It used to be that you could tell what category a vehicle belonged in just by looking at it. A truck was a truck. A sport/utility was a sport/utility. There was no such thing as a crossover, so anything that didn't fall into those 1st 2 categories was a car. Along those same lines, when looking at 4- versus all-wheel drive, it used to be easy: 1 was serious, and the other was a gimmick. All that has changed. Today, categories have become so fractured that there are vehicles that could be referred to as a car, a 4x4, or a crossover--depending on whom you ask. The weird part is that each answer could be right.
The manufacturers themselves aren't helping matters much when their respective marketing departments are more concerned about what sounds good, rather than what's accurate. And what role does the Society of Automotive Engineers, supposedly the premier authority on all things automotive, have to play in this confused world of vehicle traction systems and the name game? It used to be simple -- if you owned a pickup truck with big lever shifter on the floor and had to get out of the truck to turn the front hubs from "open" to "lock," then you had a 4-wheel drive.
Then along came the SUVs in the 1980s and 1990s, with electronic in-cab 4-wheel-drive controls and shift-on-the-fly capability. With the touch of a button from the comfort of your heated driver's seat you could switch from 2-wheel drive into either an all-wheel-drive mode (with a center viscous clutch) or a mechanical locking 4-wheel-high-lock mode, giving both front and rear driveshafts equal amounts of engine torque. And your front hubs engaged automatically. Some OEs went so far as to offer limited-slip or locking differentials in both front and rear axles to give their vehicles true 4-wheel drive, meaning all 4 wheels could deliver traction at the same time no matter what the surface.
Jeep's Grand Cherokee uses a full-time 4WD system with a dedicated transfer case that offers several terrain settings. Selec-Terrain has a setup similar to Ford (with Snow, Sand & Mud, Rock, and Normal) with an added Sport setting. This allows the Grand Cherokee to be a better, more spirited street driver a
Tremendous advancements in traction control technologies were also being made. The same computer software that controlled the fast-improving anti-lock-braking systems could also control both slow and higher-speed tire acceleration response. These systems used super-smart sensors to monitor and control wheel speed during stops and starts, whether in low-range, high-range, or in standard rear drive.
At the same time, vehicles offering the driving dynamics of a car plus the space and carrying capacity of large or midsize SUVs were born. We struggled for a while on what to call them, eventually settling on the term "crossover." This segment exploded in the late 1990s, with even more vehicles using new technologies coming to dealers in the early 2000s. Each featured weight-saving, non-truck-originated (often car-based) traction technology with some of the protective capabilities of their older (and heavier) cousins, without the rough-riding, mpg-killing penalties.
For most of this new breed, starting with a car platform and trying to adapt some kind of added front (if rear drive) or rear (if front drive) drive capability was the norm. To further confuse the segment definitions and separate their brands from the competition, marketing gurus came up with various names for the different types of all-wheel-drive and 4-wheel-drive systems. None of the names made it easier to know whether they were good 4-wheel drives or all-wheel-drive systems. In fact, some AWD systems act more like 4WD systems, while some 4WD systems act like AWD systems. And to confuse things even further, various manufacturers were technically allowed to call their all-wheel-drive vehicle 4WD because the SAE's definition says any vehicle that can deliver power to both driveshafts at the same time for an extended period can be called a 4WD.
Where does that leave us now? Unfortunately, not much smarter than we were before. Our suggestion is to consider AWDs all-weather players, whereas 4WD refers to more of an all-terrain-capable vehicle. That may be an oversimplification, especially because there are 4-wheel-drive vehicles that can do both, but this is the best way to keep things understandable.
There are plenty of manufacturers on both sides of the fence who need to be more honest about their capabilities, and the limits of those same capabilities. We'll take a look at the basic assets and strengths of 4WD and AWD systems, discuss the major players in each category, and tell you what to keep in mind when using these systems to the best of their capabilities.
The new 4Runner offers 2 different 4-wheel-drive systems, with this being full-time 4WD model. The dial is set to all-wheel-drive mode, designated at "H4F" for high range power to all wheels, all the time. Other settings include high range with engine power equally split to front and rear axles (center differential locked), and low range with engine power equally split to front and rear axle.
The 1st 4-wheel-drive vehicles were easy to understand. The output shaft from the transmission went into a transfer case where the engine power was split between front and rear driveshafts. In 2WD, the rear driveshaft turned a pinion inside the rear differential, spinning a ring gear that turned the right and left axle shafts, eventually turning each wheel. With the transfer case lever engaged in 4High or 4Low, a fork would engage a chain drive, turning the front driveshaft and pinion (at the same speed as the rear driveshaft) in the front axle's differential, also turning the ring gear and right and left front axle shafts, then the wheels. This is also referred to as a "part-time" 4-wheel drive system because the 4-wheel "driveness" must be manually engaged, recommended for use only in difficult low-grip terrain or driving surfaces. Naturally, 4-wheel-drive systems are more complicated now, but the principles are the same. As you might expect, there are more computers and advanced electrical sensors in most of today's 4WD systems, but there are still a few brand-new vehicles that aren't much different from the originals, like the Jeep Wrangler and Ram HD Power Wagon.
