The number of four-wheel-drive vehicles grows every year for cars, trucks, and those crossovers in between. And as that crossover line blurs, so does the one that defines four-wheel drive. So many "modes" and "tracs" are on new trucks that even we have to climb underneath to sort them all out. A study of these various drive systems will help you decide what advantages one offers over another--if any at all--list who has what, and ponders if Newton really was on to something.

If you learn nothing else here, learn this: Four-wheel drive does not nullify the laws of gravity, physics, or common sense. Regardless of drive, only the tire patches connect you to the ground, high-centering or falling over notwithstanding. If you blister down a snow-covered grade with only tenuous grip, you'll learn the hard way that for every action there is a reaction, as you thud to a stop in a snow bank.

Do You Need It?
Four-wheel drive will help in acceleration and to a lesser extent in steering, because the front wheels are pulling in the direction preferred. But there's nothing to suggest a 4WD will stop better, as this is generally a function of brakes, tires, and vehicle weight. Four-wheel drives often come with better tires for adverse conditions and, in some cases, with better brakes (either upgraded to four discs or with ABS standard), but 4WDs weigh more as well, with some of that weight added over the already heavier front end. On the same tires with no brake differences, the 2WD will usually stop better. A 4WD will offer more suitable traction, sometimes better ground clearance, occasionally improved ride quality derived from the larger tire sidewall, may sit higher for a superior view, and might win a "mine is bigger" driveway battle. For some trucks (Wrangler, Land Rover, Hummer), 4WD may be the only way you can get them.

On the other hand, 4WD will cost more (in the neighborhood of $2500-$3500). It will also cost more to keep, use tires and gasoline faster, and the extra driveline parts may add to routine--and unexpected--service bills. It will weigh more--anywhere from 200 to over 500 lb--depending on the model, and it may have a little less power because of packaging restrictions, especially in those AWD vehicles based on a front-driver. Finally, a 4WD might cost more to insure, though this is becoming less of an issue.

What Is It?
At Truck Trend, we consider anything with four driven wheels and low-range gearing as four-wheel drive and anything without the low-range as all-wheel drive. Where four-wheel drive is mentioned hereafter, much of the information will apply to all-wheel drive as well (keep in mind gearing is where we make the distinction). We also reserve the term "part-time" for 4WD systems that operate as two-wheel drive (usually on the rear wheels) until the driver changes it by switch, pushbutton, or lever. The term "full-time" describes a 4WD system that always operates as a four-wheel drive, but with no driver-selectable 2WD option. It isn't necessary to agree with our definitions, but at least understand our slightly arbitrary classification system. See glossary for further clarification.

For example, we call the Range Rover and M-class full-time 4WD. They have low-range but not a 2WD option. The BMW X5 and Pontiac Aztek both qualify as AWD. Neither has low-range or driver-selectable 2WD, even though the Aztek operates normally as front-drive only and the X5 biases output to the rear. The Jeep Wrangler and Ford pickups are part-time 4WDs--the driver has to engage 4WD and both have low-range. Not coincidentally, neither can run 4WD on pavement.

These part-time 4WD systems are generally inexpensive, the least complex, and most likely manually operated. All part-time systems and many full-time systems also offer the ability to put the transfer case in Neutral, effectively disconnecting the engine and transmission from the wheels. This makes flat towing (with all four wheels on the ground) much simpler when using your truck as a motorhome dinghy, trail-only toy and makes it harder to steal.

Low-range gearing in any 4WD system is for torque multiplication. San Francisco streets are steep and require lots of throttle and torque converter slip to scale, but they pale in comparison to climbs in off-road conditions or the power required to turn a 100-lb tire buried in 200 lb of mud. Low-range reduction gearing, normally in the 2.00-2.7:1 range, is located in the transfer case, works on all forward and reverse gears and may be engaged by lever or switch. However, the gear change must be done with the vehicle stopped or nearly so.

Why Differentials?
If we didn't have to turn and weren't concerned with noise and tire wear, trucks wouldn't need differentials. But every time a truck changes direction, the front wheels scribe a larger arc than the rear's, and the outside tires do the same relative to the inside tires. This can be accomplished without differentials, but makes steering difficult and locks everything together.

A differential allows wheels to operate with some independence, so that the inside tire doesn't travel the same distance as the outside tire or the rear tires the same as the front. In turn, the tire doesn't bark while it turns in place and the truck doesn't feel like it's trying to jack itself up (often referred to as "binding" or "driveline windup").

A part-time four-wheel-drive system that will not be used on pavement needs only two differentials, one for each axle. In theory, a full-time four-wheel- or all-wheel-drive vehicle will need three differentials, one for each axle and one between the front and rear axles. However, as a wider variety of drive systems make their way into trucks and crossovers, a center differential is not always necessary.

Some AWD and full-time 4WD vehicles use torque split to maintain the driving dynamics of a 2WD. In vehicles derived from front drives, many are front drive normally and add rear drive as required: The amount of output transferred to the rear is often a function of strength--the Saturn VUE four-cylinder can direct 69-percent power to the rear, while the V-6 sends a maximum of 57 percent. Conversely, vehicles like the Escalade and X5 run a rear-biased torque split, usually near 1/3 or 2/3 percentage, which maintains the dynamics of a rear-drive system, while adding some front drive for directional stability and low-traction steering improvement. Systems like Infiniti/Nissan's All-Mode are designed to work in "auto" for best handling and performance and offer the "2WD" position primarily for dynamometer and emission testing.

In vehicles derived from most two-wheel-drive trucks and sport/utilities, primary drive is to the rear. When four-wheel drive is selected, drive is added to the front axle from the transfer case. The connection between front and rear driveshafts in the transfer case is mechanically locked in part-time systems, which is why they can't be used on pavement where there is no tire slip. In full-time and AWD systems, the connection must have some means of allowing a small difference in front and rear driveshaft speeds so the truck can get around a corner without binding.

This can be accomplished by using another (center) differential in the transfer case, so front and rear drive can operate with some independence. In many cases this center differential may be locked (sometimes only in low range) for off-pavement use, as with the Hummer, earlier Range Rovers and Discoverys, and Chrysler's Selec-Trac.

Making Connections
The connection may also be made by a viscous-coupling unit (VCU), a sealed canister or barrel-shaped device that's filled with thixotropic silicone fluid and a series of interwoven plates resembling CDs in a changer magazine. One set of these plates is attached to the drum shell, and the others are attached to a driveshaft through the middle. As the speed varies between the two sets of plates, because one driveshaft is rotating faster than the other (either by turning or by wheels slipping), the silicone reacts to the heat being generated by thickening, which in turn starts to limit the speed differential between the two sides of the VCU. This allows the slip necessary to avoid binding, without allowing one axle to spin while the other does nothing. Some VCUs have a locking function for off-road use (Montero, Range Rover) while others do not (Escalade).

Another alternative is an electro-magnetic clutch, initially used on Ford's 90's Aerostar AWD and now found in its ControlTrac (Expedition, Explorer, Navigator). This system uses an electromagnetic mechanism controlled by a computer receiving input from driveshaft speed sensors to variably apply pressure to a clutch pack in the transfer case. In "auto" this system defaults torque split 98 percent to the rear, adding front drive only as slip is detected. While the system can vary up to 100 percent output to the front, complete lack of traction at the back would imply at least partial loss at the front, so the front-axle assembly will not have to cope with all the engine's output. This is an important consideration--even a small V-8, run through low-range first gear at full throttle, could generate 10,000 lb-ft of torque to one axle, and many front axles simply won't take it.