We have heard of all-wheel-drive or AWD, but what does that mean? And how does it differentiate from four-wheel driver or two-wheel drive? Here are some definitions.
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.
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. With most four-wheel drives the driver is able to typically 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, slippery surfaces, or loose gravel.
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.
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 (allowing you to shift into 2WD on dry pavement for example), and may or may not have Hi and Lo speed ranges.
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.
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 under 60 mph.
All Wheel Drive vehicles are often described as “full time” 4WD that may be used on dry pavement without damaging the differentials, although the term may be abused when marketing a vehicle. AWD can be used on dry pavement because it employs a center differential, which allows each axle to rotate at a different speed. (“Full-Time” 4WD can be disengaged and the center differential can be locked, essentially turning it into regular 4WD). On the other hand, AWD cannot be disengaged and differentials cannot be locked. Another 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 ice) 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.
Cars known for their impressive All Wheel Drive are numerous. A few include the Subaru Impreza, Ford V6 SE, Acura RL, Toyota Sienna LE, Subaru Outback, Subaru Forester, Ford Edge, Acura MDX and the Audi TSS.
Your driving needs and talking with your car dealer can help you determine which vehicle is best for you.