by Jim Kerr

Off-road adventures, slippery boat ramps, rain-soaked roads and even winter’s ice and snow can be a test of your driving skills, but technology is available for those that feel they need more control. It is called all wheel drive.

Propelling a vehicle though all the wheels is not new. Even when motor cars were first being invented, there were models that put the power to the ground through all the wheels. This type of system is called 4 wheel drive, which is a good system, but a little different than all wheel drive (AWD).

In both 4 wheel drive and all wheel drive systems, the power flows from the engine into the transmission, and then into a transfer case. The transfer case may be part of the transmission, or may be bolted onto the end of the transmission. Its job is to transfer the power to both ends of the vehicle through driveshafts. The driveshafts then turn the differential units in the axles. The differentials split the power between the 2 wheels at one end of the vehicle, causing the wheels to drive.

Where the drive systems are different is mainly in the transfer case. Four wheel drive vehicles lock the front and rear axles together so they must operate at the same speed. All wheel drive vehicles use a coupling unit inside the transfer unit so the front and rear wheels can rotate at different speeds. This may sound unusual, but there are times when it is desirable to have this happening. One time is when you are turning corners. Another is when you are driving on slippery roads.

Many 4 wheel drive vehicles have controls so the vehicle can be switched from 2 wheel drive to 4 wheel drive. In 2 wheel drive, the drive wheels at one end of the vehicle are mechanically disconnected. This can improve the fuel mileage slightly and decrease wear on the drive-train of the vehicle. When the driver switches to 4 wheel drive, the transfer unit sends power equally to both ends of the vehicle at the same speed. If the tires at one end start to spin, then the tires at the other end may also start to spin. With all the tires spinning, there is no connection between the vehicle and the road, and control of the vehicle is extremely difficult.

All wheel drive vehicles have no controls that allow the driver to switch into 2 wheel drive. This is not required because the design of the transfer unit permits wheels at one end of the vehicle to spin slightly without the other wheels losing traction. The improved contact with the road gives the driver better control of the vehicle, and better traction for acceleration and braking.

Power inside the AWD transfer case is typically divided to both ends of the vehicle through a viscous coupling. This is a mechanism filled with a silicone based fluid that thickens when it is heated. The heat is created when two surfaces in the viscous coupling move at different speeds. When the road surfaces are good, everything inside the transfer unit rotates at the same speed, and the viscous coupling doesn’t have to do any work. When the tires start to spin on a slippery road, the viscous coupling fluid thickens very quickly and the power is transferred to the wheels with the most traction. This takes place automatically, and much faster than you could shift a 4 wheel drive vehicle from 2WD to 4WD.

Last winter a major snow storm gave me an excellent opportunity to test drive a Subaru Legacy equipped with all wheel drive under very poor road conditions. Subaru is the world’s largest manufacturer of all wheel drive vehicles and this system is available on every model they manufacture. My experience with it is that it works well! Pulling away from a stop sign at a slippery intersection was so easy it was astonishing. The car accelerated like I was on dry pavement. There was a secure feeling that I had complete control of the vehicle. I know that on my own front wheel drive car it would not have been possible to have this feeling of control.

Subaru markets all wheel drive as an “Active” safety system. This means it is a part of the vehicle that helps the diver prevent accidents, unlike airbags, that only protect passengers after an accident. Good vehicle control under all road conditions can help prevent many accidents.

Many other manufacturers make one or two models equipped with all wheel drive. Ford has their Aerostar van, GM has their Astro van, and imports like Audi, BMW, Mercedes-Benz, and Volvo make all wheel drive available. If you are the type of person who has to drive under less than ideal conditions, then try out an all wheel drive equipped vehicle. They ride and handle as good as 2 wheel drive vehicles on good road surfaces, and I am sure that if you have an opportunity to drive on slippery road surfaces, you will be as impressed as I am with their driving performance.

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