In this graph, each box represents a AWD/4WD system category. A dotted arrow from system A to system B means that the two system are comparable and that the latter system provides better traction on slippery roads. Not every two systems are directly comparable, for example, it does not make sense to compare an AWD system with a part-time 4WD system. However, arrows are transitive, for example, the Full-time 4WD (Torsen) system is better than on-demand AWD systems.
Under normal driving conditions, only one axle (typically the front) gets most of the power. When slipping is detected, more power is diverted to the other axle.
On-demand AWD (transverse): Most economical, low-end all-wheel-drive systems fall into this category. The system is typically based on a front-wheel-drive (FWD) platform. AWD is usually an optional feature on these cars. The engine and transmission are transversely mounted so they can send power directly to the front axle. When needed, intermittent power is transmitted to the rear axle via several indirections.
Example: Most AWD cars and crossovers built on a FWD platform.
On-demand AWD (longitudinal): These are better performing all-wheel-drive systems. Engine and transmission are mounted longitudinally. Power is first transmitted to the center of the vehicle, then split to front and rear axles. The drivetrain layout is designed to distribute torque more evenly between front and rear axles when AWD is activated.
Examples: Subaru Active AWD, BMW X-drive (latest), Infinity Intelligent AWD.
Limitations: typical on-demand AWD systems cannot maintain continuous power output to both axles during normal driving, because the power splitting mechanism is designed for intermittent operation and may overheat during extended use.
In a full-time AWD system, power is constantly split between front and rear axles at a fixed ratio using a mechanical center differential
. During normal driving, each wheel gets less torque compared to two-wheel-drive or on-demand systems so they are less likely to slip and lose traction.
Full-time AWD (v-lsd/open): These are the simpler full-time AWD systems that maintains constant power split between front and rear axles in normal driving conditions but have little ability on reacting to slipping conditions: when one set (front or rear) of the axle do lose traction, the other side gets little to no power output due to the use of an open differential. Viscous limited-slip differential are introduced to reduce slipping, but typically takes longer time to react. Some recent systems use ABS brakes to reduce wheel slipping.
Examples: Subaru Continuous AWD, Mercedes 4-Matic (4ETS).
Full-time AWD (e-lsd): These systems provide the best of two worlds: the mechanical center differential provides continuous power split between front and rear axles in normal driving conditions, but the power distribution ratio is also electronically controlled when slipping occurs just like on-demand AWD systems.
Example: Subaru VTD AWD.
Full-time AWD (Torsen): This system uses a mechanical, Torsen center differential which proactively adjusts the power distribution between axles. It performs better than electronically-controlled slip limiting mechanisms because the Torsen differential is designed to be proactive; there is no delay in detecting the slipping condition and taking time to react.
Example: Audi Quattro (most models).
Part-time 4WD: In harsh or off-road conditions, nothing beats the simplicity, reliability and predicability of a 4WD system. The driver can manually select a locking gear setting that binds the front and rear axles together with hard links and forces them to always rotate at the same speed. The system can transmit 100% power to either axle as needed; slipping or traction loss in one axle will not affect the other. However, this locking 4WD mode can be dangerous to use on dry roads because the vehicle is difficult to make turns when the front and rear axle are forced to rotate at the same speed.
Example: Most 4x4 pickup trucks.
Full-time 4WD is essentially the combination of a full-time all-wheel-drive system and a part-time 4WD system. In addition to part-time 4WD systems, a mechanical center differential is used to provide continuous power split between both front and rear axles without binding them together. Such systems usually have selectable gears between full-time 4WD mode (for on-road and light off-road use) and locking 4WD mode (for harsh off-road conditions). Depending on the type of center differential and slip limiting mechanism used, full-time 4WD has similar categories with full-time AWD:
Full-time 4WD (v-lsd/open): the center differential is either an open differential or a viscous limited-slip differential.
Full-time 4WD (e-lsd): the center differential is equipped with electronically controlled mechanisms that adjusts power split between front and rear axles.
Example: Jeep Grand Cherokee (Quadra-Drive II), Jeep Liberty Renegade (Selec-Trac II).
Full-time 4WD (Torsen): a Torsen center differential is used to optimally distribute power between front and rear axles.
Examples: Toyota FJ Cruiser (Manual), 4Runner Limited, Sequoia, Land Cruiser, Lexus GX and LX. A similar but more advanced example is HMMWV which uses a lockable, open center differential with two Torsen differentials on both front and rear axles.
Which is better for me?
- Cars and crossovers use AWD. They don't need off-road capabilities. AWD pvodides better traction in less than ideal weather conditions.
- SUVs and trucks are better with 4WD for their purpose. Full-time 4WD can do everything AWD can do, but 4WD vehicles can be heavier in weight due to added gears and heavier duty parts. Trucks don't drive like cars.