The all-wheel-drive (4WD) system is the soul for 4x4 vehicles, and the key feature that makes a true 4x4 goes off-road.
There are four types of AWD systems, covering not only 4x4 vehicles like the old rugged mighty FJ40 but also performance vehicles, such as famous Subaru STI series.
- Part-time all wheel drive
This is a "temporary" all wheel drive system. In normal driving conditions, just one axle (the rear axle normally) is driven. In slippery conditions, another axle is engaged by the driver, whether by a lever or a button. This type of all wheel drive does not have a center differential - when all wheel drive is engaged, the front and rear driveshafts are mechanically connected and rotate at the same speed.
Because the part-time all wheel drive system does not have a center differential, the front wheels cannot go faster than the rear wheels. This type of all wheel drive cannot be used on pavement. Turning on pavement (even on a wet pavement) with all wheel drive engaged causes transmission windup and increases the chances of the transmission breakdown. When all wheel drive is engaged, the vehicle heavily understeers and this can lead to an accident.
The all wheel drive mode should only be used on surfaces with low traction (mud, snow, ice, sand), for short periods, and at low speeds. In these conditions the transmission windup is eliminated by slipping of the wheels.
Most of the modem SUVs are part-time AWD, such as VW Tiguan. The part-time AWD sacrifices some off-road capabilities for better fuel efficiency. And with the help of Electronic brakeforce distribution (EBD) and Electronic Stability Control (ESC)systems , even some of the high-end two-wheel-drive systems are able to perform closely to a AWD system.
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