Here's a comparative analogy for understanding the similarities and differences between all-wheel drive (AWD) and four-wheel drive (4WD): Miller Lite is to tequila as all-wheel drive is to four-wheel drive. Both are means to an end, but while beer and all-wheel drive are acceptable for everyday use (never at the same time, of course), tequila and four-wheel drive should be reserved for special occasions, when you need something stronger.
How All-Wheel Drive Works
AWD is optimized for on-road use. It has the capability to send the engine's power to all four tires all of the time. It will help 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, all-wheel drive 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 all-wheel-drive systems seamlessly shuffle torque between the front and rear tires as needed and automatically switch to two-wheel drive when appropriate to improve fuel economy.
All-wheel drive 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—so all-wheel drive is a better system than four-wheel drive for the average driver seeking bad-weather security. For this reason, all-wheel drive is what you'll find on most modern SUVs and passenger cars. It's even increasingly offered in pickup trucks, the longtime domain of four-wheel drive.
How Four-Wheel Drive Works
Intended only for use off-road or on extremely slippery surfaces, 4WD is a part-time system, meaning the driver must shift into and out of four-wheel drive 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 four-wheel drive is not designed to be used on paved roads and is reserved for vehicles with serious off-road capability, like the Jeep Wrangler and Gladiator and pickups such as the Ford F-150 Raptor and Chevrolet Colorado Bison.
The difference between all-wheel drive and four-wheel drive is like the difference between beer and tequila.
Restrictions on Four-Wheel Drive Use
In 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 with a slightly different length. To allow the front and rear tires to spin at different speeds, all-wheel-drive 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 all-wheel-drive systems.
A four-wheel-drive 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 four-wheel drive 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 four-wheel drive is regularly used on paved roads. More important, using four-wheel drive on pavement can be dangerous, 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 order a Miller Lite with a shot of tequila, it's possible to buy a vehicle with both an all-wheel-drive and a four-wheel-drive system. Optional in many modern full-size pickup trucks, these systems allow the driver to choose all-wheel drive with an Auto or 4Auto mode and four-wheel drive with the 4High setting. (These vehicles typically also include a rear-wheel-drive 2High mode and low-range 4Low mode.) A Mercedes-Benz G-wagen, for instance, behaves like an all-wheel-drive vehicle until you lock its center differential, at which point it becomes a four-wheel-drive truck.
System Confusion
Auto manufacturers don't make it easy for buyers to decipher the technology underneath their vehicles. Many vehicles with Subaru's Symmetrical All-Wheel Drive actually split torque between the front and rear axle asymmetrically. Audi's Quattro system means three different things if you're talking about an Audi A3, an A4, or an A4 Allroad. Chevrolet brands its all-wheel-drive trucks as four-wheel drive, while four-wheel-drive models are four-by-fours. Ford also insists on using four-wheel drive to describe its many all-wheel-drive SUVs. One of the easiest ways to know if your vehicle has all-wheel drive or four-wheel drive is to crack open the owner's manual. If it warns against activating the system on dry pavement, you have four-wheel drive. Barring that, the system is likely all-wheel drive. And if you're considering a new vehicle, you'll want to know these distinctions before you buy. Nor do all-wheel-drive systems from different manufacturers perform exactly alike, as illustrated by this video.
This content is imported from youTube. You may be able to find the same content in another format, or you may be able to find more information, at their web site.
4MATIC vs QUATTRO - Mercedes W205 C200 vs Audi A6 2.0 TDI - 4x4 test on rollers
Watch on4WD and AWD In the Ice and Snow
After 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 all-wheel drive and four-wheel drive 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 whether on a car with two-wheel drive, all-wheel drive, or four-wheel drive. 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 a proper set of winter tires.
This content is imported from youTube. You may be able to find the same content in another format, or you may be able to find more information, at their web site.
This content is imported from youTube. You may be able to find the same content in another format, or you may be able to find more information, at their web site.