By Jim Kerr
All-wheel drive is becoming a very popular choice for Canadian drivers. While we may be used to seeing all-wheel drive on sport utility vehicles, there are now more manufacturers than ever offering all-wheel drive as standard equipment or as an option on passenger cars too. Acura, BMW, Chrysler, Mercedes, Porsche and Volkswagen are but a few of the companies joining traditional AWD stalwarts, Audi and Subaru.
So, why are these systems being offered, and are they all the same?
Traction to get unstuck is the first thought of many drivers when they think of all-wheel drive. Yes, there is the extra traction to help get you out of that snow bank or mud puddle, but auto manufacturers have introduced all-wheel drive to provide safety and more balanced handling too. The traction of all-wheel drive works when cornering, so the vehicle turns into corners better. The additional grip helps when unexpected slippery spots occur on the road, such as ice, water patches or even loose gravel.
As well, handling and balance can be tailored by the manufacturer with an all-wheel drive system. Most full-time systems provide more torque to the back wheels, giving the vehicle a more sporty rear-wheel drive feel. On some systems, more torque can be automatically transferred to the front wheels during cornering to help prevent understeer - where the nose of the vehicle tends to "push" out on the corner.
There is a difference between full-time and part-time all-wheel drive systems. Part-time all-wheel drive vehicles operate with front-wheel drive most of the time. Computer-controlled clutches, or speed-controlled clutches on some systems, will engage the rear drive wheels when necessary to provide all-wheel drive. Computer controlled systems "anticipate" vehicle acceleration by monitoring gas pedal position, and can engage the rear axle before the vehicle moves. This reduces wheel spin on takeoff acceleration. Speed-controlled systems typically have to see a difference in front wheel to rear wheel speed before the clutches are engaged to drive the back wheels. In may only take one turn of the front wheels before the rear wheels engage, but that can be enough to allow wheel spin during acceleration, and to become stuck on deep loose surfaces such as sand beaches.
Don't confuse part-time all wheel drive with four-wheel drive systems. Four-wheel drive automatic systems are similar in operation to part-time all-wheel drive systems, but unless your tires are exactly the same size and tread depth, four-wheel drive systems can be damaged at highway speeds even when in automatic mode. All-wheel drive is designed for all speeds.
Full-time all-wheel drive systems use a centre differential or clutches and planetary gearset in the transfer case to distribute torque to the front and rear axles. If an open centre differential were used, the torque would be transferred to the wheels that are spinning, so many manufacturers use a viscous coupling in the transfer case centre differential. This viscous coupling allows the front and rear wheels to turn at different rates, such as when turning a corner, but if the speed difference becomes too high, such as spinning a wheel, then the fluid begins to heat and becomes thick. The thick fluid causes the transfer case parts to transfers torque to the rest of the wheels.
Other manufacturers use multi-disc clutches (similar to automatic transmission clutches) in the transfer case differential to vary the torque between the front and rear axles. By engaging the clutches partially, the amount of torque transfer can be varied. Vehicles such as Subaru's WRX STi even allow the driver to select the amount of clutch application to vary the torque between the front and rear axles. This allows the driver to change the handling of the vehicle for different road conditions. Ever wonder why Subaru is such a strong competitor in World Rally Championships? Now you know part of the reason.
Full-time all-wheel drive vehicles can provide excellent traction and handling, but it takes power to turn all the extra parts and it takes fuel to make power. The same vehicle equipped with two-wheel drive will get better fuel economy. Part-time all-wheel drive is a compromise between traction and fuel economy. I prefer the sporty handling and traction of full-time all-wheel drives, but your driving needs may be met with a part-time system. When buying a new all-wheel drive vehicle, be sure to ask how the system works. Many compact SUV's and CUV's use part-time systems, and many passenger cars now use full-time systems. Both work well but suit different needs.
Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).
