The Drive Train

The drive train is the system that connects the engine and the transmission to the wheels. There are three basic categories of drive train systems: front-wheel drive systems, rear-wheel drive systems, and fourwheel drive systems. Each system is named according to the wheels that are driven by the engine. Let's look at each of these drive systems.

FIGURE 6 - In a frontwheel drive system, the two front wheels of the vehicle are driven by the engine. The two rear wheels rotate freely.

The front-wheel drive (FWD) system is the most common drive train used today. In this system, the two front wheels of the vehicle are driven (rotated) by the engine, and the two rear wheels rotate freely (See Figure 6). Front-wheel drive systems are most often used in compact and midsize automobiles. In these cars, the engine and transmission are normally located at the very front of the vehicle, above the front wheels. Because of the engine's location, the majority of the vehicle's weight is positioned over the front wheels. The front-wheel drive system takes advantage of this weight to help provide better traction on slippery roads. Also, because the entire drive system is located at the front of the vehicle, the rest of the car can be designed to have more space inside for passengers and cargo.



FIGURE 7-In a rearwheel drive system, the engine drives only the two rear wheels of the vehicle. The two front wheels rotate freely.

In a rear-wheel drive (RWD) system, the engine drives only the two rear wheels of the vehicle (See Figure 7). The front wheels in this type of vehicle rotate freely. Rear-wheel drive systems are most often used on larger passenger cars and some light trucks. In a larger vehicle, the rear wheels carry much of the machine's weight. Rear-wheel drive systems can be made slightly larger and stronger, so they can withstand heavier loads than front-wheel drive systems. This makes the rear-wheel drive system the better choice for larger vehicles.

The final category of drive trains is the four-wheel drive (4WD) system (also sometimes called an all-wheel drive system). In this type of system, all four wheels of a vehicle are driven by the engine (See Figure 8). Because all four wheels are driven, this type of drive system offers better traction on slippery roads than either an FWD or an RWD system. In fact, four-wheel drive vehicles have such good traction that they're often used in off-road conditions, where no established road is present. Four-wheel drive systems are commonly used on light trucks and sport utility vehicles.

FIGURE 8-In an allwheel drive system, all four of the vehicle's wheels are driven by the engine.