Four-wheel and all-wheel-drive (AWD) can be somewhat confusing terms. It’s as if AWD vehicles have some number of wheels other than four, but that’s obviously not the case. Particularly with new technology constantly emerging, the difference can turn into marketing semantics from different manufacturers to an extent. Nonetheless, the industry as a whole recognizes some general characteristics of each type.
“Think of full-sized trucks and SUVs and you pretty much have the concept of four-wheel-drive,” said Mike Calkins, manager of approved auto repair at AAA. “They typically have a separate transfer case, a real low-range gear ratio for real low-speed, off-road work. Some of the newer systems do have an automatic switch to put it into four-wheel drive. It’ll activate the four-wheel drive when you need it.”
Although all-wheel-drive systems can handle light off-road work, they really shine on-road in challenging weather conditions.
“All-wheel systems are typically designed to power just two of the wheels most of the time and then they automatically sense when the driven wheels start to slip. When they sense that, they automatically transfer some of the power to the other axle,” Calkins said. “Some all-wheel-drive systems actually power all four wheels all the time, to different degrees, as it varies with the system. The key factor is that they automatically apportion torque as it is necessary.”
The mechanics of a vehicle’s differential also come into play. Simply put, an open differential allows the wheels on the same axle to turn at different rates. In a normal turn, the outside front wheel needs to turn more, because it has farther to travel than the front wheel on the inside of the turn’s arc. This technology works great until one wheel is parked on a patch of ice and the other is not. The design of an open differential unfortunately sends all the power to the spinning wheel on the ice and leaves the wheel with traction motionless. The option of limited-slip differential for four-wheel-drive vehicles eliminates this problem by sending some power to both wheels.
For AWD cars, this distinction is a moot point. “All-wheel-drive systems kind of eliminate the need for a limited-slip differential because the electronics that control the all-wheel-drive detect that slip and control it either through regulating the wheel brakes or through clutches that control the flow of power through the system,” Calkins said.
THE BOTTOM LINE
Although the mechanical systems are quite different, a driver can still follow the same winter-driving advice for four-wheel or AWD vehicles.
“There are a lot of urban myths out there about different driving techniques for different drive configurations, but the reality is that if you’re using good technique, all drive configurations handle about the same,” said Mark Cox, director of the Bridgestone Winter Driving School in Steamboat Springs, Colo. “If you’re using bad technique, it can be very different, because the driver’s mistakes become evident in different ways.”
Traditional four-wheel-drive is favored by those who plan for serious off-roading, but simply for on-road treks up to ski resorts, many drivers prefer AWD. “I’ve owned both types of vehicles over the years and my experience is that an all-wheel-drive is superior in 90 percent or more of (weather) situations,” Calkins said. “There are a few extremely heavy-going situations where a traditional four-wheel-drive system locking the transfer case and locking the axles will get you through where an all-wheel-drive system won’t. But, to my way of thinking, if the roads are that bad, it’s just better to stay home.”