Which Wheel Drive Is Best For You?

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Many car buyers are confused by the matter of which wheels actually drive the vehicle, and which is best for their particular needs. There are four main choices: front-wheel drive, rear-wheel drive, four-wheel drive or all-wheel drive. Each has its advantages, and no single layout is best for all situations.

It will help to understand a device known as the differential. If you imagine a pair of wheels on an axle going around a turn---and a turn, for this discussion, is anything that is not absolutely straight---then it becomes apparent that the wheel on the outside of the turn must travel a greater distance than the wheel on the inside. Therefore, it must also rotate faster. To allow this "differential" rate of speed between the wheel on the inside of the turn and the wheel on the outside, there is a device called, appropriately enough, a differential. It is comprised of a set of gears, arranged so that power will be delivered to both wheels, while still allowing them to rotate at different speeds if need be. Another characteristic of a common, "open" differential is that it will deliver power equally to both wheels as long as they both have equal traction. But, if one wheel is easier to turn than the other---as, for example, if one wheel is on ice---the open differential will send the power to that wheel which is easiest to turn, causing that wheel to spin and resulting in the vehicle not having enough traction to keep moving. Thus, there are a variety of systems, under the term of "limited-slip differentials," which "limit" the "slip" that might occur if one wheel is on a slippery surface. These limited-slip differentials are popular with people who must drive in conditions in which they frequently encounter poor traction, such as ice or snow.

Another thing to consider is what engineers call weight transfer. As a vehicle accelerates forward, its weight is transferred to the rear, onto the rear wheels. As it stops, its weight is transferred to the front, onto the front wheels. This is why any vehicle "squats" at the rear when the driver steps on the accelerator, or "dives" at the front when the driver steps on the brakes.

With those things in mind, let's discuss some characteristics of various drive layouts.

Rear-wheel drive
With rear-wheel drive the rear wheels drive the vehicle. For decades, rear-wheel drive was the system of choice, primarily because it is easy to manufacture, simple, inherently robust and reliable. The typical rear-wheel drive layout consisted of an engine in front, connected to a transmission, then the power went through a driveshaft to the rear-axle gears and then the rear wheels. Almost all trucks---except a few light-duty models---have been rear-wheel drive. If you look under the rear of a typical pickup truck you will see the rear axle housing, which has a big lump, or bulge, in the center, roughly the size of a pumpkin or basketball. Inside that bulge will be found the ring-and-pinion gears---which transfer power from the driveshaft to the wheels, provide the appropriate gear ratio and also allow the power to make the right-angle turn from the driveshaft to the wheels---and the gears and assembly that make up the differential. By the way, the ring-and-pinion gears---also known as the final drive---are not the differential, and you can have one without the other.

With rear-wheel drive the rear wheels move the vehicle and the fronts provide steering. Thus, there is somewhat of a division of labor. The advantages of rear-wheel drive are based upon the application. For trucks and heavy-duty vehicles, rear-wheel drive offers rugged durability and, as the load is increased, the traction also increases, because that load pushes down on the driving wheels. For passenger cars, rear-wheel drive offers the capability to deal more effectively with higher engine outputs and higher vehicle weights. Luxury cars, for example, tend to have rear-wheel drive. All true sports cars have rear-wheel drive, and all purpose-built race cars, such as those raced in Formula One Grand Prix racing, or in NASCAR, have rear-wheel drive. For performance applications, a primary advantage of rear-wheel drive is that weight transfer causes traction to be increased with acceleration---the more acceleration, the more weight transferred to the rear wheels and the more available traction, all of which enhances acceleration.

A rear-wheel drive vehicle also has a more equitable balance of the vehicle's weight front-to-rear, so each tire carries a more equal share of the load, which leads to improved cornering response and higher potential cornering limits. Finally, a rear-wheel drive vehicle can offer potentially superior braking performance because, when the brakes are applied, the weight is more equitably allotted among all four wheels.

By the way, in case you're interested, the ideal weight distribution, as it is known, for maximum performance---as with a pure race car---is to be rear-wheel drive and tail-heavy. This helps acceleration, because there is more weight on the rear, driving wheels, and also helps braking because, under the extreme weight transfer that occurs during braking in racing conditions, the vehicle's weight then becomes more evenly distributed among all four wheels, so each of the four wheels can make a maximum contribution to stopping power. A high-powered, pure race car, such as those that race at Indianapolis or in Formula One, will have roughly 35 percent of its total weight on the front wheels and 65 percent on the rear wheels.

Front-wheel drive
With front-wheel drive the engine, transmission, final drive gears and differential are in a single unit and drive the front wheels. Most modern sedans, and particularly those in the medium and lower price ranges, or with more moderate power levels, have front-wheel drive. There are two basic reasons for front-wheel drive: better fuel economy and enhanced space efficiency. By combining the entire powertrain into one unit, the remainder of the vehicle can be made much lighter in weight. It can also be made roomier inside for passengers and cargo, because the engine, transmission and other powertrain components intrude less into that available space. Therefore, all minivans are front-wheel drive; without front-wheel drive they simply could not offer the space efficiency that makes them so popular.

