Off Road

FOUR WHEEL DRIVE SYSTEMS

Tom Hagin

We had fun watching the many 4WD vehicles negotiate Highway 88 in California's snow country last Sunday. Chain controls were in effect, but most rigs on the road that day were exempt. Those hardy enough to install chains themselves were cold, bitter and watched by hundreds of warm, cozy passengers inside four-wheelers who would reach the slopes first, with plenty of time to get "first-tracks" in the fresh powder.

With the recent proliferation of sport utility vehicles (SUVs), and the popularity of 4WD pickups and cars among those who wouldn't otherwise buy one, it's no wonder I receive blank stares when I ask my ski shop customers what kind of 4WD system their car or truck uses.

There are several. And they all operate in different ways to achieve the same goal: to increase traction.

So today we'll go over those that I know of, and maybe we'll hit on the system that gives your Subaru - or Chevy, the grip it needs when the roads are covered with a half-foot of fresh powder.

There are a several types of vehicles using 4WD. One is light-duty that's more suited to traversing muddy puddles in the parking lot of the local Little League baseball field. This system is complex, using an array of electrical solenoids, clutch plates and fluid pumps to undetectably transfer power to the tires that need extra grip. The other is a rugged system, using a transfer case (a device that looks like a short, squat transmission filled with gears, chains and shafts), connected to a lever or dash-mounted button to "lock-in" the system for maximum traction. One is active (the driver controls it), the other passive (the driver simply pushes the gas pedal).

PART-TIME 4WD - Part-time 4WD is found in many of today's vehicles, but it's use is waning. Part-time 4WD is the least expensive system and is meant to be used off-road (dirt) or on slippery roads (snow or ice). The typical part-time system features either manual locking front axle hubs, (someone must get out of the vehicle and twist the hubs into a locked position) or automatic locking hubs, where the driver shifts into 4WD and the hubs lock via a sophisticated sliding mechanism. When in use, part-time 4WD essentially locks the front and rear axles together, but the transfer case lacks any mechanism to control the speed differences between the front and rear axles. When it's engaged, part-time 4WD will produce tires scrub and wheel "hop" if the wheels are turned on dry pavement. This puts unnecessary bind on expensive driveline components, as well as premature tire wear. While some vehicles equipped with part-time systems must be slowed to a crawl for engagement, many can be shifted "on-the-fly" at speeds of up to 60 mph. Part-time 4WD is best suited for serious off-road use.

FULL-TIME 4WD - A full-time system, sometimes called all-wheel- drive, sends engine power to both the front and rear axles on all surfaces at all times. It uses a center differential, which acts as a limited-slip unit, to compensate for the speed differences between the front and rear axles. This eliminates tires scrub and hop, but may compromise some fuel mileage. Most use a viscous coupling - a device with sets of clutch plates running in a bath of silicon fluid. This system is complicated, expensive and used heavily in the more genteel SUVs, where driver input is minimal.

SKI TIP OF THE DAY - Many of my customers complain of cold feet. And that's logical, since they're skiing in freezing weather, and boots are only effective if they're a snug fit - which reduces blood flow.

Here's what I do. Take the boots from the back of the car/truck/ van/SUV - then turn the vehicle's heater to full blast/floor setting. Open the boots and direct hot air as deeply as possible into the boots, warming the toe area. You can do this while dressing in the parking lot, but be careful not to breathe exhaust fumes. If you start with warm boots, you'll ski with warm boots. Or you can buy boot heaters, at around $120 - but the heat they produce is worth every penny.