All About Front-, Rear-, Four- and All-Wheel Drive
How AWD, 4WD and Rear-Wheel Drive Work
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 half of all passenger cars and trucks sold in the U.S. are equipped with either all-wheel-drive (AWD) or four-wheel-drive (4WD) systems. Clearly, the numbers suggest that two-wheel drive just isn’t enough for many buyers.
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 driven wheels — are all in the front of the vehicle. It’s called front-wheel drive, and this system has become ubiquitous in cars since the late 1970s. But most trucks, as well as many SUVs and some cars, still use rear-wheel-drive systems, in which a long driveshaft transmits power from the engine in the front of the vehicle to the driven wheels at the back.
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 is buried under ice and snow all winter and slick with mud in the spring. Nor is a raised 4WD vehicle with huge knobby tires ideal for the driver whose daily commute consists of nicely paved city streets. Here’s a look at the basics of each system, plus some pros and cons.
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 availability. But fuel efficiency is the main reason most cars today are front-wheel-drive models.
When the federal government instituted the Corporate Average Fuel Efficiency (CAFE) program in the mid-1970s, automakers realized that the front-wheel-drive system was a quick way to gain a few miles per gallon.
Most vehicles already had their engines up front, so it was relatively simple to connect the powerplant to the front wheels with a couple of short drive axles. The 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 separate differential.
Keeping the motor’s weight directly over the driven wheels also improves acceleration and traction on roads made slippery by 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 too fast.
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 the other under heavy acceleration. Additionally, a front-wheel-drive vehicle’s turning radius can often be greater than the same vehicle with rear-wheel drive. 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.
Front-wheel-drive systems also tend to wear out faster than the less complex rear-wheel-drive systems. And 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.
Almost everything about front-wheel drive is reversed for vehicles with rear-wheel drive. Standing-start performance generally 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 for maximum acceleration on dry surfaces. And, as a result, the front wheels focus on directing the vehicle, eliminating torque steer. You can also “aim” a rear-wheel-drive car with the gas pedal by applying power and sliding the rear end through a corner, although this is a tactic best reserved for expert drivers on racetracks.
Downsides of Rear-Wheel Drive
This performance advantage doesn’t necessarily make rear-wheel drive the better configuration. Rear-wheel-drive 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 the driveshaft to the rear wheels. These components generally make rear-wheel-drive cars less fuel-efficient than front-wheel-drive vehicles.
Rear-wheel drive also is more challenging in inclement weather. Although traction control, standard on most cars and trucks today, can help rear-wheel-drive vehicles maintain control in slippery conditions, these models are still usually at a disadvantage compared to their front-wheel-drive counterparts.
Rear-Wheel vs. Front-Wheel Drive
Front-wheel drive reduces weight, decreases production costs, and improves fuel economy compared to a rear-wheel-drive system. It also improves traction since the weight of the engine and transmission is directly over the driven wheels.
Rear-wheel drive generally offers better initial acceleration than front-wheel drive because weight is transferred to the rear of the car upon accelerating, which boosts traction. Rear-wheel drive 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 back, a rear-wheel-drive car usually has weight distribution closer to the optimal 50 percent front and 50 percent rear, which improves a vehicle’s overall balance and handling.
In two-wheel-drive trucks, rear-wheel drive is essential because the back of the truck is so light that putting the entire drive system up front would make an empty pickup more difficult to drive. The rear wheels would almost be floating and would easily lose contact with the surface on even moderately bumpy roads. Conversely, adding a load to the back of a rear-wheel-drive truck or SUV, such as hauling cargo or towing a trailer, improves traction. Having the driven wheels close to the point where the trailer is connected to the vehicle also helps improve steering while towing.
The development of antilock braking and traction control systems has greatly improved the handling characteristics of two-wheel-drive systems. And for many drivers, a two-wheel-drive vehicle with traction control is all they’ll ever need.
The Four-Wheel World
For those who do need more, 4WD and AWD systems offer increased traction and handling capabilities under many circumstances. 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.
