 |  | | | In simpler times, it was pretty easy to decide on what type of drive system to have on your vehicle. If you planned to head off road at all, you needed to get a four-wheel drive (4WD) truck or Jeep. These early 4WD systems were not very advanced in that they simply put power to all four wheels, and were purely for tackling rugged terrain.
Choices
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Today, however, in addition to a number of advancements in 4WD, there's a dizzying array of other drive systems to pick from, with variations within those types. All kinds of drivers are buying 4WD and all-wheel drive (AWD) vehicles. Although we often use the terms interchangeably, they're not the same. In fact, when the going is tough, four-wheel drive may not really power all four wheels. Meanwhile, some all-wheel-drive systems are effective because, when necessary, they get all of the vehicle' s torque to just one wheel.
To help sort through them, we'll provide some basic descriptions and applications, so you can decide what's best for you. Traditional 4WD systems are part-time, and not meant for use on dry pavement. Due to their drawbacks, if they were all we had today, there would probably never have been an SUV boom. That's because engaging/disengaging these systems requires shifting an often-balky lever and locking the hubs by hand. Also, when on a surface with good traction, having the front and rear drivetrains linked together makes the vehicle jerk and lurch, especially while turning (called driveline windup or tight-corner braking). These systems will wear out quickly or break if used extensively on clean pavement.
With a part-time four-wheel drive (4WD), you only engage the system when you need traction on loose terrain, and in deep mud or snow. Simple and reliable, when combined with limited-slip or locking differential options, it can be nearly unstoppable in extreme conditions.
Part-Time
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Part-time four-wheel drive systems typically have a two-speed transfer case, with "low" and "high" settings that can be switched using an interior lever or switch. The "low" setting provides even greater torque for pulling or climbing in an off-road environment. The "high" is useful for slippery on-road situations like packed snow or ice. 4WD might also use a locking center differential to avoid unnecessary slippage between the right and left wheels while off-road.
Few new vehicles offer full-time or part-time 4WD in pure form. Part-time variants include automatic locking hubs, electric or vacuum connection/disconnection of front halfshafts, electrically shifted transfer cases, and electromagnetic front driveline spool-up clutches.
Full-time systems have been enhanced with viscous couplings or hydraulic clutch packs to progressively lock front and rear drivetrains or with manually lockable center differentials for rough going (in effect, adding part-time 4WD to full-time 4WD).
Even though part-time four-wheel drive is the most common system found on SUVs and trucks, AWD is becoming more popular on a wide range of vehicles because it can be regularly driven on paved surfaces. In fact, that's its primary application, though it can come in handy in moderately challenging "soft-road" conditions. The chief purposes of AWD are to improve everyday driving feel on dry pavement, increase power delivery to the ground and enhance traction in routine slippery driving, such as rain-slicked or snowy highways. This isn't king of the hill for off-road trailblazing.
Full-Time
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You can think of All-Wheel Drive as "permanent" four-wheel drive. The basic system employs a differential that transfers force to the front and rear drive wheels, allowing them to rotate at different speeds as needed, such as when they travel varying distances around curves or on slippery pavement. The result is very good traction and handling on most any surface—dry, wet, muddy or slushy.
In general, there are two AWD system types: either a fixed or a variable front-to-rear torque split (referring to the amount of drive force going to each end of the vehicle). Varying the front to rear torque split in a passive variable AWD system requires using a center viscous coupling in addition to the center differential.
Both the part-time 4WD and AWD system have performance tradeoffs. Part-time systems are relatively inexpensive and quite reliable. They provide most people with more than enough off-road traction. The low-range lever provides more torque when needed, and helps control vehicle speed on steep downhill terrain, a benefit not available with most AWD systems. The main disadvantage to part-time 4WD is having to deactivate the system when driving on hard, high-friction surfaces.
On Demand
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The dominant advantage of a passive, i.e., mechanical AWD system is its ability to run on dry pavement, in addition to providing better acceleration, cornering, throttle steering/braking and high-speed stability than a vehicle driven only by the front or rear wheels. And while fine for road-going high-performance lightweight sports cars, passive AWD systems are generally not robust enough for vigorously driven off-road vehicles.
That's not necessarily the case on every AWD system, however, such as with Isuzu's hybrid Traction On Demand (TOD) system found on its Axiom, Trooper and VehiCROSS models. This sophisticated electronic system has stability and traction abilities that surpass passive mechanical AWD systems, along with better durability than part-time systems. TOD's performance has been tested and proven since 1988 in off-road racing trucks that produce between 800 and 1,000 lb.-ft. of torque. According to Isuzu, TOD has scored more points and won more races than any other drive system in off-road racing. Even though TOD is an electronically controlled system, it's still tough enough to handle everything that Baja, and a powerful racing engine, can throw at it. As with a part-time system, its two-speed transfer case lets you shift into super-low, fully locked four-wheel-drive mode when needed. In fact, TOD's pushbutton control allows you to shift from two-wheel-drive to four-wheel-drive high-range and back again without stopping at speeds up to 60 mph.
In addition, the system monitors operating conditions and makes corrections in response to driver inputs. For example, if the system senses that the throttle is opening quickly, it knows the vehicle will be accelerating and the traction demands will change. After quickly evaluating input from all its sensors, it decides how to divide the torque split even before it detects a front-to-rear slip-percentage difference. It anticipates that the wheels may slip and alters the torque split to avoid that condition, thus improving traction, handling, and acceleration in extremely low-friction conditions. So the bottom line is that you end up getting the flexibility of AWD, but the durability of a 4WD system.
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