Four wheel drive vs all wheel drive

The terms "all-wheel drive" and "four-wheel drive" are often used interchangeably, but they really shouldn't be. There are key differences between the two. Perhaps you've never known the differences between all- and four-wheel drive and were afraid to ask, or maybe you didn't realize they mean different things. This video from Engineering Explained sums it up succinctly, just for you.

Basically, an all-wheel drive system can send a variable amount of torque to each axle, and the driver doesn't typically control this. A four-wheel drive system sends a fixed amount of power to each axle, and it can be switched on or off by the driver. So, how do they go about doing their jobs? Generally, an all-wheel drive system uses a center differential to distribute the engine's torque between the two axles, while four-wheel drive relies on a transfer case, which functions like a locked differential.

At this point you might be wondering why there's a need for both of these types of systems, since the aim of both is to distribute power to all four wheels. Four-wheel drive is great for off-roading and other low-traction scenarios, since the system sends a fixed amount of power to each tire. Whichever tire has the most traction is guaranteed to get the power it needs, helping prevent the vehicle from getting stuck.

But four-wheel drive often doesn't work on the road for the exact same reason it works so well off road. In a corner, the wheels of a car spin at different speeds. With four-wheel drive selected, the system tries to get each wheel spinning at similar speeds, which makes on-road cornering difficult. That's why most four-wheel drive systems can be operated in a two-wheel drive mode, which is recommended for road use.

All-wheel drive systems are great for road usage because they can actively send power to the wheel (or wheels) that need it most. Some all-wheel drive systems have a fixed torque split between the front and rear axles, but those don't have the same issues with cornering as four-wheel drive systems since they rely on a differential rather than a transfer case.

Of course, there are lots of different types of all- and four-wheel drive systems that buck these conventions. Engineering Explained uses the new BMW M5 as an example, which has a four-wheel drive system that uses clever hardware to behave more like an all-wheel drive system. And pictured at the top of this post is a Ford F-150 Raptor, which has a system that combines four- and all-wheel drive.

Car terminologies can often get confusing, very confusing. Few other illustrations stand true to this fact other than the concepts of Four-Wheel drive and All-wheel drive.

So let’s try and understand just what is the difference between Four-wheel drive and All-wheel drive in modern cars?

For starters to put it simply, as the name suggests, All-wheel drive powers both the front wheels as well as the rear wheels at the same time. On the other hand, Four-wheel drive refers to the situation wherein a driver-selectable system mechanically engages the drive to all four wheels.

As such, a layman definition would summarise the Four-wheel drive as a ‘part-time system’ for a car while an All-wheel drive would mean a car with a ‘full-time system’ meaning all four wheels of the car are always ‘powered.’

Typically, the concept of Four-wheel drive is most commonly seen in modern-day SUV’s- both family SUVs and the mid-size SUVs. That means it applies more commonly to cars that are designed to use the extra traction of 4WD in off-road situations.

Furthermore, the All-wheel drive system has become an accepted description for a car that drives all of the wheels, all at the same time. The system of a car-in this case- must include some mechanism such as an electronically controlled clutch that allows for a rotational difference between the front and rear axles of a car.

Time for more clarity? Very well then!

In a Four-wheel drive- the power is sent to the rear wheels, which means that the back-wheels of a car drive the vehicle forward whilst the front wheels function or spin freely. In almost all contemporary cars, in order to activate the Four-wheel drive system, the driver has to push a button; or pull a lever to engage the 4WD system. But it must be remembered that the 4WD isn’t meant to be ‘on’ at all times.

Coming over to All-wheel drive, the system is always ‘on’ at all times instead of a driver opting to put it on at a certain juncture during a journey. Typically speaking, if one is driving on a plain highway or a regular highway to speak, then for maximum fuel efficiency, the system of the car will send more power to the rear wheels of a wagon. On the other hand, if the road condition changes, for instance, during times of rains or when navigating through snow, then the system of the car equitably distributes the power across all four wheels, hence the name – All Wheel Drive. This process enables better traction for the vehicle and to avoid wheelspin. This concept is also known as ‘Variable Torque Distribution.’

Now let’s speak of advantages that a Four-wheel drive have over an All-wheel drive?

When voyaging over challenging terrains or extreme surface conditions, think typically of flooding or deep muddy terrains, a Four-wheel drive system can provide more traction and control for the driver.

On the other hand, when speaking of general driving conditions, then All-wheel drive is always the preferred mode of operation.

But that said, it’s important to understand that the same advantage that a Four-wheel drive provides a machine during difficult terrains is exactly what is discounted on normal terrains. Let’s take the instance of a corner.

When you are approaching a corner then in making the turn, all four wheels spin at different speeds but if you were to put the Four-wheel drive system on, then the system would try to get all wheels spinning at exactly the same speed, which would basically make on-road cornering difficult.

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