Four Wheel Drive vs AWD

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I've tried to use the Internet to do research on the difference between Four Wheel Drive and All Wheel Drive. While All Wheel Drive explains what it does well, Four Wheel Drive requires some qualification. The concepts of Four Wheel Drive and All Wheel Drive are both poorly understood.

Lets start with Four Wheel Drive. If you crawl under a old Jeep, say a CJ5 you'll find a driveshaft running from the engine and transmission, but instead of going to a front or rear differential the driveshaft first goes into a transfer case. A transfer case is made up of gears and/or chains, and is typically a very simple device. For those with some car and bike experience, picture the primary drive on a Harley Davidson with two outputs. It sends roughly half the power of the engine to the front and the other half to the rear wheels or axle.

Two driveshalfts exit the transfer case, each going into a differntial usually near the center of the front and rear axle. These use a ring and pinion gear to turn the driveshafts 90 degrees, where they attach to individual axles to drive the wheels. Most differentials only drive one wheel at a time. The reason for this, and why they are called differentials is that as a car drives the wheels go different distances. When a car goes around a corner the inside wheels drive a shorter distance than the outside wheels. The simplest way to make up for this is to drive one wheel on an axle and have the other axle not be driven (free wheeling).  To make matters worse, the way they are designed, most standard differentials send the engine torque to the wheel with the LEAST traction.

Open Differential

This means that in a normal, relatively inexpensive Four Wheel drive system, one wheel drives on the front axle and one wheel drives on the rear axle. A simple, standard 4WD system is in fact 2 wheel drive. They work better than the typically rear wheel drive system because a wheel turns on both axles. It also means that at least one wheel will turn on the front axle that has the motor and 55%-60% of the cars weight. The front axle provides most of your traction in low traction conditions because of having a larger percentage of the cars weight over its tires, given them more traction.

Better Four Wheel drive (4WD) systems come with Limited Slip differentials. These are differentials that more evenly adjust the torque to go out of its two axles. They are more complicated, and more expensive. Limited Slip come in many types, including fixed (split) value, torque sensitive, speed sensitive, and electonically controlled. They use different methods to run and adjust the torque split based on traction, speed etc. 4WD with limited slip is a very effective step up from standard 4WD. Since there is still some slip, limited slip diffentials can go around corners while bringing 4 driven wheels into play. This is the first time you're getting all 4 wheels working to provide traction when you need it. A 4WD system with limited slip is where the systems capability, driving all four wheels, matches the name Four Wheel drive. Its also the most similar to All Wheel Drive.

Limited Slip Differential

Real off road racers and heavy duty off roaders move up to differentials called lockers. This actually locks both wheels together on either axle. The good news is both wheels rotate at the same speed. The bad news is these systems cannot operate in all conditions on a normal car. They require a slippery surface to allow the wheels to slip and travel at different speeds.  When you drive a locked differential on dry ground however, the wheels will "chuck" at you. They skip over the sticky pavment with more force, which you can feel in the steering and even the car.  This makes the vehicle very hard to drive and will wear tires and driveline parts out quickly. Some systems that run full time 4WD have unlocking rings on the front wheel hubs to lock and unlock the front wheels from the front driving axles. When the wheel hubs are unlocked, the wheels don't attach to the turning axles and each wheel can travel at their own speed. You would re-lock the hubs when the weather gets bad.

Another way you can accomplish locked axles is to use a switched locked diffential. These are typically available on very expensive and technical vehicles. The Ford Raptor, a special offroad version of a Ford pickup, comes with a remotely lockable rear differential. It can be unlocked to allow the back wheels to move easily at their own speed on high traction surfaces like dry concrete and be locked remotely to have both wheels turn at the same speed on surfaces with low traction.

ALL WHEEL DRIVE

All Wheel Drive is typically a more sophisticated system than Four Wheel drive. Companies like Subaru or Audi have a differential or clutch packs in the middle of the car, similar to the transfer case in function that it distributes power as needed to the front and rear axles. But its far more sophisticated than a 4WD transfer case. Torsen gears combined with computer controls can vary the output from locked to variable between the front and rear axles.

When the power gets to the axles, many of these systems continue to use computers and sophisticated differentials to control the exact torque that goes to each wheel. So if one wheel has traction in the snow, the computer can switch most of the power to the axle with the most traction and the differential can send most if not all of the power to the wheel with the most traction. This type of system has additional benefits to bad weather performance. For example, systems like Acura's SHAWD (Super Handling AWD) use computers and sensors to shift the power to the appropriate wheels to keep the car from skidding off the road and to help the car handle better. For exmaple, if an Acura with SHAWD is making a left turn, it passes more power to the right side tires to help the car rotate to the left, and those wheels to travel the longer distance they have to in a left turn. 

Torsen Differential

All Wheel Drive comes in many flavors and has many benefits. In some hybrids, it can even be accomplished by a traditional gas or diesel engine powering one axle and one or two electric motors powering each of the other two wheels. These are of course are all controlled by computer to allow them to work smoothly together! The advantage to this type of system is lighter weight - there is no heavy center differential or transfer case. The small dissadvantage to such a system is that you cannot put the power from all of the motors out from any one wheel/tire.

I hope this helps to explain the difference between Four Wheel Drive (4WD) and All Wheel Drive! Please send me your feedback!