All About Front-, Rear-, Four-Wheel and All-Wheel Drive
The number of trucks, SUVs and cars in which all four wheels get power from the engine has grown remarkably since the 1980s, to the point that almost one-third of all passenger cars and trucks sold in the U.S. in 2013 had either all-wheel-drive (AWD) or four-wheel-drive (4WD) systems.
The numbers suggest that two-wheel drive just isn't enough for many consumers.
Still, most passenger cars sold in the U.S. today use a two-wheel-drive system in which the entire drive package — engine, transmission, differential and the wheels that are driven by the engine — are all in the front of the vehicle.
It is called front-wheel drive (FWD) and has become ubiquitous in cars since the late 1970s.
Trucks, though, as well as many SUVs and some cars, still use rear-wheel-drive (RWD) systems, in which a long driveshaft transmits power from the engine in the front of the vehicle to the driven wheels at the back. A differential is used to let the power from the driveshaft make a 90-degree turn so it can get to the wheels. These are still two-wheel-drive vehicles.
With all those choices, what's right for you?
It all comes down to what you need most from your vehicle in terms of passenger and cargo capabilities, as well as what kinds of terrain and weather conditions you deal with on a regular basis.
A low-slung sports car with rear-wheel drive is not a good choice if you live at the top of a steep hill accessible via a rutted dirt road that usually is buried under ice and snow all winter and slick with mud all spring. Nor is a raised four-wheel-drive utility vehicle with huge knobby tires ideal for the driver whose daily commute via a nicely paved highway is from a suburban house to a high-rise office in a metropolitan downtown area.
Rear Wheel Versus Front Wheel
Compared to rear-wheel drive, front-wheel drive reduces weight and production cost, improves fuel economy and typically delivers better traction. That's because the weight of the engine and transmission is directly over the driven wheels.
Rear-wheel drive offers better initial acceleration than does FWD when a quick start is of the essence. That's because weight is transferred to the rear of the car upon accelerating, thus boosting traction. RWD also permits expert drivers to use various techniques to slide the rear end around corners, which is a skill most useful in racing. Additionally, by keeping part of the drivetrain in the rear, RWD cars usually have weight distribution closer to the optimal 50 percent front/50 percent rear than can be achieved with a FWD system: Equal weight distribution improves a vehicle's overall balance and handling.
In two-wheel-drive trucks, RWD is essential because the back of the truck is so light that putting the entire drive system up front would make an empty pickup nearly impossible to drive. The rear wheels would almost be floating and could easily lose contact with the surface on even moderately bumpy roads. Conversely, adding load in the rear of a RWD truck or SUV that's hauling cargo or a towing a trailer or boat improves traction. Having the driven wheels close to the point where the trailer is connected to the vehicle (via an articulated hitch) also helps with steering while towing.
The development of anti-lock braking and traction control systems has greatly improved two-wheel-drive systems' handling characteristics, however. And for many drivers, a two-wheel-drive system with traction control is all they'll ever need.
But for those who do need more, four-wheel and all-wheel drive offer increased handling capabilities. They're especially helpful on slick, loose or slippery surfaces, because they can direct power to the wheels that have the most traction. They can also help with some towing chores, such as pulling boats up wet, slippery launch ramps.
Good Tires Are Critical
In most cases, a vehicle's tires can be more important than the number of wheels being driven. For example, many sporty AWD cars boast good dry traction but are sold with summer tires that make them horrible to handle in the snow and ice — even with all four wheels working.
Here's another way to think about it: What would perform better in the snow, a front-wheel-drive car with winter tires or an AWD car with all-season tires?
Michelin tested this snow-tires-versus-AWD scenario a few years ago. The front-wheel-drive car with winter tires outperformed the AWD car with all-season tires in nearly every test. The all-wheel-drive vehicle had the edge in acceleration, but when it came time to hit the brakes, its braking distance was significantly longer than the FWD car. Of course, if the AWD vehicle had a full set of winter tires, it would be the hands-down winner, but this test goes to show you the importance of good tires.
Don't Buy a 10-Percent Car
People sometimes buy an AWD or 4WD vehicle for the occasional off-road outing or ski trip, despite the fact that 90 percent of the time they'll be sitting in traffic or using the vehicle on paved roads. These drivers would be better served by buying a two-wheel-drive vehicle for their daily use and renting an AWD car or 4WD truck for their ski trips. This would save them money on the price of the car as well as overall fuel and maintenance costs.
More About All Four
Choosing between AWD and 4WD used to be fairly simple — one was for sporty cars on pavement and one for trucks and SUVs on rugged mountain trails.
"But, really, today there's no clear line between all-wheel and four-wheel drive" from a technical viewpoint, says Dave Herzog, lead engineer on Chrysler's Jeep Grand Cherokee. Most car shoppers — and automakers — still adhere to the distinction of 4WD for off-road and AWD for on-road, he says, but Chrysler, for instance, sells an all-wheel-drive Durango SUV and a 4WD Jeep Cherokee with nearly the same systems.
Four-wheel-drive systems are usually found on vehicles with raised ground clearance, shielded underbodies, tow hooks and big, knobby tires. AWD systems can be found on everything from low-slung sports cars to SUVs of all stripes. That includes SUVs that are mild models designed for soccer moms and pops and wild line-toppers aimed at rock stars and celebutantes. They also can be high-clearance, knobby-tired models for the flannel-shirt-and-hiking-boots crowd.
All-wheel-drive systems come in two styles. There are those with part-time or automatic AWD, in which the vehicle typically operates in front-wheel-drive mode with power delivered to all four wheels only when needed. And there's full-time AWD that delivers power to all the wheels all the time, much like a 4WD system except there's no extreme low range for heavy-duty off-roading.
