Evo MR in the snow
In the past, consumers only really had two drivetrain choices, RWD (rear wheel drive) and 4WD (4-wheel drive). Passenger cars and trucks were driven by the rear wheels, and a few specialty trucks got the 4-wheel drive option.
Now it seems hard to find a RWD sedan, and the lines between 4WD and AWD (all-wheel drive) can be a bit fuzzy. Front-wheel drive (FWD) has become the standard for many of the cars on the road today, but it has its pros and cons, depending on your driving needs. Let’s take a brief look at how each system works, so you can determine which one will suit your needs.
2010 Toyota CorollaEnlarge Photo
Front-Wheel Drive (FWD)
I’ll start with FWD, since it’s probably the most practical choice for the majority of today’s drivers. If you haven’t guessed, FWD works exactly as is sounds – the motor turns the car’s front wheels to get you going. The motor is mounted in a transverse orientation, which means the axes of rotation of the motor and wheels are parallel. A transaxle is attached to the engine, and acts as a transmission and differential combined. I’ll explain later how this is different from a RWD system. One advantage of the transversally mounted engine and transaxle is its compact size. A FWD system--such as that used in the 2010 Toyota Corolla--can also provide good traction during acceleration in slippery conditions. Eliminating the solid rear axle also makes it easier to give vehicles an independent rear suspension, although it’s not uncommon for modern RWD vehicles to have independent left and right rear axle shafts. For the enthusiast, a FWD car can be less appealing for a few reasons. First of all, it forces the front tires to do a lot more work. They have to handle turning and acceleration. The uneven weight distribution caused by moving the transmission and differential to the front can upset cornering performance too.
2010 Chrysler 300C SRT8Enlarge Photo
Rear-Wheel Drive (RWD)
Although it’s still available on plenty of new vehicles, the RWD system seems to have become less popular, due to the practicality of the FWD arrangement. The engine in a RWD car--like the Chrysler 300, for instance--is mounted longitudinally. In this case, the axis of the motor’s rotation is perpendicular to the rear axle’s axis of rotation. The transmission is usually mounted directly behind the engine, instead of below, or next to it. This is an improvement in weight distribution over the FWD system, but is less efficient, since there are a few more parts. Rear drive cars and trucks generally don’t do well in slippery conditions, since the rear wheels don’t carry as much weight as the front. This will make it easier for the rear wheels to spin, possibly causing the car to fishtail. This type of drivetrain is ideal for high performance cars, since it can generally handle more torque and provide better handling. In some exotic cars, the engine is mounted towards the rear end of the car. This makes steering extremely responsive, and can improve traction in the rear wheels.
2010 Subaru OutbackEnlarge Photo
All-Wheel Drive (AWD)
The introduction of the AWD system seems to solve some of the problems associated with FWD and RWD cars. Vehicles equipped with all-wheel drive transmit power to all four wheels, all the time. Some of these systems have been adapted from FWD arrangements, others began as RWD platforms. For this reason, the specifics of the layout can vary quite a bit between manufacturers. Modern traction control technology has been used to optimize this style of drivetrain by monitoring wheel slip and directing torque to the wheels with the best traction. When you’ve got four to choose from, instead of only two, your chances of getting some grip improve quite a bit. AWD vehicles, such as those from Subaru, do very well in slippery conditions, but tend to sacrifice a small amount of fuel economy, and add weight.