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Four-wheel drive isn't just for trucks and SUVs anymore. Anyone driving on wet or icy roads can appreciate this safety feature, thoughmany driversmay not understand how the technology operates.
Contemporary four-wheel-drive and all-wheel-drive systems have made enormous strides beyond the first manually locked hub-engagement systems that sent equal power to all four wheels of work trucks. The drive technology available in today’s trucks, SUVs and cars ranges from simple mechanical arrangements to complex electronic management systems that constantly alter the mechanical distribution of power to send it to the wheels with the best traction. These new systems are tailored for all types of vehicles and driving conditions. Here’s a look at how they work.
Different technologies
A basic four-wheel-drive system may start with a transfer case, which engages only two wheels during normal operation and can be shifted to spread power to each of the four wheels. A truck in four-wheel-drive mode has all four wheels churning through snow or mud, but the drawback is an awkward, binding feeling whenever you negotiate a tight turn because the outside wheels have more distance to travel than the inside wheels.
The car industry, led by such companies as German automaker Audi (with its quattro system) and Subaru in Japan, devised and perfected systems whereby all four wheels get power at all times, with no switching necessary. The power can be varied to the two axles and then to the specific wheels on each axle—similar to a limited-slip system on twowheel- drive axles.When a wheel spins in slippery conditions, the system diverts the torque to the wheels that aren’t slipping. Such systems are available in almost unlimited variations in cars, pickups, SUVs and even vans.
Among the most versatile systems are those with an electronic control switch on the dash or console. With these systems, the driver can choose two-wheel drive (either front or rear) for optimum fuel economy during routine highway or urban driving in normal weather; in foul weather, the driver can switch to allwheel-drive for maximum traction.
One such alternative is an automatic 4x4 setting that runs through either a viscous or an electronic coupling to distribute the power after sensing which wheels have the best traction. A third setting on the most versatile systems offers “4x4 lock,” which is similar to the equal power- to-all-wheels mode. That system is least useful in normal conditions, but in deep snow or mud or during off-road excursions such as hill-climbing and rock-crawling, the ability to simply let all four wheels churn their way is best.
Almost every manufacturer now offers several forms of four-wheel or all-wheel drive. New pickup trucks and SUVs are available with more advanced systems, and some compact crossover SUVs now offer switchable controls. Others simplify the system of automatic all-wheel-drive mode. The newest systems require only a tiny decrease in fuel economy and provide the security of knowing that the vehicle can instantly react if a wet spot on the highway happens to be frozen.
Vehicles' variations
The new Jeep vehicles offer virtually every kind of all-wheel power. Quadra-Trac I is a full-time four-wheel-drive system with a single-speed transfer case, whereas Quadra-Trac II has a two-speed transfer case with control to transfer torque to the wheels with the best traction for differing road conditions.
Jeep also offers Quadra-Drive II, which has a full-time transfer case working with electronic limited-slip differentials and electronically controlled clutch pads to go fromslip to lock at either axle. This system is designed for improved on-road and moderate off-road use.
Audi’s newest quattro all-wheeldrive system has been altered from a basically equal apportionment of power in the front and rear to a 60 percent rear bias for normal driving, and it is capable of switching instantly to send more power to the front if the rear wheels start to slip (or vice versa). Volkswagen (which is part of the same company as Audi) has a similar system. Depending on the type of vehicle, this system can be adapted for high-performance on-road use or serious off-road applications.
German competitors BMW and Mercedes have added all-wheel drive to their basic rear-wheel drive, and both companies now offer systems that can send the majority of power to the front when necessary. The new BMW X-drive ismore sophisticated and flexible than its previous system, whereas the Mercedes system is quite simple — almost like a reversal of antilock brake technology, it rapidly applies and removes power.
Sweden’s Volvo features the Haldex system, which swiftly and precisely apportions power to each of the four wheels; the system is also used on the company’s SUV, station wagons and sedans with about the same seamless results as Audi’s quattro. Ford, which owns Volvo, uses the Haldex system on its latest SUVs and cars, including the new Taurus and Taurus X (formally the 500 and Freestyle). In the United States, the truck-based systems prevail for pickups and large SUVs, whereas medium-size SUVs and crossovers may reach all-wheel-drive levels from either a basic four-wheeldrive or rear-wheel-drive system, some automatically, some switchable as conditions warrant.
Japanese trucks, SUVs and cars also have varied systems. Subaru’s wagons and sedans have continuous all-wheel drive with viscous couplings that split power equally to the two axles via a center differential; electronic management controls vary the power from front to rear and side to side. This system provides excellent traction and stability on the road and for light-duty off-road use in the Outback.
Toyota uses a Torsen torque-sensing limited-slip center differential with the capability to measure steering input and speed before the computer chooses the power split.Vehicles such as the 4Runner have locking capability. In the RAV4 compact SUV, power can switch from four-wheel drive to all-wheel drive. In the automatic setting, torque is decreased to the rear for smoother cornering.
Honda’s RealTime four-wheel drive, which is used in the Ridgeline, runs at 70 percent power to the front and 30 percent to the rear in normal mode. Power is allocated by two internal hydraulic pumps, one operated by the driveshaft and one by the rear wheels. Fluid circulates through an internal multiplate clutch. Any wheel slippage increases the flow rate to the shafts, which force the clutch to progressively engage up to 70 percent
power to the rear.
The company’s newest SH-AWD (Super Handling all-wheel drive) and its luxury Acura RL sedan and MDX and RDX SUVs send almost all of the power to the front wheels but transfer power to the rear immediately whenever slippage occurs or when hard acceleration causes a weight shift toward the rear. During hard cornering, these vehicles have a remarkable ability to shift 100 percent of whatever torque is going to the rear axle to the outside rear wheel, which pushes the vehicle around the curve.
Perhaps the most impressive characteristic of computerized automatic switching systems is their seamless operation. When road conditions change, these vehicles “think” far ahead of the driver, assessing the conditions as well as the driver’s reactions and then distributing power to avoid slippage. The driver may not even realize the vehicle shifted power. Sometimes the best technology is the kind that works so well you don’t even notice.
For more than 35 years, John Gilbert has filtered the facts about new cars and trucks for various media outlets.