How many driven wheels does a vehicle really need? The proliferation of SUVs on America's highways and byways over the last ten years would suggest that two simply isn't enough. But what exactly are the advantages of four-wheel drive (or all-wheel drive) versus front-wheel drive and rear-wheel drive?
First of all, before we're "driven" to confusion by all of this terminology, let's lay a few ground rules. In most automotive circles (and for the purposes of this discussion) the following acronyms will hold true:
AWD = all-wheel drive
4WD = four-wheel drive
RWD = rear-wheel drive
FWD = front-wheel drive
Each of these drive systems has its own advantages and disadvantages which I'll cover in greater detail.
Since the majority of vehicles sold today are front-wheel drive, we'll start here. As the name suggests, FWD vehicles use only the front wheels to deliver power from the engine to the pavement. While relatively rare on the streets of America during the first half of the 20th century, FWD cars now account for approximately 70 percent of new car offerings. Why the recent switch from RWD to FWD in new cars? Well, since most vehicles carry their engines up front, it is a simpler task to get that engine power to the driven wheels when they are both on the same end of the car. Essentially, you turn the engine sideways and connect a short driveshaft between either side of the engine and the two front wheels. Less distance and fewer parts between the engine and the driven wheels means less horsepower loss through mechanical inefficiency. Additionally, fewer parts means reduced production costs and, ultimately, a lower sticker price. One final advantage of FWD is that it puts the engine weight directly over the driven wheels which can improve traction on slippery or snow-packed roads.
Despite its practical advantages, FWD has several disadvantages when related to vehicle performance. First of all, since vehicle weight shifts to the rear of a car during hard acceleration, FWD cars will always be fighting a losing battle when it comes to straight-line acceleration. Handling performance also suffers on FWD platforms since the option of using "throttle-induced oversteer" or sliding the rear end doesn't exist (unless you get really creative with the emergency brake).
In the end FWD is a very practical way to power a vehicle but its inherent design is contradictory to performance. Look at it this way: Your car came with four wheels, why put the stress of steering, braking, and acceleration on only two of them?
Take everything I just said about FWD cars and reverse it for RWD cars. Performance goes up because now the two rear wheels handle the duties of acceleration, leaving the front wheels to focus solely on directing the vehicle (plus the majority of braking). This means that when you punch it, vehicle weight transfers to the rear end, just where you want it during acceleration. You can also "steer" a RWD car with the gas pedal by applying power and sliding the rear end while in a corner. If you don't think this is an advantage, try driving a Mazda Miata and a Chevy Cavalier convertible through the same set of backroad twisties. Even though they're both top-down, four-cylinder compact cars, the experience will be very different with a clear victory in "fun factor" going to the Miata.
This doesn't necessarily make rear-wheel drive the "better" configuration since it has disadvantages too. As mentioned earlier, RWD cars require a driveshaft (and dreaded interior "hump") and a differential to get power from the engine to the rear wheels. These components add extra cost and weight to a vehicle while robbing horsepower and making RWD cars generally less efficient than FWD vehicles. This layout is also more challenging to drive in inclement weather where, without the aid of traction control, a RWD car can more easily end up on somebody's front lawn or stuck in a ditch.
I personally have an affinity for RWD because its biggest disadvantage is also its biggest advantage; it's less practical and, consequently, more fun.
Both of the previous drivetrain layouts are based on using only half of a vehicle's wheels to deliver power to the ground. But those other wheels are just sitting there, why not use them too? Actually, the concept of using all-wheel drive is not new. A Dutch manufacturer named Spyker had one of the first full-time 4WD vehicles on display at the Paris Motor Show in 1903.
While we're here let's go over what the supposed differences are between four-wheel drive and all-wheel drive. 4WD was basically the original term used to describe all four wheels being driven on early Jeeps and trucks. These systems were not very advanced in that they simply put power to all four wheels, usually after an interior shift lever was moved and/or the vehicle's hubs were manually "locked" by the operator. During this period 4WD was thought of as a purely off-road utility. Today 4WD still refers to vehicles that can opt for two- or four-wheel drive. These vehicles often have a 4WD "low" and "high" setting that can be switched using an interior lever. 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 also uses a locking center differential to avoid unnecessary slippage between the right and left wheels while off-road.
AWD generally operates all the time with no "low" or "high" gearing options. While 4WD is basically an SUV term, associated with off-road use, AWD is a car/wagon/minivan term that means traction improvement for performance or bad weather reasons. Some vehicles, like the Mercury Mountaineer or the Lexus RX 350, are tougher to classify because they are SUVs in appearance but use AWD instead of 4WD. Many of these types of vehicles are being called "crossovers."
The "all" part of all-wheel drive is a bit misleading since the majority of AWD vehicles use the front wheels primarily and only direct power to the rear wheels when a sensor detects front-wheel slip. Other cars, like the Porsche 911 Turbo, use AWD for performance purposes like getting from zero-to-sixty in 3.4 seconds. Even with sufficient horsepower, a Porsche with only two-wheel drive would be traction-limited to high 4- or low 5-second zero-to-sixty times. AWD also helps keep Subaru wagons and Toyota minivans from sliding on wet or snowy roads by redirecting power from "the wheels that slip to the wheels that grip."
So, it seems like 4WD/AWD is the best way to go, right? Well, not exactly. Both of these systems add substantial weight, complexity and cost to a vehicle. They also reduce gas mileage because of increased drag on the drivetrain. When you consider that traction control is becoming more and more common on front- and rear-wheel drive cars, the supposed advantages of 4WD/AWD start to wane.
By now it should be obvious that there is no "best" drivetrain layout; just different designs with different characteristics. Hopefully, after reading this column, you'll have a better idea which one is right for your circumstances and not get suckered into buying an all-wheel drive Subaru for your daily commuter in Phoenix or a BMW M3 for exploring the Yukon.
