by Jim Kerr
Much has been written about all-wheel-drive and four-wheel-drive, but there are many variations on these systems and I still get many questions from owners and new car buyers on how they work. Here is a brief primer on typical systems found on current vehicles that may help you to understand why a vehicle handles the way it does.
Four-wheel-drive (4WD or 4×4) systems have been with us on trucks for decades and is the simplest system on the market. Some use a lever to shift between two wheel drive (2wd) and four wheel drive, while other manufacturers may use a switch and electric motor or vacuum actuator to shift modes. These systems drive the rear wheels only in 2wd. Engage 4×4 mode and now all four wheels drive. Both the front and rear axle are locked together by the gears in the transfer case so 4×4 mode should be used on surfaces where tires can slip. Do not use 4×4 mode on hard packed or pavement surfaces because slight differences in tire sizes cause the axles to turn at different speeds and this causes what is referred to as binding or wind-up in the transfer case. Continue to drive with the driveline binding and soon the axles or transfer case can be destroyed.
Everybody thinks 4×4 mode is great for ice and snow. Not really true. While 4×4 mode may get you moving, the binding in the driveline makes turning corners tricky. During a turn, the tires follow different paths. This aggravates driveline binding and the vehicle can slide out of control suddenly. Cornering in two wheel mode is much safer.
Safer cornering and braking are advantages of the current generation of automatic transfer cases offered in many trucks and SUV’s. These systems can be identified by the “AUTO” position on the shift control. These systems typically have 2WD (rear-wheel-drive) and 4×4 mode as well, but the Auto mode should be selected when driving on mixed traction surfaces. They will operate in 2wd until tire spin occurs and then automatically switch to 4wd mode. As soon as tire slip ends, the system reverts back to 2WD.
Because these systems only operate in 4×4 mode when tire slip occurs (with auto selected), cornering does not usually cause binding in the driveline and the ABS system can optimise brake operation during slippery stops. Differences in tire size can still cause transfer case problems when in AUTO mode because the computer monitoring system operation detects different tire sizes as tire slip. If the tire size difference is large enough or vehicle speeds are high, the computer can engage the transfer case. Binding in the transfer case will burn the clutches inside it, so 2wd mode should be selected for highway travel.
All wheel drive (AWD) is found on some trucks and SUV’s and many car models. Jeep often calls this “Full Time”. These systems use a transfer case that allows front and rear axles to turn at different speeds yet both can still provide traction. There are many variations on AWD. Some vehicles such as Porsche, Jaguar, Volvo and Subaru split the torque front to rear with the majority provided to the rear wheels. These systems offer great traction and handling on all surfaces. Subaru even uses one in their WRX STi model that allows the driver to vary the torque split for different road surfaces.
Some AWD systems use a silicone fluid filled clutch inside the transfer case that will allow some differences in axle speeds but start to lock up when axle speed differences are too great. Other AWD systems are mainly front-wheel-drive with auxiliary rear-drive axles. Even here there are some differences. Honda’s CRV is typical of one type where the front wheels have to slip before the rear wheels are engaged. The reaction time is not long but it can make a difference when starting out on slippery surfaces. Other vehicles, such as Nissan’s Pathfinder couple the rear axle all the time at start up and then adjust the torque split to the front if no tire slip occurs. The differences are subtle but noticeable on slippery starts.
There are many variations in 4WD and AWD systems. Some can be operated on hard surfaces; some not. Some systems always drive all four wheels and others operate mostly as front-wheel-drive vehicles. I find that even many automotive salespeople don’t clearly understand the differences. The service department could be a better source of information!