4WD vs AWD

What’s the difference?

Four wheel drive. Every car and (most) trucks have four wheels. So it must be interchangeable with all-wheel drive. Right? The logic may be sound, but four wheel drive and all-wheel drive vehicles work in very different ways. We all know that four wheel drive vehicles are better suited for rough terrain, but why is that?

What marks the difference between the two is how the differential works. The differential is a collection of gears responsible for distributing power to the wheels. (Fig. 1) Envision driving in your car or truck, and envision going around a corner. To turn that corner, the wheels farthest from the corner (outside wheels) must spin faster than those on the inside of the turn. Without the differential to supply extra power to the outside wheels, your tires would slip and spin, losing traction as you went around the corner.

This is one way the differential distributes power between the wheels of your vehicle. Now that we have a sense of what the differential can do, let’s look at how it functions differently between four wheel drive and all-wheel drive applications.

Four wheel drive applications distribute torque evenly between the wheels. This is especially helpful in traversing rough terrain because vehicles enjoy superior traction when left and right wheels spin at the same rate.

Four wheel drive applications are typically only in four wheel drive mode part of the time. The four wheel drive capability is turned off and on via a lever or button inside the cabin of the vehicle. This allows the vehicle to enjoy superior traction while driving through mud or snow, (Fig. 2) but also allows the vehicle to benefit from the efficiency of two wheel drive in normal driving conditions.

All wheel drive applications split the power to each of the wheels, but it does so in an uneven way. Power is supplied to the wheels with the best traction. In doing so, the tires with the best grip on the road get the most torque. This means all-wheel drive applications perform better in slipper condition than other vehicles. (Fig. 3)

So, in terms of driving, four wheel drive and all-wheel drive are actually nothing alike. When employed, four wheel drive engages all four wheels – as all four wheels are engaged in all wheel drive. However, four wheel drive can be turned off and on as needed and shares power evenly, while all-wheel drive selects wheels to give extra torque to.