Many different types of vehicles including pickup trucks, SUVs and even some cars boast about including four-wheel, or all-wheel, drive. This system can be an improvement over two-wheel drive systems. However, there are certain cases where each type of drive configuration is best. Understanding the differences between four-wheel and two-wheel drive is essential to knowing what type of vehicle to shop for.
The difference in function between two-wheel drive and four-wheel drive is one of the key defining characteristics for any vehicle. Two-wheel drive cars and trucks feature an engine that is mechanically linked to only two of the wheels, referred to as the drive wheels, via the transmission and the rest of the drivetrain. Four-wheel drive vehicles have the engine attached to all four wheels, sending the power to move the vehicle forward to all of the wheels simultaneously. In the case of four-wheel drive vehicles, the front wheels serve double duty, both providing forward traction for driving and side-to-side traction for steering.
There are several different types of two- and four-wheel drive systems. Two-wheel drive vehicles can be either front-wheel or rear-wheel drive. In the case of front-wheel drive, the engine is attached to the front wheels using spindles instead of an axle, and the rear wheels are used only for stabilization. Rear-wheel drive vehicles have a transaxle that runs the length of the car, transferring power from the engine and transmission to the rear axle to spin the wheels.
Four-wheel drive vehicles fall into two categories: part-time four-wheel drive and full-time four-wheel drive (sometimes called all-wheel drive). All-wheel drive vehicles have the engine permanently attached to all four wheels, while part-time four-wheel drive systems allow the driver to select between two- and four-wheel drive modes using a button or lever.
Each type of driveline has its own set of advantages. Four-wheel drive systems provide better traction on slick surfaces like snow, ice and mud. This makes it preferable for vehicles that drivers intend to use off-road or during winter road conditions. At the same time, two-wheel drive systems are mechanically simpler and more lightweight, allowing vehicles to achieve better fuel economy and reducing the overall price of the drivetrain.
There are significant drawbacks to each type of drive system. Four-wheel drive requires additional components to attach all four wheels to the engine, resulting in added cost, added weight and more opportunities for mechanical failure. This can make four-wheel drive vehicles less fuel efficient and less reliable over time. Two-wheel drive vehicles may not handle as well on snow or ice, making them dangerous in winter conditions. In the case of front-wheel drive vehicles, the phenomenon known as torque steer results in poor handling when the front tires are overtaxed between their two duties of steering and driving.
Safety concerns also weigh into a decision between two-wheel drive and four-wheel drive. Some automakers tend not to produce many vehicles with rear-wheel drive due to public perception about its lack of safety, since there is little weight placed over the drive wheels to produce traction as there is with a front- or four-wheel drive vehicle. The introduction of electronic traction control and anti-lock brakes has made rear-wheel drive vehicles much safer. Even so, four-wheel drive is best for avoiding skidding and loss of control on less-than-ideal roads. Four-wheel drive is also safer for vehicles towing or hauling large weights, since the presence of four drive wheels provides greater stability if the load shifts.
- Photo Credit pickup truck image by Tonda from Fotolia.com
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