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.
Honda's RealTime 4WD on the...
Honda's RealTime 4WD on the CR-V uses a gerotor-pump to apply pressure to a clutch pack. This "engages" the rear axle only when necessary and under load; under braking it disengages, and therefore has no effect on ABS.
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").