4WD vs AWD and the Fallacy of Snow Traction

As winter settles in and the snow falls, I've noticed a number of amusing things regarding drivers and their misconceptions about the capabilities of an SUV. While driving home in my Jeep Wrangler on I-95 the other night, I watched as drivers [New York, New Jersey and Connecticut] slipped and slid through the ice in their expensive cars. I listened to my police scanner to the numerous accidents around me, so many accidents in fact that one particular transmission caught my ear.

Anyway, I digress. My point is that many people assume that because their vehicle has AWD (all wheel drive) and some form of traction control, that the car will always have traction thereby meaning they can continue to drive normally. Wrong Wrong WRONG! To begin with, it is necessary to understand what options exist.

1. Part Time 4WD
2. Full Time 4WD
3. AWD
4. 2WD Rear
5. 2WD Front

Starting with the worst, two wheel drive obviously limits the number of tires actually contributing to traction. It's generally understood that front wheel drive handles better in slippery conditions than a rear wheel drive vehicle. AWD differs from 2WD only in that two additional tires are turning. The misconception is that all AWD means that four tires always turn. That is not how your car works.

The drive shaft from the engine connects to a differential which distributes power to the axles and in turn rotates the wheels. But what is a differential? Put simply, it is a collection of gears that allow the two axles to rotate at different speeds. This is necessary in order for a car to turn (as the outside tire must traverse a greater distance than the inside tire) without having the wheels skip. Obviously, wheel skip introduces a loss of traction and thereby control. While the differential serves this crucial purpose, it is also the differential that creates the "one wheel turns, the other doesn't" problem. The differential gearing transfers the power into the path of least resistance, which is often the wheel that has no frictional contact with the ground.

Back to all wheel drive. So long as all four wheels have equal footing, it is true, all four tires will turn with an equal amount of power and traction. However, just as two wheels on an axle are connected by a differential, the front and rear drive shafts are connected by a differential as well. This means, if one wheel of an AWD vehicle loses traction, the problem is amplified because ALL of the power goes to that one wheel while the other three sit stationary. This is why, without a form of traction control, an AWD vehicle is useless in any condition other than dry pavement.

A full time 4WD vehicle is essentially just an AWD vehicle that has the ability to "lock" the center differential on command. What this means is that when the lock is engaged, the front and rear drive shafts can be thought of as a single, solid shaft. Hypothetically, if one front wheel were to lose traction, it would spin while the opposite front wheel remained stuck. However, because the rear drive shaft is locked, both rear wheels will turn which will free the vehicle and - ideally - move the car forward such that the spinning wheel regains traction. The center drive shaft should only be locked in conditions that warrant such behavior because it will have negative traction effects on dry pavement when the vehicle attempts to turn (for the reasons noted above about differentials).

A part time 4WD vehicle is essentially a 2WD vehicle for all normal conditions. When 4WD is engaged, the front and rear drive shafts are locked immediately. Part time 4WD vehicles do not have the option of having the transfer case operate as an open differential. The drive shafts are either engaged and locked together or only one shaft is engaged. Typically, part time 4WD vehicles also have additional low-gearing options giving them tremendous amounts of torque to free themselves or tackle difficult obstacles.

Up to this point, it is obvious how part time 4WD, full time 4WD and AWD each have their strengths and weaknesses given particular road conditions. Additional technologies contribute to increase the likelihood of having traction in the event of wheel spin. The most fundamental device is called the locker. The locker principle was alluded to in the discussion on full time 4WD for locking the center drive shaft. Lockers also have their place in the differential, preventing the axles from spinning separately by locking them together "like a spool." A variety of lockers exist that can engage automatically or manually on demand in order to guarantee that two wheels will always turn. This sort of option typically only exists for off-road 4x4s and trucks.

The next tool for preventing wheel spin is called the limited slip differential. These differ from the standard open differential in that a coupling device is incorporated to allow axles to spin at different speeds, as necessary for a differential to function, but not at vastly different speeds, which is indicative of traction loss. A limited slip differential can be coupled by viscous fluids or a series of clutches. Ultimately, however, the limited slip differential keeps the wheels with traction turning allowing a vehicle to continue moving. As an example, take a standard AWD vehicle. If the right rear wheel loses traction, it receives all the power while the remaining three wheels do not turn. In a Jeep Wrangler Unlimited, if the right rear wheel loses traction, the limited slip differential will transfer power to the left rear wheel and the locked transfer case will transfer power to both front wheels allowing the 4x4 to continue moving.

Lastly, AWD vehicles regain some of their usefulness in slippery situations with the addition of traction control. The most common implementation is called electronic traction control which, counter-intuitively, uses the brakes. This system will be employed on vehicles with anti-lock brakes because a computer already has sensors monitoring the wheel rotation. In the event that a wheel begins to spin beyond a normal threshold, the electronic traction control system will actually begin to activate the brakes on that specific wheel - independently pumping up to 100% braking power. This has the effect of stopping the errant wheel's spin completely - which, utilizing the path of least resistance, will transfer power through the open differentials to the remaining wheels in contact with the pavement. The system works fine for mildly slippery conditions, however on ice it can be dangerous because the car itself is repeatedly applying full brake force on the sliding wheels which prevents them for regaining rolling traction during a skid. A mechanical alternative to electronic traction control exists in the form of the Torque Biasing Differential [pdf]. Through a series of worm gears, torque biasing creates a mechanical advantage to the high-traction side of the differential which helps to prevent wheel spin. These differentials are often found on the front axle of performance tuned road cars. Torque bias differentials are not suited to 4x4 applications because they do not handle having one wheel lifted from the ground completely. (Notably, the HMMWV uses a torque bias differential and compensates for the raised wheel by applying both throttle and brakes simultaneously like a manual EDL.)

In summary, contrary to what owners believe, an SUV with AWD and traction control really is nothing more than a mini-van with big tires when it comes to slippery surfaces like snow, ice and off-road conditions. As such, it is necessary to continue driving them in winter conditions with the same due diligence as any driver of a vanilla 2WD vehicle. While true 4x4s and off-road vehicles have a definite advantage in the snow and ice, they too must take due care as an out of control SUV will cause them just as much harm whether they're firmly planted or not.