We describe how all-wheel drive (AWD) and four-wheel drive (4WD, a.k.a. 4x4) work: similarities, differences, and ways they can be confused.
Here's a comparative analogy for understanding the similarities and differences between all-wheel drive and four-wheel drive : an espresso is to Irish coffee as all-wheel drive is to four-wheel drive.
While espresso and all-wheel drive are acceptable for everyday use, Irish coffee and four-wheel drive should be reserved for more special occasions , the times when you really need something with an extra bit of strength. And to clear things up, the term 4x4 is synonymous with 4WD, much as 2x4 is analogous to two-wheel drive .How All-Wheel Drive WorksAWD is optimized for on-road use. It can send the engine's power to all four tires all of the time. Due to this, AWD helps keep your vehicle moving forward better than front-wheel drive or rear-wheel drive on snow-covered or rain-slicked roads. In high-performance vehicles, AWD helps transfer the engine's torque to the ground while cornering at high speed or when launching from rest with the engine's full oomph. Many AWD systems seamlessly shuffle torque between the front and rear tires as needed. These systems are often capable of switching to two-wheel drive in certain driving conditions in order to improve fuel economy. In such instances, these setups will typically revert to sending power to all four wheels at the first sign of slip from one or both of the two driven wheels. AWD can be used on pavement with no ill effect because it is engineered to enable each tire to rotate at its own speed in turns—inboard tires rotate slower in corners. This makes AWD a better system than 4WD for the average driver seeking bad-weather security. For this reason, AWD is what you'll find on most modern SUVs and passenger cars. It's even increasingly offered in pickup trucks, the longtime domain of 4WD. How Four-Wheel Drive WorksFour-wheel drive systems are part-time setups intended only for use off-road or on extremely slippery surfaces. Vehicles with these setups require the driver physically shift into and out of 4WD by turning a knob, pushing a button, or yanking a lever. Doing so locks the front and rear driveshafts together, keeping the front and rear axles turning at the same speed. In sand, mud, and snow, this guarantees that, at a minimum, engine torque is always being sent to at least one front and one rear wheel, without relying on computers to predict or detect wheelslip. And that means there's more power being put to the ground in ultraslippery conditions to get you moving and keep you going. But 4WD is not designed for use on paved roads. It's typically reserved for vehicles with serious off-road chops, such as the Jeep Wrangler and Gladiator SUV and pickup.Restrictions on Four-Wheel Drive UseIn a curve, a vehicle's four wheels all rotate at different speeds. You can see this in the tracks left when a vehicle turns through fresh snow as each wheel traces a unique arc. To allow the front and rear tires to spin at different speeds, AWD systems use either a center differential or a clutch-pack coupling between the front and rear axles. We've covered the complexities and nuances of the common hardware in our overview of AWD systems. A 4WD system locks the front and rear driveshafts together so that they rotate at the same speed and receive equal amounts of torque. Try driving in a tight circle on dry pavement with 4WD engaged and you'll both feel and hear an unnerving shudder as the driveline binds up. Sometimes the front tires will also hop and chirp. This binding stresses the driveline and will cause the tires to wear unevenly if 4WD is regularly used on paved roads. Using 4WD on pavement can be dangerous, too, as the locked driveline can make turning difficult. On low-grip surfaces such as snow, ice, dirt, or mud, binding isn't a problem because the tires can slide freely over the surface to reconcile the locked driveshafts with the need for each tire to trace a unique arc. Best of Both Worlds Just as you can down separate shots of espresso and Baileys simultaneously, it's possible to buy a vehicle with both an AWD and a 4WD system. Such systems allow the driver to choose AWD with an Auto or 4Auto mode and 4WD with a separate 4High setting. A Mercedes G-class, for instance, behaves like an AWD vehicle until you lock its center differential, at which point it becomes a 4WD truck. System ConfusionOne of the easiest ways to know if your vehicle has AWD or 4WD is to crack open the owner's manual. If it warns against activating the system on dry pavement, you have 4WD. Barring that, the system is likely AWD. And if you're considering a new vehicle, you'll want to know these distinctions before you buy. Note that AWD systems from different manufacturers do not perform exactly alike, as illustrated in the video below. 4WD and AWD In the Ice and SnowAfter getting stuck or losing control in slippery conditions, some drivers become convinced that they need a vehicle with AWD or 4WD. That's not necessarily true. Because AWD and 4WD send power to all four wheels, rather than two, they do provide roughly twice as much traction as a two-wheel-drive vehicle for accelerating in slippery conditions. But these systems help only with accelerating the vehicle on snow-covered or icy roads. They do nothing to help you turn or stop better in those conditions. Your tires provide the same traction in slippery turns, while brakes always work on all four wheels to slow the vehicle—no matter if two or all four wheels are used to accelerate. If you want to improve your vehicle's ability to corner and brake as well as accelerate in adverse winter conditions, you'll often be better served by fitting it with a proper set of winter tires.
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