all wheel drive explained (Čeština, Deutsch, English, Polski, Русский)

part-time all wheel drive(Edit)

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.

When a vehicle is turning, the front wheels travel greater distance than the rear wheels.

Figure: Wheels rotate with different speeds and travel different distances when vehicle is turning

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.

Note: "Part-time 4wd" mode of the Jeep Cherokee's SelecTrac transmission means "locking of center differential". Jeep's SelecTrac is a selectable all wheel drive system.

full-time all wheel drive(Edit)

This is a permanent all wheel drive or permanently engaged all wheel drive system. All wheels are powered at all times. The vehicles with full-time all wheel drive are equipped with a center differential that lets all wheels travel different distances while turning. This type of all wheel drive can be used both on and off road. In slippery conditions, the center differential can be locked, whether manually or automatically, depending on the vehicle.

When a manual center differential lock (available on off-road vehicles and some SUVs) is engaged, the transmission's behavior is similar to part-time all wheel drive, i.e. the front and rear driveshafts rotate at the same speed. The use of full-time all wheel drive with locked center differential is limited to surfaces with low traction.

In case of an automatic lock, a Torsen differential, viscous coupling, multi-plate hydraulic clutch, or similar traction device is used in conjunction with the center differential. When a wheel slip occurs (one driveshaft rotates faster than the other) the device locks the center differential and the torque is transferred from the axle that slips to the other axle that has traction. As soon as the wheel slip is eliminated, the device unlocks.

Some vehicles (Land Raver Discovery II, pre-xDrive BMW X5) do not have a locking center differential, but are equipped with an electronic traction control system (known as Electronic Differential Lock - EDL) on all four wheels. This electronic system detects slipping wheels by reading ABS sensors, then it applies brakes to the slipping wheels and the torque gets transferred to the wheels that have traction. While it performs well on slippery roads, the system cannot compete with a real mechanically locking differential when driving off-road.

automatic all wheel drive(Edit)

This is an "on-demand" all wheel drive system. Under normal driving conditions, only one axle is powered. When wheel slipping occurs (the driving driveshaft rotates faster than the driven driveshaft), a multiplate hydraulic clutch, viscous coupling, or other similar traction device locks and engages another axle. The torque gets transferred to another axle. As soon as the difference in the front and rear axle speeds is eliminated, the device unlocks and the vehicle goes back to the two-wheel drive mode.

The difference between the traction devices that are used in full-time all wheel drive and automatic all wheel drive systems is that the device used in automatic all wheel drive system replaces the center differential.

Advanced electronically controlled all wheel drive systems can be proactive and lock the traction device even before wheels start to slip - the need of all wheel drive is determined in real-time, based on the information that is collected from different sensors (i.e. g-force sensor, accelerator pedal position, etc.).

Some vehicles let the driver to lock the multiplate hydraulic clutch manually when the driver feels that he needs all wheel drive engaged permanently and before wheels start to slip. For example, in Nissan X-Trail, this is accomplished by pressing a button on the dashboard console. In Subaru Legacy, the clutch is locked when the automatic transmission gear shift lever is at the position "1".

selectable all wheel drive(Edit)

In this category fall Mitsubishi Pajero(Montero) with its Super Select transmission, Jeep Grand Cherokee with SelecTrac transmission, and a few other off-road vehicles. Mitsubishi, for example, has in fact a full-time all wheel drive transmission with two wheel drive possibility. In Mitsubishi, the driver can choose between the 2wd mode, 4wd mode with automatic distribution of torque via viscous coupling (acts like the full-time all wheel drive ), 4wd with locked differential (acts like the part-time all wheel drive) and 4wd with low gearing (low range part-time all wheel drive).

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