Some AWD systems also offer what is commonly called torque vectoring, in which sensors direct engine power to the wheels with the most traction, regardless of which end or side they are on. AWD systems typically are used for cars and crossovers and are most efficient on pavement and well-maintained dirt and gravel roads.
Four-wheel-drive systems also come in part-time and full-time versions and typically are used in trucks and SUVs. They are designed primarily for off-road use. Part-time systems operate in two-wheel-drive mode until the driver — or an on-board computer that monitors traction — decides it is time for all four wheels to share the work. Full-time 4WD is just what its name suggests.
The High and Low of ItMost 4WD systems have low and high ranges that can be selected by the driver, usually with an electronic switch. Jeep's venerable Wrangler, however, still uses a floor-mounted, mechanical lever.
Whether electronic or mechanical, the 4WD's low setting provides even greater torque for pulling or climbing in an off-road environment. The setting's low gearing also makes it easier to descend steep slopes on unstable surfaces without using — and burning up — the brakes.
The 4WD's high setting is the default setting and is useful for slippery on-road situations, such as packed snow or ice or loose sand or gravel.
How Tto Pick Your Drive
Despite their higher costs, these four-driven-wheel systems do make sense for a lot of people. They offer greatly improved traction on all kinds of surfaces, and, depending on the system type, can improve towing ability.
Both AWD and 4WD introduce greater complexity to the drive system, adding both weight and cost. The extra weight robs the AWDs and 4WDs of some fuel efficiency: typically 4-9 percent in cars and 5-10 percent in trucks. Cost premiums for these vehicles typically run from about $1,000 up to nearly $4,000.
Because of their additional maintenance needs and lower fuel economy, they also increase total ownership costs compared to two-wheel-drive versions of the same vehicles.
But those costs can pale in comparison to the cost of repairing damage to an ill-equipped two-wheel-drive vehicle that's been operated in rugged terrain or on nasty snow- and ice-slicked roads.
Also helping to offset those extra costs is the fact that vehicles with AWD or 4WD system usually have higher resale or trade-in values than their two-wheel-drive counterparts, especially in areas where inclement winter weather and rough terrain make such systems popular.
More About Two-Wheel Drive
Front-wheel-drive systems are less complex and thus cheaper to make than other drivetrain systems, so economics has played a role in their growing use. But fuel efficiency is the main reason most cars today are FWD models.
When the federal government instituted the Corporate Average Fuel Efficiency (CAFE) program in the mid-1970s, automakers realized the FWD system was a quick way to gain a few miles per gallon.
Most vehicles already had their engines up front, so it was a relatively simple ask to turn the motor sideways and connect it to the front wheels with a couple of short drive axels. That more-compact drivetrain improves fuel economy by reducing the vehicle's overall weight and eliminating the energy losses inherent in delivering power from the front to the rear via a long, heavy driveshaft and differential.
Keeping the motor's weight directly over the driven wheels also improves acceleration and traction on roads made slippery with things such as water, ice, sand, gravel or snow. Finally, using the front wheels to pull the car around corners also helps reduce a common problem in rear-wheel-drive vehicles: loss of traction, or "fishtailing," when entering a curve at too much speed.
The Downside of Front-Wheel Drive
Despite its practical advantages, front-wheel drive has several performance disadvantages. Some exhibit a characteristic called "torque steer," in which unequal power application to one of the front wheels causes the vehicle to pull to one side or another under heavy acceleration. Additionally, a FWD vehicle's turning radius can often be greater than the same vehicle with RWD. That's because cramming all that powertrain and drivetrain equipment under the hood doesn't leave enough room for the front wheels to be turned as sharply as in a rear-wheel-drive application.
FWD systems also tend to wear out faster than the less-complex RWD systems. Further, the lifespan of front tires can be compromised because so much weight is placed on them and they have to handle all of the acceleration and steering forces as well as much of the braking.
Ups and Downs of RWD
Everything about front-wheel drive is reversed for vehicles with rear-wheel drive. Performance goes up: When you punch the accelerator pedal in a rear-wheel-drive vehicle, the weight transfers to the rear end, just where you want it during acceleration. This leaves the front wheels to focus on directing the vehicle. You can also "steer" a rear-wheel-drive car with the gas pedal by applying power and sliding the rear end while in a corner, although this is a tactic best reserved for expert drivers on racetracks.
This doesn't necessarily make rear-wheel drive the better configuration. RWD has its own disadvantages. RWD cars require a driveshaft and to accommodate it, they have that space-robbing interior hump down the middle of the passenger cabin. They also need a rear differential to make the 90-degree turn necessary to transfer engine power from drive shaft to the rear wheels. These components add extra cost and weight to a vehicle while robbing horsepower and making RWD cars generally less fuel-efficient than front-wheel-drive vehicles.
Rear-wheel drive also is more challenging in inclement weather. Without the aid of traction control, a RWD car can more easily end up on somebody's front lawn or stuck in a ditch. Fortunately, traction control is standard on most cars and trucks today.
Putting It All Together
Both 4WD and AWD systems add substantial weight, complexity and cost to a vehicle. They also reduce gas mileage because of increased drag on the drivetrain. As traction control becomes more common on both front- and rear-wheel-drive cars, the advantages to the average motorist of 4WD and AWD have pretty much disappeared, as have those of RWD.
In the end, there is no "best" drivetrain layout. It's just what's best for you from among several different layouts with different characteristics.
A good car dealer can help by working with you to determine the type of system that best meets your needs.
But nothing beats going into the dealership already understanding the differences. That's what will help you decide which type of drive system is right for your circumstances and help you avoid mistakes such as buying a four-wheel-drive vehicle for your daily commute in downtown Los Angeles, or a rear-wheel-drive sport car for exploring the ski areas of Vermont.