The terms “four-wheel drive” and “all-wheel drive” and are often used interchangeably. The problem is that they shouldn’t be. There are key differences between the two that today’s car enthusiasts should be cognizant of. Let’s jump right into this with some technical details:
Four-wheel drive systems are typically seen on trucks and large SUVs. The systems divide up the power coming from the vehicle’s engine and sends a fixed amount to each of the four wheels. The device that does the splitting of the power is called the transfer case. Transfer cases are usually mounted in the center of four-wheel drive vehicles and power two small drive shafts that connect to front and rear differentials. Another thing you should know, according to Westborn Chrysler, Dodge, Jeep, Ram of Dearborn, MI, is that the transfer cases in four-wheel drive vehicles usually provide high- and low-speed gearing. Also, one of the key points with four-wheel drive is that it can be turned on and off by the driver.
All-wheel drive systems are typically seen on smaller SUV and sporty model cars. The way they work is that they send a variable amount of torque to each of the four wheels depending on driving conditions. It is important to note that the driver doesn’t typically control this, the vehicles CPU does. The design of an all-wheel drive systems is different than four-wheel drive designs. Instead of a transfer case, all-wheel drive systems split the engine power between all four wheels with a center mounted differential driving controllable clutch packs on each wheel.
How they compare
Four-wheel drive is great for off-roading and other low-traction scenarios because the system sends a fixed amount of power to each tire. Whichever tire has the most traction is guaranteed to get the power it needs, helping prevent the vehicle from getting stuck. But four-wheel drive often doesn’t work on the road for the exact same reason it works so well off-road. In a corner, the wheels of a car spin at different speeds. With four-wheel drive selected, the system tries to get each wheel spinning at similar speeds, which makes on-road cornering somewhat difficult. All-wheel drive systems are great for road usage because they can actively send power to the wheel (or wheels) that need it most. Because the power can be adjusted on the fly, all-wheel drive systems typically corner very well.
While four-wheel drive and all-wheel drive systems are common designs in today’s cars, others exist. Take the Chrysler Corporation’s RAM truck line, for example. RAM trucks have typical part-time four-wheel drive drivetrains centered around two-speed transfer cases that offer three operating ranges: 2HI (two-wheel drive), 4HI (four-wheel drive) and 4LO (low-range reduction four-wheel drive). RAM transfer cases direct engine torque to both the front and rear differentials. In the case of the RAM Heavy Duty, electro-magnetic actuators mechanically lock both front and rear differentials. The Heavy Duty also has front axle disconnect system that allows front drivetrain components to be disconnected from the drive system to reduce parasitic loss and improve overall efficiency. Available in the Renegade, Jeep Active Drive is a fully automatic 4WD system that seamlessly shifts in and out of four-wheel drive at any speed once slip is detected. This system requires no driver intervention and delivers yaw correction during dynamic events. A fully variable wet clutch housed in the rear-drive module utilizes the Jeep brand’s proprietary controls to provide the proper amount of torque for any driving condition, including low-traction surfaces and dynamic off-road driving.