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Driving Wheels

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On: Fri, Sep 30, 2011 at 11:16AM | By: Peter C Sessler


Driving Wheels

Over the past twenty years, there has been a proliferation of four-wheel drive vehicles on the road. It would seem that good old two-wheel drive isn't adequate anymore, but that really depends on a lot of factors. Do you need four-wheel drive and what are its advantages over front-wheel and rear-wheel drive?

Each system has its advantages and disadvantages and we'll take a look at all of the systems currently available—they are usually referred to as:

FWD - Front-Wheel Drive

RWD - Rear-Wheel Drive

4WD - Four-Wheel Drive

AWD - All-Wheel Drive

Until the mid-1970s or so, the vast majority of cars sold in this country were RWD. Today, about 70% of the vehicles sold are FWD. FWD was an outgrowth of the 1970s gas crisis. FWD provides better packaging. FWD has fewer parts, better efficiency, more interior room, is less expensive to produce, and because the engine is directly over the driving wheels, better traction on slippery roads.

On the other hand, FWD has several disadvantages, especially as it relates to performance. Because the car's weight shifts to the rear during acceleration, this aspect of a car's performance is reduced. Second, with over 65% of the vehicle's weight over the front tires, handling suffers. Because all that weight is on the front, the front tires are far more stressed than other drive types—they handle all the steering, most of the braking, and all of the acceleration tasks.

FWD is an excellent system for the typical passenger vehicle, but the design layout is contrary for high performance. The fastest high performance cars—as well as race cars—are all RWD.

In a RWD situation, the front wheels do the steering and most of the braking while the rear wheels provide the acceleration. There is much better rear weight transfer during acceleration and you can steer a RWD car through a turn by using the gas pedal. The RWD vehicle has better front/rear weight distribution, too. Everything being equal, a RWD car is more fun to drive than a comparable FWD car.

There are disadvantages. Getting the power to the rear wheels requires a driveshaft and a differential and, of course, a large hump in the interior. The system is less efficient, and without traction control, a RWD car is more difficult to drive in bad weather.

When 4WD was first arrived, it was basically for off-road use. The system distributed power to all four wheels after the vehicle's front hubs were locked and a shift lever engaged the system. The typical 4WD system used today has two settings, a high and low. The low setting is used for pulling while the high setting provides better traction in slippery situations.

AWD operates all the time and has generally been used by Chrysler's minivans and Subaru's vehicles. The system directs most of its power to the front wheels in normal driving (in Chrysler vans it sends 90% to the front and 10% to the rear). It doesn't have the high and low settings of 4WD and its advantages become obvious in slippery conditions. Its best feature is that it is able to (as the commercials say) send power from "the wheels that slip to the wheels that grip".

The disadvantages of 4WD and AWD is an increase in vehicle cost, complexity, weight, and poorer gas mileage. As traction control becomes more and more popular with FWD and RWD vehicles, the advantages of 4WD and AWD lessen.

So which is best? It all depends on your needs. Using a Chevrolet Tahoe for commuting is not really practical, nor is driving a Corvette during winter in Colorado. On the other hand, driving a 4WD or AWD vehicle in Florida is equally impractical. There is no single "best" system—just different drivetrain configurations for differing needs.




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