by Jim Kerr
A few all wheel drive vehicles have been around for years. Changes in the marketplace are putting more on the road everyday. Truck-based 4-wheel drive vehicles are evolving into all wheel drive sport utilities and luxury trucks. Now, “special purpose” crossover vehicles and passenger cars are using all wheel drive.
Some people confuse All wheel drive with 4-wheel drive vehicles, which have been built for much longer than All wheel drives. So what is the difference? All wheel drive vehicles are designed to operate with all wheels propelling the vehicle under all driving conditions, while four wheel drive vehicles should only be operated in 4-wheel drive when driven on poor traction surfaces. Operate with 4-wheel drive engaged on hard, smooth road surfaces and you will soon be spending a lot of money on repairs. Everything from tires, to drive axles, to transfer cases can be damaged by the binding of the drivetrain.
All wheel drive has another advantage over 4-wheel drive. Try cornering in 4-wheel drive. Because the front and rear axles are locked together but turning through different arcs, the tires on one of the axles has to slip. If you are on hard surfaces or ice, one axle looses traction and the vehicle can get out of control easily. All wheel drive systems allow the tires to turn at different speeds from the front to the rear axle, so traction is maintained and vehicle handling is stable.
Many of the newest all wheel drive systems are based on front wheel drivetrains, with auxiliary rear drive axles. Look at Jaguar’s new X-Type sedan. Here, the all wheel drive system was selected to balance the handling of the car. The added traction and stability were side benefits. This car uses a transverse mounted engine and front transaxle with an auxiliary rear drive gear unit bolted onto the transaxle, which sends power down a driveshaft to the rear axle. Inside the auxiliary drive, there is a planetary gear set and a viscous coupling.
The ratio of the gears in the planetary gear set is what determines how much torque is delivered to each axle. On Jaguar’s X-Type, the planetary gear set ratios split the drivetrain torque, with 60% to the rear wheels and 40% to the front wheels. This is under normal driving conditions, but get on slippery surfaces and the viscous coupling comes into operation.
The viscous coupling is connected between the gears of the planetary gear set. As long as traction is good, the gears are all turning very slowly in relationship to each other. When tires start to spin, the silicone fluid is forced to slip very quickly. Then the Silicone fluid becomes thick, forcing the gears to turn at similar speeds, and the engine’s torque is transferred to the axle with the best traction. The system is compact, light, and operates seamlessly. You wouldn’t know it is an all wheel drive vehicle.
Buick’s Rendezvous and Acura’s MDX provide all wheel drive with a different design from the Jaguar X-Type. Both the Rendezvous and the MDX operate as front wheel drive vehicles with an auxiliary drive unit on the transaxle that turns the driveshaft going to the rear axle. The rear axle on both these vehicles is along for the ride until wheel spin occurs. Then the rear axle drives in addition to the front wheels.
While the concept is the same on the Rendezvous and the MDX, there are major design differences in the operation of the rear axle. The Rendezvous system is called “VersaTrak” and it can also be found on the Pontiac Aztec and will be optional on smaller 2002 GM vans.
VersaTrak is a mechanical system that uses two oil pumps and hydraulic pressure to apply clutches on each side of the axle. Each side of the rear axle is similar, so we only need to look at one side. One part of an oil pump is turned by the differential. The other part of the pump is turned by one axle. If the axle is turning the same speed as the differential (such as on good traction surfaces), then no oil pressure is developed and the clutches are not applied. The axle is not being driven. When tire slip occurs, the two parts of the pump turn at different speeds and the oil pressure developed applies the clutches. This engages the axle to the differential so it will drive. The other side of the rear axle has the same components. This unit is compact, light, and simple.
Acura’s MDX also uses clutches on each side of the differential to drive the individual axles, but these clutches are applied electrically. An electro-magnet is used to engage a tapered ball-cam device inside the axle housing. When the ball-cam is turned, pressure is applied to the appropriate clutches, causing them to drive the axles. Acura uses electronics to control the operation of the rear axle for the MDX. The computer monitors wheel speed sensors and can apply the electro-magnets and clutches when it anticipates that all wheel drive is desirable. Just like Jaguar’s all wheel drive system, GM’s VersaTrak and the MDX all wheel drive units operate seamlessly without any input needed from the driver.
All wheel drive provides many benefits in handling and safety. New technologies and materials have made the units smaller, lighter, and more responsive. Subaru may have the most experience with all wheel drive, but manufacturers such as Acura, Porsche, Jaguar, GM, BMW, and Mercedes are making it a common sight on the road today.