Four-Wheel-Drive Demystified

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All-wheel drive, four-wheel drive, part-time 4WD or permanent 4WD: Which one of these systems is just the right one for you?

So, you've decided your next vehicle should have four-wheel drive. Maybe you've decided it should be all-wheel drive.

Will it be quattro, Autotrac, Versatrak, 4Matic, 4Motion, Control Trac or Command Trac? Should you care?

The marketing names may not be important, but consumers may find it useful to understand the variety of systems available and how they are both similar and different. Here is a rundown:

PART-TIME 4WD
This is the most basic system; it operates after a driver either moves a shift lever or presses a button inside the vehicle.

Otherwise, the part-time four-wheel-drive vehicle travels in two-wheel-drive mode; typically in these cases, the power that's moving the vehicle is sent to the two rear wheels and the two front wheels are pushed along.

When a driver shifts from two- to four-wheel drive in a vehicle with part-time four-wheel drive, he or she locks together the front and rear axles, so the front and rear wheels rotate at the same speed. This improves straight-line traction.

Since the part-time system has no center differential, there is no way for the two axles to rotate at different speeds in a corner. So, part-time systems shouldn't be engaged unless the vehicle is on very slippery road conditions such as deep snow and mud where wheels can slip as necessary for turning.

If you try driving one of these vehicles on dry pavement with four-wheel drive activated, you will likely feel an awkward, binding sense as you turn a corner. It's also possible to damage drive system components on these vehicles and cause premature tire wear if you travel in four-wheel drive on dry pavement.

Distinguishing Characteristics

• In normal mode, the vehicle travels in 2WD, typically with rear wheels receiving power and pushing along the front wheels.
• Driver is responsible for engaging 4WD and disengaging it.
• Activating 4WD sends power to the front wheels and continues power to rear wheels as well.
• In rigorous, off-road terrain, a driver can engage an extra-low gear for improved torque while in 4WD.
• Requires a transfer case, two drive shafts, two axle differentials and two powered axles.
• Does not include a center differential.
• 4WD should not be used on dry, hard surfaces.

Advantages
Since this system is basic, it's less complicated and less costly to build than some other systems, resulting in a lower vehicle price for car shoppers. This helps explain why it can be found on lower-priced, entry-level vehicles.

The system has shown itself, in general, to be durable under heavy stress. Since four-wheel drive is used only when needed by the driver, components don't get constant wear and tear.

Two-wheel-drive mode, which these vehicles operate in most of the time, can be quieter than four-wheel drive, especially if a vehicle doesn't have sufficient sound insulation.

Another advantage of the part-time system is a very slight increase in fuel economy, compared with what you'd see in a comparable vehicle where all four wheels receive power all the time.

Drawbacks
Older vehicles with part-time four-wheel drive may require drivers to stop their vehicles before shifting from two-wheel drive to four-wheel drive. Some older vehicles also mandated that the driver get out and manually lock the front hubs at each of the front wheels. It was not the most convenient process for consumers, especially for those who shunned snowy or muddy conditions. Most, if not all, of today's part-time, four-wheel-drive vehicles are no longer built with manually locking front hubs.

Part-time four-wheel-drive systems make the driver responsible for deciding whether road conditions warrant engaging four-wheel drive. The driver also must decide when to deactivate four-wheel drive.

Trend
In recent years, automakers have been offering fewer vehicles with part-time systems.

Some prominent models that still have it include the Jeep TJ and Nissan Xterra sport-utility vehicles that are popular with off-roaders.

PERMANENT 4WD
Vehicles with permanent four-wheel drive (also known as full-time four-wheel drive) have no two-wheel-drive mode. They always operate with power going to all four wheels, so drivers benefit all the time from four-wheel traction and don't have to shift or push a button to activate it. There also is no way to turn off the four-wheel drive.

In contrast with part-time four-wheel drive, vehicles with permanent four-wheel drive do have a center differential, so they can travel on dry pavement without damaging components or having wheels bind in turns and corners.

In some vehicles, the center differential also can be locked when more traction is needed, such as during rugged off-road driving.

There can be other differences, especially in electronic features. For example, Mercedes-Benz M-Class models and Land Rover's Discovery Series II and Range Rover use traction control to help control wheel slip in all conditions.

Distinguishing Characteristics

• All four wheels are powered all the time.
• A driver has no way of switching to 2WD.
• In rigorous off-road terrain, a driver can engage low range for improved torque.
• Requires a center differential, transfer case, two drive shafts, two axle differentials and two powered axles.

Advantages
Since drivers in vehicles with permanent four-wheel drive don't need to activate the system, they can concentrate on driving, not judging road conditions and pushing buttons or shifting into four-wheel drive.

There is no lag time for four-wheel-drive activation in these vehicles, as there could be in vehicles with automatic four-wheel-drive systems that engage only after sensing that road conditions have changed.

Drawbacks
Vehicles with permanent four-wheel drive can have less attractive fuel economy than you might get from a comparable vehicle with part-time four-wheel drive or automatic four-wheel drive.

In some circumstances, driving with the center differential locked may make steering more strenuous because the vehicle will tend to want to travel straight forward, rather than turn.

Vehicles with permanent four-wheel drive tend to have more heavy-duty drive train equipment and might be priced higher than other four-wheel-drive vehicles.

Trend
The number of vehicles offered with permanent 4WD has remained rather steady in recent years, and Land Rover remains one brand whose entire product line, from the compact Freelander to the opulent Range Rover HSE, has this feature.

