Advice:
4WD ignorance can cause expensive damage
by Phil Bailey

The time has come, to start to think about driving in mixed traction conditions. Sometimes snow, sometimes ice, and sometimes a mixture of the two, with dry pavement in between. You can do a lot of damage, expensive damage, to your four wheel drive system if you are not thoroughly familiar with the manner in which each of the different systems works. And more importantly, the way in which they DON'T work.

In other words, these systems can still get you into a lot of trouble unless you understand their limitations. Taking the engine's power and splitting it between four wheels rather than two is basically a good idea. Whether on an icy road, a muddy field or in deep snow, the spinning wheels of a stuck two wheel drive car make a convincing case. Many of the off-roaders that stand out like transport trucks in the supermarket parking lot are rarely used off-road, but the validity of the case for getting drive to all four wheels is much the same on or off the pavement.

How this four wheel power is transmitted varies between manufacturers - but not as much as they'd have you believe from the confusing array of trade names such as Selectrac, Active-trac, and Control-trac. Of course some so-called "4x4s" don't operate in four-wheel drive mode at all until you select it. And when you do, you must be sure to de-select it when it is not required, especially on hard surfaces. There are really only three types of four-wheel drive system.

There's driver selected control, whereby the vehicle is normally in two-wheel drive but the driver can select 4x4 with a lever or a button when required. This system is cheap and simple, but the driver must remember not to use it on dry roads. Part time four wheel drive indicates that the vehicle is usually in two-wheel drive mode but has various devices (typically a viscous coupling) that gradually stiffen the coupling between front and rear drive shafts when there is any significant difference in axle speeds. For example, if the front wheels are spinning, drive is gradually introduced to the rear axle. It sounds elegant, but despite quick reaction times one axle has to spin before the other comes to its rescue, and there are times when you need instant grip.

With full time four wheel drive, the front and rear axles are driven all the time. If you're going to have 4x4 at all, this is the best choice, especially on-road, because it is always ready for the unexpected slippery patch. Off-road, even with a selectable system, you can usually see problems in advance and shift into 4x4 when you need to.

So if full time 4x4 is best and you have a vehicle with selectable 4x4, why not select all-wheel drive and leave it that way? Well, imagine you're first in the car park after it snows and turn through 180 degrees into your parking spot. You'll notice that your front wheels have traced a different (longer) path in the snow than the back wheels, which tend to cut the corner. This shows that the front and rear axles travel different distances in the same time. In other words front and rear drive shafts need to be able to turn at different speeds, all the time, on every corner. With permanent 4x4, because both drive shafts are driven all the time, a differential is fitted between front and rear axles to permit the dissimilar speeds. A vehicle with selectable 4x4 has front and rear drive shafts driven only when 4x4 is selected. When the lever is moved, it simply locks the front and rear drive shafts together - no centre differential, or means of accommodating the slight front-rear speed differences. So on a hard, grippy surface, there is some "fight" between front and rear axles trying to go different distances. This scuffs the tires, stresses the driveline and, because it introduces slip, can actually reduce the vehicle's grip on slippery corners.

Differentials are wondrous inventions, permitting the drive from the drive shaft to be split 50/50 between left and right drive shafts, or 80/20 according to load - or even 100/0 if one wheel is on ice and the other is on dry pavement. Reducing that extreme left/right or front/rear split of power (to quell wheel spin) or eliminating it altogether is done by a limited slip differential or a lockable differential or by putting a viscous coupling across it, e.g. between front and rear drive shafts or across an axle. Sometimes the wheel spin of the unloaded wheel (sensed by the ABS wheel speed sensors) is prevented by the automatic application of brake - so-called traction control. Ignore the clich