Most cars and trucks made today have front wheel drive, but some have all-wheel drive (AWD) or four-wheel drive (4WD). While most consumers have a vague notion that all-wheel drive and four-wheel drive offer better performance in certain conditions, they might not understand the specific mechanics, benefits and drawbacks of front-wheel drive and all-wheel drive.
Front-wheel-drive vehicles apply the torque from the engine to the front wheels. This differs from rear-wheel drive, which distributes the driving power to the back wheels. Modern cars typically are equipped with front-wheel drive to provide better fuel efficiency and traction. By consolidating the transmission, engine and differential into the front of the car, the bulk of the weight is moved to the front of the vehicle, which gives tires a better grip on the road. The rear space can then be used for cargo or passengers, making front-wheel drive more suitable for passenger vehicles.
The differential in a vehicle is designed to allow both wheels to receive an equal amount of power while still allowing them to rotate at different speeds. This is to compensate for the different rotation rates when turning a corner. Because the inside tire covers less distance, it needs fewer rotations to make a turn than the outside tire (think of a rotating column in a marching band; the players near the inside of the line end up taking much smaller steps compared to those on the outside, who may have to run to keep up). By allowing the wheels to rotate at different rates, the differential vastly reduces the amount of wear on the tires.
When you are driving in slippery conditions---on snow, ice or mud---power would normally be distributed to the wheel that has the least amount of traction. Because of this, you would end up spinning one wheel while the other remained stationary. A limited-slip differential ensures that both wheels receive an equal distribution of power in such a situation while still allowing wheels to rotate at different rates while turning corners.
Four-Wheel Drive and All-Wheel Drive
A four-wheel-drive vehicle has power distributed to both the front and rear wheels. This power is equally distributed to the front and back axles, meaning that the differential will divert power to only one front wheel and one back wheel. However, a "locking differential" allows all the wheels to receive the same amount of driving power, which is good for off-road driving.
All-wheel drive differs from four-wheel drive because it allows power to be distributed among the wheels in any configuration. Rather than picking one of the front wheels and one of the back wheels, a vehicle with all-wheel drive can choose whichever wheels have the most traction, giving a driver optimum road grip. For example, an all-wheel-drive system could transfer power to only the front wheels or only the back wheels, whereas a four-wheel-drive car could only choose between the left and right side.
Four-wheel-drive and all-wheel-drive vehicles tend to get slightly less gas mileage than front-wheel-drive vehicles. This is due to the added weight from the multiple-wheel-drive components as well as the extra power required to engage more wheels.
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