The more I talk to people about four wheel drive, the more I realize that it is probably one of the most MISunderstood automotive concepts second only to perhaps carburation. This is understandable, considering the fact that the vast majority of supposed 4WD vehicles are really 2WD, or more accurately, 2AD (two axle drive). By the time you get to the bottom of this page you should have a better understanding of just what *is* going on underneath the floor in your "4WD" truck.

Let me get this out of the way right here:
if you are driving any stock or even a modified truck, unless you have locked differentials front and rear with the transfer case engaged, you are *not* driving with all four wheels at any given moment. However, "Four wheels with the potential of driving" somehow isn't quite as catchy, so although we call it 4WD, it really isn't.

Transfer cases do exactly what the name implies. They transfer the torque output from the transmission to the front and rear axles via the driveshafts. You can think of it as a secondary transmission. There are two main types of transfer cases, namely part-time and full-time.

Part-time cases (such as the Dana 20 and NP 205) are what most people think of when they discuss 4WD systems. These xcases are exactly what they say they are. Part-time meaning that the front and rear axles are only both engaged on a part-time basis. Most of the time this also means that locking/unlocking hubs are included on the front axle.

Part-time xcases can only be used on slippery surfaces that allow the front and rear tires to move at different speeds.
(Mud,snow,etc.) They can only drive the driveshafts at the exact same rate since the front and rear outputs of the part-time cases are locked together in a 50/50 torque split.

Keep in mind that the only time your axles are moving at the same speed is in theory going in a straight line with equal gear ratios and equal tire size/wear/pressure. Take the example of making a turn-the wheels on the outside of the turn move faster than the wheels on the inside because they have to travel farther. The same is true of your axle/driveshaft speeds.

Your part-time case is going to try to force the driveshafts/axles to move at the exact same rate front and rear. However, due to turning/road irregularity/etc., your axles are going to try to turn at different speeds while the xcase is trying to force them to turn at the same speed. That's why it's so hard to turn a part-time xcase truck on pavement with the xcase in "4WD" This is known as wind-up, and unless there is some provision (tire slip in mud or snow) for the driveline to dissipate this force, something will break.

This is where full-time cases come in. A full-time xcase (such as the QuadraTrac) is always in "4WD". There is a provision in these cases for the dissapation of wind-up, usually some kind of differential or torque coupling. Full-time xcases are great for mixed condition driving, (dry/slippery) and members of the family who may be challenged by yet another confusin' lever in the cab.

The disadvantages to full time cases are that in some full-time set-ups it is possible to immobilize the vehicle by getting only one tire (but not QT) on a slippery surface, all of them that I can think of are chain driven, and since you are always driving two axles, MPG and tire life suffers. One last comment on the chain drive is that the QT has the beefiest chain that I know of and you will probably snap a U joint before a serviceable QT chain. There are some guys flogging QTs
on hill climbs with 35" tires with success.

In Part 2, I will get into what it means to "lock" your xcase, and open/posi/locker differentials. I will also touch on "shift on the fly" vacuum actuated locking hubs that add alot of confusion to the mix for many guys I talk to. Until then, just remember that it isn't as mysterious as the marketing folk in Detroit would have you beleive.