For any full-time 4WD system or AWD (those 2 labels are basically interchangeable), there needs to be a good way to modulate the power to the front and rear wheels, either through mechanical means or electronic sensors and computer controls. The new 4Runner full-time system, like many other AWD vehicles sold around the world, uses a Torsen differential to split engine power by using offset helical gears that ramp up locking force progressively depending on changing front and rear axle speeds.
Still, as we define it, a basic 4-wheel-drive system will have some kind of dedicated transfer case with a separate low-range gear that allows the driver to multiply the transmission, and ring-and-pinion gearing in order to provide for more (and slower) control of the vehicle. The slower speeds of this crawl ratio (1st gear x axle gear x low range) allow for more usable tractive force (or torque) to each tire, whether on rocks, snow, gravel, or a steep incline. The low-range gearing effectively acts as a torque multiplier, giving the vehicle more grunt at the wheels to climb over nastier terrain or obstacles.
A typical AWD or 4WD drivetrain in a pickup truck or large SUV has the engine sending power through the transmission and into the transfer case, where the torque is split between the front and rear driveshafts, with transfer to the front usually handled by a chain.
Of course, it doesn't hurt that many vehicles equipped with a dedicated transfer case also have a good amount of ground clearance and larger wheels and tires. Many drivers who enjoy this type of driving for recreation often remove some air pressure from their tires at the trailhead for a larger contact patch and more sidewall flex. Some OEs have gone so far as to include advanced computer controls that can take over throttle and braking control on steep hill climbs or, more often, on extreme descents (i.e. Toyota's 4Runner and Land Cruiser, and their Lexus cousins) where all the driver has to do is concentrate on steering the vehicle away from dangerous areas. We should note that many of the more sophisticated 4-wheel-drive systems have some kind of separate all-wheel drive or 2-wheel setting for the drivers. More often they are simply all-wheel drive in normal operating conditions. Additionally, these changes are usually invisible to the driver, as the computers constantly (and in some cases hundreds and thousands of times per second) determine the optimum amount of traction the front and rear wheels need for a given situation.
Some of the smartest systems (Jeep Grand Cherokee and Land Rover LR4) have a predictive algorithm that allows the computer to control traction the same way a high-tech camera instantaneously adjusts lens focus, based on current and changing inputs. This means the system tries to predict, based what has been happening, what to do next. However, as smart as these systems may seem, they have their downsides.
Traditional 4-wheel-drive systems have a weight and cost penalty. And they usually come with bigger wheels and tires, more electronics, a liberal use of heavier metals, and in many cases protective skidplating. The weight penalties alone can add up to several hundred pounds and can significantly affect fuel economy and driver handling dynamics. Other drawbacks include added maintenance, more expensive initial cost, and the silly belief you can drive anywhere when the weather or terrain gets bad. We've seen more 4-wheel-drives on the side of the road during a snowstorm than just about any other type of vehicle. It doesn't matter how good the 4-wheel-drive system is, 1 unknowledgeable driver can override any well-designed technology.
Returning to the idea of proper definitions, we need to be careful about this 1. Just because it can theoretically or otherwise drive all 4 wheels at the same time doesn't necessarily mean we should call it a 4-wheeler. There are many very sophisticated, electronically controlled all-wheel-drive systems nowadays that can do a much better job of maintaining traction to the wheels than many of the early 4-wheel-drive systems, but they too have their various strengths and weaknesses.
According to SAE, an all-wheel-drive vehicle is 1 that has an on-demand feature that occasionally sends power to the non-primary powered wheels (rear in most cases, front in others). Everything else, with this definition, can be called a 4-wheel-drive system. (We're not sure that makes sense, but more on that later.) All-wheel-drive systems have also been around for a long time but became popular with European performance cars in the 1980s and 1990s for their added pavement and gravel road grip and improved handling characteristics.
