Front Wheel Drive - Rear Wheel Drive and Four Wheel / All Wheel Drive Explained

Before going into the functionality of each type of drive configuration and the advantages it may have on your driving experience, we need to firstly understand what a vehicle drive train does. A vehicle drive train is the power transmitting system of your car and is responsible for transmitting the power generated by the engine to the wheels of your car. The components that essentially provide this bridge between the engine and the wheels are the clutch, the torque converter, the transmission, the drive shaft , the U-joint, the CV joints, the differential (also known as the diff) and the axles. The torque (pulling power) that is transmitted from the engine travels through the transmission to the differential in the wheel axle. The torque is then transmitted to the wheels.

The transaxle is the main role-player in this type of system. The transaxle incorporates the transmission and drive axle or differential in a single complete unit. As the name suggests, all components of the front wheel drive are located at the front of the vehicle. The transaxle though, is located at the bottom and more towards the rear of the engine. The wheels are connected directly to the drive shaft by means of a CV joint that facilitates the smooth transmission of power. The functionality of a front wheel drive ensures that the wheels turn left and right (as per the driver’s intended instruction) and move up and down (caused by differences in road surface). Both cornering and engine acceleration forces are therefore handled by the front wheels. Due to the comparatively easier and cheaper means of manufacture, front wheel drive cars are produced quite commonly in places where demand for less expensive production cars is high (e.g. India).

Advantages: Traction is increased as all weight is located in the front and suspended on top of the front wheels. This allows for more grip in unfavorable weather conditions such as snow and ice. Another clear advantage of a front wheel drive car is the availability of more interior space for legroom and design flexibility. Basic mechanics of required power through a comparatively shorter distance (i.e., the engine is much closer to the wheels) would also imply that a front wheel drive is more economical in terms of fuel consumption as well.

The engine power of the car is transmitted from the engine (located in the front) to the back wheels. The transmission is located behind the engine, and the torque of the engine is sent by the driveshaft to a differential unit located at the rear axle. The rear axle then sends this torque straight to the wheels causing them to rotate. Drive shafts in rear wheel drive cars are connected to the rear axle by Universal Joints or U-Joints. U-joints allow for suspension variation in the up and down movement of the car. Rear wheel drive is common amongst high performance racing cars.

Advantages: The weight of the car is more evenly distributed and balanced allowing for better acceleration, better road handling, and better stopping. Torque steer (that is common in front wheel drive cars) caused when the accelerating force of the car effects the steering is non-existent in rear wheel drive cars. This is because the functionality of the two sets of wheels is specific to their design and not entirely isolated to just the front set. Finally, the fact that each wheel set is separate in function and design makes rear wheel drive cars much cheaper and easier to repair.

The main difference between four wheel drive and all wheel drive cars is that an all wheel drive always have all wheels engaged and a four wheel drive can alternate between two wheel drive (for normal driving) and four wheel drive for extreme driving conditions such as off-road driving.

The drive train of a four wheel drive vehicle is in some aspects common to the drive train of a front wheel drive. There is, however, the presence of a driveshaft, a centre differential and a rear differential. These additional components are used together with a transfer case to allow easy shifting between the 2 modes of driving. A transfer case locks the front axle driveshaft to the rear axle driveshaft causing all wheels to spin at the same speed. Ideally, the four wheel drive function should only be engaged when the vehicle is moving on low-traction terrain. Four wheel drive on a normal tar/concrete road surface could lead to jerky turns and extra wear on the tires and drive train. There is also the presence of an additional set of gears in some transfer cases that give the vehicle low range (ie. extra torque and slow output speed) enabling the vehicle to climb slowly and smoothly up steep hills.

The car types that most commonly use this type of drive functionality are trucks and SUVS. The presence of locking hubs may be found on some trucks that utilize the four wheel drive function. Locking hubs are located on the front wheels and can be locked when engaged or disengaged to enable a free-wheeling position. Performance cars with four wheel drive have a computer controlled transfer case to route power between the wheels.

Advantages: The reduction of wheel slip is perhaps the most obvious advantage of four wheel drive vehicles. Improved traction allows for enhanced steering control and maneuverability as well as a more safer, efficient and reliable overall driving experience.

References: www.autotips.plentycar.com, www.technetprofessional.com, www.familycar.com, www.howstuffworks.com