Both AWD and 4WD introduce greater complexity to the drive system, adding both weight and cost. The extra weight generally means a reduction in fuel economy, although this can be mitigated with today’s more efficient engines, including hybrid, electric and diesel options. The inclusion of AWD or 4WD to a vehicle typically raises a vehicle’s price by about $1,500 up to nearly $4,000.
People sometimes buy an AWD or 4WD vehicle for the occasional off-road outing or ski trip, even though 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 or SUV for their ski trips. This tactic would save them money on the price of the car as well as overall fuel and maintenance costs.
Because of their additional maintenance needs and lower fuel economy, 4WD and AWD also increase total ownership costs compared to two-wheel-drive versions of the same vehicles. Despite their higher cost, systems that drive all four wheels 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.
In addition, the extra cost can pale in comparison to the burden of repairing damage to an ill-equipped two-wheel-drive vehicle that’s crashed in rugged terrain or in snowy, icy or slippery conditions. Also helping to offset those extra costs: Vehicles with an AWD or 4WD system usually have higher resale or trade-in values than their two-wheel-drive counterparts. This is especially true in areas where inclement winter weather and rough terrain make such systems popular.
AWD vs. 4WD
So you’ve decided you need power to all four wheels. But which system do you want? Choosing between AWD and 4WD used to be fairly simple &mdash one was for sporty cars on pavement and the other for trucks and SUVs on rugged mountain trails. But today those distinctions have become blurred. Although many car shoppers, and some automakers, still adhere to these traditional labels, manufacturers tend to define the terms to suit their own model lineups and marketing strategies.
Nevertheless, the 4WD label is still generally applied to vehicles intended for heavy-duty or off-road use. AWD systems can be found on everything from sports cars to trucks and SUVs of all stripes — including milder SUVs or crossovers that are designed for soccer moms and pops, as well as line-topping luxury models. AWD also can be found on high-clearance, knobby-tired models for outdoorsy people.
AWD 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 serious 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.
The 4WD systems also come in part-time and full-time versions and are typically found in trucks and SUVs that are designed for both on-road and more rugged off-road use. Part-time systems operate in two-wheel-drive mode until the driver — or an onboard computer that monitors traction — decides that it’s time for all four wheels to share the work. Full-time 4WD is just what its name suggests, with all wheels being driven all the time.
The High and Low of 4WD
Most 4WD systems have low and high ranges that can be selected by the driver, usually with an electronic switch. Some, like Jeep’s venerable Wrangler, however, use a floor-mounted mechanical lever.
Whether electronically or mechanically activated, 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 for descending steep slopes on unstable surfaces without overstressing the brakes. The default 4WD high setting is useful for slippery on-road situations, such as packed snow or ice or loose sand or gravel.
A Word About Tires
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-road traction but are sold with summer tires that make them handle poorly in the snow and ice, even with all four wheels working. So a front-wheel-drive car shod with winter tires will perform better in icy, snowy conditions than an AWD car with summer or even all-season tires.
Michelin tested this scenario a few years ago. The study found that a front-wheel-drive car with winter tires outperformed the AWD car with all-season tires in nearly every test. The AWD vehicle had the edge in acceleration, but when it came time to hit the brakes, its braking distance was significantly longer than the front-wheel-drive vehicle. 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 choosing the proper tires.
Putting It All Together
Both 4WD and AWD systems add complexity and cost to a vehicle as well as reduce fuel efficiency because of increased weight and 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 been reduced somewhat. But sales of these vehicles demonstrate that large numbers of buyers still opt for the extra security of four driven wheels.
In the end, there is no “best” drivetrain layout. The only thing that matters is what’s best for you when choosing from among several different layouts with different characteristics. And remember to assess how your tires will play in the system you choose.
A good car dealer can help sort out the pluses and minuses 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 heavy-duty 4WD vehicle for your daily commute to downtown Los Angeles or a rear-wheel-drive sports car for exploring the hilly ski areas of Vermont.
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