all wheel drive explained

Why is it important to know how all wheel drive works on your car? First, it may appear that its all wheel drive system is not meant to be used on a road. For example, part-time all wheel drive cannot be used in non-slippery conditions - you'll have to drive such car in rear wheel drive mode, even when it is raining or snowing - in the weather conditions where all wheel drive might be needed. Second, depending on the type of all wheel drive, your car behaves differently when driving and cornering in slippery conditions. You might want to know what to expect.

Don't get confused by abbreviations that manufacturers use: "AWD" is not necessarily a full-time all wheel drive, "4WD" is not just for off-road vehicles. There is a dozen of brands car manufacturers are using to distinguish their four-wheel drive vehicles - "quattro", "4motion", and so on. However, these rarely indicate the type of all wheel drive system that is used on a particular vehicle.

In fact, just four types of all wheel drive systems exist:

• Part-time all wheel drive
• Full-time all wheel drive
• Automatic all wheel drive
• Selectable all wheel drive

Note: On this web site, when we describe details of the all wheel drive system used on a particular vehicle, we use the definitions that are listed here.

part-time all wheel drive Next

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, front wheels travel greater distance than rear wheels.

Figure: Wheels rotate with different speeds and travel different distances when vehicle is turning (23KB)

Because part-time all wheel drive system does not have a center differential, front wheels cannot go faster than 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 on a vehicle with part-time all wheel drive should only be used on surfaces with low traction (mud, snow, ice, sand), for short periods, and at low speeds. When driving on such surfaces, 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.

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 each wheel 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 automatic lock, a Torsen differential, viscous coupling, multi-plate hydraulic clutch, or similar traction control device is used in conjunction with the center differential. When a wheel slips on one of the axles (one driveshaft rotates faster than the other) the device locks the center differential and 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 Rover 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 slipping wheels and 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.

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 control device locks and engages another axle. Torque gets transferred to another axle. As soon as difference in front and rear axle speeds is eliminated, the device unlocks and the vehicle goes back into 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 control device even before wheels start to slip - the need for all wheel drive is determined in real time, based on the information that is collected from various sensors (i.e. g-force sensor, accelerator pedal position, etc.).

Some vehicles allow the driver to lock the multiplate hydraulic clutch manually when the driver feels that he needs all wheel drive engaged permanently, for example, to drive off road. In the Nissan X-Trail, for example, 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".

A known problem with multiplate hydraulic clutches, used in automatic all wheel drive systems, is traction device overheating. For example, this can occur with some compact SUVs when they are driven off-road, through a thick layer of snow, etc. ECU detects overheating, disengages the clutch, and puts the vehicle into 2WD mode. A warning light is displayed on the instrument panel.

In this category falls the 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 transmission that is similar to the one used in full-time all wheel drive vehicles, but 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 (vehicle behaves just like the one with full-time all wheel drive), 4wd with locked differential (behaves like part-time all wheel drive) and 4wd with low gearing (low range part-time all wheel drive).

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