AUTOMATIC OR FULL-TIME 4WD
This system is designed to automatically decide for drivers when and where to apply torque to different wheels when added traction is needed.

Marketed by a variety of names including Autotrac, Control Trac, RealTime 4WD or Versatrak, this system works via a center differential or viscous coupling or some other kind of coupling to transfer torque, or power, in fractions of a second when the system senses a need for extra traction.

Distinguishing Characteristics

• In a number of models, the driver can set an "auto 4WD" function, usually via a button on the dashboard, and the vehicle decides when to activate and deactivate 4WD, as necessary; driver has no further input. In several other vehicles, the system is entirely automatic and has no control switch or button whatsoever.
• When the vehicle engages 4WD, it manages and transmits the power, as necessary, to both front and rear wheels or just to the wheels with the most traction.
• On certain automatic 4WD vehicles only, the driver can also switch to low gear for improved torque in rigorous off-road conditions, for instance.
• Requires a center differential or some kind of viscous coupling or multi-plate clutch, a transfer case, two drive shafts, two axle differentials and two powered axles.

Advantages
The appeal of these automatic four-wheel-drive vehicles is they monitor and sense their own traction needs as they travel, and automatically adjust how the power is delivered to the wheels. The driver can concentrate on driving, rather than having to decide whether to shift into four-wheel drive.

Drawbacks
Some four-wheel-drive experts say that in severe off-road conditions the automatic system can hamper a driver's efforts. For example, because the system is quick and automatic, a driver may find the power shifting from one axle to another abrupt and unsettling while the vehicle attempts to climb over rocks or traverse a gulch. In this case, a driver should set the vehicle in 4WD high or 4WD low, rather than automatic 4WD.

The automatic system is more complex, typically with more components, than a part-time four-wheel-drive system, and it can be more costly.

Trend
Automatic four-wheel drive has been a growing offering on vehicles and is available on models such as the Ford Escape and Honda CR-V.

ALL-WHEEL DRIVE
All-wheel drive works like a permanent four-wheel-drive system by providing continuous power to front and rear axles. There is no way to turn off the all-wheel-drive feature, and a driver doesn't have to do anything to activate it.

The mechanicals in these vehicles lack a transfer gearbox (also referred to as a 'transfer case') that would allow engine torque to be geared down to a very low range for rigorous off-roading.

Following the lead of all-wheel drive pioneer Audi, with its vaunted quattro system (a version of which is used on some of parent company Volkswagen's models and branded as 4Motion), all-wheel-drive systems tend to be packaged compactly, with less bulk and weight than in traditional truck-like applications and are thus suitable for use in a variety of smaller, lighter-weight vehicles such as cars. This also explains why all-wheel-drive vehicles don't necessarily have to be positioned higher above the road as four-wheel-drive vehicles are.

In some all-wheel-drive vehicles, much of the power during everyday, normal driving is channelled primarily to one of the axles. For example, the normal power distribution under ideal traction conditions in the Porsche 911 Carrera 4 and Turbo is 5 per cent front and 95 per cent rear. More common is an even 50/50 power and torque distribution between axles, or a close approximation.

Vehicles with all-wheel drive use a center differential, viscous coupling or clutch pack, depending on the vehicle's engineering, to maximize traction when needed. As wheels begin to slip in these vehicles, more and more power is automatically directed to the wheels where there is no slippage. The power shift can be sizable, if necessary, and again, it's done without any input from the driver.

Distinguishing Characteristics

• Front and rear axles receive power all the time.
• A driver has no way of switching to 2WD.
• Vehicle does not have low gear for strenuous off-road terrain.
• Requires a center differential, two drive shafts, two axle differentials and two powered axles.
• Does not include a transfer case.

Advantages
All-wheel-drive vehicles tend to offer respectable fuel economy compared with other types of four-wheel-drive vehicles.

All-wheel-drive systems often can be packaged compactly, with less bulk and weight, and thus are suitable for use in a variety of smaller, lighter-weight vehicles.

The systems also can be less pricey than those in vehicles with heavy-duty four-wheel-drive equipment.

Typically, there is little need to position the vehicle higher above the road to accommodate the all-wheel-drive mechanicals.

In all-wheel-drive vehicles, drivers don't have to activate the system. They have continuous traction from all four wheels.

Drawbacks
All-wheel-drive vehicles do not come with a transfer gearbox (also referred to as a 'transfer case') that provides a low range to gear down the engine torque for the extra control that off-road enthusiasts often demand.

All-wheel-drive vehicles can cost more than comparable vehicles with two-wheel drive.

Trend
All-wheel drive is showing up in more models each year and can be found in a wide variety of vehicles, from Toyota's fully-redesigned 2004 Sienna minivan to the most recent Mercedes-Benz C-Class, E-Class and S-Class models. BMW has also been offering all-wheel drive "xi" versions of its class-defining 3-Series sedans and Touring sport wagons for the past few years. Jaguar will soon also be launching a wagon (estate) version of the X-Type, its first-ever 4WD model.

Subaru made a strategic decision, years ago, to market all-wheel-drive vehicles exclusively. This approach has brought the Japanese brand great success and continues to this day.

These days, several luxury brands other than all-wheel drive pioneer Audi are effectively seeing this type of drive train as the next "must have" feature.

In a country and a climate such as ours, this is great news indeed.