The most basic systems often begin with a front-wheel-drive platform with some kind of power-splitting differential in a separate box, or it can be housed inside the transmission, and is able to send rotational power through a prop-shaft to a rear differential where power is then sent to each rear wheel. In some cases, a small or large percentage of engine power is sent to the rear wheels until the computer sensor detects front wheel slip, and then it can redirect or adjust power. Many smaller crossovers or small SUVs were originally developed from passenger car platforms, so the AWD systems were adapted to work in different designs. Although this type of system does not offer an extra low-range gear, it does provide a good amount of traction in snow, rain, or icy conditions when the computer system can detect small and large amounts of wheel slip between front and rear wheels. It's worth noting, but probably not too surprising to anyone, that just as 4-wheel-drive systems have benefited from the advancement of added computer power, so have all-wheel-drive systems, especially in the last decade or so.
This cutaway gives a great look at what goes on inside a transfer case. Although this particular model (from an Audi Q7) is AWD, the only thing missing from a 4WD transfer case is the extra reduction gear and engagement forks. A lot of torque is running through this chain, so the parts and pieces have to be strong; that’s why this kind of system has the additional weight penalty.
1 of the best systems is Subaru's Symmetrical AWD system, seen in the Forester. The setup starts with a well-balanced chassis and platform, keeping all the powertrain parts flat and relatively low to the ground. The transmission type (manual or automatic) determines which type of center differential strategy is used to best distribute power to the front and rear wheels. The front axle shafts are driven directly off the transmission, while the rear driveshaft is separately driven from the transmission through a center differential.
A viscous coupling in the 5-speed manual transmission basically keeps the front-to-rear split at 50/50, while the 4-speed automatic uses a series of multi-clutch packs to continuously vary the front-to-rear split from 60/40 in normal driving to as much as 2/98 if more rear traction is needed where the power is eventually split to both wheels. The traction control will limit any torque losses through wheel slip. The key to this system is how quickly the computer and wheel sensors detect that slip, shut it down with the traction control, and route the remaining engine torque to the wheel with the most grip. It happens almost instantaneously, usually without the driver knowing. This system is a leader in its segment and a popular choice for those who live in harsh winter climates.
Import aficionados and high-performance junkies already know both Subaru and Mitsubishi have vehicle packages where the driver can adjust how much grip and power the center differential will distribute between the front and rear wheels. Where most all other AWD systems control the distribution electronically, these 2 companies trust their enthusiast buyers (who love the WRX STi and Evo X) to make their own choices. Choices for the center diff modes are typically separated as gravel, snow, and pavement, with the resulting amount of grip in each mode (on the appropriate surface) providing a stunning amount of grip and control.
The Acura SH-AWD system uses sensors and computer power to keep all the wheels gripping wherever the traction exists, even if very little is to be had. This AWD system can also minimize the dangerous effects of under- and oversteer by sending power to an individual rear tire.
Another impressive piece of AWD technology comes from Acura in the form of what it calls its Super Handling All-Wheel Drive (SH-AWD) system, included on several of its SUVs and passenger cars. This super-smart computer system not only detects wheels speeds and slip in real time, but also has a sophisticated predictive understanding of what's likely to happen, and makes power distribution changes in order to provide the vehicle with more traction.
The heart of this technology allows the rear differential to seamlessly distribute all or a portion of the power coming through the rear driveshaft to either the right or left wheel. What this means is that the SH-AWD system not only can control under- or oversteer situations with braking or engine management, but it can also provide more power to an individual outside wheel for more turning grip in the front wheels. This makes the Acura MDX 1 of the most balanced and confident performers on snow and ice (and pavement) during high- and low-speed spirited driving. The system has no speed parameters, so it will work when cornering around a decreasing radius turn at the local track or in an icy downpour near the local Wal-Mart when a minivan shoots out in front of you.
All-wheel-drive systems, although typically not viewed as being as safe or sophisticated as 4-wheel-drive systems, can still be quite advanced and provide exceptional safety and fun-factor benefits. These types of vehicles offer a great buffer for those who don't want to bother with as many buttons or levers or switches to get all the benefits of extra traction. These systems are usually less expensive, easier to maintain, and don't typically require any study to operate.
This graphic of the FJ Cruiser and 4Runner A-TRAC system shows how, through computer-monitored traction control, the vehicle will automatically get power to the tire that has grip (torque and traction vary). Without smart traction-control, front and rear lockers would be necessary to overcome torque losses on ice.
HOW TO CHOOSE
Once you understand what's available, the choice that best suits your needs should become clear. Are you most worried about bad weather, or might you need to navigate nasty ruts in a washed-out dirt road late at night? Either way, there are plenty of choices. Beware of anyone tells you you'll never need 4- or all-wheel drive, so you may as well just buy a car and be done with it. As good as their intentions might be (and we know there will always be haters out there), keep in mind that whatever the cost of your drive system, if you live in an area where it might come in handy each winter or may save your bacon when you go camping or on vacation, the extra cost is justifiable. Even if you look at these systems as another safety feature on your vehicle (like airbags, ABS, or seatbelts) and you never have to use the technology, it's still nice to know you have it just in case.
