There are many automotive terms that the average motorist may not be familiar with. In most cases, they can confuse one for the other. Others may interchange different terms despite the fact that they shouldn’t. Two of the automotive terms that most people often use in an interchangeable manner are Four-Wheel Drive (4WD) and All-Wheel Drive (AWD). Much of the confusion stems from the fact that AWD or 4WD vehicles come with four wheels. When you say “all” as in all-wheel drive, then you are referring to “all” the four wheels of the vehicle putting power to the ground. This is why most folks use AWD to mean 4WD and vice versa. But these two systems are not one and the same.
The Differential
Before we start talking about the differences between an AWD and a 4WD system, it is important to have a basic idea about the principles at work.
When we drive, the engine delivers power to the axles so that wheels and tires will spin. As they spin, the vehicle moves. If we are driving in a straight line, the power delivered to each wheel is constant. However, when we turn our vehicle, the outside wheels will have to travel farther compared to the inside wheels. Since the outside wheels travel farther, they need to spin faster. The front wheels of your vehicle will also travel farther than those in the rear. As such, the front wheels will need to spin at a much faster rate than those at the back.
There must be a mechanism that will allow the outside wheel to spin faster than the inside wheel. This mechanism should also allow the front wheels to spin faster than the rear wheels. This mechanism is what we know as the differential. As such, you will need a differential to manage the distribution of power or torque along an axle. You will also need another differential to manage torque delivery between front and rear axles.
The differential will always convey torque or power to the path of least resistance. In this case, it’s the wheel that has less traction. For example, if you will drive up a snowy slope, you may find that one tire will be slipping or free-spinning while the other isn’t. No matter how you depress the gas pedal, you will never go anywhere. This is because one of your tires does not have traction. The differential will allow the vehicle to lock both wheels on the same axle so that they will both turn.
The Four Wheel Drive
Whenever we talk about a 4WD system, we always conjure images of off-road vehicles. These are terrain-hugging conquerors that make treacherous roads their favorite playground. You have monster trucks and SUVs that come with humongous tires. They blaze the dirt roads of the backcountry and climb the steepest hills with ease. They are the undeniable kings of the wilderness. They go where no other vehicle will dare go.
A 4WD system will split the torque and distribute it in an even manner to all the four wheels. This means that each wheel will be spinning at the same speed as the other wheels. The engine transfers the power to the transmission. A transfer case will then divide the torque for equal distribution between front and rear axles.
Four Wheel Drive systems lock the speed of both the front and rear axles. If your right front wheel has less traction, it will spin only as fast as the right wheel on the rear axle. This occurs because of the equal distribution of torque to all the wheels.
If you drive a 4WD vehicle on concrete roads, there’s a chance that the vehicle can spin out of control. This is because the differential or transfer case is forcing both axles to spin at the same speed. We already know that during cornering, you need the outside and front wheels to spin at a faster speed than the inside and rear wheels. This can put unnecessary strain on your vehicle’s powertrain. Over time, it can sustain costly damage.
It is for this reason that modern 4WD vehicles are, by default, Two-Wheel Drive (2WD) systems. In a 2WD system, only one axle receives torque from the engine; the other axle is free-spinning. Modern vehicles come with either Front Wheel Drive (FWD) or Rear Wheel Drive (RWD) system.
When on the road, modern 4WD vehicles operate as either a FWD or a RWD vehicle. When the same vehicle leaves the full traction of the pavement, the driver can switch the drive to a 4WD. This locks both axles to spin at the same speed while also doubling traction. We call such vehicles as being a Part-time 4WD. You can activate the 4WD system when you need to. If not, the vehicle will be operating on a 2WD system.
To complicate matters, 4WD systems can also come in two modes, either as Four-High (4H) or Four-Low (4L). The 4H operates more like an All-Wheel Drive system. It allows you to drive at normal speeds in road conditions that require additional traction. You can use the Four-High 4WD system on loose gravel but level roads. It can also work on wet, icy, or snowy roads as well as muddy ones.
The Four-Low 4WD system is your quintessential off-road workhorse. It helps you drive your vehicle in deep sand, mud, and snow. If you are going to cross water or navigate a steep hill, the 4L system can get you to your destination. It’s also perfect for climbing rocks or scaling an uneven slope. Whenever you engage the Four-Low system, it is imperative that you do not go faster than 40 MPH. The 4L 4WD system is best for situations that require maximum power and maximum traction.
The All-Wheel Drive
Some say that the All-Wheel Drive system works like a Part-time 4WD. What it does is that it directs power to the tire that has the least grip. In a conventional 2WD system, the car can only deliver power to either of the wheels on the axle that is part of the system. For example, in a Front Wheel Drive, the differential can only send power to either the right or left tires of the front axle. In an All-Wheel Drive system, it always looks for the path of least resistance.
It is a bit trickier than a 4WD system, though. If the right front wheel is spinning faster because it doesn’t have enough traction, the system cannot send power to this wheel. Doing so will only make the wheel spin a lot faster. What it can do is to send more power to the other wheels so that they match the speed of the problematic wheel. In some AWD systems, the electronic activation of brake vectoring can slow down the slipping wheel. This allows it to match the speed of the other three wheels.
To put it in simple terms, an All-Wheel Drive system will transfer torque from the slipping wheel to the wheel or wheels with traction. AWD systems deliver torque to both axles while also adjusting the amount of power that reaches the wheels. This amount of power depends on whether the wheel is losing or maintaining traction. If the system senses that the wheel is losing traction, then it reduces the amount of power to this wheel. At the same time, it increases the power to the other wheel on the same axle.
You may think that the AWD is a better system than the 4WD in off-road conditions. On paper it seems like it. In the real world, however, AWD systems cannot match the raw power delivery of 4WD systems in terms of low-speed off-roading activities. You cannot expect an AWD to crawl rocks with the same efficiency of a 4WD.
This doesn’t mean there are no advantages of an AWD vehicle. One of the most important advantages of an AWD system is its extensive use of computer data. Each wheel on an AWD vehicle comes with a number of sensors. These monitor traction and wheel speed as well as other data points. The Engine Control Unit analyzes these data and decides which of the four wheels should receive power. They call this technology as torque vectoring. This can improve the vehicle’s overall handling as well as all-weather capability.
Since the monitoring of traction parameters is electronic, there is no need for driver inputs. The system is automatic. It delivers power to the wheel that needs it most. You can drive on slippery roads without having to think whether you are losing traction or not. You can also drive on dirt roads or on snow-covered streets. With the AWD system, you can drive on a variety of road conditions without switching or activating anything else.
The Difference
In a nutshell, an All-Wheel Drive system delivers variable power to the different tires. It’s perfect for various road conditions from the pavement to slippery roads. Meanwhile, a Four-Wheel Drive system delivers equal power to both front and rear axles so that all the wheels spin at the same speed. They are best for off-roaders.
Sources:
- 4WD vs. AWD: What’s the Difference? – Outside Online
- AWD vs. 4WD: What’s the Difference between the Two? – Digital Trends
- What’s the Difference between 4WD and AWD? – Car Sales
