the differential abides by the laws of physics which state that energy always takes the easiest route. So, if one wheel is on a slippery surface (like sand) then all the energy will be sent to that wheel and away from the wheel with traction. The end result is that you lose all forward motion!
When four-wheel drive mode is engaged, the front and rear axles are locked together, so at least one wheel on each axle can be driven by the engine effectively.
You can force a 4×2 vehicle to act similar to a 4×4 on occasion by gently pressing the brake pedal to slow down the wheel that’s spinning and transfer energy to the wheel with traction.
4 x 4 (4WD) – Describes a vehicle with four-wheel drive.
The first figure is the number of wheels. The second is the number of powered wheels.
4 x 2 (2WD) – Refers to a two-wheel drive vehicle with four wheels. The first figure is the number of wheels. The second is the number of powered wheels. With a 4×2, engine power is transmitted to only two wheels, usually the rear.
Part-Time 4WD – Refers to a four-wheel drive system that operates on-demand and drives all four wheels by locking front and rear axles together via a shift lever. It usually includes two speed ranges (Hi and Lo). Part-time 4WD systems must be operated in 2WD mode on dry pavement, as they’re designed to be used only in specific situations when extra traction is required. Jeep Wrangler, Nissan Patrol, Isuzu Trooper & the Toyota Land-cruiser 80 & 100 series GXR are typical examples.
Full-Time 4WD – Describes a four-wheel-drive system that can be operated continuously on all surfaces. A full-time four-wheel-drive system may include the option of part-time operation, this is one of the best setups to have both on road & off, and may or may not have Hi and Lo speed ranges, most of the modern high end vehicles such as the Toyota Land-Cruiser 100 series VXR, Toyota Land-cruiser 200 series, Toyota Prado, Jeep Grand Cherokee, the Land Rover & the Range Rover are all good examples.
Automatic Four-Wheel Drive (A4WD) – This type of drive system automatically engages 4WD as needed. When internal monitors sense differences in individual wheel speeds, indicating that a tire is slipping, then 4WD is automatically engaged, almost all the modern SUV’s fall under this category the Honda CRV, Nissan Murano etc.
Shift on the Fly – This type of system allows manual shifting from 2WD to 4WD Hi without coming to a stop. Most systems have a speed limit at which you can engage the system; typically it’s less than 60 mph.
All-Wheel Drive (AWD) – A full-time single-speed system designed to supply drive power to all four wheels. The percentage of front/rear power delivery varies from system to system.

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