Front-wheel drive has some operational advantages, as well. With all that weight concentrated over the driving wheels, it offers very good traction on slippery surfaces at relatively lower speeds. Thus, front-wheel drive cars perform very well in snowy conditions, or on ice. People who live in places with frequent bad weather understand these benefits of front-wheel drive.

A typical front-wheel drive car will carry about 65 percent of its weight on the front wheels, so a major disadvantage of front-wheel drive is related to weight transfer. Under harder acceleration, weight is transferred off the driving wheels, so traction is reduced. Under harder braking, even more weight is transferred to the front wheels, so they have to do the vast majority of the work. Therefore, on a front-wheel drive car, the front tires and the front brakes wear out much faster than those on the rear. But, on the flip side, a front-wheel drive car, with all that weight on the front, will generally be very good at what engineers call "directional stability," or the tendency to keep going straight. This can tend to make a front-wheel drive car very stable at highway speeds during crosswinds, for example.

It is true that there are several front-wheel drive cars which offer quite exemplary levels of performance but, in absolute terms, and assuming equal levels of power, weight, expense and engineering sophistication, a front-wheel drive car will never match the maximum performance capability of a rear-wheel drive car.

Four-wheel drive
To understand the benefits and limitations of pure "four-wheel drive," it's important to go back to that initial discussion regarding the wheels on a given axle rotating at different speeds as that axle---or the vehicle in question---goes around a turn. And remember, a turn is anything that is not absolutely straight. So consider: Not only do the outside wheels travel a greater distance than the inside wheels, the front wheels travel a greater distance than the rear wheels. If you think not, drive any four-wheeled vehicle through a puddle of water to get the tires wet, then immediately turn the steering into a sharp turn, and observe the four paths of the four tires. You can do this with a child's wagon, or a shopping cart.

In pure, basic four-wheel drive, such as is found in many trucks, sport utility vehicles and military vehicles, the vehicle operates in two-wheel drive until four-wheel drive is engaged by the driver. Typically, this means the vehicle has rear-wheel drive until four-wheel drive is in operation, and until that time the front-wheel drive portion of the system is just along for the ride. This engagement is handled in a device known as a "transfer case." In the most basic systems, when four-wheel drive is engaged the transfer case positively, mechanically locks the front and rear drive systems together and, therefore, the front and rear wheels are mechanically forced to rotate at the same speed. But, remember that bit about front and rear wheels traveling different distances in a turn? If you lock a basic four-wheel drive system into four-wheel drive, you must do it---and this is a very emphatic must---only if the vehicle is on a surface that is slippery enough to allow the front and rear wheels to slip a little bit, such as snow, ice or bad mud. Pavement on a rainy day is not slippery enough, so the basic four-wheel drive system is not for rainy days on paved surfaces. It is, instead, specifically for very low-traction surfaces. If you put a pure, basic four-wheel drive system into four-wheel drive while on a good surface you will---not might; will---either destroy the tires or destroy something in the driveline. The transfer case, for example, or perhaps driveshaft universal joints, will simply have a catastrophic failure.

But four-wheel drive is great for those very low-traction situations, as shown by its capabilities in deep snow or for serious off-roading. And there are many, more modern four-wheel drive systems, which have automatic engagement. One typical method is for the system to monitor individual wheel speeds and thus "sense" when one or more wheels is slipping, and then electrically or mechanically engage the system. With these systems, the driver doesn't have to do anything and, in fact, may not even be aware the system is engaged.

All-wheel drive
To deal with the limitations of four-wheel drive, and to offer all-wheel traction all the time, there are various all-wheel drive systems. These are too numerous in design details to list every variation here but, in very general terms, they are roughly similar to a four-wheel drive layout but with some assembly or device that allows a certain level of slip between the front and rear wheels. In very simple terms, think of a four-wheel drive system, and then add a differential in that transfer case, configured so as to allow differential rates of speed front-to-rear. If it went no further than that, however, it wouldn't work, because it would send the power to the wheel that was easiest to turn (remember that bit about the open differential?) and therefore all four wheels would have to have good traction, all the time. So, to that differential-type device in the transfer case, add a limited-slip capability, and you have the fundamental concept of all-wheel drive. Think of it as a system that can send power to all four wheels, all the time, and still allow the necessary different rates of wheel speeds.

The benefit of all-wheel drive is that it is, typically, "on" all the time, so the driver doesn't have to do anything, doesn't have to move a lever or engage the transfer case, to make it work. If the vehicle is in motion, the system is working. However, for very heavy-duty off-road situations, all-wheel drive doesn't quite match pure four-wheel drive. To deal with this, many modern all-wheel drive systems have the capability to be locked into a pure, four-wheel drive mode, when the going gets really tough.

Which should you pick? For most people, most of the time, front-wheel drive, with its benefits of fuel economy, space efficiency and good traction in slippery conditions, is the best choice. For ultimate performance in decent weather, it's rear-wheel drive. For the serious off-roader or the tough work in really bad weather, it's four-wheel drive. And for the all-time, all-around, all-weather maximum traction, it's all-wheel drive. You have to decide what's best for the driving you do.

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