By Jim Kerr
With the number of variations in powertrain systems provided by manufacturers, it is easy to be confused with terms such as all-wheel drive, four-wheel drive, 4×4 systems and part-time and full-time systems. Let’s look at the variations on the market to help understand how these vehicles react in various situations.
All-wheel drive or full-time systems drive all the wheels but allow for a difference in speed between the back and front axles. This is an important attribute. If the tires couldn’t turn at different rates, they would create driveline bind on corners, where the wheels travel different distances. The driveline would also bind if the tires are different sizes (even new tire diameters vary slightly) when driving on hard road surfaces. Binding will wear out drive axle gears and transfer cases in just a few thousand kilometres. All-wheel drive designs eliminate binding so they can be engaged all the time, regardless of road conditions.
Some all-wheel drive systems engage both drive axles all the time. Subaru, Audi, Porsche, Mercedes and some Jeep vehicles are examples of this. All these systems are built to offer maximum traction without the driver having to do anything.
Other “all-wheel drives” are front-wheel drive based vehicles that use a rear axle assist. Many compact SUV’s and mid size crossovers use these systems. The rear axles only drive when the front wheels start to spin. Many use computer controls to anticipate front wheel spin during acceleration and can engage the rear axles before slip occurs. These systems provide better fuel economy, as the rear axle is only driven when needed. Some of these types of systems will have a control to “lock” the transfer case so the rear axle can be driven all the time, but it usually only can be engaged at slower speeds.
Four-wheel drive systems can lock the front and rear axles together. This provides better traction in loose surfaces such as deep snow, sand, mud or deep gravel. However, it can give less traction on ice. Because four-wheel drive creates some driveline bind, tires on one axle have to slip to reduce this binding. The slipping tire has no traction, especially on ice, so steering control and stability are hindered. Almost all four-wheel drive systems can be shifted so they only operate in two-wheel drive and this is preferred when the roads are icy or you are driving on hard surfaces such as pavement. This feature of being able to shift between four-wheel drive and two-wheel drive is sometimes referred to as a “part-time” system because it is operated in four-wheel drive only part of the time.
Some four-wheel drive systems have a low range for slow speed off-road traction. The same rules apply here as they do to operating in high range. Low range can be used for loose surfaces but don’t use it unless you need it. Get stuck in 4 Low and you will be walking for a tow. Serious off-roaders carry a winch and anchors to help when they really get stuck!
Many pickup trucks have automatic mode in their four-wheel drive systems. Automatic mode uses a computer to engage 4WD when there is a speed difference between the front and rear drive axles. When the drive axles are turning at the same rate, the system automatically changes to rear-wheel drive only. This system automatically switches between 2WD and 4WD and has many advantages of both, but tires do have to be even in tire tread depth. If tread depth varies by more than 2/32 inch, the system will be continually try to engage 4WD while driving on hard surfaces and this can cause wear.
On any all-wheel or four-wheel drive system, the tires have to have traction before the vehicle will move. Systems with a differential in the transfer case can have one wheel spinning on ice and no torque being transferred to any other tire. The vehicle is stuck! Traction control systems can apply the brake to the spinning wheel so torque is applied to the other wheels to get you out. If you are stuck with a rear wheel on ice, an old trick is to apply the parking brake slightly to slow the spinning tire. This will cause the other rear tire to turn.