What is the difference between All Wheel Drive & Four Wheel Drive?

4WD and AWD Explained

If you do a lot of driving, odds are, you've been in some situation where your wheels have started to slip, and maybe you got stuck or your car started to slide. Ice, snow, mud, and other slippery surfaces can prevent your tires from getting good traction with the road. In that circumstance, it doesn't matter how much power your engine can generate. If your tires can't grip the road, they can't push hard enough against it to give a good push in the right direction. To make things worse, if your drive wheels are connected by an open differential (for an explanation of what differentials do, and the different types, see our article about differentials), all the power will go to the slipping wheel, since the power follows the path of least resistance and there's less friction at that wheel. The wheel with traction won't spin at all, which leaves you stuck, or unable to recover from your slide.

Most cars only send power to two wheels, either at the front (in front wheel drive or FWD) or the rear (in rear wheel drive or RWD). To deal with the kind of traction loss situations described above, automakers developed four wheel drive (4WD) and All Wheel Drive (AWD) systems. In these systems power is sent to both the front and rear wheels, which affords more opportunity for one of the wheels to find traction and get you out of your slippery situation.

Every automaker seems to have its own system and they tend to use terms like AWD and 4WD more or less interchangeably. We're going to walk through the basic ideas of the different systems, describe how they work, and detail the benefits and drawbacks of each.

Part Time 4WD

In many ways, part time 4WD is the most straightforward of these systems. This is the type of system used in many pickup trucks and SUVs. Normally, the vehicle operates in RWD. The transmission sends power to the rear axle via the rear driveshaft. However, between the transmission and the rear driveshaft is a transfer case. The transfer case is basically a second gear box. One of the gears connects to a second driveshaft that leads to the front axles. The gear that connects to the front driveshaft is attached, by another gear or by a chain, to a gear that can be meshed to yet another gear that is connected to the rear driveshaft. When that gear is meshed, it spins with the rear driveshaft, which turns the chain (or connecting gear), which turns the gear for the front driveshaft. Power goes to both the front and rear wheels.

The method for selecting 4WD may vary from one vehicle to another. In some, there is a second shift lever that engages the gears. In others, the gears can be meshed by the push of a button. That sends a signal to an electric motor, the transfer case shift motor that shifts the gears. In most trucks the shifting can be down while moving as long as you aren't moving faster than the speed recommended by the manufacturer.

Many part time 4WD systems also have a low gear that can be engaged. The low gear decreases your speed, but increases your torque, greatly. It's useful for very tough off-road maneuvering or very heavy snow, but isn't something you would use much for every day driving.

In these systems, most of the time, the front wheels are allowed to spin freely on their axles. That allows for easier turning in RWD. To get the benefits of 4WD, though, you have to connect the wheels to the front axles. Older trucks tended to have manual locking hubs that connected the axles to the wheel hubs. The driver would have to stop the truck and get out to turn the hubs to lock them.

Many newer trucks have automatic locking hubs that engage due to forward motion from the axles. Some newer trucks have the front wheels permanently connected to the axles, but have one front axle disengaged from the differential. When 4WD comes on, vacuum hoses pull a fork that engages the axle to the differential. If the vacuum lines failed, you would only have 3WD.

When 4WD is engaged, the front and rear wheels are locked together and spin at the same speed. That's the strength and the weakness of this system. In slippery situations, it keeps all the wheels working. It's not very good on dry pavement and can make cornering difficult, though.

When you take a turn, your rear wheels cover a greater distance than your front wheels. Picture the arc made by each set of wheels during a turn. The arc of the rear wheels is wider. That means those wheels cover more distance. So, they need to spin a little bit faster than the front wheels. Since the front and rear wheels are locked together in 4WD, things can bind up while turning which can lead to a sort of herky-jerky, hopping motion. This doesn't occur as much in low traction conditions though, because the wheels are naturally slipping somewhat.

This is why 4WD systems like this one have to be able to be turned on when needed and back off otherwise.

Full Time 4WD

Engineers wanted a way to get the benefits of part time full wheel drive, but without the need for driver input. So, they developed full time 4WD. These systems have more or less the same layout as part time 4WD with the addition of a differential. Many of them still have a low gear that can be engaged. After this second gearbox, power is sent to the front and rear driveshaft through a differential. This could be an open differential or any sort of limited slip differential. The differential allows the driveshafts to rotate at different speeds and makes cornering smoother. By avoid the hopping problem, power can be sent to all four wheels all the time. That also eliminates the need for locking hubs or an axle engage fork. Because the parts are heavy and power is always being sent to all four wheels, full time 4WD vehicles tend to use more gas than RWD or FWD vehicles.

Mechanical AWD

The AWD systems used by Audi and Subaru are a further advancement from full time AWD. These systems do not have a front driveshaft. Instead, the front wheels are driven by a transaxle. Essentially the transmission also contains the front differential. The front differential also connects to a limited slip differential that sends power to the rear wheels. This called the center differential.

Under normal driving conditions, power is evenly distributed to all four wheels. The center differential allows the front and rear wheels to spin at different speeds for proper cornering. If there is significant slippage between the two sets of wheels, though, the limited slip center differential will send power to the other wheels.

Subaru likes to advertise how their AWD system is symmetrical. In 4WD systems, the front driveshaft has to be offset to one side or the other. That means one front axle is longer, so the power distribution might not be even. Also, all the extra parts that have to transfer the power add weight and can lead to power losses through friction. The symmetrical layout also provides for better handling.

The main difference between the Audi and Subaru systems is the type of limited slip differential used. Audi tends to use Torsen differentials, while Subaru tends to use viscous couplings.

AWD cars tend to face the same gas mileage problems as full time 4WD ones.

Electronic AWD

Some cars use electronic systems to engage AWD. These systems are similar to mechanical AWD systems except that instead of a limited slip differential, the rear driveshaft is connected via an electronically controlled clutch pack. The vehicle's computer can detect slippage via the wheel speed sensors. If slipping is detected, the clutch pack can be engaged to send power to the rear wheels. This is type of system used in cars that are available as either FWD or AWD. The computer can control how much power is sent to the rear wheels, but sometimes, it can achieve the same distributions as mechanical AWD systems. Because the AWD may not be engaged all the time, these systems can sometimes use less gas than mechanical AWD ones.

A Word About Snow Tires

Many people buy AWD or 4WD cars because they want better handling in the snow. Indeed, sending power to all of your wheels can help. If you lose traction at one wheel, you can still put down power at the other wheels. That can help you get out of a skid. Snow tires also provide huge benefits for winter driving. Snow tires are designed to provide better traction, which may mean that you don't have a wheel slipping to begin with. Even with AWD or 4WD, snow tires can make a big difference in your winter weather handling.


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