How drivetrains work
Are you looking for a vehicle with four-wheel drive, all-wheel drive or sometime drive? Today's sophisticated drivetrains can put power where you need it on a slippery road, and give drivers and passengers peace of mind and a safer ride. But the car marketers don't always make it easy to know what's best for you.
How traditional drivetrains work
This is a basic "foot bone connected to the ankle bone" view of the drivetrain. We'll keep it simple and leave out some of the components and technology that may vary by vehicle make and age. The traditional basic two-wheel, rear-wheel drivetrain components consist of the engine, which transmits power through the transmission and is connected to the drive shaft.
The drive shaft is connected to the rear differential, which turns the axles that drive the wheels. The differential allows the wheels to turn at different speeds through a turn. This component comes up again later. In two-wheel, front-wheel drive, the front wheels are driven by the drivetrain and also steer the vehicle. There are shafts from the transmission that run to each wheel; the back wheels are just along for the ride.
How four-wheel drive works
With four-wheel drive (4WD), things start to get complicated. A true 4WD system adds a component called a transfer case. This piece splits the power from the drive shaft between the front and rear wheels. In older vehicles, the system split the power 50/50. If you have an older Jeep or off-road vehicle, you know that this system is best for dirt, mud, gravel and other slippery surfaces because it makes it hard to turn the front wheels on asphalt. In some vehicles you had to get out and lock the front hubs to engage the four-wheel drive when going off road, or throw a lever inside. It’s worth noting that 4WD systems also come in two options: full-time and part-time.
4WD: Full-time vs. part-time
In many full-time 4WD systems, power is routed to two wheels most of the time and the 4WD lies dormant, waiting for wheel slip from a wet or icy road to jump in and help out. SUVs and pickups designed for serious off-road travel will have a four-wheel low and high setting. The low setting is usually reserved for slow-speed, off-road travel only.
Some systems may also have locking differentials to prevent one wheel from slipping if the other wheel has traction. That's great if you're driving on dirt or gravel when the wheels slip. On asphalt, the locked differentials make it hard to turn the steering wheel, and driving on a road with a locked differential could cause catastrophic damage. The 4WD high setting is used for on-road driving and faster off-road excursions (e.g., a two-track dirt path).
Some vehicles offer part-time 4WD that must be engaged when needed, either mechanically with a lever or electronically by pressing a button. These systems are lighter and cheaper than full-time 4WD options, but can't be used on dry pavement because of the locked differential. However, they can still deliver some serious dirt-gobbling power in a rear-wheel-drive pickup or SUV.
As handy as they are, 4WD systems have a few downsides. For example, these systems add weight to the vehicle and cut into fuel economy and towing capacity. In addition, there's maintenance involved with changing the oil in the transfer case according to the manufacturer's recommended schedule.
How all-wheel drive works
Many crossover SUVs and cars offer options called all-wheel drive (AWD). One of the main differences when compared to 4WD is the lack of a low range and locking differentials for heavy-duty dirt trekking. However, AWD systems are usually found on front-wheel-drive vehicles and come in part-time and full-time options, too.
AWD: Full-time vs. part-time
Full-time AWD sends power to all four wheels at all times, and proportion is based on input from sensors. Sophisticated systems use torque-vectoring technology to route the engine's power to one wheel at a time if that's where the traction is needed. Full-time AWD systems deliver sure-footed driving in inclement weather.
With part-time AWD, 100 percent of the power is shunted to the front drive wheels until the sensors detect enough wheel spin to switch to AWD mode. While these systems aren't designed for off-road use, part-time AWD systems can be incredibly helpful when driving in bad weather.
AWD systems can bump up the sticker price and add a layer of electronic and mechanical complexity that can make maintenance and repairs more costly. There's no single best system; the right choice depends on your driving habits, the type of vehicle you're looking for, and your purchase and operating budget. Remember, these systems aren’t a catchall for safety. Above all else, driving safely per the weather conditions (e.g., slowing down, braking early) is key. Wheels can lose traction in ice and snow, or even because the whopping horsepower output can overwhelm the grip of the tires.
4WD vs. AWD: A few more things to consider
AWD systems basically share the maintenance costs of a 4WD system – checking and replacing transfer case and differential fluids per the manufacturer's recommendations. Inspection and replacement schedules vary widely among manufacturers, and heavy-duty use will require more frequent fluid changes.
Also, here's another factor to consider: tire replacement. Because each wheel could potentially be the main drive wheel, the tire treads should be close to the same depth. That means if you have to replace one tire, the tire shop may recommend replacing all four tires to ensure there's the same level of grip on all four corners.
Gary Wollenhaupt is an experienced automotive writer, writing for Ford, GM, Saab and others. He’s also a shade tree mechanic tasked with keeping the family fleet on the road.