4WD versus AWD

The term four-wheel drive describes truck-like 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, slippery surfaces, 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. 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.) 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 6x6, the M35 2-1/2 ton cargo truck being one of the best-known examples (dual wheels on the rear axles are not counted as additional drive wheels).

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, and Volkswagen's 4motion, for example, can mean either an automatically-engaging "on-demand" system with Borg-Warner ITM 3e magnetic or Haldex Traction hidraylic clutch, or a continuously-operating permanent 4WD system with a Torsen (torque-sensing) or other type of a differential.

Design

Differentials