When cars were first invented, they had one-wheel drive. Engine power was delivered to one wheel only, which wasn’t very effective for traction, and new designs were quickly implemented. Nowadays when buying a car, you are faced with all kinds of drivetrain options that are optimized to suit your distinct driving needs. Most people have an idea of what differentiates 2-wheel drive (2WD), but what about 4-wheel drive (4WD) vs all-wheel drive? Although the terms are often used interchangeably, this isn’t actually the case. When it comes to 4WD vs AWD, it pays to know the difference.
A Differential Approach
Normal passenger vehicles have two axles — one in front and one in the rear — with wheels on the ends of each. When making turns, they need to be able to spin at different speeds, so the inside wheel can go slower than the outside wheel. That’s what a differential is for. In 2WD cars, drive power is sent from the transmission evenly to only one axle, which uses the differential to compensate at turns. The other two wheels are merely following along.
4WD vehicles are capable of sending equal torque to both front and rear axles simultaneously through a transfer case. If one wheel slips, the vehicle has more traction on the other wheels, and power still goes to the slipping wheel when it does finally make contact. Normal 4WD locks front to back and usually requires active engagement from the driver. To take 4×4 to the next level, lock side to side, so the wheels on the same axle rotate together. You can do this by engaging a locking differential and putting the vehicle in a low gear. This provides even torque to all wheels and should only be used in extreme off-roading conditions and very slowly.
What is All-Wheel Drive?
By contrast, all-wheel drive vehicles usually operate as 2WD until there is a problem. At which point, the vehicle is capable of sending power to non-primary wheels through a bit of electro-mechanical magic. Sensors detect loss of traction and send notice to the computer, which, in turn, sends power to the wheels that need it. There are a few design approaches that accomplish this. However, AWD is becoming standard on most newer models. AWD is highly responsive and considers the wheels individually, which means it senses and makes changes quickly and accurately.
So, What Do You Need?
Both 4WD and AWD add weight to the vehicle. 4WD especially comes with extra poundage that adds up quickly where mileage is concerned. Also, because of the way 4WD operates, it pulls force from the engine and is generally difficult on steering and suspension components under normal driving conditions. It really should be reserved for vehicles that spend most of their time off road. For normal driving, even in snow, rain and ice, AWD is the smoother, more responsive option.
Remember not to become too bold if you’ve upped your drive train game. It’s easy to be lulled into a false sense of security, but neither 4WD nor AWD help with braking or cornering. Something to keep in mind, as well, is the real difference that good winter tires make. AWD isn’t going to do much for you on ice if you’re driving with summer treads.
Check out all the drivetrain parts
Photo courtesy of Flickr.
Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter. In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.