All-wheel drive (abbreviated AWD), sometimes called full-time four-wheel drive, is similar to four-wheel drive in that it provides power to all four wheels. The difference being that it has three differentials, one on each axle and a third between the two axles. This allows it to provide anywhere from 0%-100% of the total power to any wheel. While AWD is considered by most to be the optimal way for a motor vehicle to maintain traction on pavement, it is not as good off pavement and can be a problem during off-road driving when one wheel is in a slippery spot and absorbs all available power. This can be avoided by the use of limited slip differentials, like those found in high-performance AWD vehicles(such as the Subaru systems). Also, almost all AWD systems lack a low-range gear which makes them much less capable in challenging off-road situations like rock crawling or deep mud. All-wheel drive can be used on dry pavement, unlike true 4WD which locks the drive line front to rear and doesn't allow for variations in drive line speed using a differential.
Disadvantages and Overcoming Them
Every vehicle has its own unique challenges, whether it lacks certain abilities, is more demanding of its operator, or has limits of capacity, all vehicles are imperfect. The following entries address overcoming these obstacles.
- Unavailable Torque can be a problem for AWD systems. The simplest way to overcome this obstacle is to modify the vehicle's differentials with either limited slip or locking models.
Proprietary AWD Implementations
All-wheel drive implementations come in many different proprietary version, each with their own unique peformance features.
Unlike the performance-oriented AWD systems of companies like Subaru, the Acura AWD system is designed for on-road traction & stability, with the occasional need for 4WD when stuck in deeper snow or mud. The system on the 1st Gen MDX models, for example, is only available in Automatic 5-speed transmissions (the only offering for these vehicles), and acts as a Front-Wheel drive-only drive train until wheel slippage (at the front) is detected, usually in about 1/60th of a second. Then the computer controlled center differential will begin to apply more and more lock-up until either the wheel slippage at the front ends, or it reaches a 50/50 balance. This all happens so quickly and seemlessly, it is almost impossible to detect.
The 1st Gen MDX AWD system also has a feature that allows you to lock the front and rear driveline for help extricating stuck vehicles, like in deep snow or mud. The vehicle will be limited to 17mph when this is engaged to minimize any possible damage to the drive line while it is locked.
While many manufacturers offer various AWD systems today, Subaru has been refining their AWD system for over 20 years, and has what many consider to be among the best out there in commercially-available passenger vehicles, especially for high-performance use. Since Subaru has been competitive in the FIA World Rally Championship (WRC) since the early 1990's, (and due in part to the Group N homologation requirements) much of their race technology is available to the average consumer. High performance limited slip differentials for the front and rear are available through the factory performance company, Subaru Technica International (STi) as well as various after market manufacturers. While the STi ones are typically a gear-type (Torsen style) LSD, clutch types are also available and would be better suited to off-road driving, since gear-type will send all the power to a wheel that has no traction at all (such as if it is off the ground). Clutch-type center LSD units are available, as well as the DCCD unit mentioned above, which is essentially a computer-controlled electronically locking center differential. The driver is able to set what percentage the center DCCD will lock up at, from 0% (which makes the vehicle behave like a RWD) to 100% (which essentially completely locks the front and rear drive line and makes the vehicle behave like a true 4WD, albeit without a low range gear).
Since Subaru drive lines are pretty much universally interchangeable between models, many people will mix-and-match these parts to put together the optimal drive line for their use, which will vary from pavement racing to dirt-road racing, to actual off-road driving. Many people have modified 1970's and 1980's model BRAT and GL Wagon models into serious off-road vehicles, due to the availability of a transmission with a low-range gear. These low-range transmissions were also offered in non-turbo Subaru Foresters from the late 1990's and on, but unfortunately they were not available in the US market.
The AWD system offered in the Subaru Automatic Transmission models differs somewhat. It is very much like the Acura system described above, but works kind of like the DCCD as well. Normally, it is 90% front / 10% rear biased for torque split, until slippage occurs. When detected (usually in 1/60th of a second)the computer will start to cycle the electronic LSD until either the slippage stops or a 50/50 balance is reached. Once stability is restored, it will go back to its 90/10 settings (which is actually *off*). All of this happens so quickly and seamlessly, it is almost impossible to detect.