In a world of constantly changing technology, with OEs continuously trying to outperform their nearest competitor, you can bet that more advancements and smarter systems are coming. From what we're seeing now, there are some interesting patterns emerging. Land Rover was the 1st to market with a smarter 4-wheel-drive system that gave different driver choices based on the type of terrain.
Others have followed (Ford and Jeep) but we'd guess more will head that direction as well. Interestingly, Jeep uses it on their impressive Grand Cherokee models, called Selec-Terrain, while Ford includes it on its new Explorer model, but only in an all-wheel-drive configuration. Ford calls it the Terrain Management System, and if it looks similar to Land Rover's all-wheel-drive system in the LR2, that's probably because the chief engineer on the new Explorer came from Land Rover. (Sometimes the world is even smaller than we think.) Our only problem with the Explorer system is that it has been embedded onto a platform that was essentially designed to be a front-drive car, based off of the Ford Taurus chassis. Yes, there are too many changes to count between the Explorer and Taurus, but the fact remains, it's an all-wheel-drive system adapted from a car platform and a bit of a letdown for a vehicle with a name that implies it can go anywhere -- the vehicle cannot.
Regardless, we expect to see more crossovers and new entries to follow the Explorer's lead, especially since it is selling quite well. Certainly, not everyone needs the capabilities offered from these types of systems, but we think there is still a place for them in the evolving world of pickups, SUVs, and crossovers. Who knows, maybe the all-electric family hauling crossover that can climb mountains and pull large trailers is just around the corner. Well, maybe not.
4WD, AWD, or Both
Many vehicles offer more than 1 type of 4WD or AWD system; some even offer different systems depending on the transmission. The complexity and type of traction system largely determines what kind of terrain or weather the vehicle is best suited for. And OEs have figured out that the more adaptable your system is, the more people it will accommodate. All-wheel-drive systems are typically designed to cope with changing or severe weather conditions. Of course, there are several examples of high-performance vehicles that offer a strong rally or track racing experience that have sophisticated AWD systems as well. Generally speaking, though, AWD is a lighter-duty "just in case" safety system for your vehicle. The more ruggedly designed, heavier, and extra-geared 4WD system with an extra low-range gear is more likely to be described as an "all terrain" choice with quite a bit more "just in case" bandwidth than an AWD. Of course, a bad driver making questionable decisions can get anything stuck or drive into a ditch. And the reverse is true as well. If you put an accomplished and observant driver in just about any vehicle (2WD, AWD, or 4WD), he may be able to drive places a bad driver in a 4WD could only dream of getting to. Below is a list and description of some good examples of both 4WD and AWD vehicles.
Chevrolet Silverado 1500 -- Transfer case offers 2WD, AWD, 4High, 4Low, N, and is the only pickup truck to do so.
Ford Explorer -- Terrain Management System gives the driver 3 choices, but this is more like an all-wheel drive system pretending to be 4-wheel drive.
Ford Raptor -- Multiple range and parameter settings of 2WD, AWD, 4WDH, 4WDL, with a smart computer that will figure out if you want high-speed settings or not.
Honda Ridgeline -- 1 of the few all-wheel-drive pickups with a center locking diff only in 1st gear. Adapted from the Odyssey minivan.
Honda CR-V -- A good example of a small SUV/crossover all-wheel-drive system, previously adapted from a front-drive car platform.
Jeep Grand Cherokee -- Brand-new for Jeep; the company considers all its traction systems to be 4x4 systems, above AWD and 4WD. Selec-Terrain gives both on- and off-pavement settings.
Land Rover LR4 -- The 1st to chart this new direction in terrain determinant 4-wheel-drive settings. Terrain Response works in both high and low range.
Mitsubishi Evo X -- A heavy-duty AWD system is needed to deal with monster horsepower for such a light car. Programmable center diff allows for varying percentages of traction front to rear.
Ram HD Power Wagon -- As old-school as it gets with front and rear electric lockers and electronically controlled anti-roll bar disconnects. Transfer case can be ordered with dash dial switch or floor-mounted lever.
Subaru Forester -- Symmetrical AWD combines overall balance and more than 20 years of trial and error. The result is the best traction system in its class and beyond.
Subaru WRX STI -- Also a car that delivers big power through an all-wheel-drive system originally designed for high-speed rally driving. Also includes a 3-position center diff.
Toyota 4Runner -- The Multi-Terrain Select system combines adjustable shocks, active anti-roll bars, traction control, a center diff lock, and other computer controls to make this about the most complicated and sophisticated 4-wheel drive system around.