If you put a record on a record player (or a CD, if you're under 30) and spun it slowly you would notice that the outside travels a much farther distance than the inside. This is essentially what happens when you drive around a corner in your vehicle; the outside wheels must turn faster and travel farther than the inside wheels.
This difference is accommodated by the differential gears in your vehicle's axle housings. Without a "diff", your inside wheels would creak and groan trying to keep up, and you'd wind up replacing your axle shafts about as often as you change your oil.
Your vehicle already has a differential. So far, so good...but what happens when you lose traction under one of your wheels? The power is transferred to both wheels through the axle in equal amounts. A spinning wheel has no torque, so no torque is supplied to the wheel on the other end of the axle. Suddenly, your four-wheel-drive vehicle is a two-wheel-drive.
If it happens that two wheels spin on both ends of your vehicle - a fairly common occurrence in slippery conditions or on uneven trails - you're down to no-wheel-drive.
This is particularly a problem with Part-Time four-wheel-drive because there is no center differential to send power between the front and rear axles. Both front and rear axles receive power side-to-side, but not front-to-back, and there is no "give", so you'd better be driving straight ahead or in very slippery conditions, enabling your inside wheels to slip and slide as they try to catch up with your outside wheels.
Despite this limitation, some drivers prefer Part-Time four-wheel-drive because it's easy to disengage and revert to a standard two-wheel-drive system for better fuel economy. It's also a relatively simple, and thus inexpensive, system. Many people also believe that it is more durable under stress, and can output more torque, pulling and climbing power than center diff systems.
If you want a center differential to transfer power between both axles, you can choose either Full-Time or Permanent four-wheel drive.
Full-Time is the most common system because it allows the driver to select between two-wheel drive (in some cases), "Auto 4WD" for changing conditions and a 4WD lock in high and low range for really challenging conditions. A "fluid" coupling in the center diff reacts on its own to changing conditions, sending more power to the wheels that need it. Unfortunately, if a wheel suddenly looses traction, this system will send it more power, which is useless. Many proponents of this system argue that such conditions are extremely temporary and often go completely unnoticed in most driving situations.
Enter...Land Rover. After half a century of tinkering around with four-wheel-drive, the old boys from Solihull believe they've created the ultimate system: Permanent four-wheel-drive. None of this two-wheel-drive nonsense; the system always sends power to all four wheels. (What would you expect from Land Rover?) With Permanent four-wheel-drive, you get the traction and control of the Part-Time system - with locking center differential capability (no slippage) and high and low range gearing - plus the viscous coupling advantages of the Full-Time system for both dry pavement and inclement weather driving.
Land Rover is not the only manufacturer using Permanent four-wheel-drive, but with the plethora of SUV's, all with their own marketing terminology, it's not always easy to tell who offers what.
Understanding the difference between differentials is a good place to start.
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